13. C37.110 Guide for the Application of Current TX Used for Protective Relaying Purpose

IEEE Transactions on Power Delivery, Vol. 14, No. 1, January 1999

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C37.110 Guide for the Application of Current T Used for Protective Relaying Purpose PSRC Working Group Members: Chairman M.W. Conroy, Vice Chairman B.D. Nelson, B. Bozoki, J. L.L. Dovrak, 1.0.Hasenwinkle, J.D. Huddleston , W.C. Kotheimer, J.R. Linders, M.J. G.R. Moskos, G.C. Parr, R. Ryan, E.T. Sage, D.W. Smaha, K.A. Stephan, J.E. Step J.T. Uchiyama, S.E. Zocholl ABSTRACT

Relay engineers have had to rely on many sources to compile information on ct application. Typically, they have drawn on relay application and transformer textbooks, manufacturer publications, and standards. Recognizing the need for a comprehensive one source document, the Power System Relaying Committee has produced the C37.1101996 Guide for the Application of Current Transformers Used for Protective Relay Purposes.

Key Words: current transformer application, ct rating, ct saturation INTRODUCTION

The working group F7 of the Relay Input Sources Subcommittee of the Power System Relaying Committee has assembled a comprehensive guide on the application of current transformers. In the past, relay engineers have had to rely on diverse sources that included relay application textbooks, transformer textbooks, transformer standards and manufacturers' publications. The single source document is the newly published C37.110 Guide for the Application of Current Transformers Used for Protective Relaying Purposes. The body of the Guide is organized under the major sections on current transformer characteristics and classification, general application of current transformers, and the effects of current transformer saturation on relays. PE-260-PWRD-0-03-1998 A paper recommended and approved by the IEEE Power System Relaying Committee of the IEEE Power Engineering Society for publication in the IEEE Transactions on Power Delivery Manuscript submitted December 19, 1997, made available for printing March 9, 1998

This paper is a summary the Guide and presents section.

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CT CHARACTERISTICS

The Guide discusses the equivalent circuit explains the use of the excitation curve to performance. These curves show th excitation voltage ( V , to the excit typical curve for a C400, 2000:5 ct is shown in Fi The curve is developed from test dat log coordinates. Definition 3.10 and defme the knee-point of a ct with a point of maximum permeability o plotted on log-log paper with squ tangent of the curve makes a 45" angle with the absciss The knee point (A) is shown in Figure thorough in contrasting the IEEE and IE Definition 3.11 gives the IEC international standard defmition of knee point defined as the point on the excitation curve where a 10% increase in excitation voltage causes a 50% change in excitation current. The IEC knee point on the excitation curve of Figure 1 occurs at point B. Section 4.4 is an important part of the Guide which explains current transformer accuracy according to IEEE Std C57.131993, clause 6.4.1. The ANSI accuracy class is determined by a letter and a secondary terminal voltage rating that describe the steady-state performance. The voltage rating is the secondary terminal voltage that the ct will deliver, when connected to a standard burden, at 20 times rated secondary current without exceeding a 10 % ratio correction. The dc component of an asymmetrical current greatly increases the flux in the ct. When the dc offset is at a maximum the flux can potentially reach 1+X/R times the flux resulting from the sinusoidal component, where X and R are the primary system resistance and reactance up to the point of the fault. The difference between the non-offset and the offset flux is shown in the waveforms of Figures 2 and 3 taken fiom the Guide. Section 4.5 of Guide presents the criteria to avoid saturation in terms of secondary current Is, the total secondary burden Zs, the primary X/R ratio, and the ct saturation voltage Vx. The definition 3.2 1 defines the

0885-8977/99/$10.00 0 1998 IEEE

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100

Secondary rms excitation volts

vs

10

1

Secondary rms excitation amperes Is

Figure 1. Excitation Curve with IEEE and IEC Knee Points I

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I

I

1

1 100

Sec Amperes

Sec Amperes

Flux Odensity (tesla x 0 01)

Flux Density (tesla x 0 01)

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I 008

-loot

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Time (Seconds)

Time (Seconds)

r

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Primary Current 200

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OM

OM

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Figure 2 Relation between ct current and flux without saturation

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Figure 3 Relation between ct current and flux with saturation

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saturation voltage Vx as the symmetrical voltage across the secondary winding for which the peak induction just exceeds the saturation flux density. To avoid ac saturation:

v,

> I,

x

300 I

I

1

I

I

I

z,

To avoid saturation with a dc decrement in the fault current:

These equations are usefid ct selection criteria to avoid saturation when Vx is taken as the ANSI voltage rating. Time (Seconds)

The important subject of remanence is covered in section 4.6. Figure 4 shows three waveforms taken from the Guide showing the output of a ct with and without remanence. These waveforms relate to a C800 1200:5 with a burden of 1.6 +j0.7 ohms. The waveforms show the ct behavior with remanent flux of O%, 50%, and 75% respectively where the fault current is 24000A and the time constant of the dc offset is 0.05 seconds ( N R = 19). The output deteriorates earlier in time with higher remanence.

