Protective Relaying for Transmission And

PROTECTIVE RELAYING FOR TRANSMISSION AND DISTRIBUTION LINES

(Basic concept, terminology functions of CTs & PTs, important types of relays etc, with reference to transmission & distribution system protection). - Er. Y.S.Jadaun, Chief Engineer, U.P.S.E.B. 1.0 INTRODUCTION The power system equipment employed in generation, transmission and distribution of electrical power is prone to accidents, some of which can be very disastrous. Such accidents are inevitable due to deterioration of insulation, lightning stroks, entry of birds and rodents into the equipment and human error in system operation. The capital investment put in a power system is large. To save this equipment from abnormal system running conditions and breakdowns generally known a “faults” and to ensure satisfactory operation of power supply system, constant watch is required to be had on the equipment and running conditions. On development of abnormal conditions / faults a very quick assessment / analysis of condition and remedial action is required to be taken so much so that it generally becomes humanly impossible to do so before taking place of damage to equipment / property and life. As such, special equipment referred as protective Gear equipment is required to be employed to sense the trouble and disconnect the faulty equipment section from the rest of the system in time to minimize the trouble. The power supply system consists of generating stations, transmission lines, substations, distribution lines, distribution substations and the distribution mains etc. Different relays / schemes are employed for the various equipment used in the power system based on principles most suited to the particular circumstances. The scope of the present assignment is to discuss the protection relays schemes and related subject as applied to the transmission & distribution lines and substations.

2.0 TYPES OF FAULTS The types of faults can be broadly classified into (i) three phase, (ii) double phase to ground (iii) phase to phase (iv) phase to ground and (v) three phase to ground majority of faults are phase to ground in general.

3.0 ESSENTIAL OF PROTECTION 3.1 The protection is required to be reliable to ensure safety of equipment, property and life and proper functioning of the unfaulted system. There are many components in a protection in – operative or mal – operative. Though complete reliability is impossible to achieve but it can be increased to great extent by taking following measures: (i) Use of very good quality relays. (ii) Appropriate application (iii) Proper application of CTs & PTs. (iv) Proper maintenance and frequent & regular checking and testing. (v) Employing well trained staff to handle the equipment. (vi) Use of ferrules & numbering to Identify wires / cables and terminals to ensure proper handling during maintenance / testing. (vii) Proper &.C. supply arrangement and its frequent maintenance. (viii) Analysis of trippings and takings appropriate measures such as changing the settings as demanded by the situation". (ix) Supervision of PT, CT supplies and trip healthy circuits. (x) Maintaining proper environment e.g. temperature, moisture etc. (xi) Proper coordination of protective gear schemes.

3.2 Selectivity

Overlapping zones of protection are arranged to ensure protection of each & every part of the system. While proper selectivity on the part of these overlapping zones / protection schemes may ensure proper functioning of the power system, improper selectivity may cause unnecessary plant outage. The schemes are classified into two on this basis:- (i) Unit System: (Such as differential protection)  These are absolutely selective.

(ii) Non – Unit Systems (Such as over current)  These are relatively selective.

Some of the points which may increase selectivity if taken care are:-

(a) Proper application of C.Ts., P.Ts. and protection schemes. (b) Ensuring desired operating characteristics. (c) Proper co-ordination of overlapping zones.

3.3 Speed of Operation:- The protection should operate quickly to limit the extent of damage. Fast clearance of fault improves systems stability also. But, this should not be at the cost of selectivity as in that case mal -trippings will take place causing unnecessary and frequent supply interruptions and at times, deteriorate the system stability as well.

3.4 Sensitivity Sensitivity, is a measure of impedance presented by protection to the current and voltage transformers. If this current high, the ratio & phase angle errors of instrument transformer will increases and the effective settings of protection will go up. The protection should be sensitive enough to ensure its reliable operation under minimum fault conditions and stable during maximum load / through fault condition.

3.5 Zones of Protection: Zone of protection is the part of system protected by a certain protection scheme. These zones are arranged to overlap each other to avoid blind spots and ensure complete protection. The zone boundaries generally correspond to the locations of CT& Where the CTs are located on both the side of C.Bs., the zones naturally overlap but otherwise blind spots occur. This problem is overcome by some of zone provision. Fig:1 Location of C.T CZ= Circuit Protection zone, BZ= Bus Bar Protection Zone, F= Fault.

