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…

• Insulate secondary circuits from primary

• Provide currents/voltages proportional to the primary

ROLE OF CT, PT IN POWER SYSTEM PROTECTION

• Protection 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 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 core.

SERIES MODE

• Primary 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 MODE

Shunt 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 core

Series 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 ACCURACY

• If 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 CURVE

• Exciting 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 CT

• If 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 current

Ip = Actual primary current

• Phase angle error– Angle by which the

secondary current vector when reversed differs from the primary current.

• Composite error– r.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 ERRORS

• Metering CTs • Protection CTs

CT RATINGS

• Accuracy limits– Limit up to which the specified accuracy could be

retained. Accuracy Limit Factor

• Rated short time current– r.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 Burden

– the 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 RATIO

• Secondary 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 CT

• Transient 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