15/529 Instrument and control transformers: applications and 15 selection
Contents Section IV: Non-conventional methods of current measurement 15/569 15.1 Introduction 15/531 15.11 Current Sensors 15/569 15.2 Types of transformers 15/531 15.11.1 Resistive shunts 15/569 15.2.1 Voltage transformers (VTs) 15/531 15.11.2 Hall effect current sensors 15/570 15.2.2 Current transformers (CTs) 15/531 15.11.3 Faraday effect optical sensors 15/570 15.3 Common features of a voltage and a current transformer 15/531 15.11.4 Zero flux current sensors 15/571 15.3.1 Design parameters 15/531 15.11.5 Rogowski coil current transducers 15/571 15.11.6 Digital optical instrument transformers 15/573 Section I: Voltage transformers 15/531 Relevant Standards 15/574 15.4 General specifications and design considerations for voltage transformers (VTs) 15/531 List of formulae used 15/574 15.4.1 Instrument voltage transformers 15/531 Further Reading 15/575 15.4.2 Electromagnetic voltage transformers 15/534 15.4.3 Residual voltage transformers (RVTs) 15/534 15.4.4 Capacitor voltage transformers (CVTs) 15/538 15.4.5 Control transformers 15/540 15.4.6 Summary of specifications of a VT 15/543 15.5 Precautions to be observed while installing a voltage transformer 15/543 Section II: Current transformers 15/544 15.6 Current transformers (CTs) 15/544 15.6.1 General specifications and design considerations for current transformers 15/545 15.6.2 Measuring current transformers 15/549 15.6.3 Interposing current transformers 15/550 15.6.4 Summation current transformers 15/550 15.6.5 Protection current transformers 15/551 15.6.6 Special-purpose current transformers, type ‘PS’ 15/553 15.6.7 Core-balanced current transformers (CBCTs) 15/563 15.7 Short-time rating and effect of momentary peak or dynamic currents 15/563 15.8 Summary of specifications of a CT 15/564 15.9 Precautions to be observed when connecting a CT 15/564 Section III: Testing of instrument and control transformers 15/566 15.10 Test requirements 15/566 15.10.1 Voltage transformers 15/566 15.10.2 Current transformers 15/568
Instrument and control transformers: applications and selection 15/531 15.1 Introduction meet the requirements for a switchgear assembly, except for small variations in the test requirements. For more details refer to the following publications: Transformers are used in an auxiliary circuit, linked to a 1 For voltage transformers power circuit, to indicate, measure and control its voltages IEC 60044-2 (for two-winding transformers such as and currents. They find application in a switchgear or a CVTs) controlgear assembly and a switchyard. It would be 2 For current transformers impracticable to produce indicating and measuring IEC 60044-1 and IEC 60044-6 instruments or protective devices to operate at high to very high voltages or currents. The universal practice, therefore, is to transform the high voltages, say, 415 V and above, and currents above 50 A to reasonably low SECTION I: VOLTAGE values, as discussed later, for these applications. Indicating TRANSFORMERS and measuring instruments and protective devices are designed for these reduced values. The transformers used to transform voltages are known as voltage transformers* 15.4 General specifications and and those to transform currents as current transformers. design considerations for Below we discuss their classifications, basic requirements voltage transformers (VTs) and design parameters. These transformers develop a voltage on the secondary, substantially proportional to the voltage on the primary 15.2 Types of transformers (there being no knee point saturation, as is sometimes required in CTs (Section 15.6.1(viii)). 15.2.1 Voltage transformers (VTs) These may be classified as follows: 15.4.1 Instrument voltage transformers 1 Instrument voltage transformers 1 Rated primary voltage (i) Conventional two-winding, electromagnetic voltage transformers This will generally be the nominal system voltage, except (ii) Residual voltage transformers (RVTs) and for transformers connected between a phase and the ground (iii) Capacitor voltage transformers (CVTs). These or between the neutral and the ground, when the primary may be used for metering or protection, with voltage will be considered as 1/ 3 times the nominal very little difference between the two as noted systems voltage (Vr). later. 2 Control transformers 2 Rated secondary voltage In Europe and Asian nations this is generally 110 or 110/ 15.2.2 Current transformers (CTs) 3 V, (63.5 V) for phase-to-phase or phase-to-ground auxiliary circuits respectively. In the USA and Canada These may be classified as: these voltages are 120 or 120/ 3 V for distribution 1 Instrument current transformers systems and 115 or 115/ 3 V for transmission systems. (i) Measuring current transformers (ii) Protection current transformers and 3 Rated frequency (iii) Special-purpose current transformers, class ‘PS’. This may be 50 or 60 Hz as the system may require. The 2 Interposing current transformers permissible variation may be considered as ±2% for 3 Summation current transformers measuring as well as protection VTs. These limits are 4 Core balance current transformers (CBCTs) based on the recommended variations applicable for a switchgear assembly (IEC 60439-1) or an electric motor (Section 1.6.2). 15.3 Common features of a voltage and a current transformer 4 Insulation systems These transformers may be PVC taped, thermoplastic 15.3.1 Design parameters (service conditions (polypropylene) moulded, fibreglass taped, polyester resin and likely deratings) cast or epoxy resin cast depending upon the system voltage and the surroundings. HV indoor transformers, for instance, These are similar to parameters for a switchgear assembly are generally polyester or epoxy resin cast, and are as discussed in Section 13.4. Since they are directly economical with good dielectric properties. They are resistant associated with the same power system and interrupting to humid, chemically contaminated and hazardous areas. devices as a switchgear assembly, they should generally Outdoor HV transformers, however, may be epoxy resin cast, oil or SF6 insulated and oil or SF6 cooled. Epoxy insulation provides better mechanical and constructional * Potential transformer (PT) is not the appropriate word to identify qualities. They are resistant to humid, contaminated and an instrument voltage transformer. corrosive atmospheres and are suitable for all HV 15/532 Electrical Power Engineering Reference & Applications Handbook systems. They are mechanically strong and can bear 9 Short-time rating shocks and impacts. This is not material in voltage transformers, as neither the voltage measuring instruments nor the protective relays 5 Creepage distances will carry any inrush current during a switching operation For outdoor installations the recommended minimum or a fault. No short-time rating is thus assigned to such creep distances for all types of voltage or current transformers. transformers are given in Table 15.1, according to IEC The electromagnetic unit, however, as used in a residual 60044-1 or IEC 60044-2. VT (Section 15.4.3) or a capacitor VT (Section 15.4.4) should be suitable for carrying the heavy discharge or 6 Tappings inrush currents during a capacitor discharge or switching respectively. Tappings are generally not necessary, as a transformer is designed for a particular voltage system. If and when such a need arises (as in a control transformer (Section 10 Accuracy class 15.4.5)) they can be provided on the primary side of the The accuracy of a VT depends upon its leakage reactance transformer. and the winding resistance. It determines the voltage and the phase errors of a transformer and varies with the 7 Rated output VA on the secondary side. With the use of better core material (for permeability) (Section 1.9) and better heat The standard ratings, at 0.8 p.f. lagging, may be 10, 15, dissipation, one can limit the excitation current and reduce 25, 30, 50, 75, 100, 150, 200, 300, 400 or 500 VA or as the error. A better core lamination can reduce the core the auxiliary circuit may demand. The procedure to size and improve heat dissipation. determine the total VA burden of a circuit is described in Section 15.4.5. Typical values of VA burden for a few • Measuring voltage transformers Standard instruments are given in Table 15.2 from data provided accuracy class may be one of 0.1, 0.2, 0.5, 1 or 3. by the manufacturers. The recommended class of accuracy will depend upon the type of metering and generally as noted in 8 Rated burden Table 15.3. This is the maximum burden the transformer may have • Protection voltage transformers Generally, a to feed at a time. The preferred values will follow series measuring voltage transformer may also be used for R-10 of ISO-3 (IEC 60059) and as noted in Section the purpose of protection. A protection transformer, 13.4.1(4). however, is assigned an accuracy class of 3 or 6, which defines the highest permissible percentage voltage error at any voltage between 5% of the rated voltage up to the voltage obtained by multiplying the rated voltage Table 15.1 Recommended values of minimum creepage by the rated voltage factor of 1.2, 1.5 or 1.9. And distances for a VT or a CT when the secondary has a burden between 25% and 100% of the rated burden at a p.f. of 0.8 lagging. This Pollution level Minimum creepage distance between phase accuracy class is followed by a letter ‘P’ such as 3P and ground mm per kV (r.m.s.) and 6P etc. The voltage and phase displacement errors (phase to phase) should not exceed the values noted in Table 15.6. Light 16 Notes Medium 20 1A low voltage of 5%, at which the transformer is required to Heavy 25 maintain its accuracy limit, is of great significance. A protection Very Heavy 31
Table 15.3 Recommended class of accuracy for VTs for different types of meters Table 15.2 Typical values of VA burdens of instruments Application Class of accuracy Instruments Maximum burden (VA)a 1 Precision testing or as a standard 0.1 VT for the testing of other VTs 1Voltmeter 5 2 Meters of precision grade 0.5 2Voltage coil of a watt-meter or a power 3 Commercial and Industrial meters 1.0 factor meter 5 4 Precision industrial meters (Indicating 0.2 or 0.5 3Voltage coil of a frequency meter 7.5 instruments, recorders and electronic 4Voltage coil of a kWh or a kVAr meter 7.5 integrating meters) 5 Recording voltmeters 5 5 General industrial measurements 1 or 3 6Voltage coils of recording watt-meters and (Indicating instruments and recorders) power factor meters 7.5 6 Purposes where the ratio is of less 3 7Voltage coil of a synchroscope 15 importance aThese VA burdens are for moving iron instruments. For electronic Note meters these values would be of the order of 0.1 to 0.5 VA. To choose a higher class of accuracy than necessary is not desirable. Instrument and control transformers: applications and selection 15/533
transformer is required to operate under a fault condition, during Z1 Z 2¢ which the primary voltage may dip to a value as low as 5% of X the rated voltage. R1 1 I 1 I 2¢ R 2¢ X 2¢ 2 It is possible to have two windings in the secondary circuit of a VT when it is required to perform the functions of both I n measurement and protection.
11 Rated voltage factor V Z This is the multiplying factor which, when applied to the V1 1¢ V2¢ rated primary voltage, will determine the maximum Im¢ I m voltage at which the transformer will comply with the thermal requirements for a specified time as well as with the relevant accuracy requirements. This factor carries a greater significance, particularly on a fault, when healthy phases may experience an over-voltage and the protection V1 – Primary voltage VTs may all the more be required to accurately sense V1¢ – Primary induced emf this and activate the protective circuit. Such a situation V2¢ – Secondary induced emf referred to the primary side may arise on a ground fault on an isolated neutral system (V2 being secondary induced emf not shown) or a high impedance grounded system (Sections 20.6 I n – Excitation or no-load current and 21.7). Table 15.4, following IEC 60044-2, suggests Im¢ – Loss component of current supplying the hysteresis and eddy the recommended voltage factors and their permissible current losses to the voltage transformer core (it is the active component) durations for different grounding conditions. I m – Magnetizing component producing the flux ‘f’ (it is the reactive component) 12 Circuit diagram R1 – Primary circuit resistance To illustrate the important features of a VT, let us analyse R 2¢ – Secondary circuit resistance referred to the primary side its equivalent circuit diagram. Refer to a simple diagram X1 – Primary circuit reactance as in Figure 15.1 which is drawn along similar lines to X 2¢ – Secondary circuit reactance referred to the primary side those for a motor (Section 1.10, Figure 1.15). For ease Z1 – Primary circuit impedance of analysis, the ratio of primary and secondary turns has Z 2¢ – Secondary circuit impedance referred to the primary side been considered as 1:1. Then from the circuit diagram, Z – Load (burden) impedance the following can be derived: Figure 15.1 Equivalent circuit diagram for a voltage transformer
VIRX1111– ( + ) = V 1¢ and
VIRX122– ( + 2 ) = V 2 ¢¢¢ ¢ ¢ KVn ◊ 21**– V Voltage error = ¥ 100% and this is drawn in the form of a phasor diagram V1* (Figure 15.2). The phase displacement between phasors V1 where Kn = rated transformation ratio and V2¢ is the phase displacement error ‘d’ as discussed later. V1 = actual primary voltage (r.m.s.) V2 = actual secondary voltage (r.m.s.) 13 Voltage error or ratio error This is the error in the transformed secondary voltage *Note as generally caused by the excitation current I1, and as Only the r.m.s. values and not the phasor quantities are considered shown in Figure 15.1. It is the variation in the to define the voltage error. The phase error is defined separately. actual transformation ratio from the rated and is expressed Together they form the composite error. Refer to Table 15.5 for by measuring and Table 15.6 for protection VTs.
Table 15.4 Rated voltage factors
Sr. no. Rated voltage factor Rated time Method of system grounding Method of primary connections
1 1.2 Continuous All types of system grounding (i) Between lines or (ii) Between transformer star point and ground 2 1.2 Continuous An effectively grounded system Between line and ground 3 1.5 30 seconds An effectively grounded system Between line and ground 4 1.2 Continuous An ineffectively grounded system Between line and ground 5 1.9 30 seconds An ineffectively grounded system Between line and ground 6 1.2 Continuous (i) An isolated neutral system or Between line and ground (ii) A resonant grounded system 7 1.9 8 hours A resonant grounded system Between line and ground 15/534 Electrical Power Engineering Reference & Applications Handbook
I 2¢ IZ V2 V1¢ 22¢ ◊ ¢ ¢ I 1 ¢ ·R 1 ◊ IR ¢ 22 22¢ ◊ I X Im¢ ¢ d V 1 1 ·X d I 1 Z1 I 2 I1· V1 I ¢ I m I I m
I 1 Im¢ f d = Phasor displacement error
Note The phasor diagram is drawn taking the applied voltage V1 as the reference phasor. It can also be drawn taking the primary induced emf V1¢ as the reference. The logic to the diagram and the subsequent results shall however, remain the same.
Figure 15.2 Phasor diagram for a voltage transformer
14 Phase displacement error, d errors at about 2% of the rated voltage, the limits of This is the difference in phase between the primary and VA burden and p.f. remaining the same. the secondary voltage phasors (d ). The direction of the phasors are so chosen, that the angle is zero for a perfect 15.4.2 Electromagnetic voltage transformers transformation. Refer to the phasor diagram, Figure 15.2, and Table 15.5 for measuring and Table 15.6 for protection These are single-, double- or three-phase wound-type VTs. transformers with windings on both primary and secondary sides (Figures 15.3(a) and (b)). 15 Limits of voltage and phase displacement errors • At rated frequency, these should not exceed the values Application given in Table 15.5 for measuring VTs, at any voltage They are used for both measuring and protection purposes. between 80% and 120% of the rated voltage and a As a measuring VT, they are used to feed a voltmeter, burden of 25–100% of the rated burden at a p.f. 0.8 kW, kWh or a kVAr meter, a power factor, frequency lagging. meter or a synchroscope. As a protection VT they are •For protection VTs these should not exceed the values used to feed a protective circuit, incorporating voltage given in Table 15.6 at any voltage between 5% of the sensing protection relays. To save on cost and mounting rated, up to the voltages obtained by multiplying the space, they may also be wound for one common primary rated voltage by the rated voltage factor as in Table and two secondary windings, one for metering and the 15.4, and a burden between 25% and 100% of the other for protection. For markings, see Section 15.10.1(2) rated load at a p.f. 0.8 lagging. At voltages lower than and Figure 15.35. 5% of the rated, the limits of error may increase disproportionately and become up to twice the specified 15.4.3 Residual voltage transformers (RVTs) When the primary of a three-phase two-winding Table 15.5 Recommended limits of voltage and phase transformer, having its secondary wound for a three- displacement errors, applicable for all types of measuring VTs phase open delta, is connected across an unbalanced supply (only electromagnetic and capacitor VTs). (A residual VT is basically a protection VT) system, a residual voltage across the open delta will appear. This is the principle on which this transformer is based (Figure 15.4(a)). As discussed in Section 21.2.2, and Class of accuracy % voltage (ratio) Phase displacement (d) error a minutes illustrated in Figure 21.7, the phasor sum of all the three ± ± line to ground voltages in a three-phase balanced system 0.1 0.1 5 is zero, i.e. 0.2 0.2 10 0.5 0.5 20 VVVRYB+ + = 0 1.0 1.0 40 When this balance is disturbed, due to either an unbalance 3.0 3.0 Not specified in the loads or due to a ground fault, a residual or zero phase sequence voltage in the neutral circuit will appear. As in IEC-60044-2 When one of the phases in the secondary of a three- a These errors are valid only when the voltage is between 80% and phase transformer is open circuited and a three-phase 120%, burden 25–100% of the rated burden and p.f., 0.8 lagging. supply is applied to its primary windings, there will appear Instrument and control transformers: applications and selection 15/535
Transformers with HV fuse
Figure 15.3(a) Typical indoor epoxy resin cast instrument voltage transformers up to 11 kV (Courtesy: Kappa Electricals)
Table 15.6 Recommended limits of voltage and phase displacement errors, applicable for all types of protection VTs (electromagnetic, capacitor and residual VTs)
Class of accuracy % voltage (ratio) Phase displacement ( ) a d error± ± minutes 3P 3.0 120 6P 6.0 240
As per IEC-60044-2 aThese errors are valid only when the voltage is between 5% to ‘rated voltage factor ¥ 100%’, burden 25–100% of the rated burden, and p.f., 0.8 lagging. At voltages lower than 5%, the limits of error may increase. They become up to twice the specified errors at about 2% of the rated voltage, the limits of VA burden and p.f. remaining the same. (a) 33 kV single-phase outdoor (b) 11 kV three-phase indoor Figure 15.3(b) Typical HV instrument voltage transformers Note (Courtesy: Prayog Electricals) The choice of class 3P or 6P will depend upon the application and the protection scheme of the system. The following may be considered a residual or zero phase sequence voltage across the as a rule of thumb when making this choice. open terminals at the secondary. This represents the residual or the zero phase sequence voltage, whatever (i) Class 3P may exist in the main supply system. This voltage will This class may be selected for protective devices that operate be zero when the main primary system is balanced and on the basis of phase relationship between the voltage and the current phasors, such as in a directional overcurrent protection, healthy. reverse power or directional distance protection. (ii) Class 6P Important parameters This class may be selected for protective devices where their operation does not depend upon the phase relationship between 1 Residual voltage The residual voltage appearing the voltage and the current phasors, such as for an over- across the secondary windings will be three times the voltage, over-current or an under-voltage protection. For zero sequence voltage, if it existed in the primary windings. instance, a residual VT should have this accuracy class. This is due to an open magnetic circuit in the secondary (iii) When a residual VT is employed for capacitor discharges it open delta winding having no return path through the requires no accuracy class. 15/536 Electrical Power Engineering Reference & Applications Handbook third magnetic limb. This phenomenon does not exist phasors in the open delta windings will be as illustrated when three single-phase transformers are used, as each in Figure 15.5(a). The phasor sum of these phasors is transformer core winding will form a closed magnetic zero. Therefore Ve = 0. circuit of its own. In a normal three-limb transformer the resultant flux, Ground fault on one phase on a ground fault, of the two healthy lines limb will System neutral grounded return through the transformer limb of the grounded line, inducing a heavy short-circuit current in that winding. Consider a ground fault on phase R. The voltage across This will induce a voltage which will be reflected in the this phase will become zero and the phasor diagram will corresponding secondary winding, and the voltage across be as illustrated in Figure 15.5(b). The other two phasors the open terminals of the delta will not be a true residual. will remain the same as in a healthy system and add to This situation is overcome by providing a low reluctance give the residual voltage Ve, i.e. return path, suitable for carrying the maximum value of unbalanced flux without saturation. This is achieved by 2 2 VVVVVe = T + T + 2◊∞TTcos 120 the use of a five-limb transformer (Figure 15.4(b)). The two additional outer limb are left unwound. where VT is the phase voltage across the secondary windings 2 Residual voltages under different operating conditions To extend the ease of application of this = 2VV2 – 2 = V device, consider the following circuit conditions to T T T determine the quantum of residual voltages: = 3 ¥ zero phase sequence voltage drop. • Healthy system: System neutral, grounded or The voltage across open delta is thus the same as the fall ungrounded. in the voltage of the faulty phase. It will lead the current • Ground fault on one phase caused by the ground circuit impedance. – System neutral grounded – System neutral isolated System neutral isolated Healthy system When the system neutral is isolated, the voltage across the faulty phase R will be the same as the ground potential In this case, all the three phases would be balanced and and the ground potential will become equal to the phase the residual voltage, V , will be zero. The three voltage e voltage VT as illustrated in Figure 15.5(c). The voltage across the healthy phases will become 3VT , i.e. 3 RBY times more than the normal phase voltage. The phasors VY and VB will thus be 60∞ apart than 120∞ and 180∞ from the primary.
