What is Current Error and Phase Displacement in C.T

Current Error

The error which a transformer introduces into the measurement of a current and which arises from the effect that the actual transformation ratio is not equal to the rated transformation ratio.

The current error expressed as a percentage is given by

current error %= (Kn*Is-Ip)x100/Ip

where,

Kn=rated transformation ratio.

Ip=actual primary current

Is= actual secondary current when Ip is flowing under the conditions of measurement.

Example,

Ip=100

100/5=20

(20x4.98-100)x100/100=current error= 4%

Phase Displacement

The displacement in phase between the primary and secondary current vectors, the direction of vectors being so chosen that the angle is zero for a perfect transformer.

The phase displacement is said to be positive when the secondary current vector leads the primary current vector and negative when it lags behind the primary current vector. It is usually expressed in minutes or in cent radians.

What is Burden, Accuracy Class and Rated Accuracy Limit Factor in CT

Accuracy Class

A designation assigned to a current transformer the errors of which remain within specified limits under prescribed conditions of use.

Burden

The value of the impedance of the secondary circuit expressed in ohms or in Volt-Amperes at the rated secondary current at the relevant power factor.

Rated Accuracy Limit Factor

The ratio of the rated accuracy limit primary current to the rated primary current.

Rated Accuracy Limit Primary Current

The value of primary current up to which the transformer will comply with the requirements for composite error.

Standard Accuracy Limit Factors

The standard accuracy limit factors are 5-10-15-20-30

Standard Accuracy Class

The standard accuracy class for protective current transformer are 5P and 10P

Composite Error

Under steady state conditions , the rms value of the difference between the instantaneous values of the primary current and instantaneous value of the actual secondary current multiplied by the rated transformer ratio.

What is Current Transformer?

Measurement of alternating current is one of the most frequent operations in electrical systems because it is necessary in determining other parameters of electrical circuits. Sample current is required for:

1. Indication instruments

2. KWh and KW meters

3. Tele metering

4. Protective relays

A current transformer is intended to operate normally with the rated current of the network flowing through the primary winding which is inserted in series in the network. The secondary winding of the current transformer is connected to measuring instruments and relays.

*CT (Current transformer) is always installed in series

*PT (Potential Transformer) is always installed in parallel

A CT is similar in construction to the single phase power transformer and also obeys the same fundamental laws. However the primary current of the CT is not controlled but is imposed on the transformer by the primary supply system.

The primary ampere turns produce a magnetic flux in the iron core which in turn induces an emf in the secondary winding. This causes a current to flow through burden connected.

For a CT,

I1N1=I2N2

or

I1/I2=N2/N1

and

V2=I2Z2

where internal drop,

E2-V2=E2-I2Z2=I2Z1 or

E2=I2(Z2+Z1)

Magnetic flux depends on E2, with variation of Z2, the flux density changes.

Where,

N1=Turns in primary winding

N2=Turns in secondary winding

I1=Current in primary winding

I2=Current in secondary winding

z2=Secondary load circuit

z1=internal impedance which consists of resistance and leakage reactance of  secondary winding

E2=voltage induced by the magnetic flux

Fi=Flux common to both windings

I0=no load current which is exiting current

In case of power transformer there is always another current, I0 known as no load current for maintaining magnetic flux and iron losses which disturbs this ideal equation and therefore I1N1=I2N2 is not fully realized. In current transformers the current flows in full through the primary winding. Hence a part of this current, called no load current is consumed to produce the required flux.

 

The voltage induced in secondary winding and the flux density of the magnetic core will depend on the secondary current and load impedance.

What is an Instrument Transformer?

Electrical current or voltage can be measured by bringing it to a suitable instrument, but in many electrical power circuits the current and voltage are both so high that it is desirable for both cost and safety to bring the circuits to the primary winding terminals of a potential transformer to measure voltage or through the primary winding of a current transformer to measure the current.

Standard secondary voltages are 100V, 115V, 120V with output ratings between 10A and 300VA, standard secondary currents are 1A, 2A and 5A.

They includes current transformers and potential (voltage) transformers.