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CURRENT TRANSFORMERS AND VOLTAGE TRANSFORMERS. INTRODUCTION. Function of the protection relays Information to the protection relays have to be accurate and reliable Currents and voltages How to overcome this difficulty. WHAT DOES CURRENT AND VOLTAGE TRANSFORMERS DO…. - PowerPoint PPT Presentation

### Transcript of CURRENT TRANSFORMERS AND VOLTAGE TRANSFORMERS

• CURRENT TRANSFORMERS AND VOLTAGE TRANSFORMERS

• INTRODUCTIONFunction of the protection relays

Information to the protection relays have to be accurate and reliable

Currents and voltages

How to overcome this difficulty

• WHAT DOES CURRENT AND VOLTAGE TRANSFORMERS DO

Insulate secondary circuits from primary

Provide currents/voltages proportional to the primary

• ROLE OF CT, PT IN POWER SYSTEM PROTECTIONProtection systems depend heavily on information provided

CTs and PTs a constituting part of the protection system

No great difference between a measuring voltage transformer and a protective voltage transformer

However, Current transformer is different

• BASIC PRINCIPLES OF OPERATION

Transformer can be operated in

Shunt mode

Series mode

• SHUNT MODEVoltage applied across the primary terminalsWhen secondary is on open circuit, Induces an EMF equal to the voltage appliedCurrent flowing will be that required to excite the core When secondary is loaded Ampere turns of the primary winding will exceed that of secondaryDifference is the flux needed to excite the core.

• SERIES MODEPrimary winding is connected in series with a circuit.

Its current is determined by the sources and loads.

Its current is determined by its sources/load conditions

A component of this current is used to excite the core sufficient to induce an EMF to drive a current in the secondary

• COMPARISON-SHUNT MODE AND SERIES MODEShunt mode

Voltage applied across the primary terminals

When secondary is on open circuit, Induces an EMF equal to the voltage applied

Current flowing will be that required to excite the core

When secondary is loaded Ampere turns of the primary winding will exceed that of secondary Difference is the flux needed to excite the coreSeries mode

Primary winding is connected in series

Its current is determined by its sources/load conditions

A component of this current is used to excite the core sufficient to induce an EMF to drive a current in the secondary

• STEADY STATE THEORY OF CTS

• CT ACCURACYIf the CT is ideal

Primary AT = Secondary AT

Secondary current Primary current

Every CT requires a certain AT to induce in it the magnetic flux

With secondary impedance fixed exciting AT form a larger proportion at smaller primary currents Exciting current required to magnetize the core is the cause for th errors in CTs

• AC PERFORMANCE OF A CT

ES = 4.44 f A B 10-8 Volts

ES= Secondary induced volts

f= Frequency in cycles/second

N= Secondary number of turns

A= Core cross sectional area in sq.cm

B= Flux density in lines per sqcm.

ES = Is( Zb + Zs + Zl)

• CT MAGNETIZATION CURVEExciting current is the source of errors

Value of exciting current depends on the core material and the amount of flux.Ankle point and knee point,

• CT EQUIVALENT CIRCUIT

• OPEN CIRCUITING OF CTIf the secondary of a CT is kept open with a current flowing in the primary, then

No secondary current

No MMF to oppose the flux

Core driven to saturation

High rate of change of flux

High voltages

• CT ERRORS

Ratio error

(KnI s Ip ) x 100/ Ip

Kn = Nominal ratio I s = Actual secondary currentIp = Actual primary currentPhase angle errorAngle by which the secondary current vector when reversed differs from the primary current.Composite errorr.m.s. value of the difference between the ideal secondary current and the actual secondary current. Includes the current and phase angle errors

• LIMITS OF ERRORSMetering CTsProtection CTs

• CT RATINGS

Accuracy limitsLimit up to which the specified accuracy could be retained. Accuracy Limit Factor

Rated short time currentr.m.s. value of the a.c. component of the current which a CT is capable of carrying for the rated time without being damaged by thermal or dynamic effects CT Burdenthe load applied to the secondary of the CT. Normal practice is to express the burden in terms of VA and power factor, the VA being what would be consumed in the burden impedance at rated secondary current

• CHOICE OF CT RATIOSecondary current 5 Amps or 1 Amp

Primary current to be equal to or greater than the normal full load current of the protected circuit

Maximum ratio of a CT is made about 3000/1 and secondary CTs of 20/1 are used.

• TRANSIENT RESPONSE OF CTTransient conditions, prevail within the first few cycles of a fault.

System conditions are very different to that under steady state conditions.

Power systems expansions increases fault currents and time constants of dc transient components.

It is mandatory for the protective equipment/circuit breakers to clear the faults within few cycles

This makes CT duties more onerous

Hence, behaviour of CTs during transient conditions is of great importance.

• TRANSIENT CONDITIONS IN A POWER SYSTEM

Sudden application of a sinusoidal voltage to a series R/L circuit will cause a current to flow in the circuit.

This is equivalent to a fault condition of a power system,

• Equivalent circuit