2 2 \ VVVeT=(3)+( 3)T + 23◊◊◊ VVTT3 cos 60 ∞ G
a1 b1 c1 2 2 2 = 3VVVT + 3+T 3T Residual voltage = 3VT a 2 b c 2 2 i.e. three times the healthy phase voltage.
Figure 15.4(a) Connections of a residual voltage transformer Important requirements • Grounding Based on the above, it is essential that Limbs the primary windings of the transformer have a a1 b1 c1 grounded neutral, without which no zero sequence exciting current will flow through the primary windings. Although the open delta will develop some 14253 voltage on an unbalance in the primary, it will only be the third harmonic component, as would be contained by the primary windings’ magnetic flux and not the zero sequence component. • Voltage factor Since this transformer may have to perform under severe fault conditions, it should be suitable for sustaining system switching surges as well a b c as surges developed on a fault. A voltage factor as 2 2 2 high as 1.9 (Table 15.4) is generally prescribed for these transformers. Figure 15.4(b) A five-limb transformer to carry unbalanced flux • Short-time duty When this transformer is required Instrument and control transformers: applications and selection 15/537
R R R R
N N N N
Y B Y Y B Y R B ¢ B R ¢ R Y B ¢ ¢ ¢ N R ¢ Y ¢ B ¢ ¢ Primary V Primary N ¢ VP P VP winding B winding ¢ Y ¢ N¢ N ¢ B ¢ Y ¢ ÷3·VT R≤ V = 0 R R Y B T ÷3·VT ≤ ≤ ≤ ≤ 30 30 Ter tiary ∞ ∞ ∞ VVVVeryb = + + ∞ R Y B V ¢¢ ¢¢ ¢¢ VT ≤ ≤ ≤ T = 0 winding V Ter tiary sin30
=0
sin 30 V V sin 30 T T e T T V T ∞ T winding VT V x V Y V V = V B≤ ≤ V 2 = T e T 2 Ve B ≤ Y≤ Ve = 0 2 2 VT VVVVVe = T + T + 2TT ◊◊ cos 120 ∞
= VT Yٞ ٞ Y ¢≤ B N B N ¢ ¢≤ ¢ Secondary Secondary V N V N winding S ¢ winding S ¢ Rٞ Yٞ B ٞ R ¢≤ Y ¢≤ B ¢≤ R ٞ R ¢≤ Figure 15.5(b) An RVT under ground fault on a 3-f four-wire Figure 15.5(a) An RVT in a healthy system grounded neutral system R R
N N
Y B Y
B
N 3V ÷ p B ¢ ÷3Vp R¢ Y¢ B¢ VP VV = 3
p Primary P yb¢¢ p V V
3 winding 3 120∞
÷ ÷ Y N 3V ¢ N ¢ ÷ p V R ≤ Y ≤ B ≤ e b¢ b≤ T
T
T 60 3·V ∞ Ter tiary V V ÷
3· winding 3·
÷
÷ ÷3·V T a¢ a≤ Ve = 3VT 2 2 2 VVe = = ( 3 VT ) + ( 3 VT ) + 2 ( 3 VT ) cos60 ab¢¢¢ ÷÷ ◊ ÷ ∞ = 3VT N¢ B ٞ
s Secondary s ÷3·Vs V V
3 winding 3
÷ ÷ Vby¢¢¢ ¢¢¢ = 3 ◊ Vs ∞R ٞ Y ٞ B ٞ 120 N Y ٞ ÷3·V s ÷ 3·V s Figure 15.5(c) Ground fault on a 3-f, three-wire delta or 3-f, four-wire ungrounded star system 15/538 Electrical Power Engineering Reference & Applications Handbook
to discharge a charged capacitor bank it should be which is reasonably low compared to the high system capable of withstanding heavy inrush discharge voltage. This helps restrict the phase error, on the one currents (see below for its application). The following hand, and facilitates an economical intermediate wound may be considered when designing a transformer for transformer Tr, on the other, to perform the same duty as discharging purposes: a normal wound voltage transformer. The purpose of line capacitors is thus to step-down the high to very high 1 The size of the capacitor banks, their voltage and system voltages to an economically low value. Through the impedance of the capacitor circuit. the tapping point A is connected a conventional and a 2Rate of discharge of the trapped charge. less expensive wound-type intermediate or auxiliary 3 The temperature of the primary windings after the voltage transformer T , rated for the intermediate voltage discharge. r V1 in association with a reactor L (Figure 15.6(b)). The 4 The magnitude of electrodynamic forces on the use of the reactor is to almost offset the heavy capacitive primary windings, which may be developed by voltage component. If possible, the reactor and the the discharge currents. transformer may be combined in one unit to make it • Applications They may be used to carry out the easier to operate. The secondary of the transformer is following functions: rated for the required standard voltage, say, 110/ 3 (63.5 V), to feed the auxiliary devices and components fitted 1To detect a ground fault or operate a directional in the auxiliary circuit. ground fault relay (Section 21.6.4). The inductive reactance of the combined transformer 2To operate a neutral displacement relay (Figures and the reactor is chosen so that it will balance the 26.4 and 26.9). capacitive reactance of the line capacitors at the rated 3To detect an unbalance in a three-phase normally frequency and thus achieve a near-resonant circuit. Since balanced capacitor bank (Section 26.1.1(8)). a frequency variation may cause a de-tuning of the resonant 4To discharge a charged capacitor bank over a very circuit, tappings are generally provided on the intermediate short period, particularly when a fully charged voltage transformer to facilitate adjustment of the circuit capacitor is interrupted. See also Section 25.7. reactance at different frequencies, to achieve a near- 5To discharge an interrupted HV circuit before a resonant condition even on other frequencies. There is a reclosing. An HV system, say, a transmission line voltage drop across both the capacitor units V and the or a cable when interrupted, develops high transient C reactor VL. Figure 15.7 illustrates a simple equivalent voltages as discussed in Sections 23.5.1 and 20.1. circuit for the CVT of Figure 15.6(b) for more clarity. Unless these transients are damped to a reasonably These voltage drops, being 180∞ out of phase, are low level so that they are not able to endanger the detrimental in influencing and adding to the phase error terminal equipment and devices on an automatic of the intermediate voltage transformer T . At higher reclosing, the equipment and devices installed in r frequencies, the summation of these voltages (+VV ) the system may become damaged due to the CL resulting switching transients. The normal practice may become very high and cause high phase errors, leading to deal with this is to damp the transients through to erratic behaviour of the instruments, devices and an electromagnetic transformer such as this. The components connected on the secondary of the transformer, however, may have to be designed intermediate voltage transformer. It is therefore, imperative for such a duty to sustain the electrostatic stresses that these voltage drops be contained as low as possible, arising from such transients, the discharge time on the one hand, and must offset each other, i.e. (+VVCL ) and impedance of the interrupting circuit up to the 0, on the other, to remain almost ineffective even at transformer. higher frequency variations, in influencing the phase error of the intermediate VT. 15.4.4 Capacitor voltage transformers (CVTs) Frequency variations are usually caused on a fault or a switching operation (Sections 20.1 and 23.5.1) and This type of voltage transformer is normally meant for a also during the changeover of the tapping of the high to an ultra-high voltage system, say, 110 kV and above. intermediate VT or the reactor. When the voltage drops While a conventional wound-type (electromagnetic) voltage VC and VL are not large enough compared to V1, the content transformer will always be the first choice, it may become of phase error is contained. An intermediate voltage of costlier and highly uneconomical at such voltages. almost 12–24 kV is found to be realistic in restricting The size and therefore the cost of a conventionally the voltage drops across C and L, to a reasonably low wound voltage transformer will be almost proportional value compared to V during normal operation. Further, to the system voltage for which it is wound. As a cost it is essential to offset the reactances XC and XL through consideration, therefore, a more economical alternative a variable reactor to achieve a near-resonant circuit when is found in a capacitor voltage transformer (CVT) (Figure the CVT is in service. This makes the whole system 15.6(a)). behave like a normally wound VT in terms of its rating A CVT consists of a capacitor divider unit in which a and class of accuracy for both metering and protection primary capacitor C1 and a secondary capacitor C2 are purposes. The same error limits will apply as for a normal connected in parallel between the line and the ground VT (Tables 15.5 and 15.6). The output for a given accuracy (Figure 15.6(b)). A tapping at point A is provided at an is dependent on the range of frequency variation over intermediate voltage V1, usually around 12 to 24 kV, which the voltage transformer is required to operate. Instrument and control transformers: applications and selection 15/539
Primary voltage terminal EHV line
V Line voltage
V1 Intermediate voltage C1 = V ◊ CC12+
C1 Primary line capacitance
Capacitor C2 Secondary line capacitance divider unit L Variable tuning inductance SA Surge suppressor C 1 Z Load (burden) impedance Intermediate voltage terminal R Damping resistor to prevent ferro-resonance effects A Tr Conventionally wound intermediate VT (electromagnetic unit, EMU)
C2 1S1–1S2 2S1–2S2 Secondary tappings 3S1–3S2 V /÷ 3
Tr
1S1
V1/÷3 1S2 2S1
L 2S2 3S1
Secondary 3S2 voltage circuit
Z SA 110 V 3 Secondary R voltage Figure 15.6(a) Capacitor voltage transformer (CVT) rated voltage Low voltage or ground terminal GG 36–420 kV and above (Courtesy: ABB) Figure 15.6(b) Schematic diagram of a basic capacitor voltage transformer (CVT)
Note by inserting a damping resistance R in the EMU circuit, as illustrated Ferro-resonance: This phenomenon may occur in an isolated in Figure 15.6(b). neutral system employing a CVT, similar to an RVT (Section 20.2.1(2)). The core of the non-linear electromagnetic unit (EMU) may saturate momentarily during a ground fault or even during a Application healthy operation, under certain circuit conditions. For instance, low-frequency transients or a fault on the secondary side may A CVT may be used to carry out the following functions: cause momentary saturation of the magnetic core of the EMU, 1To measure as well as protect a high-voltage system, which may, in turn, resonate with the ground capacitive reactances generally 110 kV and above. To save on cost and Xcg and give rise to sub-harmonic oscillations. These may be detrimental to the insulation of the EMU of the CVT as well as mounting space, the electromagnetic unit may be the terminal instruments and devices connected to it. These wound for two secondary windings, one for metering oscillations must be damped as far as possible. This can be achieved and the other for protection. 15/540 Electrical Power Engineering Reference & Applications Handbook
C = C + C 1 2 L R1 R 2¢
VC VL I n
Z
V 1/÷ 3
R l m¢ l m
During resonance, when:
XC =XL the circuit would behave like a normal transformer VC :Voltage drop across the line capacitors VL :Voltage drop across the inductance R1 :Primary resistance representing losses across ‘C ’ and ‘L’ and the intermediate voltage transformer (EMU)
R 2¢ : Secondary resistance of the intermediate VT referred to the primary side. Figure 15.9 A typical outdoor type oil-filled 11 kV control : Loss component Im¢ transformer Im :Magnetizing component Z : Load (burden) impedance R : Damping resistor to prevent ferro-resonance effects These transformers do not require a high accuracy and can be specified by the following parameters: Figure 15.7 Equivalent circuit diagram of a CVT 1 Rated primary voltage The normal practice for an HV system is to provide a separate LV feeder for the 2To feed the synchronizing equipment. auxiliary supplies. The primary voltage will be the 3 As a coupling unit for carrier signals (Section normal system voltage, V , when the transformer is 23.5.2(D) and Figure 23.9(b)). r connected line to line or Vr /3 when connected line 4To damp the transient voltages on the primary side. to neutral. For markings refer to Section 15.10.1 and Figure 15.35. 2 Rated secondary voltage This is 24, 48, 110, 220, 230, 240 or 250 volts, or according to the practice of 15.4.5 Control transformers a country. Tappings, if required, can be provided on the primary side. Refer to Figures 15.8 and 15.9. These transformers are 3 Rated burden This is the maximum load the quite different from a measuring or a protection transformer may have to feed at a time. The preferred transformer, particularly in terms of accuracy and short- ratings will follow series R-10 of ISO-3 (Section time VA ratings besides their application. They are installed 13.4.1(4)). to feed power to the control or the auxiliary devices/ 4 Short-time VA burden This accounts for the components of a switchgear or a controlgear assembly maximum switching inrush VA burden of the various not supposed to be connected directly to the main supply. auxiliary devices connected in the switching circuit
Figure 15.8 Typical single-phase and three-phase control transformers (Courtesy: Logic Controls) Instrument and control transformers: applications and selection 15/541
such as contactors, timers and indicating lights. Unless transformer may feed more auxiliary components and specified, the short-time VA burden of the transformer devices consuming power compared to an instrument will be a minimum of eight times its rating at 0.5 p.f. VT, the VA rating of such transformers is generally higher lagging. It can be expressed in terms of VA versus than of an instrument, metering or protection VT. cos f and drawn in the form of an inrush curve, for Algebraic summation will lead to a higher VA easy selection of a transformer rating (Figure 15.10). requirement than necessary. The transformer should not 5 Voltage regulation In view of heavy currents during be too small or too large to achieve better regulation in the switching of an auxiliary circuit, the reactance addition to cost. From Figure 15.11 the following may and the resistance drops of these transformers should be derived: be designed to be low to ensure a high degree of regulation during a switching operation. Regulation VAT = W + VAr
of up to 6% for control transformers rated for 1.0 22 kVA and above and up to 10% for smaller ratings is or VAT = W+ VAr considered ideal (NEMA Standard suggests these 22 values as 5%). = ((VA cos ff ) + ( VA sin ) ) For brevity, only the more relevant aspects are dis- cussed here. For more details, refer to IEC 60044-2 where for instrument voltage transformers and IEC 60076-3/ VAT = Total VA burden IS 12021 for control transformers. VA = VA burden of individual component
Application W = W1 + W2 + …
These may be used to feed the solenoid or the motor of and VA r = VAr1 + VAr2 + … an interrupting device (such as an electrically operated W1, W2, VAr1 and VAr2 are the active and reactive breaker), indicating lights and annunciation circuits, components respectively of the VA burden of a device at auxiliary contactors or relays, electrical or electronic a p.f. f1 and f2. timers, hooters or buzzers, and all such auxiliary The following may be ascertained when selecting the components and devices mounted on a controlgear or a rating of a control transformer: switchgear assembly requiring a specified control voltage. • Maximum hold-on (continuous) VA burden and the Procedure to determine the VA rating of a control corresponding p.f. of all the devices likely to be in circuit service at a time. • Pick-up VA or short-time VA: An electromagnetic The total VA burden of a control or an auxiliary circuit device such as a contactor or a timer carries a high is the phasor sum of the VA burdens of each individual inrush current, also known as ‘sealed amperes’, during component and device connected in the circuit, and a switching operation and it is associated with a high consuming power. It is advisable to add the VA burdens momentary pick-up VA burden on the circuit and the vectorially rather than algebraically. Since a control feeding control transformer. The effect of the maximum momentary pick-up VA burden and the corresponding inflow p.f. of all the components likely to be switched 1600 at a time must be calculated. • Maximum lead burden of the connecting wires under 1400 the above conditions. The control transformer to be selected may have a 1200 rating nearest to the maximum hold-on VA burden so calculated and must be suitable to feed the required
ansformer 1000 inrush current at the p.f. so calculated without affecting its regulation. So long as these two points fall below the inrush curve of the control transformer, its regulation 800 will be maintained within the prescribed limits. Figure
600 W = VA cos f
f 400
% VA rating of the control tr rating VA % 200
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Cos (of control circuit) VA f T VAr = VA sin f
Figure 15.10 Inrush characteristics of a control transformer Figure 15.11 Phasor representation of a load (VA burden). 15/542 Electrical Power Engineering Reference & Applications Handbook
15.10 illustrates this requirement and Example 15.1 The following data have been assumed: demonstrates the procedure to determine the required System: 415 V, three-phase, four wire. VA of a control transformer. Control voltage: 110 V a.c. 2 Example 15.1 Control wire: 2.5 mm (resistance of wire = 7.6 W/1000 m, as Consider the control scheme of an auto-control capacitor in Table 13.15). panel as shown in Figure 23.37. The scheme shows the control voltage as being tapped from the main bus. But for Approximate length of wire for each feeder up to the power our purpose, we have considered it through a control factor correction relay (PFCR): 35 m. transformer 415/110 V.
