Theory and Technology of Instrument Transformers

THEORY AND TECHNOLOGY OF INSTRUMENT TRANSFORMERS

TRAINING BOOKLET: 2 The information in this document is subject to change. Contact ARTECHE to conﬁ rm the characteristics and availability of the products described here.

Jaime Berrosteguieta / Ángel Enzunza © ARTECHE

Moving together CONTENTS

Theory and technology of Instrument Transformers 1. INTRODUCTION TO INSTRUMENT TRANSFORMERS

1.1. DEFINITIONS

Instrument transformers (ITs) are transformers designed to supply measuring instruments, meters, relays and other similar devices. CT VT

There are two types of instrument transformer VT › Current transformers, in which the secondary current is under normal working conditions, A V V practically proportional to the primary current and phase shifted from it by an R R R angle close to zero in the appropriate direction for connections; and › Voltage transformers, in which the secondary voltage is under normal working conditions, W practically proportional to the primary › Fig. 1.1 voltage and phase shifted from it by an angle close to zero in the appropriate direction for connections.

1.2. OBJECTIVE

The purpose of instrument transformers is to reduce the voltage and current of an electrical network to a standardized non hazardous level.

They prevent any direct connection between instruments and high voltage circuits which would be dangerous to operators and would need instrument panels with special insulation. They also do away with the need for expensive special instruments when high currents have to be measured.

Figure 1.1 shows a simple circuit diagram in which one current transformer (CT) and two voltage transformers (VTs) are included. One of the latter is connected between phases and the other between phase and earth.

4 Theory and technology of Instrument Transformers 1. INTRODUCTION TO INSTRUMENT TRANSFORMERS

1.3. GENERAL POINTS IN CURRENT TRANSFORMERS

The primary of a current transformer is made up of one or more coils connected in Kn = 100/5/5 P2 series with the circuit whose current is to be measured. The secondary supplies the current circuits of one or more measuring apparatus, P1 which are connected in series.

The primary winding may have one, two or four sections, allowing for one, two or three rated primary currents depending on how they are connected.

The secondary windings may also be one or more in number, with each wound on its own magnetic circuit. In this way one secondary does not inﬂ uence the other. Fig. 1.2 shows a CT with two independent secondaries. The core of a CT is normally ring-type, with the secondary evenly distributed to minimize the secondary ﬂ ux losses. 2S2 The primary consists of one or more coils 2S1 connected in series with the line. There are also CTs in which the primary is not incorporated; 1S2 in this case the main insulation may be in 1S1 the primary (cables, bushings etc.) or in the transformer itself. › Fig. 1.2

Figure 1.3 shows various types of CT.

› CT without primary › CT with one › CT with several incorporated primary coil primary coils › Fig. 1.3

Theory and technology of Instrument Transformers 5 1. INTRODUCTION TO INSTRUMENT TRANSFORMERS

1.4. GENERAL POINTS IN VOLTAGE TRANSFORMERS

The primary of a voltage transformer is Unlike CTs, they are therefore not independent, connected to the terminals between which and the load of one secondary inﬂ uences the the voltage is to be measured, and the accuracy of the other. secondary is connected to the voltage circuits of one or more measuring devices, connected Fig. 1.4 shows a VT with two secondaries and in parallel. a tap in each.

Voltage transformers are more like power VTs may be used to measure the voltage transformers than current transformers are. between phases or between a phase and earth. In this case one end of its primary For reasons of construction and insulation, winding will be directly earthed, inside or VTs are normally made with a rectangular outside the transformer. Fig. 1.5 shows two core and the secondaries (if there is more types of VT. Beyond around 72.5 kV., all VTs than one) are wound on the same core. are phase/earth type.

1b 2b 2a2 1a2 1a1 2a1

› Fig. 1.4 › Fig. 1.5

6 Theory and technology of Instrument Transformers 2. THEORY OF INSTRUMENT TRANSFORMERS

2.1. BASICS A transformer is made up of two windings Bearing in mind that N · = i · £ the general wound onto a magnetic core. The primary is equations for the transformer are: powered by the voltage up and absorbs the dφ di current ip. The secondary supplies the current is ______p Up = Np + Rp ip + £p = to the outside load, with voltage us. (see ﬁ g. 2.1) dt dt

If the secondary terminals are open, the dφ dis U = N ______+ R i + £ = ______primary acts as an auto-induction on the iron s s dt s s s dt core, absorbing the excitation current ipo, which comprises a magnetizing component i and a pμ Np ip = Ns is + Np ipo loss component ipw. [2.1] If the whole ﬂ ux φ created by the primary is picked up by the secondary, we can establish: And for sine wave sizes: Ū = N Ē + R Ī + jX Ī dφ dφ p p p p p p e = N ______e = N _____ p p dt s S dt Ūs = NsĒ + Rs Īs + jXsĪs

Applying Ohm's law and disregarding the NpĪp = NsĪs + NpĪpo resistance of the primary winding, we have: dφ [2.2] u - e = 0; u = e = N ______p p p p p dt u e N ______p = ______p = ______p = K where E is the electromotive force induced in us es Ns dφ a coil. u - e = 0; u = e = N ______s s s s s dt where K is the transformer ratio.

When a load is connected to the secondary terminals, secondary current is appears. This gives rise to a ﬂ ux opposed to that created by ip. To maintain up constant the primary current increases so that:

N i - N i ______p p s s = φ R

Therefore, since F = φ · R = Np · ipo, what is left is

Np · ip = Ns · is + Np · ipo φ φ In a perfect transformer, Np · ipo is insigniﬁ cant, and therefore, + Rp A i N 1 A N i = N i ______p == ______s ______p p s s i N K ip s p ep ip Up Up p

If the secondary load is Zs, we have: B B u i u u ______s ______s ______s ______p is = , and therefore: ip = ==2 ZS K KZs K Zs + Rs a a where it can be seen that the eff ect is similar is es to placing a load K2Z on the primary. On a real is s s Us Us transformer we must bear in mind not only the b excitation current ipo, but also the resistances b Rp and Rs of the windings and the leakage

ﬂ ux p and s, as shown in ﬁ g. 2.2.

› Fig. 2.1 › Fig. 2.2

Theory and technology of Instrument Transformers 7 2. THEORY OF INSTRUMENT TRANSFORMERS

2.2. EQUIVALENT TRANSFORMER To study instrument transformers it is of In the same way, from equations [2.2]: interest to refer to the secondary, whose rated values vary little in general. Ū N Ē RĪ X Ī ______p =______p + ______p p + j ______p p , from which: K KK K Let us look at how primary magnitudes are reﬂ ected in the secondary. Ū R X ______p ______p ______p Ns= Ē + 2 ( Īs + Īo ) + j 2 ( Ī s + Īo) From [2.2]: KK K

We can see that R /K2 and X /K2 are the NpĪp = NsĪs + NpĪpo p p resistance and the reactance of the primary, seen from the secondary. corresponds to: Dividing by Ns:

N N Ūp Rp Xp ______p ______p ______Ī = Ī + Ī ; K Ī = Ī + K Ī = Ū' ; Ī K = Ī' ; 2 = R' y 2 = X' p s po p s po KKp p p p K p Ns Ns

equations [2.2] are transformed into: Where KĪpo is the excitation current absorbed by the transformer if the voltage applied to Ū'p = NS Ē + (R'p + jX'p) Ī'p the secondary is Up / K. Ūs = Ns Ē - (Rs + jXs) Īs KĪpo will hence be called Īo

Ī'p = Īs + Īo Therefore, KĪp = Īs + Īo [2.3]

2.3. EQUIVALENT TRANSFORMER CIRCUIT DIAGRAM

From equations [2.3.] we can obtain the R'p X'p Rs Xs equivalent circuit diagram of the transformer. This is shown in ﬁ g. 2.3. Io I'p Is Iw Iμ

U'p Rf Xμ Es Us

› Fig. 2.3

8 Theory and technology of Instrument Transformers 3. CURRENT TRANSFORMERS

3.1. GENERAL EQUATIONS

From ﬁ g. 2.3, when the outside load Z is Z'p Zs applied, we obtain ﬁ g. 3.1. Bearing in mind equations [2.3] we can write: Io I'p Is Ēs = Ūs +Z s Īs Ī' = Ī+ Ī s s o U'p Zo Es Us Z

where Ēs = Ns Ē, and as

Ūs = Z Īs, it results that: Ēs = (Z + Zs) Īs = Zt Īs

Recalling Boucherot's formula: › Fig. 3.1 f Ē = 2,22 ______N B S 10-6 Volts eff 50 max B which is valid for sine wave currents, if we make f= 50Hz, the following results: Ē = 2,22 Gauss -6 20000 N BmaxS x 10 Hμ 10000 where: Ē = Voltage in volts 8000 2 S = Net cross section in cm HW 4000 Bmax = Induction in Gauss N = Number of turns H 2000 0 The induction required in the core of the 1000 current transformer to power external load Z 200 is therefore: 400 (Z + Z ) Ī ______s s 6 B = x 10 Gauss 200 2,22 NsS 100 from which the following conclusions may 10-2 2 4 8 10-1 2 4 8 10o H be drawn: AV/cm. › Fig. 3.2

If impedance is ﬁ xed, induction is propor- › δi φ tional to the secondary current. jXsIs › If the secondary current is ﬁ xed, induction is proportional to the total secondary load. Es β Iw Rs Is s Io ω._ Iμ 3.2. VECTORIAL DIAGRAM Us

Bearing in mind equations [2.3], from Is we can obtain the vectorial diagram of a current transformer. To obtain Io we must use the magnetizing curves of the plate used for the core, ﬁ nding Hμ and Hw from B (Fig. 3.2).

This gives: I'p H L H L Is δ ______μ ______w Iμ = Iw = Ns Ns where L is the length of the magnetic circuit. Finally, Fig. 3.3 indicates the vectorial diagram of the CT.

› Fig. 3.3

Theory and technology of Instrument Transformers 9 3. CURRENT TRANSFORMERS

3.3. CURRENT AND PHASE ERRORS

The current error εi is the error which S = Cross section of the magnetic core the transformer introduces into current [cm2]. measurements. It stems from the fact that its μ = B/H = Permeability of the magnetic transformation ratio is not exactly as rated. core [Gauss / AV / cm].

Current error εi expressed as a percentage, is which corresponds to the following formula: Once formula [3.1] is obtained, it shows the various factors involved in current transformer (K l - l ) errors, and allows the following conclusions to ______n s p εi (%) = x 100 be drawn: lp where: 1. With regard to core material:

Kn = rated transformation ratio. Figure 3.4 shows the magnetizing curves of Ip = actual primary current. various materials. Is = actual secondary current.