In case (c) fig.l fault F is not covered in circuit protection and therefore it is taken care of by covering it in Bus protection scheme.

3.6 Primary and Backup Relaying The primary and back - up protections are the first and second lines of defense. The best form of back - up protection is whom both the A.C and D.C supplies are separate. But a compromise may have to be sought from practicability and /or economic view points.

The cause of failure of primary protections may be: (i) Fai lure/faulty C.T. &P.T. supply to relay, (ii) D.C. trip supply failure / fault, Failure of protection relay itself, Failure of trip - circuit, Failure of circuit breaker.

Therefore, duplicate arrangement can ensure protection though back - up. But it is costly & at times impracticable on other counts. Hence the following may be adopted: (i) Use of separate C.T. (ii) Supervision of P.T. supply (iii) Separate trip supply circuits having separate fuses and if possible separate trip coils and supervision of trip circuits. (iv) Use of back - up protection based on different principle to primary one.

3.7 Relay Contacts: These are classified in to:- (i) Self reset (ii) Hand or electrically reset. (iii) Normally open or make contact; (iv) Normally closed or break contact.

The hand or electrically reset contacts are used whore either a maintained impulse is designed or a lock out condition is necessary. The contacts may be delayed operative on (i) Pickup (ii) drop out (iii) both at pick — up and drop out

3.8 Relay Operation Indicators These are: - (i) Mecteiieaily operated. (ii) Electrically operated: These can be :- (a) Series connected. (b) Shunt connected.

3.9 Internal Connections of Relay Contact Systems

These are several methods. Some of the common ones are: (i) Series sealing. (ii) Shunt reinforcing. (iii) Shunt repeat. (iv) Shunt sealing,

3.10 Trip Circuit Supervision Several methods are used to indicate the healthy condition of trip circuit.

4.0 TYPES OF RELAYS (ELECTRO - MAGNETIC) 4.1 Attracted Armature Type These are very simple (Fig.2) and therefore widely used. They work as 'yes* or 'no' 'magnitude' and 'ratios' relays and are used generally as auxiliary, over current, under current over voltage, under voltage and impedance measuring relays.

Fig.2 Attaracted Armature Type Relays

The relays have either a hinged armature or plunger type construction and are governed by equation: -

F = (KI – K1) F = Force K = Constant, I = RMS value of current and K = Restraining force. K Under threshold condition: I  1 constant k The relay operation is influenced by - (i) Ampere - turns developed by relay operating coil, (ii) Size of air gap between core and armature, (iii) Restraining force on the armature- These are inherently instantaneous, but small time delays of the order of 100 ms can be easily produced by magnetic damping. Slugs (bends of conducting material such as copper) are provided at armature and or at the "heel" end of the relay so as to achieve delay in build up of flux and its decay to enable delay at pick - up and at drop out respectively. 4.2 Induction Disc Type These have either shaded pole or watt - metric type electro - magnets and discs. These work on Ferraris principle (a torque is produced by two fluxed with phase displacement between them which is proportional to the produce of their magnitudes and phase displacement).

T  K 12 Sin 

If 1 and 2 are produce by I1 & I2 of the same main current I and fixed by shading arrangement, then it Follows that: Torque = K I2 K1 decreases with increase of current due to magnetic saturation. This phenomenon is made use of to obtain "Definite Minimum Time" in Time / current characteristic.

Fig:3 Induction Disc Type Relay 4.3 Induction Cup Type These relays (Fig.4) are extensions of induction disc type relays. They are used for very high speed operation along - with polarization and / or differential winding as required. 4.4 Reactance and MHO Relays Reactance and MHO type relays ate produced by manipulating current and voltage coil arrangement and relative phase displacement angles between the various fluxes. The induction cup relays can be made to measure pure reactance or impedance.

4.5 Balanced Beam Type These are similar to attracted armature type. Now a- days they are superseded by more sophisticated induction disc and induction cup type relays. The limitations of these relays were poor reset / operate ratio, susceptibility to phase displacement between the two energizing quantities.

4.6 Moving Coil Type Polarized D.C. moving coil relays are amongst most sensitive and are widely used for sensitive and accurate measurement. They are used for distance and differential protections. Being inherently suitable for D.C. only, they are used in CT / P.T circuiteries with rectifier arrangement. These relays are however more expensive then the induction cup type.