(1) A study of the control scheme
Component Total quantity nos Maximum hold-on occurs when Maximum inrush occurs when five steps of all the six steps of the PFCR the PFCR are ON and the sixth is are ON switched ON
Hold on Inrush
Main contactor 125 A 6 6 5 1 Auxiliary contactor 6 A 2 1 (auto or manual) 1 – On indicating light 8 6 (auto or manual) 5 1 PFCR 1 1 (6 steps) 5 1
(2) Approximate VA burden and cos f for each component, as available from the manufacturers’ catalogues
Component VA cos f W = VA cos f VAr = VA sin f 125 A contactor Hold-on 65 0.31 20.15 61.79 Inrush 900 0.42 378 817 6A contactor Hold-on 15 0.33 4.95 14.16 Inrush 115 0.60 69 92 Indicating light Hold-on 7 1 7 – Inrush 7 1 7 – PFCR (each step)a Hold-on 5 1 5 – aPFCRs are available in both static and electromagnetic versions. Their VA levels therefore vary significantly due to inbuilt switching relays, LEDs (light emitting diodes) and p.f. meter etc. For illustration we have considered an average VA burden of 5 VA at unity p.f. for each step. For static relays, this may be too low
(3) Computing the maximum hold-on (steady state) and inrush burden values and their cos f
Maximum hold-on values Maximum pick-up (inrush) values Hold-on for five steps already ON Inrush for the sixth step Component Qty Total Total Qty Total Total Qty Total Total WVArWVAr W VAr
Main contactor 6 6 ¥ 20.15 6 ¥ 61.79 5 5 ¥ 20.15 5 ¥ 61.79 1 378 817 = 120.90 = 370.74 = 100.75 = 308.95 Auxiliary contactor 1 4.95 14.16 1 4.95 14.16 – – – Indicating light 6 6 ¥ 7 = 42 – 5 5 ¥ 7 = 35 – 1 7 – PFCR (steps) 6 6 ¥ 5 = 30 – 5 5 ¥ 5 = 25 – 1 5 –
Total 197.85 384.90 165.70 (a) 323.11(b) 390(c) 817(d)
22 \ VA = 197.85 + 384.90 \ Total inrush, W ( a + c) = 555.7 and VAr (b + d)= 1140.11 22 433 without considering the \ VA = 555.7 + 1140.11 burden for wire leads 1268 without considering the burden for wire leads Instrument and control transformers: applications and selection 15/543
Control circuit current Control circuit current Ic = 433/110 = 3.94A 2 Ic = 1268/110 = 11.53A and lead burden = IRc ◊ 2 where R is the resistance of the connecting wires at the and lead burden = 11.53 ¥ 2.035 operating temperature (90 C, as in Table 14.5) ∞ = 270.53 W 7.6 –3 = 6¥¥ 35 [1+ 3.93 ¥ 10 (90–20)] ºW 1000 \ Total maximum inrush burden (for details refer to Table 14.4) W = 555.7 + 270.53 = 2.035 W 2 = 826.23 \Lead burden = 3.94 ¥ 2.035 = 31.59 W and VAr = 1140.11 \ Total maximum steady-state hold-on burden Maximum short-time 22 W = 197.85 + 31.59 = 229.44 \ VA = 826.23 + 1140.11 And VAr = 384.9 = 1408.0
\Maximum VA = 229.4422 + 384.9 826.23 at an inrush (short-time) cos f = = 448.1 1408.0 = 0.587 at a steady-state cos = 229.44 f 448.10 = 0.51
Rating of control transformer 1000
Select a continuous rating = 500 VA 900 Inrush characteristic of a 500 VA control transformer at a cos f = 0.51 800 and short-time rating = 1500 VA 700 at a cos = 0.587 f 600 The actual values as worked out above must fall below the inrush curve of the selected control transformer of 500 500 VA, as illustrated in Figure 15.12. % VA rating % VA 400 >1500 VA 15.4.6 Summary of specifications of a VT 300 This point should In Table 15.7 we list the data that a user must provide to lie at more than 200 1500 VA a manufacturer to design a VT for a particular application. Some of the data chosen are arbitrary to define the 100 specifications. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.587 15.5 Precautions to be observed cos f (of control circuit)
while installing a voltage Figure 15.12 Checking the suitability of the 500 VA control transformer transformer for the required duty for Example 15.1
1 Since a VT forms an inductive circuit, it generates heavy switching current surges which should be taken it may cause local discharge and heating up of the into account when deciding on protective fuses. A inter-turns, leading to dangerous fault currents and fuse with an appropriately high rating should be chosen ionization of oil in an oil-filled VT. It is advisable to avoid a blow-up during switching. to provide a Bucholtz relay to protect oil-filled VTs 2 As a result of the generally high rating of protective by detecting the presence of gas in the event of a fuses they provide no adequate protection against an fault. inter-turn fault. For critical installations, and for HV 4For lower voltages (<33 kV), any fault on the VT will VTs particularly, a separate protection may be provided be detected by the protective devices installed in the for inter-turn faults. main circuit. 3When an HV-VT develops an inter-turn fault on the 5Temperature detectors may also be provided in the HV side, there is no appreciable rise in the primary windings of large VTs as are provided in a motor circuit current and which may not be detected. But, (Section 12.8). 15/544 Electrical Power Engineering Reference & Applications Handbook
Table 15.7 Summary of specifications of VTs
Sr. no. Specifications Measuring VTs Protection VTs Control transformers
1 System voltage As in Table 13.1 (1/ 3 V for line to neutral transformers) 2 Insulation level (peak) Generally as in IEC 60044-2 or Tables 13.2, and 14.3 for series I and Tables 14.1 and 14.2 for series II voltage systems 3 Class of insulation say, E B E 4 Frequency 50 or 60 Hz 50 or 60 Hz 50 or 60 Hz
5 Nominal voltage ratio e.g. 6.6 kV/110 V for two phase or three phase transformers and 1 times this for line to neutral transformers 3 6 Output (VA) say, 500 500 1500 7 Short-time output (VA burden) – – Say, 8 times the rated VA at and corresponding p.f. 0.2 p.f. 8 Class of accuracy 0.1, 0.2, 0.5, 1 or 3 3P or 6P Not applicable 9 Grounding system Whether an isolated neutral system, an effectively grounded system or a non-effectively grounded (Section 21.7) neutral system 10 The rated voltage factor and Not applicable Depends upon the system Depends upon the system fault the corresponding rated time grounding. Refer to Table 15.4 conditions and generally as 1.9 Table 15.4
11 Service conditions ∑ Indoors or outdoors ∑ Indoors or outdoors ∑ Indoors or outdoors ∑ Whether the system is ∑ Whether the system is ∑ Whether the system is electrically exposed electrically exposed electrically exposed ∑ Ambient temperature ∑ Ambient temperature ∑ Ambient temperature ∑ Altitude, if above 1000 m ∑ Altitude, if above 1000 m ∑ Altitude, if above 1000 m ∑ Humidity ∑ Humidity ∑ Humidity ∑ Any other important features ∑ Any other important features ∑ Any other important features 12 Marking of VTs (a) 6.6 kV/110 V, 500 VA, 6.6 kV/110 V, 500 VA, 6.6 kV/48 V, 1500 VA and class 1a class 3P short-time VA: eight times the rated VA at 0.2 p.f. (b) System voltage and insulation level, class of insulation and frequency etc. for all types of VTs aWherever two separated secondary windings are provided, say, one for measuring and the other for protection, the markings will indicate all such details as are marked against (a) for each secondary winding. For more details and voltages higher than 66 kV, refer to IEC 60044-2.
SECTION II: CURRENT over-current release (Figure 12.15) where the use of a ring-type CT may appear crude and the connections TRANSFORMERS cumbersome, a bar primary CT may be used, as shown in Figure 15.14. HV CTs, as a matter of necessity and to 15.6 Current transformers (CTs) maintain correct clearances and dielectric strength between the primary current-carrying conductor and the secondary These may be one of the following types: windings, are made in the form of a bar primary or wound primary only, depending upon the primary current rating • Ring (Figures 15.13(a)–(d)) (Figures 15.15 and 15.16). In a wound primary, the primary • Bar primary (Figures 15.14(a) and (b) and 15.15(a) is also wound the same way as the secondary. and (b)) •Wound primary (Figures 15.15(a) and 15.16) Types of insulation In ring-type CTs the primary current-carrying conductor An LV CT may be insulated in the following ways, is passed through the ring and the ring forms the secondary depending upon their location and application. winding (Figure 15.13). Generally, all LV CTs are produced thus, except for very small ratings of the primary • Tape insulated (Figure 15.13(c)) For normal current, say, up to 50 A, when it becomes imperative to application and generally clean atmospheric conditions. design them in the form of a wound primary due to the • Epoxy resin cast (Figures 15.13(a) and 15.14) To design constraints discussed in Section 15.6.5(iv). provide greater mechanical strength and a better For special applications, such as for a motor protection insulation system. They are more suitable for humid, Instrument and control transformers: applications and selection 15/545
(a) LV epoxy resin cast
CBCT Relay
(b) LV fibreglass taped (c) taped LV PVC (d) Ground leakage relay with CBCT
Figure 15.13 Ring type CTs for measuring or protection (Courtesy: Prayog Electricals)
contaminated and corrosive atmospheres and for all A 1 A secondary is not recommended in higher ratios HV systems. They are mechanically strong and can due to increased induced voltage on the secondary side bear shocks and impacts. during an accidental open circuit on load. It may damage the inter-turn insulation or cause a flashover, besides Note All HV CTs are normally manufactured with epoxy resin cast. being dangerous to nearby components or a human body, if in contact. This we have illustrated in Example 15.8, •Fibreglass tape (Figure 15.13(b)). To make it more under Section 15.9. compact. • Polypropylene (ii) Rated output 15.6.1 General specifications and design The standard VA values may be one of 2.5, 5, 7.5, 10, 15 considerations for current transformers and 30 generally, depending upon the application, although a value beyond 30 VA is also acceptable. The rated frequency, insulation systems and the requirement of creepage distances will generally remain (iii) Rated burden the same as for a voltage transformer (Section 15.4.1). For the remaining parameters, the following may be noted. This is the value of the impedance of the secondary circuit (the impedance of all the devices connected to (i) Rated secondary current it), expressed in ohms and power-factor or volt-amperes, at the rated secondary current. The CTs will be selected This will be 1, 2 or 5 A, 1 and 5 A being more common. nearest to the computed total VA burden in the circuit. All measuring and protection devices are also A CT with a higher VA burden than connected will manufactured for these ratings only as standard practice. have a slightly higher error besides size. A slightly less VA 15/546 Electrical Power Engineering Reference & Applications Handbook
(a) Single-phase HV CT 11 kV CT 33 kV CT
Figure 15.15(a) Typical outdoor-type oil-filled bar primary or wound primary HV CTs (Courtesy: Kappa Electricals)
(b) Three-phase LV CT
Figure 15.14 Typical indoor-type bar primary epoxy resin cast CTs (Courtesy: Kappa Electricals) rating than the connected may normally be permissible, subject to confirmation by the manufacturer. Typical values of VA burdens at the rated current for the devices that a measuring CT may have to usually feed are as follows:
Instruments and measuring devices* Moving iron ammeters 1.5 to 5 VA Recording ammeters 2 to 10 VA Current coils of watt-meters 5 VA Recording watt-meters 5 VA kWh and kVAr meters 5 VA Thermal demand ammeters 3 VA Figure 15.15(b) 400 kV bar primary outdoor current transformer Thermal maximum demand ammeters 4 to 8 VA (Courtesy: BHEL) Power factor (p.f.) meters 5 VA Protective devices In view of the large variety of these devices such as static or electromagnetic, VA burdens *These values are typically for moving iron instruments and devices. may be obtained from catalogues or their manufacturers. For electronic instruments and devices they would be of the order of 0.1 to 0.5 VA and less. Copper flexible leads (wires) The approximate resistances Instrument and control transformers: applications and selection 15/547 Computing the VA burden 1The VA values of some of the devices used in the circuit may be available at a different current rating than the actual rated secondary current (1 or 5 A) chosen for the CT circuit. To compute the VA burden of a circuit when selecting the correct VA level of a CT, the VA values of all the devices not corresponding to the rated current of the circuit must be first converted to the rated current and only then added. This is essential because the VA level of a CT varies in a square proportion of the current passing through it, 2 i.e. VA µ I . As a result, at lower operating currents (a) Single phase HV CT its VA capacity to feed a circuit would also decrease sharply while the VA requirement of the instruments or the relays connected in the circuit will remain the same. It is therefore important that the VA level of the CT is raised in the same inverse square proportion of the current to maintain at least the same level of VA to make it suitable to activate the measuring or protective devices connected in the circuit, i.e.
VA1 VA 2 2 = 2 II1 2
where VA1 and VA2 are the VA levels of a circuit at currents I1 and I2 respectively.
Example 15. 2 Consider a 5 A secondary CT circuit connected to the following (b) HV epoxy resin cast (c) HV epoxy resin cast devices: Device I = 0.3 VA at 1 A Device II = 5 VA at 5 A Device III = 7.5 VA at 5 A Then the total burden at 5 A will be 5 2 = 0.3 Ê ˆ + 5 + 7.5 ¥ Ë 1¯ = 7.5 + 5 + 7.5 = 20 VA Therefore, one should select a 20 VA CT. Similarly, if this value was required at the 1 A secondary, then the total burden would be (d) LV epoxy resin cast (e) LV tape wound 22 1 1 = 0.3 + 5 ¥ Ê ˆ + 7.5 Ê ˆ Figure 15.16 Typical indoor-type wound primary CTs for Ë 5 ¯ Ë 5 ¯ measuring or protection (Courtesy: Kappa Electricals) = 0.3 + 0.2 + 0.3 = 0.8 VA of such conductors at 20 C are provided in Table 13.15. ∞ In this case one can select a 2.5 or 5 VA CT. They can be estimated at the operating temperature (90∞C, as in Table 14.5 or as desired). 2 The current element of a relay is wound for a wide 2 range of current settings in terms of the rated secondary VA burden = I R current of the CT, such as 10–80% for a ground fault e.g. the VA burden of a CT having a rated secondary protection, 50–200% for an over-current and 300– current of 5 A with the length of the 2.5 mm2 connecting 800% for a short-circuit protection. At lower current leads as 10 m. settings, while the VA requirement for the operation of the relay will remain the same, the VA capacity of the CT will decrease in a square proportion of the current. VA = 52–3 7.6 ¥ 10 [1 + 3.93 10 (90–20)] ¥ 1000 ¥ A CT of a correspondingly higher VA level would therefore be necessary to obtain the reduced VA level, = 2.42 VA (for details refer to Table 14.4) at least sufficient to operate the relay. At a 40% setting, 15/548 Electrical Power Engineering Reference & Applications Handbook
2 for instance, the CT must have a VA of (I/0.4I ) or Im¢ 6.25 times the VA of the relay and at a setting of 20%, e 2 (I/0.2 I )2 or 25 times of the relay. Therefore when the e 1 relay setting is low this must be borne in mind and a CT of a higher VA burden be chosen. Such a consideration, however, is more pertinent in the case d
of electro-magnetic relays that have a high VA level I 2¢ than in electrostatic (electronic) relays that have a I n I m near negligible VA level at only around 0.005 VA. I m I n Where three CTs for unrestricted or four CTs for restricted ground fault or combined O/C and G/F protections are employed in the protective circuit, the VA burden of the relay is shared by all the CTs in parallel and a normal VA CT may generally suffice. Such is the I I case in most of the protective schemes discussed in m¢ 1 = Phase displacement (phase error) between . Sections 21.6 and 15.6.6(1), except for those employing d II12 and ¢ only one CT to detect a ground fault condition, such as for a generator protection with a solidly grounded neutral Figure 15.18 Phasor diagram of a CT (Figure 21.12).