The phase shift or phase error of a current Curve I is for an old material with a high transformer, δi, is the phase diff erence silicon content, and is shown for the sake of between the vectors of the primary and comparison. Curve II represents a material with secondary currents, with vector directions a high saturation rate, and curve III one with a being chosen so that the angle is zero for a low saturation rate but high permeability at perfect transformer. low induction.

In practice, for loads with cos β = 0,8, phase Figures 3.5 and 3.6 show the values μ and 1/μ, for shift is not a limiting factor, so transformers these materials. We can see that for a minimum are calculated for the maximum ratio error, i.e. error we must use the minimum value of 1/μ, so plate I is of no interest. when Is and Io are in phase.

In this case:

B N l N l ______s o ≈ ______s o II εi = Np lp Ns ls

Be mindful of the following equations: 15000 › Boucherot's formula: I III

-6 10000 Es = 2,22 Ns Bmax S 10

› The Maxwell-Ampere law: 5000

H = Ns Io /L › Ohm's Law: 0,5 121,5 H

Fig. 3.4 Is = Es /Zt › we obtain the following:

L Z ______t εi (%) = 450000 2 N s Sμ [3.1] where:

L = Average length of the magnetic circuit [cm]. Zt = Total impedance of the secondary (internal plus load) [in ohms]. Ns = Nº of turns on the secondary winding.

10 Theory and technology of Instrument Transformers 3. CURRENT TRANSFORMERS

Curve II, for oriented grain material, is signiﬁ cant I μ ___1 when the number of ampere-turns is high μ III enough to reach the accuracy with a small cross section of iron or when a high saturation factor is sought. II Curve III is for Mumetal type material, which allows high induction at a low number of ampere-turns, and a low safety factor. The II choice of material will depend on various technical and economic requirements. III I Fig. 3.7 shows how the error varies when Is B B varies but Zt remains constant. This curve shows the variation in μ in the face of a › Fig. 3.5 › Fig. 3.6 variation in B, which remains proportional to Is.

−∈ 2. As regards to apparent power: i

The apparent power is practically proportional to the total impedance, as Zs << Zt and therefore the error is directly proportional to the apparent power. The core cross section must be made proportional to the apparent power to keep the error within permitted limits, taking into account that if the average line is increased, cross section must be increased to cancel out the eff ect.

It is interesting to note that if a current 0,1 1 1,2 ___Is Isn › Fig. 3.7 transformer is designed to work with maximum μ at rated current and load, when it works with a load Zt/4 the error is reduced to −∈i one quarter if μ remains constant, i.e. for 4 Isn. This is shown in ﬁ g. 3.8.

Since the error is always negative, in practice Zt this curve is centred on the x-axis, giving a positive advance equal to or less than the error. This is achieved by modifying the turns ratio. Fig. 3.9 shows an actual case. Zt/4

3. As regards to the number of ampere-turns: + 1 234___Is Isn › Fig. 3.8 If we maintain Is = 5A the number of ampere- turns is directly proportional to Ns, and therefore the error is inversely proportional to the square of the number of ampereturns of −Ei the secondary.

It is therefore signiﬁ cant to raise the number 1,5 of ampereturns, though there are limits Z imposed by thermal and dynamic conditions Z/4 which force the average line of the iron 0,75 circuit to be increased, which causes a loss Z 0,5 of precision. Furthermore, the increase in Z/4 the number of turns of the secondary raises the overall impedance and therefore also 0,05 0,2 1 1,2 increases the error. 0,5 Axis before compensation 0,75

With compensation 1,5 Without compensation

+Ei › Fig. 3.9

Theory and technology of Instrument Transformers 11 3. CURRENT TRANSFORMERS

3.4. CURRENT TRANSFORMERS FOR MEASURING These are current transformers designed 3.4.3. INSTRUMENT SECURITY to power measuring devices, counters and similar equipment. FACTOR (FS) To protect equipment powered by the 3.4.1. ACCURACY CLASS transformer against short-circuits in the network into which the primary is inserted, The accuracy class of a current transformer for the "rated safety factor" is taken into account. measuring is given by a number (class rate) This is calculated as follows: representing the ratio error limit expressed as a percentage of the rated primary current when Fs = Ips / Ipn the transformer is running at its "accuracy load". where,

Ips is the "rated safety current" Accuracy classes for current transformers for I is the "rated primary current" measuring are 0.1, 0.2, 0.5, 1 and 3. pn The rated safety current is the primary current Practical guide: at which the transformer begins to saturate. Class 0.1 - Laboratory. At that point the secondary current multiplied Class 0.2 - Laboratory, portable reference by the rated transformation ratio should be patterns, high accuracy counters. 0.9 or less times the primary current. Class 0.5 - Normal counters and meters. Class 1 - Panel apparatuses. This means that we can equate it as: Class 3 - Uses where great accuracy is not required. KnIss < 0.9 Ips 3.4.2. EXTENDED CURRENT Fig. 3.10 shows the ratio between the primary and secondary currents for F ≤ 5. RATINGS s In order for a current transformer to be able These are current transformers for measuring to reach a high accuracy rate with a low rated whose accuracy and heating characteristics safety factor, highly permeable, fast saturating extend to more than 120% of the rated primary magnetic plate must be used to construct current. the core. This is normally achieved, though it is not always possible, using expensive high 150-200% of the rated primary current is nickel content plate (e.g. Mumetal). Therefore usually considered as the limit of the range. before selecting Fs we must check if it really needs to be applied. If so, the manufacturer For special applications in CT with class must be consulted considering possible 0.2 and 0.5 with I = 5A, accuracy may be sn increases in transformer prices. extended to 1% of Ipn, In this case the classes are denominated 0.2S and 0.5S.

xIns 1 ___Zn 2 8 Fs ≥ 5 7 Zn 6

5 error > 10% 2 Zn 4

2 Fs ≤ 5 1

0 1 2345678 x Ipn › Fig. 3.10

12 Theory and technology of Instrument Transformers 3. CURRENT TRANSFORMERS

3.4.4. TESTING

Checking the class of a measuring current It is important to recall that the safety factor transformer includes the measure of its depends on the secondary load, and increases transformation ratio with a precision of 0.01%. proportionally to the reduction in total load.

This test can be performed only in specialized Note: Some standards permit both these test laboratories. However, comparison with properly methods, but it must be taken into account calibrated reference transformers via checking that the direct method measures the ratio bridges will show the errors in transformers to a error and the indirect the compound error. high enough standard of accuracy. However the ﬁ gures for Fs obtained by the two methods diff er very little, and the To check the rated safety factor two methods convenience of the application of the indirect can be used: method is reason enough for using it. › Powering the primary winding at the rated safety current and checking that the error in the secondary at its precision load is 10% or more. › Exciting the transformer via the second-

ary winding until Uo = Fs Isn Zt is obtained at the secondary terminals and checking

that Io ≥ 0,1 Fs Isn.

› 420 kV Current Transformers model CA. CFE, Chicoasen (Mexico)

Theory and technology of Instrument Transformers 13 3. CURRENT TRANSFORMERS

3.5. CURRENT TRANSFORMERS FOR PROTECTION These are current transformers intended for 3.5.1. ACCURACY CLASS power protective relays. They must therefore guarantee suffi cient accuracy at current levels The accuracy class of a current transformer several times higher than the rated current. for protection is given by a number (class rate) and the letter "P" (standing for "protection"). With these currents, the error to be considered The class rate indicates the upper limit of the is the "composite error", which is deﬁ ned composite error for the rated accuracy limit as the eff ective ﬁ gure for the integrated current and the accuracy load. After the letter diff erence over a period between the "P" the rated accuracy limit factor is shown. instantaneous primary current and the The normal accuracy classes are 5P and 10P. product of the rated transformation ratio by the actual instantaneous secondary currents. For protection systems in which the In percentage terms this is given by the characteristics of the transformer are an following formula: integral part of the system (such as fast acting diff erential protection devices) there are T protection classes PR and PX. ______100 ____1 2 ∑c (%) = (Kn · is - ip) dt Ip T o Class PR refers to transformers which must

If ip and is are sine wave in shape, the composite guarantee protection as a limited remanence error is the vectorial sum of the ratio error and factor (remanent fl ux to saturation fl ux the phase error. ratio) for which, in some cases, a value for the time constant of the secondary loop In this case the above formula changes to: and/or a maximum value for the secondary winding resistance can be specifi ed. Class PX is applied to low leakage inductance 2 2 transformers (without air gap) for which ∑c = ε i + δ i knowing the secondary excitation curve, the secondary winding resistance, the secondary load resistance and the turns ratio is enough to determine their performance in the protection system to which they are connected.

14 Theory and technology of Instrument Transformers 3. CURRENT TRANSFORMERS

3.5.2. ACCURACY LIMIT 3.5.3. TESTS FACTOR (ALF) In current transformers for protection The "rated accuracy limit current" is the the accuracy must be checked for the highest primary current for which the rated current, using the same system as in transformer, with the accuracy load, meets measuring transformers. The composite error the required limits for the composite error. can be checked for the accuracy limit current in two ways: The "rated accuracy limit factor" is the ratio a. By running a practically sine-wave type of the rated accuracy limit current (Ilpn)to the rated primary current (I ). current through the primary winding at an pn eff ective level equal to the accuracy limit ALF = l / l current. lpn pn b. By determining the excitation current for Remember that the accuracy limit factor the rated frequency and a practically sine- depends on the load, and if this is higher than wave type voltage with an eff ective level the accuracy load the accuracy limit factor is equal to the secondary limit electromotive lower than rated. The "safety factor" and the force. "accuracy limit factor" are similar in concept, The fi rst of these methods is hard to apply, as they indicate the multiple of Ipn at which the CT begins to saturate at its rated load. except in transformers with low primary currents and a low rated accuracy limit factor. The following formula can be used to calculate It can be used in type testing. both these new factors: For individual tests the excitation method is A the only one applicable. F = ______Z + Z s Note: We have diff erentiated clearly where A is a constant which can be obtained between current transformers for measuring from the rated fi gures for F and Z. (see fi g. 3.10) and those for protection, but the two tasks can often be performed by the same unit via two or more independent cores.

Theory and technology of Instrument Transformers 15 3. CURRENT TRANSFORMERS

3.6. CURRENT TRANSFORMERS FOR PROTECTION WHICH REQUIRE TRANSIENT REGIME RESPONSE

3.6.1. GENERAL POINTS

If a CT for protection is required to respond In high voltage lines it must normally be taken correctly during the early cycles of a short- into account that after the ﬁ rst short circuit circuit, the core must be oversized so that it does there is a rapid reconnection which increases not saturate with the non-cyclic component. the residual ﬂ ow in the CT.