(a) Rotary Moving Coil (b) Axlally Moving Coil Fig: 5 Moving Coil Relays

4.7 Polarized Moving Iron Type This is also an extension of attracted armature type with a difference that it is polarized and more sensitive. They are inverse of moving coil type as the coil here is stationary and iron armature is movable. Stationary coil arrangement affords ample winding space and a robust relay there by permitting very high with stand to pickup ratio.

5.0 TYPES OF PROTECTIONS AND USE The following types of protections are employed on transmission / distribution systems:

5.1 Over Current Protection Using: (i) Instantaneous over current (ii) Inverse time over current. * (iii) IDMT over current, (iv) Directional over current (v) Current balance, (vi) Power Balance relays. 5.2 High Speed Distance Protection - Using distance Relay with: (i) Impedance, (ii) Modified impedance, (iii) Reactance, (iv) MHO, (v) Offset MHO and (vi) Some special characteristics.

5.3 Pilot Wire Protections (i) Wire pilot. (ii) Carrier pilot, (iii) Microwave pilot protections.

6.0 SECONDARY SYSTEM PROTECTION The secondary substations ( 66kV & Below) generally vampires a row in coming and outgoing lines with step - down transformers. The 66 kV and 33 kV (Also 37.5 kV tc.) sides are generally controlled by minimum oil and bulk oil circuit breakers if the capacity of transformers is 5 MVA or more otherwise ruses are • generally employed. The 11 kV side is controlled through 11 kV switchgear.

Various makes of 1 IkV switchgears in use in UPSEB are: (a) English Electric, (b) G.E.C. (c) B.I.C.O. (d) Cromptons & Greeves (e) N.G.E.F. (F) Jyoti (g) Siemens, (h) Switchgear & (j) B.H.E.L. (k) Reyrol (l) South wales corp. (m) Voltas (n) Ferguson & Pailen .

The trip principles used are generally three: (i) Trip coils operable directly from C.T. secondary current. (ii) Trip coils operable from secondary current through relay - Called series trip . (iii) Trip coils operable from D.C. ( 24, 110V etc.) battery through relay contacts -called shunt trip.

In case (i) the tripping takes place when the current flows in trip coils exceeds the setting value forcing the plunger there by to move which strikes the tripping - rod -mechanism.

In case (ii) current, when in excess of set value , causes the relay disc to rotate , on closing of relay contacts, the instantaneous unit is energized opening there by the crip coil shorting contacts. This allows the fault current to flow through the trip coil and operate the mechanism. The trip coil ratings are 5 A and 2 or 1A for over current & Earth fault cases respectively.

The trip circuit is operated from D.C battery through current relay in cases (iii) the trip coils are of 5A & 2 or J A for over current & earth fault cases.

The over current and earth fault relays used may have ratings as below :- 6.1 Case (ii) (a) O/C (i) Current setting: 50% to 200% adjustable in 7 equal steps of 25% (ii) Time settings: O to 3 sec at 10 times current setting, (b) E/F (i) Current settings: 10% to 40% adjustable in 7 equal steps of 5% . (ii) Time setting: 0 to 3 sec at 10 times current setting.

6.2 Case (iii) (a) 0/C (i) Current setting: 50% to 200% adjustable in 7 equal steps of 25% . (ii) Time setting: 50% to 200% atjfustabie in 7 equal steps of 25% . (b) E/F (i) Current setting: 10% to 40% adjustable in 5 equal steps . (ii) Time setting: 0 to 3 sec »t 10 times current setting.

The above relays have adjustable inverse time current characteristic with a definite minimum time. The starting and operating currents are of the order of 103 to 105% and 13% of current setting respectively.

7.0 RELAY APPLICATION 7.1 Non - directional Over Current and Earth Fault

These relays are used either as main or back - up protections. (i) IDMT Relays: Limits of accuracy as set by IS: 3231 is shown in shows application to a sectional radial feeder. It seems mat with assumed relay settings and tolerances as per, the minimum permissible time grading interval for successive locations works out to 0.5 seconds. This implies that with large number of sections and increased fault levels , the clearance time towards source will be considerable with good quality relays (such as EE CDG 11) the time intervals can be reduced to 0.35 to 0.4 seconds.