(iv) Circuit diagram The phasor difference between III21n1¢ and , i.e. results This can be drawn along similar lines to those for a VT in a composite error I m¢ . The phase displacement between (Section 15.4.1(12)). Refer to the simple diagram in Figure I2¢ and I1 by an angle ‘d’ is known as the phase error. 15.17, from which we can derive the following: The current error will be important in the accurate operation of an over-current relay and the phase error in III21n1¢ = – and the operation of a phase sensitive relay. The composite error will be significant in the operation of a differential IIIn1 = mm + ¢ relay. and from this is drawn the phasor diagram (Figure 15.18). (v) Current error or ratio error
Primary side Secondary side The error in the secondary current from the rated caused by the excitation current I or the variation in the actual R X R X Z n1 1 1 I 1 I 2¢ 2¢ 2¢ transformation ratio is expressed by:
I n (KII * – *) Current error = n ¥ 2 1 100% ( = K · I ) * ¥ I2¢ n 2 I1 e e e (Kn being the rated transformation ratio.) 1 2 2¢ Refer to Table 15.8 for measuring and Table 15.9 for protection CTs. Im¢ I m
Note *Only the r.m.s. values and not the phasor quantities are considered to define the current error. The phase error is defined separately. Together e1 – Primary induced emf they form the composite error. e 2 – Secondary induced emf e2¢ – Secondary terminal voltage for bar primary e1 e2 (vi) Phase error R1 –Primary circuit resistance R 2¢ – Secondary winding resistance referred to the primary side As noted above, this is the phase displacement between X1 – Primary circuit reactance the primary and the secondary current phasors. Angle d X 2¢ – Secondary winding reactance referred to the primary side in Figure 15.18 is generally expressed in minutes. For a Z – Load (burden) impedance perfect transformer, the direction of phasors is chosen so I n – Excitation or No load current that this displacement is zero. Refer to Table 15.8 for Im¢ – Loss component supplying the hysteresis and eddy current measuring and Table 15.9 for protection CTs. losses to the CT core (it is the active component) I m – Magnetizing component producing the flux ‘f’ (it is the reactive component) (vii) Composite error I 2¢ – Secondary current referred to the primary side Refer to the phasor diagram in Figure 15.18 and Table 15.8 for measuring and Table 15.9 for protection CTs. Figure 15.17 Equivalent circuit diagram of a current transformer This error can also be expressed by Instrument and control transformers: applications and selection 15/549
Table 15.8 Limits of error for measuring CTs
a Accuracy ± % Current (ratio) error at % of rated primary current ± Phase displacement angle d (Figure 15.18) class in minutes at % of rated primary current
% rated. I1 52050 100 120 5 20 100 120 0.1 0.4 0.2 NA 0.1 0.1 15 8 5 5 0.2 0.75 0.35 NA 0.2 0.2 30 15 10 10 0.5 1.5 0.75 NA 0.5 0.5 90 45 30 30 1.0 3.0 1.5 NA 1.0 1.0 180 90 60 60 3.0 – – 3 – 3 ¨—————— Not specified ——————Æ 5.0 – – 5 – 5 ¨—————— Not specified ——————Æ As in IEC 60044-1 aThese errors are valid only when the CTs are loaded by a minimum 25% of the rated VA burden, for CTs of class 1 and 50% for CTs of classes 3 and 5 and a primary current of not less than 5% or more than 120% of the rated current. The measuring CTs may not transform correctly unless the above conditions are met.
Table 15.9 Limits of error for protection CTs
10% Accuracy Current error at Phase displacement Composite error class rated primary angle (Figure at rated
)
d f current 15.18 at rated accuracy limit Knee-point primary current primary current % minutes %
5 P ± 1 ± 60 5 10 P ± 3– 10 50% 15 P ± 5– 15 As in IEC 60044-1
T Excitation (secondary) voltage (V Composite error = 100 1/TKiit ( – )2 d % I n2◊ 1 1 Ú0 where Excitation current (Im) Kn = rated transformation ratio Figure 15.19 Knee point of the excitation characteristic of a I1 = actual primary current (r.m.s.) current transformer I2 = actual secondary current (r.m.s.) i1 = instantaneous value of the primary current i = instantaneous value of the secondary current can damage instruments connected to its secondary. For 2 measuring instruments therefore it is kept low, as it is T = duration of one cycle required to measure only the normal current and not the = 1/50 s or 20 ms for a 50 Hz system. fault current.
(viii) Knee point voltage 15.6.2 Measuring current transformers This is the point on the magnetic curve of the laminated These are employed for the measurement of power circuit core of the CT at which the saturation of the core will currents through an ammeter, kW, kWh or KVAr and start. It is defined as the point where an increase of 10% power factor meter, or similar instruments requiring a in the secondary voltage will increase the magnetizing current measurement. They must have a specified accuracy (excitation) current Im by 50% (Figure 15.19). Beyond class as in IEC 60044-1 and the secondary current this point, a very large amount of primary current would substantially proportional to the primary within a working be required to further magnetize the core, thus limiting range of about 5–120% of its primary rated current. They the secondary output to a required level. are required to commence their saturation beyond 120% of the primary rated current and saturate fully by 500%, (ix) Instrument security factor (SF) as a system is not warranted to operate on an over-load or short-circuit, and will be interrupted through its This is the ratio of instrument limit primary current to protective devices. Thus a low knee-point voltage or a the rated primary current. Consequently a high SF will low saturation level is needed to protect the connected mean a high transformation of the primary current and instruments from fault currents (over-current factor) on 15/550 Electrical Power Engineering Reference & Applications Handbook the primary side. For example, in a measuring CT of I p I s1 I s2 1000/5 A, the secondary current will be in direct proportion to the primary current from about 50 to 1200 A and the core will start saturating beyond 1200 A. Load or Over-current factor for instruments (VA L) instrument As in IEC 60051, the measuring instruments are required to have an over-current factor of not more than 120% for two hours for instruments of all accuracy classes, 200% VAC for 0.5 second for class 0.5 or less, and 1000% for 0.5 Main CT Interposing CT (VA ) second for class 1 accuracy and above. Over-currents or (VAM) I durations longer than this may damage the instruments. (a) Schematic diagram
VA Accuracy class I p I s1 VA C I This defines the maximum permissible current error at the rated current for a particular accuracy class. The 2 standard accuracy classes for the measuring CTs may be I VA Ê s1 ˆ one of 0.1, 0.2, 0.5, 1, 3 and 5. The limits of error in L Á I ˜ magnitude of the secondary current and the phase error, Ë s2 ¯ as discussed in Section 15.6.1 and shown in Figure 15.18, must be as in Table 15.8, according to IEC 60044-1, when the secondary burden is a minimum 25% of its rated burden for CTs up to class 1 and 50% for CTs of Main CT classes 3 and 5. (b) Equivalent control circuit diagram The recommended class of accuracy will depend upon Figure 15.20 Use of interposing CTs the type of application and is generally as noted below:
VA Application Class of I =VA of the interposing CTs at the primary rated accuracy current VAL =VA of the load (instrument) connected on the 1Precision testing or laboratory testing CTs 0.1 secondary of the interposing CTs, including the 2 Laboratory and test work in conjunction with high 0.2 connecting leads. precision indicating instruments, integrating meters and also for the testing of industrial CTs 3 Precision industrial meters (indicating instruments 0.5 15.6.4 Summation current transformers and recorders) These are required to sum-up the currents in a number of 4 Commercial and industrial metering 0.5 or 1 5 Use with indicating and graphic watt-meters 1 or 3 circuits at a time through the measuring CTs provided in and ammeters each such circuit. The circuits may represent different 6 Purposes where the ratio is of less importance 3 or 5 feeders connected on the same bus of a power system (Figure 15.21(a)), or of two or more different power systems (Figure 15.21(b)). A precondition for summation 15.6.3 Interposing current transformers of currents on different power systems is that all circuits must be operating on the same frequency and must relate These are auxiliary CTs, and are sometimes necessary to to the same phase. The p.f. may be different. alter the value of the secondary of the main CTs. They Each phase of these circuits is provided with an help to reduce the saturation level and hence the over- appropriate main CT, the secondary of which is connected loading of the main CTs, particularly during an over- to the primary of the summation CT. Summation is load or a fault condition. They are used especially where possible of many circuits through one summation CT the instruments to which they are connected are sensitive alone per phase. The primary of summation CTs can be to over-loads. They have to be of wound primary type. designed to accommodate up to ten power circuits easily. So that the main CTs are not overburdened they have a If more feeders are likely to be added it is possible to VA load that is as low as possible. Figure 15.20 illustrates leave space for these on the same summation CT. the application of such CTs and their selection is made The summated current is the sum of all the CT secondary on the following basis: currents of the different circuits. The rating of the instrument connected on the secondary of the summation CT should VAM = VAC + VAI + VAL be commensurate with the summated current. The error where of measurement is now high, as the errors of all individual VAM = VA of the main CTs CTs will also add up vectorially. It is necessary that CTs VAC = circuit losses between the main and the have the same ratio, secondary resistance of magnetizing interposing CTs at the primary rated current current to minimize the error. Instrument and control transformers: applications and selection 15/551
Any main CT that is under-loaded will also add to the Table 15.10 error in the measurement. Similarly, if provision is made in the primary of the summation CT to accommodate Circuit whose Current Main CT VA burden shared future circuits but is not being utilized it must be left current is being rating ratio by the main open, otherwise it will also add to the error. The impedance summed CTs of the shorting terminals will add to the impedance of the circuit and will increase the total error. Circuit 1 1000 A 1000/1 A *25 ¥ 1000 3400 As the currents of each circuit are summed by the Ӎ summation CT, the VA burden of each main CT is also 7.0 VA borne by the summation CT in addition to its own. The Circuit 2 800 A 800/1 A *25 ¥ 800 3400 VA level of the summation CT, including its own, is Ӎ 6.0 VA shared proportionately by all the main CTs in the ratio of Circuit 3 1600 A 1600/1 A *25 ¥ 1600 their primary currents. Referring to the three different 3400 circuits of Figure 15.21(b), having the ratings as shown Ӎ 12.0 VA in Table 15.10, the rating of the summation CT can be chosen as 3400/1 A. If we choose a VA level of this CT Total load Ratio of *VA of = 3400 A summation summation as 25 VA, making no provision for the future, then the CTs = 3400/1 CTs = 25 VA burden shared by each main CT will be as calculated in the last column, ignoring the losses in the connecting leads. Based on this, the VA burden of each main CT can be decided. a measuring and a protective transformer in terms of accuracy, saturation level and VA burden. Unlike a measuring CT, a protection CT will have a high saturation 15.6.5 Protection current transformers level to allow the high primary current to transform These are employed to detect a fault, rather than measuring substantially to the secondary as may be required, the current of a power system or the connected equipment. depending upon the current setting of the protective or There is a fundamental difference in the requirement of tripping relays. For protection CTs, therefore, the accuracy class is of little relevance up to the primary rated current, but a true reflection in the secondary is more important of a fault condition in the primary. Main CTs RBY Corollary Circuit no. 1 On Both requirements of measuring and protection cannot Circuit no. 2 phase B be met through one transformer generally. Thus two sets
Circuit no. 3 of transformers are required for a power circuit associated with a protection scheme, one for measurement and the other for protection. Summation CT (i) Accuracy limit primary current VA burden (a) Measuring the sum load of three circuits on phase B This is the highest limit of the primary current that can
Main CTs
R1 Y1 B1 R1
Y1
B1
R2 Y2 B2 R2
Y2
B2
Summation CTs
Burden Burden Burden
(b) Measuring the sum load of two circuits connected on different supply sources
Figure 15.21 Application of summation CTs 15/552 Electrical Power Engineering Reference & Applications Handbook be transformed to the secondary, substantially proportional, Note complying with the requirement of the composite error For similar reasons, a measuring CT of up to 50 A primary current (Section 15.6.1). For example, a protection CT 2000/5A is recommended to be produced in a wound design. represented as 5P10 means that a primary current up to ten times the rated (i.e. up to 2000 10 A) will induce a Example 15.3 ¥ Consider a protective scheme having a total VA burden of 15 proportional secondary current. The factor 10 is known and requiring an accuracy limit factor (ALF) of 20: as the accuracy limit factor as noted below. \ VA ¥ ALF = 15 ¥ 20 = 300 (ii) Accuracy limit factor (ALF) which is too large to design a CT adequately. In such cases it is advisable to consider two sets of CTs, one for those This is the ratio of the rated accuracy limit primary current relays that are set high and operate at high to very high to the rated primary current. For example, in the above currents (short-circuit protection relays) and the second for case it is all other relays that are required to operate on moderate over-loads. For example, consider one set of CTs for short- 2000 ¥ 10 = 10 circuit protection having 2000 VA = 5 The standard prescribed factors can be one of 5, 10, 15, 20 and 30. and ALF = 20 i.e. a 5P20 CT having a product of VA ¥ ALF of not more than (iii) Accuracy class 150 and the other set for all the remaining protections having, say, This defines the maximum permissible composite error VA = 10 at the rated accuracy limit primary current, followed by letter P for protection. The standard prescribed accuracy and ALF = 5 classes may be one of 5P, 10P and 15P. A protection CT i.e. a 10P5 CT having a product of VA ¥ ALF of much less is designated by accuracy class, followed by accuracy than 150. limit factor, such as 5P10. The current error, phase error d (Figure 15.18) and the composite error with the rated Note burden in the circuit are as in Table 15.9, according to For high set protective schemes, where to operate the IEC 60044-1. It should be chosen depending upon the protective relays, the primary fault currents are likely to be protective device and its accuracy requirement to extremely high, as in the above case. Here it is advisable to consider a higher primary current than the rated for the discriminate. Closer discrimination will require more protection CTs and thus indirectly reduce the ALF and the accurate CTs. product of VA ¥ ALF. In some cases, by doing so, even one set of CTs may meet the protective scheme requirement. Note An accuracy class beyond 10P is generally not recommended. Example 15.4 Consider a system being fed through a transformer of 1500 (iv) Output and accuracy limit factors kVA, 11/0.433 kV, having a rated LV current of 2000 A. The protection CT ratio on the LV side for the high set relay may The capabilities of a protection CT are determined by be considered as 4000/5 A (depending upon the setting of the relay) rather than a conventional 2000/5 A, thus reducing the primary inputs of a CT such as the primary ampere the ALF of the previous example from 20 to 10. Now only one turns AT (primary current ¥ primary number of turns), set of 15 P10 CTs will suffice, to feed the total protective core dimensions and the quality of laminations. All this scheme and have a VA ¥ ALF of not more than 150. is roughly proportional to the product of the rated output (VA) and the rated accuracy limit factor of the CT. For (v) Other considerations when selecting a normal use, the product of the VA burden and the protection CT accuracy limit factor of a protection CT should not exceed 150, otherwise it may require an unduly large and more 1 The accuracy limit factor (ALF) will depend upon expensive CT. For example, for a 10 VA CT, the accuracy the highest setting of the protective device. For a 5 to limit factor should not exceed 15 and vice versa. The 10 times setting of the high set relay, the ALF will be burden and the accuracy limit factor are thus interrelated. a minimum of 10. A decrease in burden will automatically increase its 2A higher ALF than necessary will serve no useful accuracy limit factor and vice versa. In a ring or bar purpose. primary CT, which has only one turn in the primary, the 3 It has been found that, except high set relays, all ampere turns are limited by the primary current only, other relays may not require the ALF to be more than thus limiting the accuracy and burden of such CTs. This 5. In such cases it is worthwhile to use two sets of is one reason why CTs of up to 50 A are generally protection CTs, one exclusively for high set relays, manufactured in a wound primary design, with a few requiring a high accuracy limit factor (ALF), and the turns on the primary side to obtain a reasonably high other, with a lower ALF, for the remaining relays. value of VA burden and accuracy. For larger products Otherwise choose a higher primary current than rated, than 150, it is advisable to use more than one protection if possible, and indirectly reduce the ALF as illustrated CT, or use low secondary current CTs, i.e. 1 A instead in Example 15.4 and meet the requirement with just of 5 A. one set of protection CTs. Instrument and control transformers: applications and selection 15/553