The initial level of the non-cyclic component Fig. 3.11 shows icc (a) and CT ﬂ ow (b). The varies (depending on the voltage when the oversizing coeffi cient of the core of the CT TD(K ) short circuit occurs and on the characteristics is the ratio between φT and φA. of the line) between 0 and √2 Icc, where Icc is the eff ective symmetrical short-circuit From formula [3.2], bearing the reconnection current. If we consider this maximum level, the in mind, it results that: transient short circuit current is:

t' (FRT + T ) _ ___ t' ______D -t/T1 _ ___ i = √2 I (e - cos wt) ______w T1 T2 T cc cc 2 T1 T2 KTD = e - e + 1 c T2 - T1 where T1 = L/R is the time constant of the line.

TD T w T T ______D If we consider that the secondary load is ______1 2 T T + e 2 - e 1 + 1 resistive, the ﬂ ow required to prevent the CT T2 - T1 from saturating is: [3.3] t t ______w T1 T2 T T φ = φ 1 2 - sen wt where: T A T - T e - e 1 2 T1 = is the time constant of the line. T2 = is the time constant of the CT. [3.2] t’ = is the duration of the ﬁ rst short circuit. where: FRT = is the fault repetition time (dead time).

T2 = is the transformer time constant. TD = is the time from which CT saturation

φA = is the peak level of the sine-wave is admitted. component of the ﬂ ux.

To simplify this formula, sine wt can be taken as -1. If T2 >> T1 (as is normally the case in CTs), it results that:

φT = φA (w T1 + 1)

(a)

φ KTD

(b) D T t' FRT TD › Fig. 3.11

16 Theory and technology of Instrument Transformers 3. CURRENT TRANSFORMERS

To learn how to oversize a normal protection Example: consider the oversizing factors

CT so that its behaviour during the transient of each type of CT, for T1 = 0.1s, t' = 0.08s, period can be studied, the following formula TD = 0.035s, FRT = 0.5s and f = 50 Hz. can be used:

a. TPX. Consider T2 = 10s F (R + Z ) ______n s n KTD = 26.4. K'TD = Kssc (Rs + R) b. T PY. T2 = 0,5s where: KTD = 14.7.

Fn = is the rated precision limit factor. c. TPZ. T2 = 0,07s Rs = is the secondary winding resistance. KTD = 7.6. Zn = is the rated load impedance. K = is the ratio between the symmetrical ssc Fig. 3.12 shows how a TPZ secondary (1S1- short-circuit current (Icc) and the rated primary current. 1S2) and a secondary with normal protection R = is the actual secondary resistance. (2S1-2S2) respond to a totally shifted primary current. If the actual load is not resistive, addend nº1 in formula [3.3] can be replaced by 1/cos β where

Rs also intervenes in the calculation of β. 3.6.2. CLASSIFICATION OF CTS

CTs are classed in three types:

TPX: CTs with no gap in the core, but with suffi cient cross section to respond correctly during the transient period. They reﬂ ect the non-cyclic component well. T2 is large in comparison to T1.

TPY: CTs with small gaps in the core to reduce residual induction. They reﬂ ect the non-cyclic component fairly well. T2 depends on the degree of precision required (as a guideline, it can vary between 0.3 and 1 second).

TPZ: CTs with larger gaps than in TPY. They reﬂ ect the alternating component well, but not the non-cyclic component. T2 is around 0.07 second. Due to the gaps, a high degree of precision cannot be obtained at In.

97 ms 400 ms 95 ms

29,200 A I1 28,400 A

0.104 A Ie (1S1 - 1S2) 0.815 A

267 A I2 (1S1 - 1S2)

173 A I2 (2S1 - 2S2)

› Fig. 3.12

Theory and technology of Instrument Transformers 17 3. CURRENT TRANSFORMERS

3.7. BURDEN

This is the impedance in the outside circuit VA connected to the secondary winding, expressed 50 in Ohms, with an indication of its power factor. 2 It may also be indicated by its power factor and 45 the apparent burden in volt-amperes absorbed for the rated secondary current. 40 s = 1.5 mm

2 For instance: 35 30VA precision burden for I = 5 A s = 2.5 mm sn 30 30 ______Z= = 1,2 Ohms 25 2 52 s = 4 mm 20

When secondary loads are calculated the load 2 of the connecting cables must be added to 15 s = 6 mm that of the measuring apparatuses. Fig. 3.13 is 10 2 a graph of consumption in VA for the cables s = 10 mm normally used. 5 Table 3.1 below indicated consumption in VA of normal amperometric coils. 0 25 50 75 100 125 150 175 200m › Fig. 3.13 Table 3.1. Consumption of some Apparatuses Powered by CTs

Apparatus VA at In.

Ammeters Indicators 0.25 to 2 Recorders 1,5 to 9 Counters 0.5 to 3 Wattmeters Indicators 1 to 3 Recorders 1.5 to 8 Phase meters Indicators 2 to 6 Recorders 6 to 12 Maximeters 3 Power converters 3 to 6 Relays Overcurrent, inv. time 5 to 8 Overcurrent, timed 1 to 5 Overcurrent, instantaneous 1 to 10 Directional 1.5 to 10 Power, timed 1.5 to 3 Trip switch 3 to 12 Distance 6 to 20 Regulators 10 to 150

In TP type CTs only just the power needed must be called for, and consumption in cables must be kept low. This will make up in part for the oversizing of the core in comparison with CTs with normal protection.

18 Theory and technology of Instrument Transformers 3. CURRENT TRANSFORMERS

3.8. RESISTANCE TO SHORT CIRCUITS Being connected in series to power lines, The ratio of times to currents is as follows: current transformers are subject to the same current and voltage overloads as the lines Ith1 x√t1 = Ith2 x√t2 themselves. For thermal class A transformers a current In general these overcurrents are far higher density of 180 A/mm2 is admissible in copper than the rated currents of the CTs, and have wires, corresponding to a temperature thermal and dynamic eff ects which may increase of 235 ºC (the IEEE/ANSI standard is damage transformers. somewhat more severe in this respect).

Thermal eff ects make it necessary to size the Unless otherwise indicated, CT’s are constructed

CT’s primary correctly. All the heat produced with Ith = 80 In though they may be built up to is considered as being stored in the primary Ith = 1000 In. However, bear in mind that in this conductor, the maximum heating of which is case the power and precision class which can laid down in each standard. be supplied by a particular type of apparatus will be reduced, as the rated ampere-turns will To prevent transformers from breaking under be lower (see 3.3). the dynamic stresses caused in the primary, a suitable mechanical attachment must be 3.8.2. RATED DYNAMIC ensured in the primary. These mechanical stresses are a function of the peak short- CURRENT (Idyn) circuit current. This is the peak of the ﬁ rst amplitude of the The resistance to short circuits in current current which a transformer can withstand transformers is determined by the thermal without damage when the secondary circuit and dynamic limit currents. is shorted.

The dynamic short-circuit current is obtained 3.8.1. RATED SHORT-TIME from the thermal current, taking into account THERMAL CURRENT (Ith) that the latter is given in terms of eff ective level and the former in terms of peak This is a highly eff ective primary current at level. The coeffi cient due to the non-cyclic which the transformer can withstand the Joule component is normally taken as 1.8 (IEC and eff ect for one second without damage, with the other standards). secondary circuit shorted. Therefore: Once the maximum short-circuit power in the line where the CT is ﬁ tted is known, the Idyn = 1,8 √2 Ith. = 2,5 Ith thermal current can be calculated using the following formula: where:

Idyn = Dynamic short circuit current (kA pk). P I = ______th In the IEEE/ANSI standard the two currents 3 x V are deﬁ ned separately and the dynamic limit current is expressed in eff ective kA with a where: fully shifted current, i.e.: Ith = the thermal short-circuit current (kA rms) Ipk = 2 x√2 x I = 2,83 I P = the short circuit power (MVA), and dyn dyn V = the compound voltage (kV) where I = Dynamic short circuit current (kA pk) If the short-circuit duration is other than 1 dyn second (between 0.5 and 5) the duration should be indicated after Ith.

Theory and technology of Instrument Transformers 19 3. CURRENT TRANSFORMERS

3.9. OPERATION OF AN OPEN CIRCUIT CURRENT TRANSFORMER

Let us assume that a current transformer has This problem is certainly signiﬁ cant in been built with a ratio of 1000:1 and a torus transformers for protection because of the type core with an average line length of 35 size of the core, so the peak voltage in the cm whose magnetic plate may be considered secondary terminals is sometimes limited to as saturated with 1 AV/cm. Operating with the 4 or 8 kV and the maximum operating time secondary circuit open, as from Ip = 35A = of the transformer in these conditions is

0.035 Ipn the core is saturated. determined by mutual agreement between customer and manufacturer, as in principle

As from 0.1 Ipn the ﬂ ow slope increases current transformers are not guaranteed to rapidly, as does the voltage in the secondary operate with the secondary open if the peak terminals, whose peak level is proportional to voltage is more than 3.5 kV peak.

√Ip. Losses in the core also increase, and raise the temperature to unacceptable levels.

3.10. SPECIAL VERSIONS OF CURRENT TRANSFORMERS 3.10.1. TRANSFORMERS WITH 3.10.3. TRANSFORMERS WITH SEVERAL CORES HIGH PRIMARY CURRENTS

This could almost be called a normal variant, as There is local saturation in these transformers most transformers are built with one core for due to the off -centre position and shape of the measuring and at least one other for protection. primary bar and principally to the proximity of other bars which prevent high precision from As many cores as desired may be built, being obtained. provided the overall dimensions allows to be economically manufactured. To cancel out these defects, compensation windings must be introduced which ensure 3.10.2. CASCADE constant ﬂ ow over the whole length of the core. TRANSFORMERS

These are built for high voltages, and divide the overall voltage into several steps.

Dielectrically, this is an interesting method, but from the point of view of precision it must be taken into account that the top core must supply the power for all the secondaries.

It is therefore diffi cult to make up a highly precise measuring secondary of a protection secondary with a high rated precision limit factor.

20 Theory and technology of Instrument Transformers 3. CURRENT TRANSFORMERS

3.11. CHOOSING A CURRENT TRANSFORMER

To ensure that a facility runs properly the 8. Thermal and dynamic limit currents. These current transformer must be chosen carefully, should not be set too high, as this could bearing the following points in mind: make the transformer much more expensive. 1. Type of facility: indoor or outdoor. If it is 9. Rated frequency. greater than 1000 m above sea-level, altitude 10. Number of secondaries (cores). is also a factor to be taken into account. 11. Construction details. 2. Insulation level: we recommend choosing as per the various standards. If there are TP type protection secondaries, 3. Rated transformation ratio: remember that the following should also be taken into double or triple ratios can be used and the consideration: range can be extended if necessary.

