CT wire sizing

8/10/2019 CT wire sizing

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Selection of Current Transformers

and Wire Sizing in Substations

Presented to:59th Conference for Protective Relay Engineers

Texas A&M UniversityCollege Station, Texas

April 4-6, 2006

Sethuraman Ganesan

ABB Inc.Allentown, PA


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Characteristics of CT

Metering and Protection Class

Specifications of CTs

CT Wiring and other issues

IEEE Std C57.13, Guide C37.110 IEC Std 60044-6

Discussion Paper


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CT Simplified Circuit and Phasor

IP IS1:n

a c

b d

IP/n

IE

RCT

Xm

e

f

RB

Vef

ISRCT

Vcd=n. VabIS

IE

IPn


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Metering

Metering class Typical Spec 0.3 B-0.1

Meters can be off Protection CTs

Thermal stress

Auxiliary CTs

Burdens of auxiliary CTs, accuracy Summation CTs


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Protection Class CTs

Ratings, Ratio

Polarity Class, Knee point voltage, Excitation

characteristics

Secondary Current

Magnetizing

Voltage

Vx

Vk

10A(10%)


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AC Saturation

Severe Saturation

Too large CT secondary burden,currents

Ideal Actual


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CT ratings to avoid AC saturation

Vx > If (RCT+RL+RB)

Vx = Saturation VoltageIf = CT secondary current during fault

RCT= CT Secondary Resistance- OhmsRL = CT lead Resistance- Ohms

RB

= CT Connected burden Resistance-

Ohms


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CT Transient Saturation

Caused by DC Transients in the power

system

Current

0

1

2

-2

-1

DCAC

Cycles

1 2


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CT Transient Saturation (Minimum Math!)

i = current , v = voltage = Flux in the magnetic core,all instantaneous;

i v (d /d t)where d /d t represents the rate of change of flux.

i (d /d t)Integrating,

i Rewriting,

i (Flux is decided by area under the

time function i)


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Flux during AC currents

v i d/dt

i


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Flux during DC Transients

v i d/dt

i


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CT ratings to avoid Transient saturation

Vx > If (1+X/R) (RCT+RL+RB)

Where,

X, R= Primary system reactance andresistances

Avoiding CT saturation may not always bepossible.


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Waveforms during AC+DC Transients

DC

(Ideal CT)

AC+DC Actual in CT

Saturation

I

Time

Ideal CT secondary current

Actual CT secondary current


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Specifications for ANSI CTs

Classification Letter C, K or T

C Performance can be Calculated, low leakagereactance

K- Same as C but with Knee point 70% of secondaryterminal voltage

T- Performance to be Tested

Recommended maximum secondary current100A

Error max: 10% at 100A, so 10A error


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Class C CTs

To avoid AC Saturation, in C800,

100(RCT+ 8) > If (RCT+RL+RB)

Typically

If < 100A

Connected burden RL+RB < 8 Ohms


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Class C CTs

To avoid DC Saturation, in C800,

100(RCT+ 8) > If (1+X/R)(RCT+RL+RB)

Normally If < 100A, Connected burden is lessthan design burden;

Define Ni = 100/ If ( Ideally >1)

Define Nr = (RCT+ 8) / (RCT+RL+RB ) (Ideally >1)

The equation above becomes

Ni. Nr > (1+X/R)

In other words CT saturation is avoided if

(1+X/R) < Ni. Nr


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Remanence

Remanence, Residual flux

Similar to permanent magnetism

Reduces available excursion of flux totranslate currents

If is the per unit of maximum flux remaining

as residual flux, CTs have to be oversized by afactor

1/(1-)

If = 0.9, the above factor is 10, that biggerCT is required!!!


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Remanence

Reduce

Gap in the steel core

Different core materials

Biased core

Account for remanence-

Increase the CT size- Not an option alwaysReduce the burdens, leads etc.

Make the relay faster- to operate before CT

saturation starts Increased slope

Special relays with algorithms


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CT- Time to saturate

t/T

Vx / (IRT)

0 1 2 3 4 5 60

2

1

Vx = Saturation VoltsI = Symm. Secy Current, A

R = Secy. Circuit Resist, Ie = Exciting Current, A

T = Primary Circuit Time Constant, Cycles

t = Time to saturate in Cycles


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Calculating Lead Resistance - Example

Data

CT C400, 1000/5A, RCT = 0.25 Ohms

Fault Primary = 10kA at X/R = 15

Relay burden = Negligible

Calculations:

If = 10000/CTR = 10000/200 = 50A

Ni= 100/ 50 = 2

Nr = 4.25/(0.25+RL)

Checking for adequacy,

(1+X/R) > Ni.Nr

(1+15) > 2 x 4.25 /(0.25 + RL)

RL < 0.28 Ohms


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Case Study Fig 1


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Case Study Fig 2


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Case Study Fig 3

I

t


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Case Study Fig 4


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Conclusion

A thorough understanding of the application ofCT is required

Previous experience of CT wire sizing may not

always be correct in a newer application

More than adequate CT sizes and cable sizeswaste resources

Application check is recommended, always forcritical applications

CT wire sizing

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