15.6.6 Special-purpose current transformers, R type ‘PS’ Y L1 B These are protection CTs for special applications such as biased differential protection, restricted ground fault I p I p I p protection and distance protection schemes, where it is not possible to easily identify the class of accuracy, the I f1 I f1 I f1 accuracy limit factor and the rated burden of the CTs and F1 where a full primary fault current is required to be transformed to the secondary without saturation, to accurately monitor the level of fault and/or unbalance. The type of application and the relay being used determine I p I p I p the knee point voltage. The knee point voltage and the excitation current of the CTs now form the basic design Wp parameters for such CTs. They are classified as class ‘PS’ CTs and can be identified by the following characteristics: Relay • CTR = Ip /Is I f2 I f2 I f2 • Rated test winding current F2 • Nominal turn ratio (the error must not exceed ± 0.25%) • Knee point voltage (kpv) at the maximum secondary G turns, L2 Vk ≥ 2Vft G where V = knee point voltage and F1, F2 –2 sets of identical class PS CTs k Relay – High impedance three element differential protection relay Vft = maximum voltage developed across the Wp – Windings of a power equipment or section of a power system relay circuit by the other group of CTs to be protected during a severe most through fault. • Maximum magnetizing (excitation) current at the Figure 15.22 A circulating current scheme to provide a phase and a ground fault differential protection voltage setting (Vft) of the relay or at half the knee point e.m.f. to be < 30 mA for 1A CTs for most high impedance schemes. The manufacturers select a proper
iron core to limit this to help reduce the effective I 2 relay current setting and improve its sensitivity. Magnetizing characteristics, Vf versus Im (Vf being the CT secondary voltage under rated conditions), as shown in Figure 15.19, are provided by the I 3 manufacturer to facilitate relay setting. • Maximum resistance of the secondary winding corrected to 90∞C or the maximum operating I temperature considered. In fact, it should be substituted 1 by the actual operating temperature. We discuss below a high-impedance differential I 4
protection scheme to provide a detailed procedure to I1 + I234 + I + I = 0 select PS Class CTs. Figure 15.23 Kirchhoff’s law – sum of currents entering a node is zero 1 High-impedance differential protection scheme The scheme primarily detects an inter-turn fault, a ground fault or a phase fault. It can thus protect a bus system Applying this law to a three-phase, three wire system, and windings of critical machines such as generators, IIIRYB + + = 0 transformers and reactors in addition to a ground fault. The differential system is a circulating current system and to a three-phase four-wire system between the two winding terminals of the equipment or each section of a multi-section bus system being protected IIIIRYBn + + + = 0 as illustrated in Figure 15.22. The scheme is based on When a three-phase four-wire system feeds non-linear Kirchhoff’s law, which defines that the phasor sum of or single-phase loads this balance is upset and the the currents entering a node is zero, i.e. unbalanced current flows through the neutral. The same
IIII1234 + + + = 0 relationship can be expressed as as illustrated in Figure 15.23. IIIIRYBn + + = . 15/554 Electrical Power Engineering Reference & Applications Handbook
Similarly, the balance is disturbed in the differential vary with the CT secondary voltage; scheme on a fault of any type and a spill current, which refer to Figure 15.19) is the difference between the currents drawn by the two Ic1, Ic2 = Ic = circulating currents sets of CTs, flows through the relay. Since the scheme Ire = spill or differential current through functions on the principle of balance of currents, it is the relay imperative that the two sets of CT parameters, such as Vf1, Vf2 = Vf = CT secondary voltages under rated their ratio, secondary resistance and the magnetizing conditions (these relays are defined current, should be identical, except for the permissible by both the current and the voltage tolerances as discussed in Section 15.10.2. The secondary settings) lead resistances, from the CTs to the relay terminals, Rr = resistance of the relay coil should also be the same, otherwise, spill currents may = VA where VA is the burden of the flow through the relay, even under healthy condition and I 2 cause an unwanted trip, or require a higher minimum st setting of the relay. A higher setting of the relay may relay. This may be specified in terms jeopardize its sensitivity to detect minor faults. Since it of its current rating 1 A or 5 A is not practical to produce all CTs to be identical, small or setting current Ist. Considering spill currents under healthy condition are likely and the this to be 1 VA relay at a setting of 0.05 A, minimum relay setting, Ist, must account for this. Below we consider three different cases to explain the principle R = 1 = 400 of circulating currents, along with the procedure, to select r (0.05) 2 W the CTs and carry out the relay setting. Ist = relay setting Equivalent circuit diagram and selection of class Rᐉ1, Rᐉ2 = Rᐉ =maximum resistance of the PS CTs connecting leads from the CT terminals to the relay terminals. For Refer to the control circuit diagram of Figure 15.24, calculating this, for an estimated drawn for the scheme in Figure 15.22. It is drawn on a length and size, refer to cable data single-phase basis for ease of illustration, where in Table 13.15 XCT1, XCT2 = XCT = equivalent excitation reactances of If1, If2 = If = CT secondary currents the CT secondary windings. In ring Im1, Im2 = Im = CTs’ excitation currents (these will type CTs, they are generally very low and can be ignored for ease of L1 derivation
I p RCT1, RCT2 = RCT = equivalent resistances of the CT secondary windings
F1 I f1 V f1 Healthy condition
XCT1 I m1 Refer to Figure 15.24:
R CT1 If1 = Im1 + Ic1
If2 = Im2 + Ic2 I C1 and V = (I – I ) · X R ᐉ1 f2 f2 c2 CT2 R r = Ic2(RCT2 + RI2) + (Ic2 – Ic1) (Rst + Rr) R st I re The two current through the relay are in opposite directions therefore Differential ¸ Strictly speaking, these are under protection R ᐉ2 relay I = I – I Equipment/system re c2 c1 Ô all phasor quantities but Ô = (If2 – Im2) – (If1 – Im2) only their magnitudes are I C2 Ô Ô considered for ease of R Under healthy conditions Ô CT2 ˝ illustration, as quantities X CT2 I m2 If1 = If2 Ô of similar parameters such Ô and I = I as If1, If2 and Im1, Im2 fall m1 m2 Ô almost in phase with each F V Ô 2 I f2 f2 I = 0 \ re ˛Ô other. I p Hence, in a healthy condition there will be no spill current
L 2 through the relay and it will stay inoperative.
(1) Healthy condition, I re = I c2 – I c1 = 0 Through-fault condition Figure 15.24 Equivalent control circuit diagram for a differential ground fault protection scheme of Figure 15.22 Refer to Figure 15.25(a). On a fault occurring outside Instrument and control transformers: applications and selection 15/555 the protected zone, all the CTs that fall in parallel will circuit by the other groups of CTs during a severe share the fault almost equally, depending upon the location through-fault. of the fault and the impedance of each CT circuit up to isc = maximum fault current through the secondary of the point of fault. The balance of the CTs secondary the CTs, on a severe through-fault. This may currents is therefore disturbed, but only marginally, as correspond to the fault level of the machine or the the polarities of the two sets of CTs also fall in opposition system being protected, depending upon the machine and neutralize most of the unbalanced current (Ic2 – Ic1) or the system impedances that may fall in the faulty through the relay. The small spill currents may be taken circuit. care of by the minimum setting of the relay to avoid a trip in such a condition. Hence, the relay may remain The protection must be designed to remain inoperative inoperative on a moderate fault, as illustrated in Figure in such a fictitious fault condition. This condition will 15.25(b). also determine the stability limit of the protection scheme But this may not always be true, as it is possible that and can be considered as the minimum voltage setting of one or more CTs in the faulty circuit may saturate partially the relay. In fact, this setting will have a sufficient safety or fully on a severe through-fault and create a short- margin, as the knee point voltage, Vk, of the CTs is considered quite high, of the order of Vk 2Vft on the circuit (Vf2 = 0) across the magnetizing circuits of all the ≥ CTs that are saturated. Refer to Figures 15.26(a) and (b). one hand, and the saturation of the CTs is possible only The CTs’ resistances, however, will fall across the relay under extreme conditions, on the other. Hence the level circuit. Assuming that the other sets of CTs in the circuit of Vft developed by the CTs may not be as high as thought remain functional, this would cause a severe imbalance and when the relay is set at this voltage it will provide and result in a heavy unbalanced current through the sufficient stability. relay and an unwanted trip. Under such a condition, Note Vft = isc (Rct + RI) It is advisable to choose the CTs with low secondry current, say, at 1 A, to permit a lower relay setting for the voltage and the current where trip coils. The reduced voltage across the relay will also improve Vft =maximum voltage that may develop across the relay the stability level of the protection scheme.
R L1 L1 Y I pf B 2 F1 I f1 V f1 F 1
X CT1 I m1 i f1 i f1 F1 R i f1 i f1 CT1
I C1
F R ᐉ1 Wp Relay Rr i pf R st I re
i f2
under protection R ᐉ2 Differential
Equipment/system F2 i f2 i f2 relay I C2
R CT2
X CT2 I L2 m2 G (Neutral of the winding grounded)
F2 I f2 V f2 F1, F2 –2 sets of identical class PS CTs. Wp –Windings of a power equipment or section of a power I pf system to be protected Fault location 1: L2 The relay stays inoperative for a fault occurring outside Small spill current through the relay the protected zone even if it is within the CTs zone I = I – I Fault location 2: re c2 c1 It falls outside the CTs zone.
(a) Principle of operation (b) Control circuit diagram
Figure 15.25 A through-fault condition outside the protected zone in a differential scheme 15/556 Electrical Power Engineering Reference & Applications Handbook
R Sensitivity L1 Y This is the ability of the scheme to detect the weakest B internal fault.
Stability I I F f1 f1 1 I f1 I f1 This can be defined by the most severe external fault at which the scheme will remain inoperative. It should also remain inoperative in healthy conditions. That is it should be immune to the momentary voltage or current transients Relay Wp and normal harmonic contents in the circulating current. Series LC-filter circuits are generally provided with the relay coil to suppress the harmonics and to detect the If2 fault current more precisely.
F2 I f2 I f2 Use of stabilizing resistance
It is possible that the voltage Vft may become sufficiently high to cause a spill current on a through-fault higher L2 than the relay pick-up current, Ist. To ensure that no spill F G (Neutral of the winding grounded) current higher than the relay setting, Ist, will flow through I the relay circuit under a through-fault condition, the pf impedance of the relay circuit is raised substantially. It can be obtained by using a stabilizing resistance, Rst, (a) Power circuit such that the differential circuit will act like a high impedance path for this spill current, compared to the very low magnetizing impedance of the saturated CT. L1 This resistance is shown in Figure 15.26(b). It will allow I = i I = n·i f1 sc a current of less than the relay pickup current, Ist. To pf sc fulfil this condition, the impedance of the differential circuit must be a minimum to ensure F1 V f1 V Healthy X ft CT1 I m1 (RRrst + ) ≥ CT Ist R CT1 The normal practice is to choose Rst based on the
I c1 = i sc setting voltage required. In the above equation, Vft is the minimum voltage required across the relay branch R ᐉ1 (Rr + Rst) for pushing a current equal to Ist to ensure that V ft I re the relay stays immune on a through-fault. During an internal fault, the fault current is much more than Ist, and R st R r Differential hence it is easy to detect. The equation also implies that relay R is chosen high to limit the relay current during a under protection R ᐉ st Equipment/system 2 through fault (assuming that one of the CTs is fully I C2 Magnetizing saturated) to less than its pickup current. Solving the circuit is short circuited above equation for Rst, R CT2 during the saturation Vft Vft VA of CT Rst ≥ – Rr or – 2* Ist Ist I CT X CT2 st F V = 0 2 f2 Since the additional resistance will stabilize the protective scheme during a maximum through-fault
Saturated Saturated condition without raising the relay setting, Ist, it is I pf up to I sc appropriately termed the stabilizing resistance. Figure L 2 15.27 shows an arrangement in a relay circuit and for the
V ft =i sc (R CT + R ᐉ) purpose of illustration, it is shown separately in various I sc =Fault level of the equipment under protection control circuits (Figures 15.24, 15.25(b) and Figure 25.26(b)). (b) Control scheme
Figure 15.26 Power circuit and control scheme during a very *This is relay-specific. The manufacturer may specify VA severe external fault condition corresponding to its rated current of 1 A or 5 A or setting current Ist. Instrument and control transformers: applications and selection 15/557
R Y L1 B
Isf1 Isf1 + Isf2
F1 Isf1
Relay
Isf1 Isf2 WP
F2 Isf2 Ipf
Isf2
L2 (Courtesy: Siemens) G Ipf
RNY B Relay – High impedance differential protection Stabilizing relay. It operates for the fault occurring resistors within the protection zone I –Fault current through ground for fault (1450 W each pf for Example 15.6) on phase B Non-linear (a) Power circuit resistors
L1
Ipf = n·Isf
F1 I sf1 V f1
X CT1 I m1 Figure 15.27 Three-element high-impedance circulating current relay scheme (shown with the front view of a differential R CT1 protection relay for transformers, generators, motors and busbars)
I c1
R ᐉ As standard practice, this resistance is supplied with 1 R the relay by the relay manufacturer. It is of variable type, r R st I to suit system conditions and the actual fault level. The re maximum value of the stabilizing resistance to be supplied Differential will depend upon the type of protection (ground or phase ᐉ relay under protection R 2 or both) and the relay setting. Generally, it may vary Equipment/system from 50 to 1500 W. I c2 F R CT2 Fault within the protected zone I pf X CT2 I m2 Refer to Figure 15.28(a). The balance of the two sets of F V CTs is disturbed again. The CTs now have the same 2 I sf2 f2 polarity and currents and the two sets add up to cause a high-imbalance spill current through the relay. Referring L I = I + I to Figure 15.28(b) 2 re c2 c1 (b) Control scheme These are all phasor IIIre = c1 + c2 = ( Isf2 – Im2 )¸ Figure 15.28 Fault within the protected zone quantities, but considered Ô + (IIsf1 – m1 ) ˝ linear, for ease of Ô illustration and without stability and prevents the relay from operating on moderate = II + – 2 I sf2 sf1 m ˛Ô much error external faults, while it is sensitive to small spill currents for all internal ground and phase faults, including winding The additive characteristic of the scheme now has high faults. 15/558 Electrical Power Engineering Reference & Applications Handbook 2 Current setting of the relay For a ground fault scheme, it is recommended to consider a still lower setting to ensure effective detection The relay has voltage as well as current settings. The of the ground fault current and rapid disconnection of former defines the stability limit against through-faults, the machine or the bus system being protected. A lower as discussed above, while the latter determines the setting may be desirable as the actual ground fault current sensitivity of the protected zone. may already be larger than is being detected by the relay due to a higher impedance of the ground loop than assumed If Ipf =minimum fault current through the primary (chosen on the basis of the rated full-load current previously. of the machine or the system being protected) As a rule and as recommended in IEC 60255-6, the required to trip the relay. It may be termed the POC may be chosen within 30% of the minimum estimated minimum primary operating current (POC) of ground fault current. When the scheme is required to the scheme. I , in terms of the secondary detect only a ground fault, a single-pole relay is connected pf between all the CTs’ shorted ends (Figure 15.29). All the = n ¥ Isf CTs now fall in parallel. n = turn ratio of the CTs When the scheme is required to detect the ground Im = corresponding to the Vft, to account for the most fault as well as the phase faults, a triple-pole relay is severe through-fault used, each pole of which is connected between the shorted Ist = relay current setting, i.e. minimum spill current terminals of the two same phase CTs and the neutral required to operate the relay formed by shorting the other terminals of all the CTs, as shown in Figure 15.22. The setting of all the poles is Then kept the same. In other words, the sensitivity level remains the same for all types of faults. IIIsf = m + st In the case of over-current and ground fault protection Since on a fault the p.f. is low (Section 13.4.1(5)) all the sensitivity level becomes much higher than in a single- these quantities may be considered in phase with each pole relay. Now the requirement of the minimum primary other, with little error, operating current, Ipf (Equation (15.1)), which is a measure of sensitivity, is greatly reduced. The CT on the faulty \ Isf = Im + Ist phase has to feed only one third of the CTs that fall in If there are N number of CTs connected in parallel, the parallel of each relay coil rather than all the CTs, that fell magnetizing current will flow through all of them. In a in parallel in ground fault protection using only a single- GF protection scheme all the three CTs of all the feeders pole relay. being protected together will fall in parallel, while in The CTs are designed for the worst conditions of case of a combined GF and phase fault protection scheme, fault, even when the scheme is designed to detect only a only one third of these CTs will fall in parallel. The CT in the faulty circuit must be able to draw enough current R to feed the magnetizing losses of all the CTs falling in Y parallel and the relay pickup current, Ist. The sensitivity of the differential scheme can therefore be expressed B more appropriately as
Isf = N ¥ Im + Ist (N being the number of CTs falling in parallel) and in F1 terms of the primary Non-linear resistor I = n (N I + I ) (15.1) pf ¥ m st Stabilizing Since it determines the sensitivity level of the protection resistor scheme, it must be kept as low as possible to detect even WP Relay a small fault. To achieve a high degree of sensitivity it is therefore essential
•To have the CTs with a low Im •To keep the number of CTs in parallel as small as F possible, suggesting protection of individual feeders, 2 rather than many feeders together, particularly when the equipment is critical and requires a higher level G of sensitivity for adequate protection.