4. Precision class as per the various standards. 12. Line time constant (T1).

5. Rated power as per the various standards. 13. Short circuit characteristics (t’, FRT, TD). We recommend not choosing too high 14. Precision needed at I . a power level. If there is a big diff erence n 15. Precision needed during the transient between the rated power and the power period. This may refer only to the of the apparatus to be installed, a resistor symmetrical component (TPZ) or also to in series can be ﬁ tted. the non-cyclic component (TPX, TPY). 6. Rated safety factor (if necessary). 7. Rated precision limit factor (protection transformers).

› 800 kV Current Transformers. FSK (Russia)

Theory and technology of Instrument Transformers 21 4. VOLTAGE TRANSFORMERS

4.1. GENERAL EQUATIONS

See ﬁ g. 2.3 and 3.1.

Ūs = Ēs - Rs Īs - jXs Īs δc

Ū'p = Ēs + R'p Ī'p + jX'p Ī'p and given that:

∑XlS Ī'p = Īs + Īo lS it results as: ∈C

Ū'p - Ūs = (R'p + jX'p) ( Īs + Īo) + (Rs + jXs) Īs ∑RlS β = (R'p + jX'p) Io + [(R'p + Rs) + j (X'p + Xs)] Īs [4.1] A Therefore shows that that transformer errors X'plo in load are due to: I W ∈O R'plo Io › unloaded errors, α Iμ › errors due to the secondary current O φ through the short circuit impedance.

From Boucherot's formula we know that: δo

-6 Us U'p Es = 2,22 Ns BS x 10 Volts and since the error is small:

U’p ≈ Us ≈ Es

so if U'p remains constant a voltage transformer will work at constant induction, even if the secondary load varies within admissible limits. › Fig. 4.1

4.2. VECTORIAL DIAGRAM

From equation [4.1] the vectorial diagram of a voltage transformer can be obtained, as shown in ﬁ g. 4.1.

Starting from Us, and as it is shown in the vectorial diagram of current transformers, we attain the Io of the magnetizing curves of the plate.

22 Theory and technology of Instrument Transformers 4. VOLTAGE TRANSFORMERS

4.3. VOLTAGE AND PHASE ERRORS A voltage error is an error introduced by ∈ the transformer in a voltage measurement, resulting from its transformation ratio not D 1,730 Un being exactly as the rated one. Iw tg α = ______Iμ The voltage error εu is expressed as a percentage, and is given by the formula: 0.1 Un C 0.8 Un O VA (Kn Us - Up) 100 ______A B εu % = Up 1.2 Un where,

Kn = is the rated transformation ratio. U = is the actual primary voltage. p α φ U = is the secondary voltage corresponding + δ s › Fig. 4.2 to Up under measuring conditions. -∈

The phase error of a voltage transformer εu is the phase diff erence between the vectors of F cos 0.8 H cos 0.5 the primary and secondary voltage.

Both, the ratio error and the phase error are t made up of the unloaded error plus the load error, as shown in ﬁ g. 4.1. cos β = 1 G Fig. 4.2. shows the no-load triangles, which vary according to Up. A The operating margin of the transformer in UNE, IEC and other standards is between 0.8 and 1.21 Upn.

Fig. 4.3 shows the errors given by the Kapp › Fig. 4.3 diagram according to cos β, starting from the unloaded triangle at 0.8 Upn. 200 VA cos β = 0.8 To obtain the Kapp diagram at 1.2 U , we have pn 200 VA to start from point B on ﬁ g. 4.2, etc. If the F H cos β = 0.5 variation in load is considered, we get ﬁ gure 200 VA cos β = 1 50 VA 4.4, where it can be seen how errors vary G cos β = 0.8 according to voltage, to load and to cos β of the load.

Load errors are parallel lines whose angle OVA depends on cos β of the load, as shown in ﬁ g. B 4.4. Since ratio error is negative, for a number 1.2 Upn A 0.8 Upn of turns equal to the rated transformation ratio, the error is usually centred by means of a correction in the ratio of the number of turns › Fig. 4.4 in order to make the best use of the core. - ∈ Fig. 4.5. shows how VT errors appear when the transformation ratio is correctly modiﬁ ed. 200 VA

1.2 Un

-δ +δ

0.8 Un

Compensation 50 VA + ∈ › Fig. 4.5

Theory and technology of Instrument Transformers 23 4. VOLTAGE TRANSFORMERS

4.4. VOLTAGE TRANSFORMERS FOR MEASURING 4.4.1. DEFINITIONS

These are voltage transformers designed Precision classes for voltage transformers are to power measuring devices, counters and 0.1, 0.2, 0.5, 1 and 3. similar equipment. Guide to applications: Class 0.1 - Laboratory. 4.4.2. PRECISION CLASS Class 0.2 - Laboratory, portable reference patterns, high precision counters. The precision class of a measuring voltage Class 0.5 - Normal counters and meters. transformer is given by a number (class rate), Class 1 - Panel apparatuses. representing the ratio error limit expressed Class 3 - Uses where great precision as a percentage of the rated primary current, is not required. when the transformer is running at its "precision load".

This precision must be maintained for voltages between 80 and 120% of the rated level, with loads between 25 and 100% of the precision load.

4.5. VOLTAGE TRANSFORMERS FOR PROTECTION 4.5.1. DEFINITIONS

These are voltage transformers for power Since the secondaries of a VT are inter- protective relays. dependent, it must be speciﬁ ed whether the precision powers are simultaneous or not, as If a VT is going to be used for both measuring if one of the secondaries is under load only for and protection, two separate windings are short periods of time, then loads can be taken not normally necessary as in the case of as nonsimultaneous. CTs, unless galvanic separation is required. Therefore in IEC standards VTs for protection 4.5.2. ACCURACY CLASS are required to have a precision class, the same way as VTs for measuring. Except for residual voltage windings, VTs for protection must also be specified as VTs On the same type of VT, precision power is for measuring. greater when there is a single secondary than the sum of the precision power of each Accuracy class for protection VTs is given by secondary if there are two, as the space given a number that indicates the maximum error over to insulation of the two secondaries from expressed in percentage. 5% of the rated voltage. each other must be considered. This number is followed by the letter "P".

The "residual voltage winding" is a winding The usual accuracy classes are 3P and 6P. intended to form an open triangle (together with the relevant windings of two other single-phase transformers) to supply residual voltage if there is a fault to earth.

24 Theory and technology of Instrument Transformers 4. VOLTAGE TRANSFORMERS

4.6. BURDEN It is deﬁ ned at the same way as for current transformers (see section 3.7).

Table 4.1 indicates the normal consumption of the voltmeter coils, of devices powered by voltage transformers:

Table 4.1. Consumption of some devices connected to VTs

Approx. Apparatuses Consumption in VA Voltmeters Indicators 2 - 6 Recorders 10 - 25 › 123 kV Inductive Zero meters 5 - 20 Voltage Transformers. Fingrid (Finland) Wattmeters Indicators 1 - 4 Recorders 3 - 15 Phase meters Indicators 4 - 5 Recorders 15 - 20 Meters 3 - 5 Frequency meters Indicators 1 - 5 Recorders 10 - 15 Relays Maximum voltage 10 - 15 Timed maximum voltage or current 25 - 35 Selective 2 - 10 Directional 25 - 40 Minimum voltage 5 - 15 Earth contact 10 - 30 Distance 10 - 30 Synchronoscopes 6 - 15 Voltage regulators 30 - 50

Theory and technology of Instrument Transformers 25 4. VOLTAGE TRANSFORMERS

4.7. SPECIAL VERSIONS OF VOLTAGE TRANSFORMERS

4.7.1. TRANSFORMERS WITH 4.7.3. CASCADE A SEVERAL RATED PRIMARY TRANSFORMERS VOLTAGES When the rated insulation voltage of a voltage transformer is high, it is diffi cult to achieve it These transformers can be made in four ways: with a single coil.

› series-parallel coupling in the primary; Cascade construction involves the distribution › primary coil with taps; of the primary winding in several coils, with › series-parallel coupling in the secondary; the secondary or secondaries on the last coil. This cascade construction means that each › secondary coil with taps. coil only has to withstand a fraction of the total voltage. In the ﬁ rst two ways there are problems with insulation and with the use of the core, which A cascade transformer is made up of one limit their use basically to low voltage work, or more cores with 2 coils each. The core is mainly reference patterns. rectangular shaped, and set to the potential average of the two coils. The secondary series-parallel system is used only if the two sections of the secondary The advantages of cascade voltage transformers winding have the same number of turns, include very small errors under no load, credit otherwise there is an internal circulation to the reduction of the impedance in the current which absorbs power. The two primary winding. Fig. 4.6 shows the circuit sections must be insulated between them to diagram of a cascade transformer with two at least 2 kV. cores and four coils. N The secondary with taps system is very 4.7.4. TRANSFORMERS WITH interesting specially when series-parallel a n systems are not possible or when the power SEVERAL SECONDARY › Fig. 4.6 requirements are the same for both versions. WINDINGS

Before choosing a transformer with these Voltage transformers can be built with characteristics, it is better to consult the several secondary windings on the same core. manufacturer in order to determine the most Although the load on each one will aff ect economical system. the others, the limitations found in current transformers do not apply here because of 4.7.2. TRANSFORMERS WITH the safety and saturation factors.

SEVERAL RATED SECONDARY In voltage transformers, with P2 to earth, VOLTAGES which are to be installed in networks with no neutral to earth, it is advisable to ﬁ t a These can be made in two ways: tertiary (i.e. a second secondary) to protect the transformer against ferro-resonance (see › series-parallel coupling in the secondary; point 4.9). › secondary winding with taps The increase in the cost of the transformer due to this secondary is generally small. The series-parallel coupling in the secondary can only be used when the ratio is 2:1. It retains all the characteristics of a normal transformer regarding its possibilities.

When the voltage ratio is not 2:1, normally, the more used system is the secondary with taps.

