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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