As the relay will have only one current setting for all G types of faults, it is recommended to keep it around 20– Relay – High impedance single element ground fault 40% of the rated current of the machine or the system being differential protection relay protected. This setting will be sufficient to meet the CT’s magnetizing current requirements and also trip the relay. Figure 15.29 Scheme for only ground fault differential protection Instrument and control transformers: applications and selection 15/559 ground fault. This may be a phase to phase and ground high, will mean a high POC (Equation (15.1)). A high fault, causing a severe unbalance. The iron core of such POC may not be desirable, as it may under-protect the CTs must therefore possess near-linear magnetizing system. In such cases, it is advisable to divide the system characteristics, to the extent of the fault level of the into more than one circuit and apply the scheme machine or the system being protected. This is to achieve individually to all such circuits (Example 15.6). a near-replica of the magnitude of the fault in the secondary, which may be 15 to 20 times or more of the 3 Suppressing system harmonics rated current. In generators, it can increase to 21· Ir (Section 13.4.1(5)). For the CTs, a saturation level sufficient to Such relays are normally instantaneous, highly sensitive transform the maximum primary fault condition to the and operate at low spill currents. Since they detect the secondary is therefore considered mandatory to ensure residual current of the system, the current may contain that the CTs do not saturate during the most severe fault third-harmonic components (Section 23.6(a)) and operate condition, and render the tripping scheme erratic. This the highly sensitive relay in a healthy condition. To avoid also ensures better stability of the relay, particularly during operation of the relay under such conditions, it is a normal severe most through-fault conditions (outside the CTs’ practice to supply the relay coil with a tuned filter, i.e. a detection zone) such as a bus fault, as illustrated in Figure series L-C circuit to filter out the third-harmonic 15.30. It is normal practice to define the secondary voltage components. The capacitance of the filter circuit may of the CTs by its knee point voltage (kpv), Vk. This also tame a steep rising TRV (Section 17.10.3) during a voltage will depend upon the type of relay, its VA burden momentary transient condition and protect the relay. and the required stability of the system. It is common practice to make this at least twice the relay setting voltage 4 Limiting the peak voltage on the most severe through-fault, i.e. Vk ≥ 2Vft. The most severe fault is the capacity of the machine As this is a high-impedance scheme, it can result in very or the system being protected to feed the fault, and is high voltages across the CTs and the relay, particularly determined by its fault level as indicated in Tables 13.7 during internal faults, when the CTs have the same polarity and 13.10. To consider a higher fault level than this, and the spill currents are additive. As in IEC 60255-6, it such as of the main power supply, is of little relevance as must be limited within 3 kV across the relay circuit to it would fall outside the detection zone of the CTs and protect the CTs and the relay. An approximate formula would serve no useful purpose except to further improve to determine the likely peak voltage across the relay the stability level of the protective scheme. circuit is given by Applying this scheme to system protection, where VVVV = 2 2 ( – ) (15.2) the number of circuits and hence the number of CTs are p k m k where Vp = peak voltage across the relay and Vm = theoretical maximum CT secondary voltage
G1 G2 across the relay circuit at the maximum internal fault current. (The maximum internal fault current is the level of fault of the machine or the system under protection.) This must also take into account any other supply sources that may also feed the fault, such as more than one supply bus, as shown in Section 13.4.1(5) and Figure 13.18, and illustrated in Figure 15.30. If the cumulative F F3 1 fault current is Iscc, then the maximum CT secondary voltage will be Vm = Iscc ¥ impedance of the relay circuit. This can be limited by using a non-linear resistance called Metrosil* across the relay, as shown in Figure 15.27. If R voltage reaches a dangerous level, this resistance will r provide a low-resistance parallel path to the current and limit the voltage across the relay to about 1 kV. The current I through the non-linear resistance is given by F2 = I scc b Vm = K ¥ I (K and b are constants)
F1, F3 Through faults which may be much higher than at *This is a brand name given by the manufacturer of the non-linear F2 but outside the CTs’ zone resistor, a GEC group company in the UK. General Electric, USA F2 Internal fault being fed by two sources although call it Thyrite, and similar names have been given to it by different limited by the equipment impedance manufacturers. Basically, it is a SiC non-linear resistance to provide the desired over-voltage protection. Refer to Section 18.1.1 for Figure 15.30 An internal fault being fed by more than one source more details. 15/560 Electrical Power Engineering Reference & Applications Handbook
All these values are provided by the relay supplier when a lead resistance from CT terminals to the relay to be 0.5 W this resistance becomes necessary. per lead.
\ Total lead resistance, R1 = 2 ¥ 0.5 5 I Selecting class PS CTs =1 W (presuming this to be at the Ground fault protection of a machine and setting of operating temperature) the relay. The following example illustrates the procedure Fault current in terms of the secondary, 1 to select class PS CTs for a typical G/F scheme. In practice, i sc = 16200 ¥ = 8.1 A this scheme would be more appropriate for phase and 2000 ground fault protections, as illustrated in Figure 15.22. 1 Relay voltage setting (stability limit) Example 15.5 V = i ( R + R ) (considering the resistances at the operating Consider a generator, 10 MVA, 3.3 kV, for ground fault ft sc CT 1 temperature) protection having a sub-transient reactance x d¢¢ = 12 ± 10% (Figure 15.29). = 8.1 (7 + 1) Grounding method: solidly grounded Over-load capacity: 150% for 30 seconds = 64.8 V (as in IEC 60034-1) say, 65 V or nearest higher setting available on the relay. Relay type: differential Rating: 1 A \ Minimum kpv, Vk = 2 ¥ 65 VA: 1, at the setting current, I st = 130 V 3 10 ¥ 10 I r = 3 3.3 ¥ 2 Relay current setting = 1750 A Considering a ground circuit resistance of, say, 2 W: The fault level of the system, 3.3 ¥ 1000 \ I g = 100 3 ¥ 2 I sc = 1750 ¥ (Equation (13.5)) 10.8 where I g is the ground fault current = 16.20 kA = 952.6 A (say 950 A) Let us consider a setting of, say, 30% of I : (Assuming a lower value of x d¢¢(12 – 12◊ = 10.8%) to be on g the safe side.) \ I pf = 0.3 ¥ 950 Consider CTs with a ratio of 2000/1 A and having R ct = 7 W = 285 A and the magnetizing chracteristics as in Figure 15.31. Consider Referring to Figure 15.28(a) the number of CTs that will fall in parallel, N = 6
Knee point and I m corresponds to the relay voltage setting of 65 V from the curve of Figure 15.31 = 15 mA. Vk = 130 V From Equation (15.1) 143
) 285 = 2000 (6 0.015 + I st) f 130 ¥ 285 I = – 6 0.015 \ st 2000 ¥ ted into volts) r
ve = 0.1425 – 0.09 = 0.0525 A 65 Therefore the relay can be set between 5–7.5% of 1 A.
Secondary voltage, (V Secondary voltage, 3 Stabilizing resistance (Flux density con Total desired relay circuit impedance
V ft Rz = 015I m 1.5I m I st Excitation current I (mA) m = 65 = 1238 (Ampere-turns converted into Amps.) 0.0525 W RCT = 7 W Core material – CRGO silicon steel Relay resistance R = VA = 1 Figure 15.31 Assumed magnetizing characteristic of r 22 2000/1 A class PS CTs I st (0.0525) Instrument and control transformers: applications and selection 15/561
Ӎ 363 W which is a marginal case. It is, however, advisable to provide a non-linear resistance. \ Required stabilizing resistance
Rst =1238 – 363 = 875 W 5 Specification for class PS CTs CTR = 2000/1 4 Peak voltage across the relay circuit Quantity = 6 numbers (identical) Vk ≥ 130 V I m = maximum 15 mA at a Vk/2 of 65 V. Vp = 22VVkm ( – V k ) (15.2) The CT manufacturer must provide the user with the magnetizing characteristics of the CTs, i.e. I m versus Vf. whereVm = i sc ¥ Rz (considering that there are no other feeds to the generator internal fault from other sources) II Protection of a feeder circuit
= 8.1 ¥ 1238 Example 15.6 Consider a power distribution system as shown in Figure = 10,027.8 V 15.32, where a transformer of 50 MVA, 33/11 kV, having a fault level of 750 MVA, is feeding a bus connected to six \ Vp = 22130(10,027.8 – 130) feeders of different ratings. All the CTs for a combined phase and ground fault may be connected in parallel as illustrated. = 3208 V The CTs on the primary side of the transformer will be similar
RBY
Sw
I/C feeder 7 N Circuit breaker
50 MVA 33/11 kV 750 MVA 3-element high transformer impedance instantaneous differential relay G
R Y B
SwSwSw Sw Sw Sw
O/G feeders with CTs of identical characteristics
1 2 3 4 56
R Y Control bus B N
Under healthy condition
Figure 15.32 Phase and ground fault differential protection scheme for a transformer and feeder bus protection 15/562 Electrical Power Engineering Reference & Applications Handbook to those on the outgoing feeders, except for the insulation 1 Relay voltage setting (stability limit) system and the turn ratio, to provide identical secondary current and magnetizing characteristics, as on the secondary Vft = 13.3 (10 + 1.5) (considering the resistance at the operating side of the transformer. The relay may be set for a slightly temperature) higher value to account for the slight error introduced and the = 152.95 V consequent spill currents to avoid an unwanted trip. say, 155 V or nearest higher setting available on the relay. 3 50 ¥ 10 I r = \ Minimum kpv, Vk = 2 ¥ 155 1.732 ¥ 11 = 310 V (Figure 15.33) = 2625 A 2 Relay current setting Consider a reasonably low value of I pf, say, 25% of I r, to achieve a high level of sensitivity and still feed the I to all the m I pf = 656.25 A 21 = 7 CTs I m at 155 V = 20 mA from the curve of Figure 15.33 3 \ 656.25 = 3000 (7 ¥ 0.02 + I st) \ I pf = 0.25 ¥ 2625 = 656.25 A or I = 656.25 –7 0.02 st 3000 ¥ Assume the CTs on the secondary side of the transformer to be 3000/1 and on the primary to be = 0.788 A Therefore, the relay can be set, say, at 10% of 1 A. 3000 11 1000 ¥ or A The scheme is suitable to detect both a ground fault and a 1 33 1 phase fault. and their magnetizing characteristics as in Figure 15.33. For all the CTs let Rct = 10 W and lead resistance 3 Stabilizing resistance 155 RI = 0.75 ¥ 2 = 1.5 W Relay circuit impedance, R = z 0.1 The fault level of the system = 1550 W 750 1 I sc = Relay resistance, R = 1.732 ¥ 11 r 0.12 = 39 (say 40 kA) = 100 W and in terms of the secondary (for the same relay as in the earlier example) Required stabilizing resistance, 10 3 \ i sc = 40 ¥¥ 1 3000 Rst = 1550 – 100 = 1450 W = 13.3 A 4 Peak voltage across the relay circuit
Vk = 310 V
Vm = i sc R z
Knee point = 13.3 ¥ 1550 Vk = 310 V = 20,615 V 341 310 \ Vp = 22310(20,615 – 310) ) f = 7095 V
ted into volts) r which is more than 3 kV. Hence, a non-linear resistance will
ve be necessary across the relay branch and must be ordered from the manufacturer with the relay. 155 5 Specification for class PS CTs
Secondary voltage, (V Secondary voltage, CTs : 33 kV (Flux density con CTR = 1000/1 A Qty 3 numbers CTs : 11 kV
020Im 1.5Im CTR = 3000/1 A Qty 21 numbers Excitation current I m (mA) (Ampere-turns converted into Amps.) V k ≥ 310 V
I m = as low as possible, but not more than 20 mA at 155 V. Figure 15.33 Assumed magnetizing characteristic of 3000/1 A The CT manufacturer must provide the magnetizing class PS CTs characteristics. Instrument and control transformers: applications and selection 15/563
Notes Table 15.11 Maximum short-time factors obtainable 1 In the above case the incoming feeder would trip, even economically corresponding to rated output, accuracy class, when the fault occurs in any of the outgoing feeders, accuracy limit factor and rated short-time for wound primary which may not be desirable. It is therefore recommended current transformers that this scheme be applied to individual feeders, so that in case of a fault, only the faulty feeder is isolated rather Accuracy Rated output STF obtainable, corresponding to the than the whole system. class VA rated short times up to 2 With the relay one should also order from the manufacturer (a) Stabilizing resistance of 1450 W and (b) A non-linear resistance to discharge the excess induced 0.5 s 1.0 s 3.0 s e.m.f., across the relay circuit. Both these resistances are illustrated in Figure 15.27. (A) Measuring CTs 0.5 2.5 1100 775 450 5 750 525 300 15.6.7 Core-balanced current transformers 10 500 350 200 (CBCTs) 15 375 275 150 30 200 125 75 These are protection CTs and are used for ground leakage 1 2.5 1100 775 450 protection. They are also a form of summation CTs, where 5 1000 700 400 the phasor sum of the three phase currents is measured. 10 675 475 275 The phasor difference, if any, is the measure of a ground 15 500 350 200 leakage in the circuit. They are discussed in Section 21.5. 3 30 275 200 110 2.5 1100 775 450 5 1000 700 400 10 675 475 275 15.7 Short-time rating and effect of 15 500 350 200 momentary peak or dynamic 30 275 200 110 currents (B) Protection CTs 5P 10 2.5 550 400 225 5 375 275 150 The normal practice of users when selecting a measuring 10 225 150 90 or a protection CT has been to specify only the current 15 150 100 60 ratio, the likely maximum VA burden it may have to feed 30 – – – and the class of accuracy for metering and accuracy limit 5P15 2.5 325 250 135 factor for protection CTs. 5 275 200 110 Fault level is normally not mentioned, nor it is requested 10 150 100 60 15 85 60 35 by the CT manufacturer. Generally, it should be sufficient 30 – – – to meet the likely fault level of the system and its duration 5P20 2.5 325 250 135 in most cases, particularly on an LV system. For critical 5 200 125 75 installations, large feeders and all HV systems, however, 10 100 75 40 it is recommended to check the suitability of the CTs for 15 – – – the system fault level and its duration. 30 – – – A short-circuit on a system will cause over-heating as 10P5 2.5 1000 700 400 5 750 525 300 a result of the short-time current, Isc, and its duration of 10 425 300 175 1 or 3 seconds, according to the system requirements 15 375 275 150 and its protective scheme. It will also develop 30 150 100 60 electrodynamic forces (Equation (28.4)) as a result of 10P10 2.5 600 425 250 the momentary first peak of the fault current (in CTs it is 5 425 300 175 10 275 200 110 2.5 times the short-time current, Isc, as in IEC 60044-1; see also Table 13.11). These forces may result in electrical 15 200 125 75 30 – – – as well as mechanical damage to the windings of a CT 10P20 2.5 325 225 125 depending upon the number of turns in the primary 5 275 200 110 winding and the configuration of the coil. For bar primary 10 125 75 50 CTs, having only one turn in the primary, such forces are 15 85 60 35 the least, hence the statement above. With a lower class 30 – – – of accuracy, a low VA level, and a lower accuracy limit factor (for protection CTs) a CT can easily be built to be mechanically rugged. But higher requirements of such parameters may necessitate a bulky CT, disproportionate 13.4.1(5)). For applications on an HV system, where a in size and cumbersome to instal. wound primary CT is imperative, choice of a CT from In most applications, a bar primary CT is generally standard wound primary CTs may still be possible, meeting used and a normal CT may be suitable. But for too small the minimum requirements of class of accuracy, VA burden ratings, where the use of a wound primary CT is and short-time rating. IEC 60044-1 indicates for measuring imperative, short-circuit effects must be considered, except and protection CTs the maximum short-time factors (STF) the CTs for an LV system, where the fault level for such that can be obtained economically for a normal wound small ratings may be very low and may not matter (Section primary CT where 15/564 Electrical Power Engineering Reference & Applications Handbook
instruments or give the operator a shock or even trip Short-time factor (STF) = Rated short-time current Rated primary current other relays connected in the circuit. 2 One should not allow the CT secondary to be open I = sc circuited when it is energized, for it may induce Ir dangerously high voltages. This phenomenon is Some STFs for more important CTs are reproduced in explained in Example 15.8. Table 15.11. Example 15.8 To determine the terminal voltage of a CT during an accidental Example 15.7 open circuit under an energized condition consider a metering Consider a 1.5 MVA 33/11 kV transformer having a fault level CT connected across a few instruments. Refer to the following of 750 MVA. The STF can be calculated as below with the figure based on Figure 15.17, showing an equivalent CT circuit of 11 kV: circuit referred to its secondary side. 750 I sc = 3 ¥ 11 Z l = 103.23 kW 1.0 W Ӎ 40 kA 393.7A 1.5 and I r = ¥ 1000 3 ¥ 11 Ӎ 79 A 7.5 MVA, * * 170 W J60 e2 = 508.1 kV e2¢ ** 11/÷3kV W 0.3 W STF = 40 ¥ 1000 \ 79 Ӎ 506 Consider a CT with a ratio of 100/5 A and select a bar primary CT. If a bar primary is not practicable, then for Excitation circuit an STF of almost 500, we can choose a wound-type * Typical parameters of the CT measuring CT from Table 15.11 with an accuracy class ** Resistance of instruments of 7.5VA of 0.5 and above and a corresponding VA burden of 5 for (a) Under energized and closed circuit condition. a short-time current of 1 second. If these parameters are not suitable use the measuring CT with a higher-rated primary current to meet the requirement. Z = 103.23 k l W 1.0 W Similarly, for a protection CT from Table 15.11 choose an accuracy class of 10P5 with a VA burden of 5 for a 393.7A one second short-time current. If this does not meet the need, the protection CT may also have to be selected with a higher-rated primary current. 7.5 MVA, * e 11/÷3kV J60W 2¢¢ 15.8 Summary of specifications of a CT
In Table 15.12 we list the data, that a user must provide * Approx. impedance of the excitation circuit. to a manufacturer to design a CT for a particular (b) Under energized but open circuit condition. application. Some of the data chosen are arbitrary to CT circuit referred to the Secondary side. define the specifications. We have assumed the following parameters, 15.9 Precautions to be observed Connected Load = 7.5 MVA when connecting a CT System Voltage = 11 kV Burden of all instruments connected across the 1 It is mandatory to ground the secondary circuit of the CTs (in a balanced 3f system, the current through CT = 7.5 VA the neutral will be zero; see Section 21.2.2, Figure Lead resistance = 2 0.5 = 1 21.7). It is required to eliminate the error due to ¥ W W 6 accumulation of electrostatic charge on the instruments 7.5 ¥ 10 Rated current, I r = 3 that may influence the readings. All the CTs in a 3 ¥¥ 11 10 circuit must be grounded at one point only otherwise circulating currents may raise the potential of the = 393.7 A circuit, which is dangerous and may damage Consider a CT ratio of = 400/5 A Instrument and control transformers: applications and selection 15/565
3 the CT’s own resistance and reactance. 11 ¥ 10 Load impedance Z ᐉ = Total resistance of the instruments under rated condition 3 ¥ 393.7 = 7.5/52 Ӎ 16.13 W = 0.3 2 W Z ᐉ referred to the secondary side = 16.13 (400/5) 11 400 e2 = ¥ = 103.23 kW 3 5 For ease of analysis we have ignored (without much error) Ӎ 508.1 kV
Table 15.12 Summary of specifications of a CT
Sr. no. Specifications Measuring CTs Protection CTs Special-purpose protection CTs type ‘PS’
1 System voltage As in Table 13.1 2 Insulation level (peak) As in IEC 60044-1 or Tables 13.2 and 14.3 for Series I and Tables 14.1 and 14.2 for Series II voltage systems 3 Class of insulation E B B 4 Frequency 50 or 60 Hz 50 or 60 Hz 50 or 60 Hz 5 Nominal current ratio 600/5 A 2000/5 A 2000/5 A
6VA burden¨ææææææææææææææ 2.5, 5, 7.5, 15 or 30 æææææææææææææææÆ 7 Class of accuracy 0.1, 0.2, 0.5,1, 3 or 5 (5P, 10P or 15P)a – 8 Accuracy limit factor (ALF) – (5, 10, 15)b –
9 Short-time current Isc and its 25 kA for 1 second 50 kA for 1 second – duration
10 Dynamic current Minimum 2.5 times Isc Minimum 2.5 times Isc – (in the above case 62.5 kA) (in the above case 125 kA)
11 Nominal turns ratio – – 1/400 12 Limiting secondaryc ––3 resistance at 90∞C (W) magnetizing 13 Knee point voltage (V) – – 950 ¸ curve to be 14 Excitation current at knee – – 0.05 ˝ furnished by the ˛ point voltage (or at any other manufacturer required voltage or both) (A)
15 Service conditions ∑ Indoors or outdoors ∑ Indoor or outdoor ∑ Indoors or outdoors ∑ Ambient temperature ∑ Ambient temperature ∑ Ambient temperature ∑ Altitude, if above 2000 m ∑ Altitude, if above 2000 m ∑ Altitude, if above for LV and above 1000 m for LV and above 1000 m 2000 m for LV and for HV for HV above 1000 m for HV ∑ Humidity ∑ Humidity ∑ Humidity ∑ Any other important ∑ Any other important ∑ Any other important requirement requirement requirement
16 Marking of CTs (a) 600/5 Ad 2000/5 A 6.6 kV, 2000 A, 1/400 10 VA, class 1 15 VA, class 5P10 950 ¥ 0.05 R3. (b) System voltage and insulation level, class of insulation, frequency, short-time rating and dynamic current rating etc. for all types of CTs Notes aThe class of accuracy for protection CTs is recommended to be not more than 10P as far as possible. bProduct of VA and ALF not to exceed 150. c The limiting secondary resistance is required to determine the secondary limiting e.m.f. which is = (FS) ¥ rated secondary current ¥ VA ¥ resistance of secondary windings at 90∞C or the highest operating temperature as in Table 14.5, where FS = Instrument security factor Rated instrument limit primary current = Rated primary current dWherever two separate secondary windings are provided, say, one for measuring and the other for protection, the markings shall indicate all such details that are marked against (a) for each secondary winding. 15/566 Electrical Power Engineering Reference & Applications Handbook
(a) Under energized condition when the CT’s secondary connecting leads, such as for remote measurement of is a closed circuit, voltage developed across the relay, current or other quantities. It is advisable to limit the e extra VA burden on the CTs, on account of such leads. e = 2 0.3 2¢ circuit impedance ¥ 3 = 508.1 ¥ 10 0.3 SECTION III: TESTING OF 3 ¥ (103.23 ¥ 10 + 1 + 0.3) INSTRUMENT AND CONTROL = 1.48 V TRANSFORMERS (b) Under energized condition when the CT’s secondary is accidentally open circuited, the current will have only the magnetizing path and the voltage induced 15.10 Test requirements across the CT open terminals will be the same as across the magnetizing circuit. Under this situation The following tests are recommended on a finished voltage the magnetizing circuit shall carry the same current or current transformer: as caused by the primary current, which is very high.