26 Theory and technology of Instrument Transformers 4. VOLTAGE TRANSFORMERS

4.8. LINE DISCHARGE VOLTAGE TRANSFORMERS

When a high voltage line is isolated by opening As regards to mechanical stress, the the circuit breakers, the capacitative energy maximum discharge current must be taken stored in it may cause voltage overloads on into consideration. reconnection. In the oscillation case this is: There are several procedures for discharging Rπ lines, but experience has shown that VTs do V ______4L this job well if they are sized properly. If not, i =eW1 max LW discharging may not be fast enough or the VTs 1 may be damaged by heat or dynamic eff ects. where In a simplifi ed study of this problem, considering that when the VT is not saturated, 4L - R2 C W = ______the discharge current is insignifi cant and the 1 2 L C line voltage is constant. When saturation takes place, the reactance decreases to the value is the natural pulse of the circuit. of the primary winding in air, L. In this case the circuit to be considered is shown in fi gure To calculate the times t1 (saturation of the VT) 4.7.a, where C is the line capacity and R the and t2 the following formulae may be used: resistance of the primary winding of the VT.

If R2 C > 4L, discharge is non-cyclic and slow. ______B sat x N1 x S -8 2 t = x 10 If R C < 4L, discharge oscillates, as indicated 1 V [Sec.] in ﬁ g. 4.7b. π ______As regards to heating, it is considered that all t2 = 2 W [Sec.] the energy stored in the line goes to heating 1 the copper of the VT primary. This energy is: where: 1 B sat = is the saturation induction (Gauss). __ 2 W = CV N1 = is the number of turns in the primary 2 winding. S = is the core cross section (cm2). where V is the line voltage on opening. V = is the initial discharge voltage (V).

i L

v c

v,i

v i i.máx. v

t t1 t2

b) › Fig. 4.7

Theory and technology of Instrument Transformers 27 4. VOLTAGE TRANSFORMERS

4.9. OVERVOLTAGES Like other devices installed on the high voltage side, voltage transformers are subjected to a series of overvoltages. It must withstand these without damaging its insulation. Remember that all transformers (both voltage and current) are tested for one minute at the test voltage and at industrial frequency, and are able to withstand the test voltage with shock wave, that corresponds to their level of insulation.

As an example, a measuring transformer with a rated insulation voltage of 72.5 kV rms and with a working voltage of Us = 72.5 / /√3 = 42 kV rms, is tested at 140 kV rms (3,3 Us) for one minute, and has to withstand 325 kV peak (5.5 Us) of lighting impulse.

However, in voltage transformers, series or parallel ferroresonance (depending on the network-transformer characteristics) is not an uncommon phenomenon.

This is a complex phenomenon, as it may be single or three-phase, and may occur at the fundamental, harmonic or sub-harmonic frequencies. Let us see what series and parallel ferro-resonances are.

4.9.1. SERIES FERRO- RESONANCE

Assuming that in the circuit in ﬁ g. 4.8a, where the capacity C and the saturatable inductance of the VT are in series, C is such that the straight line I/wC cuts UL at point M. (ﬁ g. 4.8b).

If the rated voltage is U1, the point of operation is A with a current of I . When there is an 1 C overvoltage, greater than U2, we go from point A through B and C to D. When the voltage drops T.T. again to U1, the new point of equilibrium is E, I where I’1 >> I1. If this new situation lasts for a long time the VT overheats, and it may even burn. U UL To return to point of equilibrium A the voltage in the network must be reduced or the VT must be loaded so that the ferro-resonance is damped. a)

This phenomenon arises in capacitative I Uc = _____ voltage transformers. It may also arise in a U WC 3-phase network with neutral to earth with one phase open if the capacity is high (e.g. a M UL circuit breaker with a distribution capacitor).

U D B U2 C A U1 E

I1 I2 I'1 I'2 I b) › Fig. 4.8

28 Theory and technology of Instrument Transformers 4. VOLTAGE TRANSFORMERS

4.9.2. PARALLEL FERRO- T.T. RESONANCE I IC IL

Fig. 4.9a shows a parallel circuit. Fig. 4.9b is U C similar to ﬁ g. 4.8b, with I and U swapped.

Now, when ferro-resonance is analyzed, we assume that equilibrium is established for I = a)

I1. Due to overvoltage or overcurrent we go, as I in the series case, to point D and then E, where IL U’1 >> U1 and there is a permanent overvoltage.

If we want this to happen in a 3-phase network, tC = UWC the neutral must be insulated. The shift of the M neutral, respect to earth, causes an overvoltage in one or two VTs which may be greater than the compound voltage. I To avoid or dampen this phenomenon, it is D necessary to place a suitable resistor in the B I2 open triangle of the tertiaries of the VTs, as C A E shown in ﬁ g. 4.10. Between 25 Ω and 50 Ω is I1 an usual value. U1 U2 U'1 U'2 U b) › Fig. 4.9

R › Fig. 4.10

› 123 kV Inductive Voltage Transformers. Transpower zzvvv(New Zealand)

Theory and technology of Instrument Transformers 29 4. VOLTAGE TRANSFORMERS

4.10. OPERATION OF VOLTAGE TRANSFORMERS WITH SHORT- CIRCUITED SECONDARIES

The "rated thermal burden" of a VT is the The VT can be protected by placing fuses or maximum power which can be supplied on circuit breakers in the secondary circuit, but it a permanent basis without the heating limits beware that if these devices fail, the substation being exceeded, when the secondary voltage protection system may operate incorrectly. is the rated one. Since most VT failures, involving secondary If the secondary load is higher than the rated short circuits, are caused by incorrect thermal burden, the VT may be damaged, connection of this circuit, it is very practical unless operating time is limited. to ﬁ t provisional fuses until it can be checked that installation is correct. When the secondary circuit is shorted, the secondary current is only limited by the internal impedance of the VT. The VT can only operate for very short periods in these conditions. Some standards require this time to be at least 1 second.

4.11. CHOOSING A VOLTAGE TRANSFORMER By choosing a voltage transformer, consider 1. Type of service: indoor or outdoor. Altitude the following points: is also a factor to be considered, when it is higher than 1.000 m above sea level. 2. Insulation level. 3. Rated transformation ratio. 4. Precision class. 5. Precision power. 6. Voltage factor. 7. Rated frequency. 8. Number of secondaries. 9. Construction details.

30 Theory and technology of Instrument Transformers 5. OTHER INSTRUMENT TRANSFORMERS

5.1. COMBINED INSTRUMENT TRANSFORMERS These are units which contain a voltage When designing combined units, the inﬂ uence transformer and a current transformer in the of CTs on the errors in VTs and vice versa same casing. This system has cost-eff ective must be considered. The standard gives the advantages, especially for high voltage units maximum admissible levels of inﬂ uence. where porcelain accounts for much of the cost of instrument transformers.

It is also important for cases where as little space as possible needs to be occupied at the sub-station.

› 72.5 kV Combined Transfores at subestation line imput. L'ONEE (Morocco)

Theory and technology of Instrument Transformers 31 5. OTHER INSTRUMENT TRANSFORMERS

5.2. CAPACITIVE VOLTAGE TRANSFORMERS (CVTS) 5.2.1. DEFINITIONS

These are voltage transformers made up of a Additionally, capacitance divider and a electro-magnetic element. C U = U ______1 P 1 C + C The capacitance divider (CD) is made up of two 1 2 capacitors, C1 and C2, connected in series as In this way we can analyze a CVT in a similar shown in ﬁ g. 5.1 to obtain an intermediate voltage way to an inductive VT (section 4). However, terminal. An inductance L1 is connected to this other factors aff ecting accuracy must also terminal, along with an inductive intermediate be considered: variations in frequency, voltage transformer (IVT). temperature and stability over time.

5.2.2. CVT OPERATION The response of a CVT in transient state is not as fast as that of an inductive VT, and in some Fig. 5.2 shows the equivalent circuit diagram cases fast protection requirements mean that of a CVT. The diagram is similar to ﬁ g. 2.3, CVTs cannot be used. given that R’p now represents the resistance of However, apart from their measuring and the windings of the IVT and the inductance L1, protection uses, CVTs also enable high voltage the iron losses of L1 and the dielectric losses of lines to be used for communication via high C1 and C2 and X’p represents the reactance due frequency carrier currents. to the capacitance C1+C2, to the inductance L1 and to the IVT primary.

› 550 kV Capacitive Voltage Transformer. UTE (Uruguay) C1 LI TTI

C2 Z

› Fig. 5.1

C1 + C2 X' R'p Rs Xs

X'p

U'p Rf Xμ Us

› Fig. 5.2

32 Theory and technology of Instrument Transformers 6. DIELECTRIC INSULATION

6.1. INSULATION OF INSTRUMENT TRANSFORMERS

From a dielectric point of view, the instrument In VTs mineral oil is still used to some extent transformer, like all electric machines, develop as an inside insulator due to the excellent as new materials and new requirements appear. impregnation of coils obtained. If resin is In our brief analysis in the development of used as an outside insulator, insulating gas instrument transformers (ITs) we shall consider (e.g. SF6) is used to impregnate the coils. To three cases: a) low voltage; b) medium voltage; prevent partial discharges the main insulator and c) high and very high voltage. must be without pores. This is hard to achieve in VT coils if impregnation is not suitable. In low voltage the dielectric problem is minimal. The insulations used depend on In high and very high voltage, either porcelain other requirements, such´as: thermal class, or an insulator consisting of ﬁ bre glass and mechanical resistance, etc. Modern materials silicon sheds are used for external insulation can be used, include insulating band (e.g. and paper-oil or SF6 gas for internal insulation. Mylar), epoxy and polyurethane resins for moulded ITs, thermoplastics (ABS, etc) and An important point in oil paper insulation is thermosetting plastics (phenolic resins, etc.) the drying of the paper and its impregnation for housing. with oil.

In medium voltage (e.g. up to 72.5 kV) for During drying vacuum is maintained, which indoor use synthetic resins have enabled the brings the moisture content of the paper to size of ITs to be reduced considerably as they below 0.2%. both insulate the primary from the core and the secondary and provide and insulating Without losing the vacuum, the paper is then surface between HV and LV in air. impregnated with mineral oil which has also been dried in a vacuum. This makes for partial For outdoor use, cyclo-aliphatic resins have discharge levels well below the limits set by come to replace porcelain in some cases due standards. Tg δ is less than 0.3%. to their high resistance to surface currents and the possibility of obtaining large leakage To improve the use of this dielectric, the paths. Experience has shown that these resins electrical ﬁ eld must be studied carefully, are suitable for outdoor service, except in avoiding areas with high gradients. heavy conductive atmospheric pollution.

