INSTRUMENT TRANSFORMERS

PRESENTED BY: MANISH SADHU(090450109039) 6TH SEM ELECTRICAL ENGINEERING

What is mean by Burden on CT?

Burden is the load imposed on the secondary of the CT at rated current and is measured in VA (product of volts and amps).

The errors in ratio and phase angle depend on the impedance connected to the secondary of the transformer. This impedance is commonly referred to as “burden”.

Burden of Instrument Transformer

Burden across the secondary of an instrument transformer is also defined as the ratio of secondary voltage to secondary current.

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Effect of change in burden and power factor on ratio and phase angle of CTs

18/03/2013

1.Effect of P.F. of secondary winding burden on errors-ratio error.

It is observed that for all inductive burdens the secondary induced voltage Es, so that δ is positive .

Under this condition, the actual transformation ratio is always greater than turns ratio.

For burdens which are sufficiently capacitive Is leads Es and so δ is negative and δ approaching - 90°

For inductive burdens,

phase angle θ is

positive for small

values of δ but

becomes negative

as secondary

burden becomes

more inductive

and δ approaches

90° 

2.Effect of change of primary winding current.

At low values of primary winding current Ip or secondary winding current Is magnetizing component Im and loss component Ie are greater proportion of primary winding current Ip and therefore error are greater.

As current Ip increases, there is a decrease in ratio error and phase angle.

Variation of ratio error and phase angle are shown in Figure 2 and 3

3.Effect of change in secondary winding circuit.An increase in secondary winding circuit

burden impedance means increase in volt ampere rating. This necessitates an increase in the secondary winding induced voltage which can be generated by an increased flux and flux density.

Therefore both magnetizing component Im and loss component Ie are increased.

Thus it is excepted that error will increased with increased in secondary winding burden.

In general, a greater burden impendance not only increases the transformation ratio, but also shifts the phase angle between primary winding current and secondary winding current reversed to more positive values as shown in Figure 2 and 3

4.Effect of change of frequency.The effect of increase in frequency will result

in proportionate decrease in the flux density.Thus, in general, the effect of change in

frequency is similar to the product by decrease in impedance of secondary winding burden.

A current transformer is seldom use at a freqency which is very different from the one or which it is designed and, therefore, consideration of this effect is not very important

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