\ Voltage developed across the CT open terminals, 1Type tests These are conducted on a finished voltage 3 or current transformer, one of each design and type, 508.1 ¥ 10 e2¢¢ = ¥ Ze (103.23 103 + Z ) to verify their compliance with the design data and ¥ e relevant Standards. where Z = Impedance of excitation circuit, 2 Routine tests These are conducted on each finished È e ˘ voltage or current transformer to verify their suitability Í 1 1 1 ˙ Í = + ˙ for the required duty. Z 170 J60 ÎÍ e ˚˙ 3 Field tests For simplicity, considering the approximate impedance of the 4 Special tests Any tests that are not covered above excitation circuit (without much error) as J60 W and are considered necessary by the user may be 3 agreed upon between the manufacturer and the user. 508.1 ¥ 10 \ e 2¢¢ = ¥ J60 (103.23 103 + J60) ¥ 15.10.1 Voltage transformers Ӎ J295 V 1 Type tests These will cover the following tests: which is approximately 200 times that of voltage under normal condition and hence highly detrimental for the insulation of (i) Temperature rise test the CT, the connecting leads and the human contact etc. (ii) Verification of dielectric properties on the primary Depending upon the system loading at the instant of CT windings To check the insulation level, as in Table circuit interruption, it is possible that the primary current is 13.2 for series I and Tables 14.1 and 14.2 for series enough to cause a saturation of the CT core. When so, it is II and Tables 14.3, 14.3(a) and (b) common for series likely that the induced voltage across the CT open terminals I and II voltage systems. may give a further momentary kick up to 2 ÷2 times the voltage calculated above, as the current and hence the voltage (a) Power frequency voltage withstand or HV test. shall undergo a rapid change from one peak to the other (b) Impulse voltage withstand or lightning impulse within one half of a cycle, Section 1.2.1. test. Since a VT is associated with a switchgear, either 3 Provision is therefore made to short-circuit all the CT with its assembly or with the switchyard, the above secondary terminals not in use (for example, in three two tests are almost the same as those for the energized measuring CTs, connected to a common ammeter through a selector switch, when either none or any one of the CTs only may be connected to the CT1 ammeter at a time, the other CTs remaining out of R CT2 circuit). In such cases, except for the CT in use, the Y remaining CTs should be shorted. The selector CT3 switches are therefore designed so that all the CT B terminals not in use are shorted automatically through the switch, even during a change-over from one CT to another. A typical circuit diagram of the switch is G shown in Figure 15.34, which illustrates the fulfilment of this requirement. It may be observed that in the OFF position, all the CT secondaries are shorted. And when any one of them is in circuit, the remaining RYB
two are shorted. All such switches must be the ‘make Selector switch A A before break-type’, so that the CT terminals are shorted 1 2 before being connected to the load (ammeter) during A the changeover. 4 One should select a lower secondary current, say, Figure 15.34 Shorting of all unused CT terminals in a CT 1 A CT, for installations requiring long lengths of secondary circuit using a selector switch Instrument and control transformers: applications and selection 15/567
switchgear assembly and as discussed in Sections Note 14.3.3 and 14.3.4. The test requirements and A repeat power frequency test, if considered necessary, must procedures are also similar. be performed at 80% of the prescribed test voltage. See also Section 14.5. (iii) Wet test for outdoor type transformers The outdoor VTs are also tested for dielectric properties under (iii) Power frequency withstand test on the secondary wet conditions. The procedure to create the wet windings This must also be conducted only on the conditions and to carry out the test are specified in electromagnetic unit of a VT, as noted above, similar IEC 60060-2. In wet conditions, the VT has the same to the control and auxiliary circuit dielectric test test voltages as specified above. (Tables 14.3 and 14.3(a) and (b)). (iv) Verification of accuracy The test results obtained (iv) Verification of accuracy As under type tests above. must comply with the values of Tables 15.5 and 15.6 for a measuring and a protection transformer 3 Field tests Power frequency withstand test on the respectively. For brevity, we have limited our primary windings (for un-grounded VTs). The value discussions as above. For more details, exact test of the test voltage and the test procedure, is almost values and test procedure refer to IEC 60044-2. the same as that for a switchgear assembly (Section 14.5). 2 Routine tests These will cover the following tests: 4 Additional tests on a capacitor VT The tests (i) Verification of terminal marking Refer to Table 15.7 discussed above refer generally to the electromagnetic and Figure 15.35, illustrating types of transformer unit only. To test the whole VT, the following tests connections. are recommended. For the test procedure and results (ii) Power frequency withstand test on the primary refer to IEC 60044-2. windings This must be conducted only on the electromagnetic unit of a VT. For example, when Type tests testing a capacitor VT it must be conducted only on the secondary circuit, i.e. the electromagnetic 1Tests on capacitors transformer. The test values and test procedure will (a) Self-resonating frequency test – applicable only remain the same as discussed above. to carrier coupling capacitors.
R Y or N
R Y or N R N
1R¢ 1Y¢ or 1N¢
R¢ Y ¢ or N¢ R¢ N¢ R ¢ N¢ (1) Single phase VT (5) Single phase residual VT 2R¢ 2Y¢ or 2N ¢ (2) Single phase VT with two secondary windings
R RNYRNB Y B
RNY B N B Y
R ¢ Y ¢ B ¢ N¢ N¢ R¢
R¢ Y ¢ B¢ N¢
R¢ Y¢ B¢ N¢ R ≤ (3) 3-phase VT
R ≤ Y ≤ B ≤ N¢ R Y B N N ≤ ≤ ≤ ¢ B ¢ (4) 3-phase VT with two (6) 3-phase residual VT ≤ Y ≤ secondary windings Figure 15.35 Single and three-phase VTs with one and two windings in the secondary 15/568 Electrical Power Engineering Reference & Applications Handbook
(b) Power frequency wet withstand test on outdoor (v) Wet test for outdoor type transformer This test capacitors. is similar to that for a VT and as discussed in (c) D.C. discharge test. Section 15.10.1(1). (d) Impulse voltage withstand test. (vi) Verification of accuracy The test results obtained (e) Partial discharge (ionization) test. must comply with the values of Tables 15.8 and 2Temperature rise test. 15.9 for a measuring and a protection CT 3 Impulse voltage withstand test. respectively. 4 Ferro-resonance test. 2 Routine Tests These will cover the following tests: 5Transient response test. (i) (a) Verification of terminal marking 6Verification of accuracy. Refer to Figure 15.36, illustrating types of transformer connections. Routine tests (b) To check the polarity of a CT It is imperative that the terminals of a CT are wired with correct 1Tests on capacitors polarity, with reference to the primary in each (a) Capacitance and tangent of the loss angle (tan d) phase. A reversal in any phase will lead to (b) Power frequency dry withstand test incorrect meter readings, in metering CTs and (c) Sealing test erratic signals to the protective relays in 2Verification of terminal marking protection CTs. Although CTs are marked with 3 Power frequency withstand test on the secondary polarities by their manufacturers, such as P1 circuit P2 for primary and S1 S2 for secondary (Figure 4Verification of accuracy 15.36) it is possible, that by sheer human error at the time of fitting the CTs, care is not taken 15.10.2 Current transformers to maintain the same polarity in all the three phases, or their connections are made 1 Type tests These will cover the following tests: inadvertently, without ascertaining their correct (i) Short-time current (Isc) test polarity. It is also possible that on a (ii) Momentary peak or dynamic current test. (This reconnection, such as at site, while reassembling must be conducted at a minimum of 2.5 Isc) the modules of a switchgear or a controlgear (iii) Temperature rise test assembly, such an omission is made. It is (iv) Verification of dielectric properties on the primary therefore advisable that the polarity of the CTs windings to check the insulation level as in Table be ascertained at site before commissioning 13.2 for series I and Tables 14.1 and 14.2 for the equipment, such as a switchgear or a series II and Tables 14.3, 14.3(a) and (b) common controlgear assembly or a switchyard utilizing for series I and II voltage systems. a few CTs. (a) Power frequency voltage withstand or HV test. D.C. voltage test to ascertain the polarity A (b) Impulse voltage withstand or lightning simple procedure to ascertain this is indicated in impulse test. Figure 15.37. A low reading d.c. voltmeter is Since a CT is associated with a switchgear, either connected across the CT secondary windings and with its assembly or the switchyard, the above a battery of 6–12 V through a switch across the four tests are almost the same as those for a switchgear assembly and as discussed in Sections Voltmeter 14.3.3 and 14.3.4. The test requirements and or procedure are also similar. Ammeter
P1 P2 –+ P1 P2
S 1 S 2 S 1 S 2 S 3 Primary (1) Single ratio CT (2) CT with an intermediate Secondary conductor tapping on the secondary winding terminals (winding) S1 S 2 C1 C2 P1 P2 D.C. source (6V–12V)
P1 P P2 1 P2
S S 1 2 1S 1 1S 2 2S 1 2S 2 (3) CT with a primary winding (4) CT with two secondary windings, in 2 sections which may be each with its own magnetic core connected in series or Switch parallel Figure 15.37 Circuit to check the polarity of a bar primary CT Figure 15.36 A CT wound in different combinations at site Instrument and control transformers: applications and selection 15/569 primary. On closing the switch, the meter needle SECTION IV: NON-CONVENTIONAL will give a momentary flicker. If the polarity is correct, the flicker will be positive on connection METHODS OF CURRENT and negative on disconnection. For HV CTs, MEASUREMENT mounted on transformer bushings, it is recommended to short-circuit the main transformer Below we provide a brief description of the non- secondary (LV) windings to reduce the overall conventional CTs to give an idea of the new generation impedance of the transformer to achieve an current measuring devices, their applications and inherent appreciable deflection of the voltmeter needle. advantages over conventional CTs. In view of their distinct (ii) Power frequency withstand test on secondary advantages their applications for current measurements windings is gradually on the rise. It it is possible that soon for very The secondary windings should be capable of high currents, voltages and special applications calling withstanding a rated power frequency, short- for higher accuracy these devices may take over duration withstand voltage of 3 kV for 1 minute. conventional CTs. Conventional CTs gradually limiting (iii) Power frequency withstand test between sections to simple current and energy measurements only. Some This test is applicable when the CT’s primary of these new devices can also be made digital IEDs and secondary windings have two or more sections. (intelligent electronic devices) capable for serial data Then the section in between will be capable of transmission and power system automation from a remote withstanding a similar voltage as noted in item control station (SCADA systems Section 24.11). For more (ii) above. details one may refer to the manufacturers or the literatures (iv) Inter-turn over-voltage test on the subject provided under Further Reading. This test is performed to check the suitability of the inter-turn insulation to withstand the high voltage developed in the secondary circuit in the 15.11 Current sensors event of an accidental secondary open circuit on load. The inter-turn insulation of the windings A conventional CT is the most used device to measure should be capable of withstanding an inter-turn current of a circuit. But its magnetizing current causes a over-voltage of 4.5 kV peak across the complete small phase displacement (d) in its secondary current I2 secondary winding. The test may be conducted ¢ with reference to primary current I1 as shown in Figure by keeping the secondary winding open circuited 15.18. This displacement introduces a phase error (Table and applying a primary current less than or equal 15.8) in the accurate measurement of primary current. to the rated primary current for 1 minute, sufficient Lower the p.f. of the circuit higher the error. It poses a to produce a voltage at the secondary terminals limitation for instruments and devices that are current equal to 4.5 kV (peak). operated and demand for accurate measurement of circuit (v) Power frequency withstand test on primary current in turn to provide reliable measurements, such as windings testing instruments, measuring current and energy. This This test is same as for item (iv) (a), under type limitation of a conventional CT is overcome through tests. electrically isolated (having no magnetic effect) current sensors or transducers. Some prominent transducers Note developed so far and available in the market are noted A repeat power frequency test, if considered necessary, must be below, performed at 80% of the prescribed test voltage. See also Section 14.5. – Resistive shunts – Hall effect current sensors (vi) Partial discharge measurement –Faraday effect optical sensors (vii) Verification of accuracy – Zero flux current sensors This test is the same as under type test item (vi). – Rogowski current transducers or Rogowski coils 3 Special tests The following additional tests may be – Digital optical instrument transformers conducted when considered necessary: (i) Chopped lightning impulse test. Refer to IEC 60044-1 15.11.1 Resistive shunts (ii) Measurement of the dielectric dissipation factor They are miniature but high precision copper shunts (tan d), applicable to only liquid immersed primary connected across the circuit whose current is to be windings, rated for 110 kV and above. measured. In high current circuits they are usually placed in a slot made in the current carrying conductor. Wound Note as low inductive coils they can have very low ohmic For lower voltage systems, say, 2.5 to 10 kV, measurement of value in the range of milli and micro ohm to contain dielectric loss factor tan d, along similar lines, to those recommended for motors and discussed in Section 9.6.1 is advisable as a process voltage drop across them. They can operate at any test to monitor the quality of an HV insulation system, during the frequency zero to MHz according to the application of course of manufacture and using the same value for future reference the device. when checking the quality of insulation at the time of energizing or Usually they have negligible burden. But not so at during a field test. higher currents because of high resistance loss and hence 15/570 Electrical Power Engineering Reference & Applications Handbook not preferred at higher currents. Being low inductive in Figure 15.38 (a corollary to Fleming’s left hand rule coils they may be associated with small contents of Figure 1.1). parasitic L and R. The ratio of L/R determines the high This philosophy is widely used in sensing numerous frequency limit of the measurement. Increasing R to reduce parameters in semiconductor circuits such as flow, r.p.m., L/R can cause heat dissipation and insertion loss problems. commutation of brushless d.c. motors, UPS (un-interrupted The basic advantage of these transducers is isolation power supply), electric welding machines, numerical from the circuit of which it is measuring the current. control machine tools, electrolysis, rectifier and The current to be measured passes through a very electroplating plants. As position sensors for vane control, low value register with low temperature co-efficient. Shunt liquid level, magnetic position, throttle or air valve resistance (R) in the current path generates a voltage position. Similarly, for various automotive applications proportional to the path current. The potential drop across (like measuring the ignition current). Measuring currents the resistor is a measure of the current to be measured. in electric and electronic circuits is just a few of the Different manufacturers may adapt to different techniques many applications these devices can perform. Figure 15.39 to measure ac and dc currents with the use of these shunts. shows a current measuring circuit. The current through the circuit produces a magnetic field which can be guided Hybrid sensors by a magnetic yoke to a linear Hall sensor. The output of the sensor is proportional to the electric current. It can When current or voltage sensors are not capable of measure the inverter loadside harmonic currents up to demonstrating the values to be measured directly and 100 kHz. It is useful where an isolated measurement of are required to be augmented through some electronic current is required, which has a d.c. component. The integrator to perform this job, the sensors are termed as device can measure both ac and dc current components hybrid sensors. Such as use of optic fibre or optic crystal but having magnetic core, is influenced by saturation sensors and obtaining the desired values through phenomenon as in a conventional CT. They also require interferometric technology. a power source and precision. If the device is not isolated Optical sensors are EM compatible and ideal for from EM environment with the use of magnetic shielding accurate measurements of currents and voltages in EM between the sensor and the electronic circuit, the sensor polluted environments. EM pollutions are on the rise may be influenced by EM interferences. with the ever-rising use of electronic technology in all fields and corrupt measurements from conventional CTs. 15.11.3 Faraday effect optical sensors Moreover with digital technologies the new generation sensors act like IEDs and are capable of serial data transfer Faradays’ law (Section 1.1) – Output signal is proportional for remote control and automation. Due to cost to the time derivative di/dt of the current ‘I’ to be consideration up to MV systems it is possible that the measured. To obtain the signal proportional to the conventional devices may continue but for HV and EHV monitored current (I ) the output signal ‘di/dt’ is required systems where cost may be of little significance the new to be electronically integrated. This is a drawback with generation devices are gradually becoming state-of-the- this transducer. Because of this feature it also falls in the art devices. category of hybrid sensors. For integration of signals use of optical crystals or optical fibres is made. In both 15.11.2 Hall effect current sensors cases a light source is necessary. The optical signals are then converted into electrical signals through light The function of Hall sensors is based on the principle of polarization using the technique of interferometry noted the Hall effect named after its inventor E.H. Hall. It briefly later. These measuring devices can be called as suggests that a voltage (Hall voltage) is generated new generation sensors and are becoming increasingly transversely to the current in a conductor if a magnetic sought after devices for measurement of currents in field is applied perpendicular to the current as illustrated electrical circuits. Optical sensors are more accurate being immune from EM interferences. They are however, sensitive to
Magnetic yoke
20 HALL A Voltage I 0 Hall sensor Ammeter I – Current to be measured
Figure 15.38 Figure illustrating Hall effect Figure 15.39 Current measurement through Hall effect Instrument and control transformers: applications and selection 15/571 temperature variations and mechanical stresses. For more eL = di details one may refer to the available literatures on the dt subject, a few mentioned under Further Reading.