(ii) Combined IDMT & High Set Instantaneous Adding of high set Instantaneous relay is advantageous on long lines with small source impedance and on transformer feeders as it make possible to reduce the tripping time under the maximum short circuit conditions. This would allow shortening of grading time between successive breakers. The effect is to reduce operating time to 0.01 sec in shaded areas. While selecting over current unit setting, care has to be taken of "transient over reach" defined as:

 A - B  transient over reach   100 %  A  Where A = pick up current in steady state r.m.s amps, B = Steady state equivalent r.m.s current just sufficient to operate the relay when the current is fully offset CAO 17 has transient over teacn from 3% to 5%, Transient over reach is not of much importance in case of applications on transformers and transformer feeders, (iii) Definite Time Over Current They are preferably where system fault current various widely (due to change in source impedance). There being relatively small change in time with variation fault current, the grading of several relays in series is easier & surer. These relays are also preferred on locations where high transient peaks of current are experienced in routine e.g. group of motors starting simultaneously and. direct on line.

The following points need attention: (a) The relay should have required setting range & transient over reach. • It should have high drop off to pick up ratio (say 80% to 90%) where it can see faults of adjoining section. • It should have a continues thermal rating not less than rated current of the circuit and a short time setting not less than maximum system fault current seen by the relay for a duration equal to the maximum time delay setting possible on the relay.

7.2 Directional Over Current And Earth Fault Directional features improve selectivity. Directional features can be introduced in ali the relays discussed under 7.1 above. This should be used where:-  Instantaneous protection is used and maximum back feed current is more than 80% of the maximum for end current.  Maximum Back feed current is more than 25% of minimum for end current and time over current units are used.  Pick up a desired below about twice the full load current, and the loan current is in the directional unit non-trip direction.

These can be used on a bank of parallel operating transformer in the absence of unit protection. This protection first for main or bank up on interconnected or ring main as (Fig.15).

8.0 RELAY SETTING CALCULATIONS 8.1 Time Setting Multipliers (TMS) Time is dependent on value of T.MS available and location from source. However, it is not kept zero to avoid spurious tripping due to vibration and for allowing safer contact gap.

Sequential setting intervals between the relays can be kept as 0.4 sec.

T T.M.S for inverse time relay  TM Where, T = Required time of operation of relay. Tm = Time obtained from relay characteristics curve at T.M.S = I & plug setting Multiplier (P.S.M) equivalent to maximum fault current. e,g.: If = 3000A, Relay setting corresponding to primary current = 150A 3000 Then; P.S.M. =  20 150 From standard curves of IDT relay; TM = 2.2 see, - If required time setting = 1.1 sec, 1.1 Then; T.S.M  0.5 2.2 8.2 Current And Time Grading

8.2.1 Current Grading

As for example: As there is sufficient margin in the C.T ratio of C.Bs (3) & (4) a 100% setting is possible and may be selected. The pick-up values of these will then be 500 & 200 A respectively.

Breaker (2) has full load current of 87 Amps. & C.T ratio 75/5 amps. We can select a 125% setting. It gives 94 amps allowing a small margin of overload.

Breaker (l) covering meter would be of normal pattern with a pick up of 105% and based upon a C.T ratio of 150/5 would represent a primary current settings of 157 amp.

As most of the relays are on 6.6 kV side, we choose 6.6 kV as voltage base. On this basis 157 amp at 415 V are equal to 9,8 amps.

It is noticed that there is ample current grading even considering pick up errors of 1.05 to 1.3 times relay setting, Relay 1: Pickup at 9.8 A. Relay 2: Pick up between 94×1.05 to 94×1.3 = 100 to 125A. Relay 3: Pick up between 220×1.05 to 200×1.3 = 210 to 250A. Relay 4: Pick up between 500×1.05 to 500×1.3 = 520 to 650A.

(Relay 4 is with dual characteristics to cater for faults close to bus-bar second setting at 40% in which case pick-up will be 520× 0.4 to 650×0.4 = 210 to 260 Amp.)

If we add an instantaneous element to relay 1, its current setting must be chosen for value above starting current of motor which is 6 times here and so setting of 8 times Hill load will be o.k. If we add an instantaneous element to (2), the choice will depend upon maximum short circuit current on L.V side of transformer so as not to reflected on H.V side. If through fault current is say 1300A, a setting of 1600A is o.k.