› Fig. 6.1

Theory and technology of Instrument Transformers 33 6. DIELECTRIC INSULATION

6.2. INSULATION TESTING

To ensure correct operation, all transformers cavity and C3 is that of the dielectric in series are subjected to various tests before leaving with it. Capacitance Ck (coupling) serves the factory. Those related with the dielectric to detect the apparent partial discharge in problem may be individual tests, type tests or impedance Z more easily. special tests. There are various procedures set down in IEC In type tests the design of the transformer 60270 standard, but the most suitable for ITs in general is checked out. These tests can be is the measuring of the apparent discharge in avoided (with the agreement of the customer) pC. At ARTECHE, we have been measuring if the manufacturer presents a test certiﬁ cate partial discharges in transformers for 50 years. for transformers of the same or similar model. This is a non-destructive test, so improvements In individual tests the insulation of each in insulation as treatments are given can be transformer is tested. These tests may feature checked out. This test also enables us to the following: check whether the dielectric test at industrial frequency has damaged the insulation, by 1. Dielectric test at industrial frequency comparing the level of partial discharges between high voltage and low voltage. before and after that test. 2. Partial discharge test and tangent of loss The tangent of angle of loss test (Tgδ) is an angle. excellent indicator of treatment quality in oil 3. Various dielectric tests at industrial paper transformers, and reveals any changes frequency between nearby insulated in insulation during service. elements. The third group of tests includes tests to The ﬁ rst test consists of subjecting the insulation check insulation between insulated coils on between HV and LV to voltage gradients far the same winding, between secondaries, etc. higher than those it will encounter in service for a short period (generally one minute). This Finally, chromatic analysis of the gases is a classic test which enables a certain safety dissolved in oils can be carried out on high coeffi cient to be guaranteed in the insulation. voltage transformers which have been in service for some time. This test can pick However transformers which have been up any thermal, partial discharge or other subjected to this test often have shorter anomaly before the insulation fails completely. lifetimes than expected. This may be due to However it can only be performed in specialist small faults in the insulation which could not laboratories and is costly. be detected, and which cause premature aging. Success in dealing with this major problem has been achieved by performing partial discharge tests.

The main test in the second group is the partial discharge test, consisting of detecting the small discharges which occur between the walls of the cavities in the insulation when it is faulty.

Fig. 6.2a shows a faulty dielectric subjected to A/C voltage. The voltage between the opposing walls of the cavity is greater than that of the contiguous insulator, due to the lower dielectric constant of the gas. Furthermore its rigidity is lower than that of the rest of the insulator, especially if there is some degree of vacuum (Paschen's law).

This results in discharges between the ends of the cavity at a working voltage well below that of the rigidity of the insulator, which discharges gradually damaging the insulator. These are high frequency discharges, and can be detected as indicated in ﬁ g. 6.2b, where C1 is the capacitance of the dielectric in parallel with the cavity, C2 is the capacitatance of the

› Fig. 6.2

34 Theory and technology of Instrument Transformers 7. STANDARDS

7.1. STANDARDS CONSULTED Consulted standards are: IEC 61869 y IEEE C57.13 7.2. INSULATION LEVELS Table 7.1 shows the insulation levels according Some standards also include chopped lighting to the diff erent standards. impulse waves and tests in wet conditions.

Lightning impulse voltage pertains to a wave of 1.2/50 μs and switching impulse is for 250/2500 μs. These are type tests.

Table 7.1 IEC IEEE

Rated Insulation Level Rated Insulation Level Highest Highest Rated voltage Power- Lightning Switching Voltage Voltage Power- Lightning Switching kVrms frequency impulse impulse (kV) (kV) frequency impulse impulse (kV) (kVp) (kVp) (kV) (kVp) (kVp) 0.72 3 – – 0.66 0.6 4 10 - 1.2 6 – – 1.2 1.2 10 30 - 3.6 10 20 / 40 – 2.75 2.4 15 45 - 7.2 20 40 / 60 – 5.60 5.0 19 60 - 12 28 60 / 75 – 9.52 8.7 26 75 - 17.5 38 75 / 95 – 15.5 15 34 95/110 - 24 50 95 / 125 – 25.5 25 40/50 125/150 - 36 70 145 / 170 – 36.5 34.5 70 200 - 52 95 250 – 48.3 46 95 250 - 72.5 140 325 – 72.5 69 140 350 - 100 185 450 – 123 185 / 230 450 / 550 – 123 115 185/230 450/550 - 145 230 / 275 550 / 650 – 145 138 275 650 - 170 275 / 325 650 / 750 – 170 161 325 750 - 245 395/460 950/1050 – 245 230 395/460 900/1050 - 300 395 / 460 950 / 1050 750 / 850 362 460 / 510 1050 / 1175 850 / 950 362 345 575 1300 825 420 570 / 630 1300 / 1425 950/1050 550 630 / 680 1425 / 1550 1050 / 1175 550 500 750/800 1675/1800 1175 800 880/975 1950/2100 1425/1550 800 765 920 2050 1425

7.3. ENVIRONMENTAL CONDITIONS In IEC standard normal temperature limits Table 7.2 gives the minimum levels for the for outdoor service go from –40ºC to +40ºC creepage distance. (–40/40; –25/40; –5/40 categories) although it is also possible, as special condition, reaching from Table 7.2 –50ºC to +50ºC (–50/40; –5/50 categories). Minimum nominal speciﬁ c Pollution level creepage distance In the IEEE the normal limits for the temperature (mm/kV between phases) are –30 and +50ºC although it is also possible I Light 16 diff erent values as special conditions. II Medium 20

In both cases the normal altitude is below 1000 III Heavy 25 m. For higher altitudes external insulation must IV Very heavy 31 be increased.

Theory and technology of Instrument Transformers 35 7. STANDARDS

7.4. CURRENT TRANSFORMERS 7.4.1. RATED PRIMARY 7.4.4. STANDARD BURDENS CURRENTS The IEC standards admit the following rated As per IEC burdens expressed in volt-amperes: a. Single ratio transformers: 2.5 - 5 - 10 - 15 - 30 and greater. Power factor cos β = 0,8 10 - 12.5 - 15 - 20 - 25 - 30 - 40 - 50 - 60 - 75 The IEEE/ANSI standard expresses the and their multiples or decimal sub- burdens in a diff erent way. They are: multiples. The preferential ﬁ gures are underlined. B-0.1, B-0.2, B-0.5, B-0.9, B-1.8, B-1, B-2, B-4 y B-8 b. Multiratio transformers: where the number appearing after the letter B The standard values given in a) refer to the (burden) indicates the impedance in Ohms at lowest values of rated primary current. 60 Hz. Burdens B-0.1, B-0.2, B-0.5, B- 0.9 and B-1.8 are used for measuring with cos β = 0.9, and B-1, B-2, B-4 and B-8 are used for protection 7.4.2. RATED SECONDARY with cos β = 0.5. CURRENTS Table 7.4 shows the approximate equivalence between IEC and IEEE/ANSI burdens. In general 1 and 5 A are considered, with the last one being the preferential level. Table 7.3 Also other levels are admited, especially 2A. and, in CTs which are to be connected in a Extended current ratings triangle, the above levels may be divided by √3. IEC IEEE/ANSI 7.4.3. CONTINUOUS THERMAL 11 CURRENTS 1.2 - 1.5 - 2 1.33 - 1.5 - 2 - 3 - 4 3 - 4 The current transformer must withstand its rated continuous thermal current without Table 7.4 exceeding in the winding the admissible Equivalence of IEC and IEEE/ANSI burdens temperature for the relevant thermal class of the insulation. IEC IEEE/ANSI

The IEC standard admits as many heating 2.5 VA B-0.1 limits as classes of insulation. IEEE/ANSI 5 VA B-0.2 admits three types of transformers from the 15 VA B-0.5 (≈ 12.5 VA) temperature rise point of view: the 55 and 65ºC temperature rise type in both 30 and 20 VA B-0.9 (≈ 22.5 VA) 55ºC ambient temperature and the 80ºC 30 VA B-1 (≈ 25 VA) increase for dry insulation. 40 VA B-1.8 (≈ 45 VA) 50 VA B-2 Table 7.3 shows continuous thermal rated heating current rating factors according to 100 VA B-4 diff erent standards expressed as number of 200 VA B-8 times the rated current.

IEEE/ANSI also speciﬁ es the variation in thermal limit current (RF) according to ambient temperature.

36 Theory and technology of Instrument Transformers 7. STANDARDS

7.4.5. ACCURACY CLASSES FOR MEASURING CURRENT TRANSFORMERS

Tables 7.5, 7.6, 7.7 and ﬁ gures 7.1 y 7.2 show the maximum errors admitted by IEC and IEEE standards.

Table 7.5 IEC standard

± Phase displacement at percentage of rated current ± Percentage current (ratio) error at percentage of shown below Accuracy class rated current shown below Minutes Centiradians

5 20 100 120 5 20 100 120 5 20 100 120 0.1 0.4 0.2 0.1 0.1 15 8 5 5 0.45 0.24 0.15 0.15 0.2 0.75 0.35 0.2 0.2 30 15 10 10 0.9 0.45 0.3 0.3 0.5 1.5 0.75 0.5 0.5 90 45 30 30 2.7 1.35 0.9 0.9 1.0 3.0 1.5 1.0 1.0 180 90 60 60 5.4 2.7 1.8 1.8

Table 7.6 IEC standard (Extended range)

± Phase displacement at percentage of rated current shown below ± Percentage current (ratio) error at Accuracy percentage of rated current shown below Minutes Centiradians class 1 5 20 100 120 1 5 20 100 120 1 5 20 100 120 0.2 S 0.75 0.35 0.2 0.2 0.2 30 15 10 10 10 0.9 0.45 0.3 0.3 0.3 0.5 S 1.5 0.75 0.5 0.5 0.5 90 45 30 30 30 2.7 1.35 0.9 0.9 0.9

Table 7.7 IEEE/ANSI standards

± Percentage current (ratio) error at percentage of ± Phase displacement at percentage of rated current Accuracy class rated current shown below shown below 100 (**) 10 100 (**) 10 0.3 0.3 0.6 15 30 0.6 0.6 1.2 30 60 1.2 1.2 2.4 60 120

(*) The permitted ratio and phase errors are inter-dependent. With the data in the table a graph must be made up along the lines of ﬁ g. 7.2a, and only those errors within the parallelogram must be admitted. (**) These ﬁ gures must be complied with also for RF (see 7.4.3). The power factor may vary between 0.6 and 1.

IEEE Standard for High-Accuracy Instrument Transformers IEEE C57.13.6 deﬁ nes higher accuracy classes that extend the accuracy range beyond the traditional IEEE C57.13 requirements stated above.

