Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Power System Protection
S.A.Soman
Department of Electrical EngineeringIIT Bombay
Introduction to CT
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
1 Introduction
2 Equivalent Circuit of CTModeling of CT
3 Classification of CTsANSI/IEEE Classification
4 Summary
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
IntroductionAll electrical measurements and relaying decisions arederived from current and voltage signals. CTs and VTs arenecessary because
They scale down line currents and bus voltages to fedinto the relays.
They are sensors for the relay.
They electrically isolate the relaying system from theapparatus.
Electrical isolation from the primary voltage providessafety for both human personnel and equipment.
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
Equivalent Circuit of CT
It is similar to that of a regular transformer.
The main difference is that CTs have current sourceexcitation.
Primary winding of the CT is connected in series withthe transmission line.
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
1 Introduction
2 Equivalent Circuit of CTModeling of CT
3 Classification of CTsANSI/IEEE Classification
4 Summary
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
Modeling of CT
It is assumed that CT primary is connected to a currentsource.
Primary winding resistance and leakage reactance areneglected.
Magnetizing impedance from primary side is shifted tosecondary side.
The load on the secondary side are the relay burdenand the lead wire resistance.
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
Modeling of CT
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
Modeling of CT
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
Total impedance on the secondary side is the sum of
Relay burden.
Lead wire resistance.
Leakage impedance of secondary winding.
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
The secondary voltage developed by the CT is given by,
E2 = 4.44fN2φm
If Bm corresponding to the peak sinusoidal flux φm isabove the knee point, the CT will saturate.During saturation, the performance of CT deteriorates.CT is to be operated always in closed condition.The magnetizing impedance falls down duringsaturation.
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
Classification of CTsCTs can be classified into
Measurement CTs.
They have lower VA capacity than protection grade CTs.It has to be accurate over a range of 5% to 125% ofnormal current.It is not expected to give linear response during largefault currents.
Protection CTs.
It is expected to give linear response upto 20 times therated current.Its performance has to be accurate in the range ofnormal currents to fault currents.In order to get linear response, CT burden has to bekept low.
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
Classification of CTsCTs can be classified into
Measurement CTs.They have lower VA capacity than protection grade CTs.It has to be accurate over a range of 5% to 125% ofnormal current.It is not expected to give linear response during largefault currents.
Protection CTs.
It is expected to give linear response upto 20 times therated current.Its performance has to be accurate in the range ofnormal currents to fault currents.In order to get linear response, CT burden has to bekept low.
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
Classification of CTsCTs can be classified into
Measurement CTs.They have lower VA capacity than protection grade CTs.It has to be accurate over a range of 5% to 125% ofnormal current.It is not expected to give linear response during largefault currents.
Protection CTs.It is expected to give linear response upto 20 times therated current.Its performance has to be accurate in the range ofnormal currents to fault currents.In order to get linear response, CT burden has to bekept low.
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
1 Introduction
2 Equivalent Circuit of CTModeling of CT
3 Classification of CTsANSI/IEEE Classification
4 Summary
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
ANSI/IEEE ClassificationANSI/IEEE standards classify CTs into two types:
Class T CTs.
Class C CTs.
Ratio ErrorIt is the percentage deviation in the current magnitude in thesecondary from the desired value.
Ratio Error =IpN − Is
Is× 100 =
IEIs× 100
During saturation, ratio errors are very large.
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
Class T CTs
It is a wound type CT with one or more primary turns ona core.It has high leakage flux.Its performance can be determined only by test.CT burden should be kept as low as possible.
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
Class C CTs
Its leakage flux is very small.
It is more accurate.
Its performance can be evaluated from standardexciting curves.
Ratio error is maintained within ±10%.
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
Power SystemProtection
S.A.Soman
Introduction
EquivalentCircuit of CTModeling of CT
Classificationof CTsANSI/IEEEClassification
Summary
Introduction to CT
Summary
Equivalent circuit and modeling of CTs.
Measurement CTs and Protection CTs.
Class T CTs and class C CTs.
Ratio error.
