Analyzing and Applying Current Transformers

105 Pages·2004·17.06 MB·English

by  Stanley E. Zocholl

Also by Stanley E. Zocholl

In Paperback

AC Motor Protection
Analyzing and Applying
Current Transformers

by

Stanley E. Zocholl

Schweitzer Engineering Laboratories, Inc.
Copyright © 2004 Schweitzer Engineering Laboratories, Ine

All rights reserved. No par of this publication may be reproduced, stored in 3
retrieval system, or transmitted, in amy form oF by any means, electronic,
‘mechanical, photocopying, recording, or otherwise, without the prior written
pemnission of the author and/or publixner. For information, coatiet Schweitzer
Engineering Laboratories, Ine.. 2350 NE Hopkins Court, Pullman, WA 99163.

Library of Congress Cataloging-in-Publication Dita
Zocholl, Stanley E.
Analyzing and Applying Current ‘transtormers/ Stanley E. Zocholl
p. em.
Includes bibliographical references (p. 95)
SBN 0.9725026-2-9
1, Electric transformers,
2. Rlectric transformers-Protection, 1. "Title.
‘TK2551 263 2008
Fins Edition: August 2004

Printed and Bound in the United States of America,

SEL isa registered trademark of Schweitzer Engineering Laborator
Contents

Introduction... 2.0.66... cece cece ee ee eee eens 1
Current Transformer Equivalent Circuit .............. 3
Volt-Time Area and Saturated Waveforms ............ 5

The Excitation Curve

Computer Simulation
Flux Density Limit of the Volt-Time Area ............ 19

The Criterion to Avoid Saturation .
Rating Current Transformers for Li
Current Transformer Selection Procedure
Calculating Current Transformer Burden
Limits of the Criterion
Statistics of Asymmetry...

ACase History .

33 KA Fault With Ideal Current Transformers 29

The Saturation Effect (attres weaneanooees edd
33 kA Fault With Maximum Asymmetry in A-Phase ....... 36
33 kA Fault With Offset in B-Phase and C-Phase ........-. 38
CT Application Rules Ad Ewe Pesos me tbe 40
Transformer Differential Relay Applications ......... 42
Generator Differential Relay Application ..... Bre

Bus Differential Relay Applications ................ 54

Percentage Slope Characteri
Alpha Plane Differential Characteristic .

Determining the Slope for a Given Degree of
Saturation ...........
Saturation Voltage . sees . 65

Bus and Machine Differential Applications .......... 66
The Effect of Remanent Flux on the Slope Setting ....69
The Impact of High Fault Current on
Overcurrent Protection ...............0-e ee eee 72
Limitations of an 80 A Instantaneous Setting ...----- 74
Defining a CT Selection Criterion 77
Maximum Fault Current With an 80 A. Instantaneous et 8

Minimum CT Rating With an 80 A Instantaneous Setting... 82

implementing Instantaneous Overcurrent Elements . . .83
Digital Implementation of Instantaneous Overcurrent Relays . 83
Digital Fitter Performance With CT Saturation

The Cosine-Peak Adaptive Filter .
‘A Simple Distortion Index

Annex A; Volt-Time Area Computer Simulation ...... 93
References .........-. see e eee eee eee eens 95
Figure 1
Figure 2
Figure 3

Figure 4

Figure 5

Figure 6
Figuie7
Figure 8

Figure 9
Figure 10
Figure 11

Figure 12
Figure 13

Figure 14

Figure 15

Figures

Toroid Current Transformer ceeerseeeeeeeee I

Current Transformer Equivalent Circuit... 4
econdary Current Waveforms for Various Levels of

Excitation (Resistive Burden) 7

Core Flux and Secondary Curent in a CT With a
20-Per-Unit Symmetiical Rating Carrying 10-Per-Unit
Current With the Offset Due to 0.010 Second Primary

Time Constant (75° Impedance Angle) 8
(C400, 2000:5 Current Transformer Excitation Curve

Showing the 300:5 Tap With Knee-Point Tangent and
Normal Lines .... vette we 10
Magnetizing Impedance cece DD
Equivalent Circuit of a Current Transformer ........... 13
Equivaleat Circuit for the Differential Connection of

Current Transformers old
B-H Loop and the Slope of the Mean Curve... 16

C400, 2000:5 Cusrent Transformer Computer Simulation 18.
C400, 2000:5 Current Transformer With 40 kA

Asymimetrival Current... see IB
Burden Voltage for Asymmetrical Fault Current 20
Event 1—33 kA ABC Fault at 1.55 Miles on a 230 kV Line

(Theoretical —No CT Saturation) ul

Secondary Current in a C800, 3000 With 39 KA OTs
‘Three-Phase Fault at 1.55 Miles With 0.066 Primary