300 I

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I

I

I

Remanent Flux =

GENERAL APPLICATION OF CURRENT TRANSFORMERS

The guide discusses the fundamental ct phenomena that may affect relay performance such as saturation, proximity effect, ct location, and suitability of polarizing sources. The ct saturation problems and their solutions are discussed in this section. The examples of ct related problems include the review of overcurrent relays, transformer and generator differential, bus differential, and distance relays. The generator differential application is an appropriate example of the information found in the guide. In this application it is impractical to size the cts to avoid saturation because of high X k ratio and fault current. The rule is to select the largest practical rating and match the terminal and neutral side cts. The pitfall is that the highest accuracy class is the C800 and that any ct with excitation voltage exceeding 800V is classified C800 no matter how high the voltage. For example, a 6000:5 ct may have an excitation voltage of 1500 V at 10 amps of exciting current and be classified ‘2800. A second 6000:5 ct of a different manufacture may have 978 V at 10 A of excitation and also be classified C800. Therefore the generator cts must have the same excitation curve with matching knee point voltage and the same excitation voltage at 10 A excitation current in order to avoid differential error occurring during an offset through fault condition.

Time (Seconds)

300 1

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aoo

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100

Amperes 0

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loo.

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I 01

Time (Seconds)

Figure 4 Seconday waveforms with inc remanent flux The guide considers the application of a generator differential relay for a 111 MVA generator. The machine has an X / R ratio of 52 and can contribute 58800 A to an external bus fault, All the cts are classified 6000.5 C800 A 6000:5 rating is selected as the first standard full winding rating above the rated load current of 5572 A. The ct and lead resistance for the generator terminal cts to be 2.6 C2 and 2.3 C2 for the neutral cts impedance in the restraint windings. Consequently, the maximum ct symmetrical voltage due to the maximum fault current is :

I

I

97 I

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I

I

I

(3) 6000:5 C800 kneepoint 500 V

(3) 6000:s c800 knee point 552 V

-50

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Figure 5 Generator Differential Application

v=-58800 (2.6) = 12W 1200

However, the ct would have to support a symmetrical current of (I+ XR)times this value or 127 x (1+52) = 673 1 V to avoid saturating during a fully offset maximum fault. However, the largest ANSI rating is C800. For this reason all the cts must be of the same manufacture with knee point voltages matched as closely as possible so as to experience the same degree of saturation during the offset. How closely should the knee point voltages be matched? The Guide considers the application shown in the schematic of Figure 5 with a set of terminal side cts having a 500 V knee point voltage and a set of neutral side cts having a 552 V knee point voltage. Figure 6 shows the response of these cts for a 58800 A fault with maximum offset due to an X/R ratio of 52. In this case, the ct at the generator terminal saturates slightly before the generator neutral cts. The slight mismatch of the knee point and burden resistance cause a 100 A pulse of current in the operating coil. Although there is a large restraint current, the pulse casts doubt as to the

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Figure 6 Secondary current in the neutral and terminal side cts with differential current

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Differential current

I 0 01

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Figure 7 Secondary current in the neutral and terminal side cts with differential current for corrected burden.

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relay’s through fault security. However, the mismatch can be eliminated by increasing the series impedance on the neutral side to equal the 2.6 R of the terminal side burden times the ratio of the knee point voltages (552/500), or 2.87 R. AS shown on Figure 7, the corrected burden forces saturation to occur at the same time in both cts and eliminates the pulse of difference current for the through fault condition. CONCLUSIONS

1. Formerly, information on the application of current transformers had to be compiled from diverse sources including relay and transformer textbooks, standards, and manufacturer publications. All aspects of ct application are now available in the new C37.110-1996 “IEEE Guide for the Application of Current Transformers Used for Protective Relay Purposes”. 2. In addition to the explanation of ct characteristics, performance, and application, the Guide includes annexes on IEC current transformer standards and C rating standard burdens. It also has a substantial bibliography of more than 35 entries.