8.2.2 Time Grading (a) The time setting of relay (1) and fuse are not adjustable and these can be plotted from manufacturers curve. (b) Pick-up time & current setting of relay (2) is greater than setting of relay (l). The relay (2) can be set at lowest T.M.S ie.0.1. A curve can be plotted by noting time of operation at various fault currents. (c) Pick-up setting of relay (3) is200A (100% setting chosen); P.S.M at 1600 Amp 1600 (Cut-off value of relay (2)  8 200 From standard curves, time setting on T.M.S of 1.0 at 8 times P.S.M is 3.5 sec. Allowing discriminating interval of 0.4 sec. to operating times of relay (2) at 1600 Amp. (which is 0.24 from standard curves); T.M.S of relay(3) 0.24  0.4 0.19 3.3 Now relay pick up range is 210 to 260 amp. at T.A1S 0.19 and, at twice the current = 400 amp, operating time = 0.19 sec at 10X200 = 200 amp. Relay operating time From curve on sec, and at 20×200 = 4000 amps it is 0.42 sec, so a curve can be plotted for relay (3) 2900 Maximum short circuit currents = 2900 amps & P.M.S. = = 14.5, from standard curves, 200 time setting on a T.M.S of 1.0 at 14.5 times plug setting = 2.5 sec. 0.480.4 T.M.S of relay (4) =  0.35sec 2.5 Where 0.48 = operating time of relay (3) at 2900 amp. A curve can be drawn now. Similarly a curve can be drawn for lower setting of Relay (3) Similarly a curve can be drawn for lower setting of Relay (4) 9.0 LINE PROTECTION BY DISTANCE RELAYS Distance relay schemes offer considerable economic and technical advantages. This form of protection is simple to supply and is of high speed class. These can be used both primary and back up protection, 9.1 Principle of Distance Relaying The relay compares local current and voltage in corresponding phase or suitable components of them. For a fault the end of impedance Z, the line drop will be 1Z. It follows that the voltage V V current ratio will be Z. For an internal fault is less than Z. Since Z is proportional to line I I length between the relay and the fault, it is also measure of distance to fault. 9.2 Relay Characteristics (i) Impedance Relay This is simplest form of distance relay and respond only to the magnitudes of the impedance given by applied voltage and current. The characteristic is a circle with radius Z on R-Z diagram. This relay as well as others to be described here may take various forms eg. A balanced beam relay, an induction disc relay , a transducer relay, a moving coil relay and so on. (ii) Directional Relay These are not strictly depend on impedance for operation but belong to the same family and they are dependent on the phase angle between V & I. On R-Z diagram the characteristic is a straight line through origin.

(a) Directional (b) Plain Impedance (c) Angle Impedance Fig: 6 Characteristic of Impedance Relays (iii) MHO Relay These combine the feature (i) and (ii) above. The operation depends upon both the magnitude and phase angle of impressed impedance. The characteristic is a circle passing through origin on R-K. diagram. (iv) Offset MHO Relay It is modified form of (iii) above and have the characteristic does not pass through origin.

(v) Ohm Relay V These have straight line characteristic on polar diagram and operate when the ratio I gives value of complex impedance which lie on side of this characteristic.

(vi) Reactance Relay It is special case of (v) and has a polar characteristic as straight line to the resistance axis.

9.3 Application Of Distance Relay (i) Impedance With Directional Impedance relay is non-directional so a directional unit is required along with to make the scheme directionally selective. The relay can operate for terminal faults except for a three phase One in which case the operation becomes uncertain. The difficulty also exists in relation to susceptibility of relays to system power swings. Further, under external fault condition. On an interconnected system it is possible for a sudden reversal of current to take place when one C.B opens on a double circuit line and unless the timings of the directional and impedance relays are properly coordinated, a spurious tripping may take place. In this case both the directional and impedance relays require slow operating and fast resetting times.

If only one impedance and one directional relay is used and switching is employed to obtain zone II and zone III by means of timer than starting must be by means of directional relays. A directional relay is liable to operate under normal load conditions and therefore such an arrangement is not very satisfactory.

(ii) Impedance With MHO This scheme is less prove to normal loads and power swings. However, some problems are encountered as regards directional control as discussed earlier. (iii) Reactance with MHO The difficulties here are also similar to the ones discussed above. The use or reactance relay is particularly suited to short lines having high source to line impedance ratio since it can operate down to a lower voltage on account of the fact that reactance relay obtains its polarization input from system current and not the voltage. The reach of relay does not vary with fault resistance theoretically. However when fault resistance are comparable, the reach of the relay is modified by the value of the load and its p.f and it may either over- reach or under-reach. Also, when the fault current is being fed from both ends of a line with two current not in phase, the fault resistance will appear as complex impedance. Tthis will positive on higher source angle side relay, causing the relay to under reach and negative on the lower source angle side causing this relay to over reach. (iv) MHO With Offset MHO Being inherently directional, the use of MHO relay avoids use of separate directional relay and therefore the difficulties associated with it. The reach point varies with the fault angle as the impedance measurement is not constant at all angles.