15.11.4 Zero flux current sensors They are employed to measure d.c. component such as for error in a CT, current in a d.c. circuit like transmission and distribution of d.c. power. Schematic of such a device is illustrated in Figure 15.39(a). The device is in the C C ¢ form of a toroidal magnetic circuit similar to a conventional CT. The flux caused by a.c. component of I the current I1 in which the content of d.c. component is to be measured, is cancelled by current I2 as illustrated in Figure 15.39(a). Current I2 is specially created for this purpose by providing a compensating secondary winding S2. I2 is adjusted automatically by an amplifier ‘A’. Accordingly, when I1 = I2 the resultant flux in the magnetic circuit (MC) is zero and hence the name of the sensor. Figure 15.40(a) Representing a Rogowski coil The remainder is the d.c. component. Because the fluxes are nullified, the error of measurement is low. The device being a magnetic core is highly susceptible to EMI and 3.5 must be well protected from EM interferences and made EM Compatible. 3.0 2.5 15.11.5 Rogowski coil current transducers (isolated current probes) 2.0 Rogowski coils were invented in 1912. They are air cored 1.5 toroidal windings, which when wrapped around a current carrying conductor produce a voltage that is proportional 1.0
Secondary voltage (V) Secondary voltage to differential of the current di/dt flowing in the conductor. 0.2 The windings are spaced around the toroidal as shown in Figure 15.40(a). Since there is no magnetic circuit they 0 cannot measure d.c. components and therefore output 0
1600
3200
4800
6400
8000 voltage is largely independent of d.c. distortions. Having 9600
11200
12800
14400 no magnetic core there is no hysteresis saturation effect Primary current (A) 16000 (no non-linearity) and bandwidth is wide. Figure 15.40(b) Figure 15.40(b) Linear characteristics of a few Rogowski coils illustrates the linear characteristics of such transducers. (Source: Larsen & Toubro) But they are susceptible to external electromagnetic interferences (EMI). To minimize sensitivity of the coil from external magnetic fields, the coil has a rigid coil carrying conductors in the vicinity. To overcome EMI former and symmetrical coil windings. Small error in effects a grounded di-electric shield to make it positioning of the windings can render the Rogowski electromagnetic compatible (EMC) is usually provided coil (RC) susceptible to fields produced by other current between the core and the winding. There are two mirror image coil layers, each consisting of two concentric sections wound in opposite directions such that the turns MC area of the inner and outer sections are equal. The current I = Current to be 1 I1 carrying conductor whose current is to be measured passes measured through the core of the coil. Electric flux generated by I2 = Secondary circuit I2 current the current in the busbars is coupled with the sensor coil. MC = Magnetic circuit The output signals of the coil are used directly for Z=Load impedance S1 measuring, protection and control/monitoring units. Figure usually low 15.40(c) shows a few such coils. These coils can also be A=Current amplifier made digital and used for serial data transfer and S1 = Secondary winding S Z power system automation from a remote control station S2 = Zero flux detection 2 winding controlling making use of Faraday’s effect and employing optical amplifier A sensors. A G=Ground G Usually two designs are available, one wound on a rigid toroidal core former and the other wound on a Figure 15.39(a) Schematic diagram of a zero current sensor flexible chord like core former to enable achieve any (Source: Merlin Gerin) size of core to fix it on to any size of a current carrying 15/572 Electrical Power Engineering Reference & Applications Handbook
Figure 15.40(c) View of a few Rogowski coils (Source: Lilco, UK) Figure 15.41 Arrangement of a Rogowski coil (Source: Power Electronic Measurements, UK) circuit. Both designs can be made openable enabling to fix it on to a line and measure current of a live circuit. In this way they can also be fixed at inconvenient locations. Rogowski principle Rogowski coil is used for high current measurements. The basic principle is to measure the electric field produced by the primary current. It is therefore susceptible to external electromagnetic interferences (EMI) compared to a conventional CT. But the coil can be protected from EMI by proper design as noted above. For details refer to the literature mentioned under Further Reading. The induced voltage at the coil terminals i e = L d dt where L represents the mutual inductance of Rogowski coil in henry (H). It is the signal level of the coil per unit Figure 15.42 A Rogowski coil probe to measure a.c. currents di/dt. The e.m.f. of the coil would vary with the variation 0.1 Hz–16 MHz (Source: Power Electronic Measurements, UK) in the primary current. Since the measurement is dependent on principle of mutual inductance by the primary current, this coil stays immune to the d.c. components that produce – Since there is total electrical isolation they are useful no induced field. These coils are now established as the where a CT is not convenient or a high d.c. component true di/dt sensors. Due to no non-linearity in the exists (high immunity to d.c. transients in primary). measurement, this arrangement is advantageous to measure – They are compatible under broadband high frequencies large to very large currents with the least error. This 0.1 Hz–100 MHz. method is quite popular for measuring large system – As a broadband measuring device they can measure currents accurately at any voltage. Figures 15.41 and sinusoidal and irregular current waveforms accurately 15.42 show a few applications. without electrical contact and also display it on an oscilloscope. Unique features – Because of non-magnetic core they have high linearity – They are environment friendly as they use no oil or and cause no saturation or ferro-resonance effects at SF6 as in conventional HV and EHV instrument fault currents. And as there are no iron losses they transformers and therefore call for little maintenance. are accurate and linear current sensing devices. – System voltage is no bar and they can be employed – They can measure currents from 50 mA–50 kA and for LV and all HV and EHV applications. They can more. be encapsulated and fixed around bushings or cables – They cause low errors and have accuracy up to 1% avoiding the need for high insulation. and less. Instrument and control transformers: applications and selection 15/573
– Can also measure capacitive discharge currents of (Section 23.5.2). These transformers also conform to any magnitude. IEC 60044-8. –Can monitor power semiconductors switching performance. Principle of operation The same Faraday effect applies – Can be used as current monitors, current probes and (to sense and integrate electric field). The current through current sensors. a conductor induces electric field that affects propagation – Fault monitoring – by measuring breaker fault currents. of light travelling through an optical fibre wrapped around the conductor CC¢ (for simplicity not shown in Figure Applications 15.40(a)). The electric field changes with the change in The use of Rogowski coils is now on the rise on HV and the conductor current – and that changes the velocity of EHV systems. They are being used on a live circuit for the polarized light waves in the sensing fibre. By high current measurements which can be rich in harmonics measuring change in light velocity in an interferometric* such as capacitor circuit currents, busbar systems, scheme and processing the information so obtained, an circulating currents in beams, columns etc. As they can extremely accurate measurement of current is obtained. operate accurately under broadband high frequencies they can also be used for measuring transient currents and Digital current integrator Since the Rogowski coil sensor testing of generators, circuit breakers, bus systems or measures the current in time derivative (di/dt), an any kind of high current system. A unique application is electronic integrator is essential to convert this to the to measure arc currents of an arc furnace to monitor arc primary current ‘i(t)’ for further processing. Usually a restrikes to optimize heating. high performance digital integrator is used to convert the di/dt signal output to the current output. It also takes Note account of EMI. 1. A Rogowski coil is a low VA device of the order of 0.001 VA during normal service and 0.25 VA on fault and can feed only low VA burdens. With the application of the state-of-the-art Additional features of a digital RC compared to microprocessor based measuring and protective devices the an analog Rogowski coil VA demand of the measuring and protective circuits is now very low and these coils are fully capable to feed these – They can combine with a capacitor voltage divider burdens. unit (CVT), Section 15.4.4 and perform current and 2. Some manufacturers have combined the Rogowski coil and voltage sensing for measurement and protection Hall sensors to achieve a current probe to take advantage of through a single device. both. Such as measurement of d.c. component (which is not possible by RCs), large a.c. components (which is not possible – Accuracy is very high and can conform to IEC-Class by Hall sensors) and measuring the current directly than using 0.2 and 0.3. an electronic integrator etc. – As real-time monitoring sensors they can also send out data for change in temperature and environmental 15.11.6 Digital optical instrument transformers conditions. (also known as electronic instrument current – Can undertake extensive metering and data acquisition – Long-term trending analysis. transformers) – Use of optical or crystal fibre eliminates elaborate A Rogowski coil (RC) by employing Faraday’s effect insulation. and using optical fibre or optical crystal technology and incorporating digital current integrator can be made Note digital to provide digital signals. They may also be called Above we have discussed Rogowski coil digital instrument transformers being the next generation instrument transformers. as Faraday effect current sensors. As digital device they Conventional digital optical instrument transformers (they are also can transmit data in digital form and can be termed as electronic instrument transformers) are, however, already in use. new generation instrument transformers. They are capable For details see Cigre paper under Further Reading or contact the of digital communication between power generation and manufacturers of conventional HV and EHV instrument distribution systems (Section 24.11.5) and compatible transformers. to interface with other IEDs (relays and measuring instruments) for real time data transfer for power systems’ monitoring and automation schemes like SCADA * Radar interferometry system is a technique for measuring interference phenomena with the use of a device called interferometer. (Section 24.11). They can conform to various protocol Interferometer separates out a beam of light by means of reflection requirements like IEC 61850 now in vogue. Optical in two beams to produce interference pattern to measure wavelength fibre or crystal sensors are not sensitive to EMI influences and index of refraction to determine the circuit current. 15/574 Electrical Power Engineering Reference & Applications Handbook
Relevant Standards
IEC Title IS BS ISO 60034-1/2004 Rotating electrical machines. 4722/2001, BS EN 60034-1/1998 – Rating and performance. 325/2002 60044-1/2002 Specifications for current transformers. 2705-1/2002, BS EN 60044-1/1999 – General requirements. 2705-2/2002, Application guide for current transformers. 4201/2001 60044-2/2003 Application guide for voltage transformers. 4146/2001, BS 7729/1995, – General requirements for voltage transformers. 3156-1/2002, BS EN 60044-2/1999 Measuring voltage transformers. 3156-2/2002 60044-5/2004 Instrument transformers. 5547/2001 BS EN 60044-5/2004 – Capacitor voltage transformers. 60044-6/1992 Protective current transformers. 2705-3/2002 BS EN 60044-6/1999 – Protective current transformers for special purpose 2705-4/2002 applications. 60044-7/1999 Instrument transformers – Electronic VTs – – – 60044-8/2002 Instrument transformers – Electronic CTs – – – 60051-1 to 9 Direct acting indicating analogue electrical measuring 1248-1 to 9 BS 89-1 to 9 – instruments and their accessories. 60059/1999 Standard current ratings (based on Renald series R-10 of – – 3/1973 ISO-3). 60060-1/1989 High voltage testing techniques. General definitions and 2071-1/1999 BS 923-1/1990 – test requirements. 60060-2/1994 High voltage test techniques. Measuring systems. 2071-2/2001 BS EN 60060-2/1997 – 60076-3/2000 Power transformers. Specification for insulation levels 2026-3/2001 – – and dielectric tests. 60255-6/1988 Electrical relays for power system protection. General 3231/2001 BS EN 60255-6/1995 – requirements. 3842/2001 60439-1/2004 Low voltage switchgear and controlgear assemblies. 8623-1/1998 BS EN 60439-1/1999 – Requirements for type-tested and partially type-tested assemblies. – Summation current transformers. 6949/2001 – – – Specification for control transformers for switchgear and 12021/2000 – – controlgear for voltages not exceeding 1000 V.
Relevant US Standards ANSI/NEMA and IEEE
ANSI/IEEE-C57.13/1993 Instrument transformers (CTs and VTs) – Requirements.
Notes 1 In the table of relevant Standards while the latest editions of the Standards are provided, it is possible that revised editions have become available or some of them are even withdrawn. With the advances in technology and/or its application, the upgrading of Standards is a continuous process by different Standards organizations. It is therefore advisable that for more authentic references, one may consult the relevant organizations for the latest version of a Standard. 2 Some of the BS or IS Standards mentioned against IEC may not be identical. 3The year noted against each Standard may also refer to the year it was last reaffirmed and not necessarily the year of publication.
List of formulae used n = turn ratio of the CTs Im =magnetizing current of the CTs corresponding to Differential ground fault protection the Vft Ist = relay current setting Current setting of the relay, N = no. of CTs falling in parallel
Ipf = n (N ¥ Im + Ist) (15.1) To limit the peak voltage Ipf =minimum fault current through the primary required to trip the relay VVVVp = 2 2 k (m – k ) (15.2) Instrument and control transformers: applications and selection 15/575
Vp = peak voltage across the relay Ray, W.F., Hewson, C.R., High Performance Rogowski Current Transducers, Power Electronic Measurements Ltd, NG96AD, Vm = theoretical maximum CT secondary voltage across UK. the relay circuit at the maximum internal fault current Klimek, Andrew, Optical Technology: A New Generation Vk = knee point voltage of Instrument Transformers, Electricity Today Magazine, Canada. Development of an electronic instrument transformer (active and Further Reading passive) Adolfo Ibero, Jose Miguel Nogueiras, Electrotecnica Arteche, Hnos, S.A. (Spain) – 12/23/34-04 (Session 2000) © Cigre. Teyssandier Christian, n∞170 – From current transformers to hybrid Protective Relays and Application Guide, GEC Measurement, General sensors, in HV, Merlin Gerin – E/CT 170 first issued March, Electric Co. Ltd, Stafford, UK. 1995.