› “High Accuracy Class 0.15” means that from 100% of nominal current through the rating factor, accuracy is guaranteed to be ±0.15%, and from 5% of nominal current through 100% of nominal current accuracy is guaranteed to be ±0.3%. › “Accuracy Class 0.15S” means that from 5% of nominal current through the rating factor, accuracy is guaranteed to be 0.15%. › “High Accuracy, Extended Range Class 0.15” means that from 1% of nominal current through the rating factor, accuracy is guaranteed to be ±0.15%

Theory and technology of Instrument Transformers 37 7. STANDARDS

IEC 61869-2 IEEE C57.13 ∈ % ∈ % +1.2 +1

0.6 1.2 +0.6 +0.5

0 0 0.5 1

-0.5 -0.6

-1 -1.2

δ δ 180 90 0 90 180 60 40 20 0 20 40 60 a) a) ε ∈ +1.2 +1.5 +0.75 +0.5 +0.6

0 0 0.05 0.2 1 1.2 x In 0.1 1 x In

-0.5 -0.6 -0.75 -1.5 -1.2 b) Class 0.5 b) Class 0.6

δ δi min

+90 +60

+0.45 +30 +30

0 0 0.05 0.2 1 1.2 x In 0.1 1 x In

-30 +30

-0.45 -90 -60

c) Class 0.5 c) Class 0.6

› Fig. 7.1 › Fig. 7.2

38 Theory and technology of Instrument Transformers 7. STANDARDS

7.4.6. PRECISION CLASSES IN 7.4.7. SHORT CIRCUIT CURRENT TRANSFORMERS CURRENTS FOR PROTECTION Network short circuit currents cause thermal The IEC standard admits the classes and and dynamic problems in transformers. errors indicated in Table 7.8. Secondaries must If I is the short circuit current of the network be loaded to their rated precision power cc and t its maximum duration in seconds The IEEE/ANSI standard admits classes C, K (between 0.5 and 5 sec.), the thermal limit and T for current transformers for protection. current of the transformer expressed for 1 sec. Class C and K transformers are those whose must meet the following condition: coils are evenly distributed, and therefore I ≥ I √t have unimportant fl ow losses. The errors th CC of these transformers can be determined I and I are rms values. by calculation. K classifi cation shall have a th CC condition in its Knee-point voltage. The dynamic eff ect is due to the maximum All (classes C, K and T) transformers must amplitude of the short circuit current wave. have ratio errors of less than 10% at 20 In. Some standards therefore link the thermal They are designated by a C, K or T followed by and dynamic currents in the worst case. a fi gure representing the secondary voltage in overcurrent operation. For instance C100 IEC requires: indicates that at 20 Isn = 20x5 = 100A, the I ≥ 2.5 I voltage in the secondary terminals is 100V dyn th (so, the burden is therefore 1 Ohm). In IEEE/ANSI standard current is expressed as the peak level of the symmetrical component of a fully shifted wave.

Therefore:

Idyn = 2 x √2 x Ith = 2,83 Ith

Table 7.8 IEC standard

Current error at rated Phase displacement at rated primary current Composite error at rated accuracy Accuracy class primary current % Minutes Centiradians limit primary current % 5P ± 1 ± 60 ± 1.8 5 10P ± 3 – – 10

Theory and technology of Instrument Transformers 39 7. STANDARDS

7.4.8. TERMINAL MARKINGS Extended range transformers must show the continuous thermal current rating factor AND DESIGNATION after the accuracy class (e.g. 15VA class 0.5 extension 150%). Terminals must be marked clearly and indelibly on the surface or in their immediate proximity. Current transformers for protection must also Terminal designation varies from one standard show the accuracy limit factor (e.g. 30VA to another. Table 7.9 shows some examples. class 5P10).

The IEEE/ANSI standard requires the letter H to be used for the primary and X for the 7.4.10. INDIVIDUAL OR secondary. If there are several secondaries ROUTINE TEST Y,Z,V, etc. may be used. Polarity is indicated by a number after each letter, e.g. H1, H2, X1, These are tests to which all transformers are X2, etc. with the odd numbers representing subjected to. The IEC standard considers the terminals of the same polarity. following as routime tests:

7.4.9. DATA TO BE SHOWN ON a. Checking of terminal markings. This consists of checking that the terminals are THE RATING PLATE correctly marked.

IEC standards require all current transformers b. Power frequency withstand test on the to show at least the following data: primary winding. The insulation must withstand the power frequency voltage a. Manufacturer’s name or an indication corresponding to its insulation level enabling the manufacturer to be easily for one minute. This voltage is applied identiﬁ ed. between the primary and the secondary b. Serial number or type of apparatus and winding(s) connected to earth (see Table manufacture date/year. 7.1). If the primary winding is subdivided c. Rated transformation ratio in the following into two or more sections, each section form: must withstand a rms voltage of 3kV K = I / I (e.g.: K = 100/5) n pn sn n between itself and all the other sections d. Rated frequency. (Hz) for one minute. e. Rated output, accuracy class and corresponding terminal designation for c. Power frequency withstand test on each winding. secondary windings. Each secondary f. Highest voltage for equipment and its winding must withstand a rms voltage rated insulation level. of 3kV between itself and the other g. Rated short-circuit thermal and dynamic secondary windings connected to earth, currents in kA. for one minute. h. Weight in kg. i. Service temperature. d. Inter-turn Overvoltage test. This involves j. Mechanical class. checking the secondary winding(s) for one minute at the induced voltage (if its peak In low voltage transformers f) and g) are not value is less than 4.5 kV), supplying the compulsory. primary winding at its rated current and with the secondary open, or vice versa. Current transformers for measuring must If the voltage appearing in the secondary also indicate, if relevant, the instrument terminals is greater than 4.5 kV peak this security factor (in the form Fs ≤ x) for the voltage is used for the test. indicated burden.

Table 7.9

40 Theory and technology of Instrument Transformers 7. STANDARDS e. Accuracy tests. In current transformers adopted by the manufacturer is no higher for metering and relaying the tests are than 180 A/mm2, if cooper and 120 A/mm2 run to check compliance with the relevant if aluminium, and in dynamic aspect if the standards. manufacturer has tests for other models of similar size and mechanical attachment to In current transformers for protection, those in question. compound error tests must be run using the excitation method, which consists of b. Temperature rise test. The transformer determining the excitation current (for the must withstand a primary current rated frequency), applying a practically equivalent to its continuous thermal sinusoidal voltage to the secondary at a current as per 7.4.3 permanently, without level equal to the product of the accuracy exceeding its temperature rise limits, limit factor by the rated secondary current under normal operating conditions. and by the vectorial sum of the burden impedance and the transformer secondary c. Lightning and switching impulse test. winding impedance. Normally, for lightning impulse test, f. Partial discharges. 15 consecutive pulses of positive and This requirement is applicable to current negative polarity with full waves of 1.2/50 transformers having Um not less than 7.2 kV. μs are applied and, for switching impulse test, 15 full positive pulses of 250/2500 μs. Table 7.10 shows the IEC limits of partial Although, when the highest voltage of the discharge level (in pC) that must not be material is ≥ 300 Kv only 3 pulses of both exceeded at the voltage indicated after a polarities are necessary in the lightning prestressing of 100 or 80% of the power- impulse test. frequency withstand voltage. d. Wet tests for outdoor type transformers. 7.4.11. TYPE TESTS These tests are aimed at checking the validity of the external insulation. These are tests to which only one transformer, of each model are subjected. These tests need According to the highest voltage of the not be performed if the manufacturer submits material, power frequency or switching test certiﬁ cates for similar transformers which impulse test are applied. are acceptable to the purchaser. e. Radio interference voltage measurement. According to the IEC standard these tests are Test for the appearance of the corona the following: eff ect (external discharges) and its limitation so as not to disrupt radioelectric a. Short-time current test. This consists of transmissions. checking the resistance of transformers to the rated thermal and dynamic currents. f. Accuracy tests. In current transformers for measuring the checking of the accuracy These tests are always expensive to class at all the levels indicated in the perform. So, they are generally taken as relevant table may be taken as a type test. passed in thermal aspect if the density

Table 7.10 Permissible PD level/pC Type of transformer Type of earthing PD test voltage Type of insulation and connection of the system (kV rms) Liquid Solid

Um 10 50 Earthed neutral system Phase-to-earth 1.2 Um / √3520 voltage and current transformers Isolated or 1.2 Um 10 50 non-eff ectively earthed 1.2 Um / √3520

Earthed neutral 1.2 Um 5 20 system Phase-to-phase voltage transformers Isolated or non-eff ectively 1.2 Um 5 20 earthed

Theory and technology of Instrument Transformers 41 7. STANDARDS

Also it must be checked that the e. Internal arc test. This test is performed transformer met the instrument security on a real transformer, equipped with all factor using the overcurrent or excitation accessories and in service conditions. An methods in a similar way as the accuracy internal fault in the transformer is made limit factor of protective transformers. and a current that will cause the explosion of the transformer is applied. Both the In current transformers for protection the value and duration of this current, the type checking of compound error by the primary of fault caused and criteria to pass this test overcurrent method or by the excitation are set out in IEC 61869-1, but some other method is treated as a type test. criteria agreed between customer and manufacturer can be used. 7.4.12. SPECIAL TESTS f. Measurement of capacitance and tg δ. Tests which must be arranged by the These two measurements are made manufacturer and the buyer and are usually together, before and after dielectric test at quite expensive. They are designer tests which 50 Hz and 10 kV and Um/√3. Capacitance justify the behaviour of a certain transformer measurement test checks consistency of family. According to the IEC standard, they production. There are no limits as each design are the following: has its own value, acceptance criteria for this test is that there should be no signifi cant a. Chopped lightning impulse. Two chopped variation in the diff erent measures. The δ waves are inserted in combination with the test checks quality of the drying process, negative polarity lightning impulse test. which infl uence in new transformers, the dielectric dissipation factor of the insulation. b. Mechanical tests. The transformers must The recommended limit in the standard is withstand static mechanical stresses (table 0.5%. In Arteche this test is a routine test for 7.11) which comprise service demands 100% of the production. including those due to wind and ice. c. Multiple chopped impulses. A test which verifi es the appropriate behaviour of electric fi eld distribution screens before high-frecuency waves derived from disconnector operations. 600 reduced value (70%) negative polarity chopped impulses are applied with gas dissolved in Table 7.11 oil testing before and after their application. Static withstand test loads The criterion to assess if the design is valid Static withstand test load or not is the in gas concentrations. Highest voltage for the FR (N) equipment Um d. Overvoltage transferred to secondaries. kV Load Class Load Class I II This test indicates the electromagnetic compatibility of a tranformer. By applying 72.5 a 100 1,250 2,500 a step impulse in the high voltage part 123 a 170 2,000 3,000 it is possible to verify which percentage 245 a 362 2,500 4,000 passes to the low voltage part. ≥ 420 4,000 6,000

42 Theory and technology of Instrument Transformers 7. STANDARDS

7.5. VOLTAGE TRANSFORMERS 7.5.1. RATED PRIMARY Table 7.12 VOLTAGES IEC standard Insulation rated voltage Rated primary voltage The values for rated primary voltage are in 12 11,000 IEC 60038. Some of these values can be seen 17.5 13,200 - 16,500 in Table 7.12 24 22,000 In single phase voltage transformers which 36 27,500 - 33,000 can only be connected between phase and 52 44,000 earth, the standardized allocated primary voltages are taken to be the ﬁ gures indicated 72.5 55,000 - 66,000 divided by √3. 123 110,000 145 132,000 In IEEE/ANSI standard other ﬁ gures are given, 245 220,000 and 5 groups of voltage transformers are available: 420 380,000 - 400,000 550 500,000 Group 1: Transformers which can be connected line-to-line in a system with voltage Up, or line- to-ground in a system with voltage √3U . p › Dielectric test in H.V. laboratory Group 2: Transformers which can be connected line-to-line or line-to-ground, both in a system with voltage Up.