Time Constant 32
Magnitude and Phase Difference of Sampled Seeondary
Current... 9 sore Ceeca cries 3
Figure 16

Figure 17

Figure 18
Figure 19

Figure 20

Figure 21

Figure 22

Figure 23
Figure 24
Figure 25

Figure 26
Figure 27
Figure 28
Figure 29
Figure 30
Figure 31
Figore 32
Figure 33
Figure 34

Figure 35

Event 2—33 KA ARC Fault at 1.55 Miles on a
230 KV Line (CT Saturation Caused by
Maximum Offset in A-Phase) :
Event 3—33 KA ABC Fault at 1.55 Miles on a
230 kV Line (CT Saturation Caused by

Offset in B- and C-Phase)
Power Plant Auxiliary

Differential Relay Restsnt Curent and
ly Is the Differential,

Harmonie Content ofthe Different Cuvrent in Figure 19
Plotted in Per Unit of the Fundamental Frequency
Secondary Currents 1) and Ip Due to a 58800 A

Through Fault Current With Offset. The Differential
Current Iy Is Also Plotted, 6000:5 CTs Have

Knee-Point Volts of 700 and 500. X/R = 52.

Secondary Currents 1y and Ip Due to an Asymmetrical
‘Through Fault of SRR00 A. Adding Burden Resistance
Decreases the Differential Current Ty. 6000:5. CTs
Knee-Point Voltages of 700 and 500, X/R = 52.
Differential Protection ......

Percentage Characteristic...

Saturation Due to DC Offset. C200 2000 5 cT.
2.00 Burden, Fault Current 10667 Amp. X/R = 14 -
Fundamental Waveforms Extracted by Digital Filter
Circle Characteristic 7

Chartcteristie Resembling a Cadioid

MATLAB Program Graphic Interface...»

VS = 20 Fault Current 106674 -

40 Fault Current 106674.

80 Fault Current 106674.

160 Fault Current 106674 .
VS = 80 Fault Current 10667A With Cardio i ss
Cras

= 160 Fait ante 10667 With Caio ci svat
Chances

yy

50
Figure 36
Figure 37
Figure 38
Figure 39
Figure 40
Figure 41

Figure 42
Figure 43
Figure 44
Figure 45
Figure 46
Figure 47
Figure 48
Figure 49
Figure 50
Figure 51
Figure $2
Figure 53

Figure sq

Figure 55

Figure 56

Slope k Versus CT Saturation Voltage
Mulirestraint Bus Differential

Vs=40,k=30 00...

Operate Signal Versus Restraint

VS = 67, k = 44, 40 Percent Remanence -
Operate Signal Versus Restraint With 40 Percent
Remanence .......ceeeereee =
Generator Auxiliary Bus With High Fault Curent.
Relay Schematic Showing Measured Signals

Signal Extracted With a 200 A Primary CT Current
CT and Relay Signals fora 40 kA Fault Using C50,
100:5 CTs...
Response to 240 KA Fault Using C200, 200:5 CTs «...
Response to 40 kA Fault Using C400, 400:5 CTs
Diagram of Bipolar Peak Detector

Bipolar Peak Detector Transient Overreach

RMS Filter Transient Overreach Conipe to Bipolar
Peak Detector
Fitter Response, Fault 40 kA, X/R = 20, C100, 200: C1 r.
0.5 W Burden : ce
Instantaneous Element Using the Cosine Peale

Adaptive Filter . _
Cosine Filter Providing Trip for a Waveform With Low
Saturation C400, 200:5, 4500 A Pault With X/R =11.31
Bipolar Peak Filter Providing Trip for a Waveform

With High Saturation C50, 200:5, 20 kA Fault

With X/R= 11.31
Cosine Filter Providing Trip for a Waveform With No

Saturation C100, 200:5, 4kA Symmetrical Fault Current .

Mathcad Plot of Current and Flux .

65
67
8
68

i)

1
B
%
16

oy
”
9

. 84

84

85

85

87

89

90

a

299
Se ee ee ee pee ee pee
Introduction

ng of how to

The purpose of this book is to develop an understan
analyze and apply current transformers (CTs) used as current
sources for protective relays. A CT appears to be the simplest of
electrical devices. For example, a bushing CT is simply a winding
on an insulated core, which becomes a transformer only when
placed over a primary conductor as shown in Figure 1

Secondary Winding
SPEEA 14
\— x

Primary Conductor

tT.
OT insulated Magnetic Core
<r

Figure 1 Toroid Current Transformer

The opening paragraph of S. D. Moreron's classic paper [| states
that the art of calculating current transformer characteristics from
ation curve data has been known for some time, Moreton wrote
this in 1943 and referred to papers written two years earlier. Today,
nearly 60 years later, there is still an abiding interest in this

al devices

scemingly simplest of elec