Hence, those are better suited to power swings and heavy load conditions limitation for terminal fault is there unless special measure is provided such as "memory" circuit. Arc resistance causes under reach.

(a) Reactance Relay (b) Conductance Relay (c) MHO Relay Fig: 7 Various Relay Characteristic

The ellipticalisation of MHO characteristic improves of the scheme on power swings and heavily loaded lines, but worsens the response to fault resistance. This scheme has found wide application due its better over 11 performance.

(v) Starting Relay The primary function is to control the timing relay for extending reach to second and other zones (Fig:8). They also provide directional discrimination when used with non- directional relay. This means that a variety of combinations can be had eg. (a) Directional arc with impedance measuring relays. (b) O/C with MHO measuring relays. (c) MHO with any distance relay including reactance type etc.

Fig.8 Three Step MHO Distance Relay Block (vi) Special Characteristics

Complex characteristics relay such as hyperbolic, rectangular, elliptical, combination of various other characteristics etc are available and may be used under various situations.

(a) Offset MHO (b) Elliptical (c) Rectangular Fig: 9 Some Other Characteristic Special characteristics are produced by manipulation of electrical quantities

Fig: 10 Schematic Voltage S1 and S2

VL = Line Voltage IL = Line Current Ma = Amplitude Comparator Mp = Phase Angle Comparator S1 & S2 = Derived Signals C = Comparator. Table: Derivative Voltages S1 and S2

S.No Distance Amplitude Comparator Phase Angle Comparator Scheme Operating Restraining Quantity Operating Quantity Restraining Quantity Quantity

1 Directional ILZR VL VL VL I L  I L  Z R Z R 2 Plain I Z V  I Z V  I L VL L R L L R L Impedance Z R

3 Angle I Z V  I Z  V VL L R L L R Impedance I  L L Z Z R R

4 Reactance V V I LZR VLSin I LZ R  I  L L L X Z R R

5 Conductance V V I L Z R VLCos I Z  I  L L L R L R R

6 MHO I I Z V  VL L VL L R L I L  Z R 7 Offset MHO I Z V Cos (   (V  nI Z ) I L VL L R L L L R  nLL Z R

8 Elliptical ' '' I L VL  Z R I L VL  Z R I L (3 input)  2a 2a

10.0 DISTANCE SCHEMES USED IN U.P.S.E.B (i) ASEA Make RYZEB and RYZOE (ii) Brown Beveri Make L3 WYAS + L6 f and L3WYS +L6 f (iii) Siemens Make R3Z24, R2Z24a, R1Z25. (iv) English. Electric Make SSRR3V, MM3V, MM3t, RR3V. (v) Static Relays of various makes.

11.0 CURRENT TRANSFORMERS These are used to provide proportional currents for protection schemes at the same time isolating the primary -and secondary circuits electricity. These are required to give fairly proportional reproduction of secondary currents over a large value of primary currents. In a C.T input tc .To C.T approximates to a current source and therefore primary winding impedance can be neglected,

C.Ts can be two types viz.; High reactance or low reactance type. The first one is wousid primary type and the second one toroidal type.

The exciting current depends upon core material used and amount of flux required for a certain burden. This may be obtained directly from magnetizing characteristic since the secondary e.m.f and therefore the flux developed is proportional to the product of secondary current and burden impedance. The core material used in CTs are

(i) Hot rolled silicon steel and cold rolled silicon steel. (ii) Nickel iron alloy. The knee point occurs at high flux density in case of (i) and (ii) and are used for protection C.T and at low flux density in case (iii) and this is used for C.T in metering / instrumentation.

A C.T is required to develop a voltage of magnitude

 I f RCT  RL  RR 

where: If = fault current.k RCT = C.T secondary resistance. RL = Load resistance

RR = Impedance1 of the relaying scheme at current If through it.

This method is O.K. If operating time of the relay is more than 0. 1 sec. But with X fast protections operating in transient and subtransient impedance regions, ratio, is to be R taken in to consideration to ta'ke in to account the effect of saturation of C.T.