Group 3-4-5: Transformers which can be only for line-to-ground connection.

7.5.2. ALLOCATED SECONDARY VOLTAGES

Many standards allow the same ﬁ gures. IEC groups them as follows:

For single phase transformers used in single phase networks or between phases of 3-phases networks: a. Based on usual practice in a group of European countries: 100V and 110V 200V for extended secondary circuits. b. Based on usual practice in the USA and Canada: 120V for distribution systems. 115V for transmission systems. 230V for extended secondary circuits.

For single phase transformers to be used phase-to-earth in 3-phase systems where the rated primary voltage is a number divided by √3, the rated secondary voltage should be one of the ﬁ gures indicated divided by √3, thus retaining the rated transformation ratio.

Theory and technology of Instrument Transformers 43 7. STANDARDS

7.5.3. TRANSFORMATION 7.5.5. TEMPERATURE-RISE LIMIT RATIOS According to the IEC standard the temperature- All the standards aim to give a simple value rise of a transformer in continuous service for transformation ratio. The IEC standard should not exceed the levels indicated for the therefore recommends that the rated relevant insulation class for a voltage factor transformation ratio be one of the following: of 1.2. If the voltage factor is 1.5 or 1.9 they must be tested at the resulting voltage for 10-12-15-20-25-30-40-50-60-80 the time indicated in Table 7.14, starting from and their decimal multiples. stable thermal conditions attained at 1.2 times the allocated primary voltage and without exceeding the admissible temperature 7.5.4. RATED VOLTAGE FACTOR increase by more than 10ºC.

This is the factor by which the rated primary voltage must be multiplied to determine the 7.5.6. RATED OUTPUT maximum voltage for which the transformer must comply with heating speciﬁ cations, The IEC standard gives the following rated during the indicated time, and the accuracy output for a power factor of 0.8 lagging, speciﬁ cations. expressed in volt-amperes:

Table 7.13 shows the standardized ﬁ gures for the 10-15-25-30-50-75-100- voltage factor admitted by the IEC standard. 150-200-300-400-500 The preferential ﬁ gures are underlined. IEEE/ANSI gives a voltage factor fo 1.1 for all voltage transformers in general, as far as IEEE/ANSI admits the standard levels precision and heating problems are concerned. indicated in Table 7.14.

For group 1 and 3 transformers voltage factors of 1.25 are reached in continuous service and 1.73 for 1 minute, under certain temperature and load conditions.

Table 7.13 Standard values of rated voltage factors Rated Rated voltage Method of connecting the primary winding and system earthing conditions time factor

Between phases in any network 1.2 Continuous Between transformer star-point and earth in any network

1.2 Continuous Between phase and earth in an eff ectively earthed neutral system (Sub-clause 4.23 a) 1.5 30 s

1.2 Continuous Between phase and earth in a non-eff ectively earthed neutral system (Sub-clause 4.23 b) with automatic earth fault tripping 1.9 30 s

1.2 Continuous Between phase and earth in an isolated neutral system without automatic earth fault tripping (Sub-clause 4.20) or in a resonant earthed system (Sub-clause 4.21) without 1.9 8 h automatic earth fault tripping

Table 7.14

Designation Voltamp. Power factor

W 12.5 0.10 X 25 0.70 Y 75 0.85 Z 200 0.85 ZZ 400 0.85 M 35 0.20

44 Theory and technology of Instrument Transformers 7. STANDARDS

Table 7.15. Limits of voltage error and phase 7.5.7. ACCURACY CLASSES displacement Percentage Phase Class voltage (ratio) FOR MEASURING VOLTAGE displacement ± TRANSFORMERS error ± 0.1 0.1 5 Under the IEC standard voltage transformers 0.2 0.2 10 must meet their accuracy class requirements 0.5 0.5 20 for any voltage between 80 and 120% of rated 1 1.0 40 voltage and with burdens between 25 and 100% of the rated burden at a power factor of 3 3.0 Not speciﬁ ed 0.8 lagging.

Table 7.15 shows the limits of voltage error IEEE C57.13 and phase displacement according to accuracy class.

The maximum admissible errors under the IEEE/ANSI standard are for points within the parallelograms of ﬁ g. 7.3. This requirement must be met for burdens from zero to rated burden and for voltages from 0.9 to 1.1 Un, with a power factor between 0.6 lagging and 1.

Fig. 7.4 shows the limits of error under the IEC standard for precision classes 0.5 and 1.

7.5.8. PRECISION CLASSES IN VOLTAGE TRANSFORMERS FOR PROTECTION › Fig. 7.3 The IEC standard admits the classes and limits shown in Table 7.16. Errors must not exceed the levels in the table at 5% of the rated voltage IEC 61869-3 and at the product of the rated voltage by the voltage factor (1.2, 1.5 or 1.9) for any load between 25 and 100% of the rated load with a factor of 0.8 inductive.

At 2% of rated voltage, the error limits will be twice as high as those indicated in table 7.16, with the same burden conditions.

7.5.9. RESISTANCE OF VOLTAGE TRANSFORMERS TO SHORT CIRCUITS

When there is a short circuit in the secondary terminals transformers suff er mechanical and thermal stress. › Fig. 7.4 Standards specify that all voltage transformers must be able to withstand a secondary short circuit for one second Table 7.16.Limits of voltage error and phase without exceeding termperature limits when displacement energized at rated voltage. Percentage Phase Class voltage (ratio) displacement ± error ± min. 3 P 3.0 120 6 P 6.0 240

Theory and technology of Instrument Transformers 45 7. STANDARDS

7.5.10. TERMINAL MARKINGS For this test, a distinction must be made between transformers for phase-phase Table 7.17 shows some examples of terminal connection and those for phase earth markings as per IEC. connection.

The designation indicated in 7.4.8. for current Transformers between phases must transformers under IEEE/ANSI also applies to withstand a test involving voltage applied voltage transformers. between the two primary terminals, joined together, and the secondary windings connected to earth, and two further tests 7.5.11. DATA TO BE SHOWN ON with induced voltage. THE ID PLATE The two latter tests may be performed by The IEC standard requires to show the applying voltage through the secondary following data: or through the primary. In both cases the voltage measured on the high voltage side a. Manufacturer's name or an indication must be the same as the speciﬁ ed test enabling the manufacturer to be easily voltage. During this test one terminal of identiﬁ ed. the secondary winding should be joined to b. Serial number and manufacture date/year. one terminal of the primary winding, and c. Type of apparatus. both should be earthed. d. Rated primary and secondary voltages. e. Rated frequency. Transformers connected between phase f. Rated output, the corresponding accuracy and earth are tested only via this second class and terminal marking for each method, taking care that the primary winding. terminal which is to be connected to earth g. Highest system voltage for the material during service (N) is the one which is and its rated insulation level, separated by connected to earth during the test. Also, diagonal strokes. N terminal must be tested as if another h. Rated voltage factor, and corresponding secondary was. rated time if necessary. i. Insulation class if diff erent from class A. c. Power frequency withstand test on j. Weight in kg. secondary windings and sections. Each k. Service temperature. secondary winding or section must l. Mechanical class. withstand an eff ective voltage of 3 kV m. Capacitance of the transformer and each with all the other windings and sections individual capacitor (only for CVT). connected to each other and to earth.

d. Partial discharges. 7.5.12. INDIVIDUAL OR ROUTINE TESTS In general the comments above 7.4.10 f) concerning current transformers apply here also. Standards regard the following: When the operating voltage is well below a. Veriﬁ cation of terminal markings. the speciﬁ ed insulation level there may be diffi culties in performing this test due b. Power frequency withstand test on the to saturation of the core. An agreement primary windings. As in the case of current between manufacturer and user is required. transformers, the insulation of the primary windings must withstand the power e. Accuracy tests. Check that the maximum frequency voltage corresponding to its admissible errors are not exceeded. This insulation level for one minute. can be done with a low number of voltages and burdens.

Table 7.17

46 Theory and technology of Instrument Transformers 7. STANDARDS

7.5.13. TYPE TESTS 7.5.14. SPECIAL TESTS

Standards regard the following: Same tests as CT are applied with some particular considerations. a. Temperature-rise test. Consists of checking that the transformer meets the Capacitance and tg δ test is considered a requirements of 7.5.4. routine test for CVT. b. Lightning and switching impulse test. The mechanical stress which a TI must withstand is shown in Table 7.18. 3 or 15 consecutive pulses are applied with positive and negative polarity, with full waves of 1.2/50 μs for lightning impulse and 15 positive pulses of 250/2500 μs for switching impulse test, depending on the highest voltage of the material. c. Wet tests for outdoor transformers. These tests are aimed for checking the validity of the external insulation.

Both power frequency and impulse waves are applied, according to the highest voltage of the material d. Short circuit test. Consists of checking that the transformer meets the requirements of 7.5.9. e. Radio interference voltage measurement. Tests for the appearance of the corona eff ect (external discharges) and its limitation so as not to disrupt radioelectric transmissions. f. Accuracy tests to check that the maximum admissible errors given in table 7.15 are not exceeded.

Table 7.18 Static withstand test loads

Static withstand test load FR N Highest voltage for Voltage transformers with: equipment Um kV Voltage Through current terminals terminals Load Class I Load Class II 72,5 a 100 500 1.250 2.500 123 a 170 1.000 2.000 3.000 245 a 362 1.250 2.500 4.000 ≥ 420 1.500 4.000 6.000

Theory and technology of Instrument Transformers 47 Moving together