The C.T are classified according to the allowable percentage errors in secondary current reproduction. The classification as per the Indian standards (IS: 2705 are given in the following tables 1 and 2.

12.0 VOLTAGE TRANSFORMERS The V.Ts are used to provide proportional voltage in secondary circuit at the same time isolating secondary circuit from the primary electrically.

The V.Ts are required to maintain protection accuracy between 5% of rated • voltage and the highest possible system voltage depending upon the system being effectively earthed and non- effectively earthed.

A secondary winding is connected in broken or open delta for use in earth fault protection whenever such a protection is required to be used. In that case two windings may be required to be had in the V.T., the second winding connected in broken delta. TABLE-1: LIMITS OF ERRORS FOR PROTECTION C.T

Accuracy Current Errors at Phase Displacement Composite Errors at Class Related Primary rated Primary Current Rated Accuracy Limit Current Primary Current Percent Minutes Percent ± 1 + 60 5 ±3 ± 10 ±5 + 15

TABLE-II: ACCURACY CLASS FOR METERING CTs

Application Class of Accuracy (a) Precision testing or as a standard for testing laboratory current 0.1 transformers. (b) For laboratory and test work in conjunction with high accuracy 0.2 indicating instruments integrating meters and also for standard for testing of industrial current transformers. (0 For precision industrial metering 0.5 (d) For commercial and industrial metering 0.5 or 1 (e) For use with indicating and graphic watt meters and ammeters 1 or 3 (f) For purposes where the ratio of mass importance, foe eg 3 or 5 ammeters there approximate values are required.

13.0 RECENT TRENDS Before 1980’s electric magnetic relays have been in use universally, through static relays had come in existence much earlier. The power system having electromagnetic and static relays still continue to have them. It is because mainly of two reasons, viz (i) The protection systems using electromagnetic and static relays are functioning in a very good manner, (ii) Replacement of these systems, which are very large in number, involves huge cost apart from extensive work required in carrying out of the same. However where ever replacement is called for, the same is being done in line with the current technology. 1980's saw a trend towards use of individual solid state relays in place of electromagnetic ones for each protective function.

With the introduction of microprocessor based multifunction digital protective relays in 1980's, the trend has changed and this technology is being preferred now a days in place of use of "solid state relays. 13.1 Advantages of Microprocessor Based Protective Relay (MPR)

(i) Self Monitoring: These arc "watch dog timers" to continuously monitor their own operating status. Any potential malfunctioning is immediately identified and reported to the distributed Computer Control System (DCS)/ Programmable Logic Controller (PLC). A protective relaying system is required to operate only in case of abnormal condition of running of power system / equipment and therefore, any malfunctioning of earlier types could be detected only on routine testing or on its failing to operate when it was required to operate. The self monitoring feature of MPR make is much advantageous, (ii) Communication: The digital relays are provided with serial data parts based on established protocols compatible with the DCS / PLC communication protocol used at the plant. This allows these relays to use digital communication scheme to communicate to plant control system. (iii) Multiple Protective Functions: The older relays required an individual relay for each protective function. The digital relays provide multiple protective functions in one relay there by reducing panel space and wiring costs, (iv) Self- Calibration: Digital relays are provided with self- calibration routine which can be initiated by selecting relay calibration mode in soft ware programming of the relay. (v) Programmable Set points: Defining zones of protection for each primary and backup relay, calculating settings, performing co-ordination analysis etc needed experienced engineers for application of previous type relay systems.

In case of MPR this is a simpler job since digital relay uses widows based soft ware programmed which provides tutorials and recommended set points for each protective relay function based on system characteristics. Most of the soft ware programmers also provide graphic display worksheets showing time current characteristics of the relays in the form of graph to simplify co-ordination amongst multi protective functions contained in one relay. (vi) Event Storage: 8 to 10 selected wave forms can be stored by digital relays on an oscillograph record. This will show the condition of each of the selected wave forms before and after the protective relay has operated. This information is valuable as it helps determining the reason of operation of relay and cause of tripping. .

13.2 Application Possibilities and Preferences Use of MPR is fast gaining acceptance especially in new plant and systems on account of the various advantages associated with it. Use of these is also being made in cases of renovation,, modernization and remote control of the old/worn out power stations/power systems, However, their use-in. existing systems is not finding .many acceptances due-, to cost and other implications, particularly in " developing countries. It appears that it will take quite some tirne in charge over and till such time old systems will continue to run concurrently with the MPRs.