Current Transformer (CT) – Construction and Working Principle

Capacitive Potential Transformers

These are also introduced as bushing type or coupling type capacitive potential transformers or capacitive potential dividers. The series arrangement of capacitors is attached to the primary or secondary sides. The output value through the secondary winding is detected. It is used for power cable carrier communication aims and is also more costly.

Capacitive Potential Transformer

Errors in Potential Transformers

In the primary instruments, the output value on the secondary side is exactly related to the value of the secondary transformer. The voltage drops due to the resistance and reactance in primary and secondary in the potential transformers, and also the power coefficient on secondary results in the phase shift errors and voltage errors.

Ratio and Phase Angle Errors of Potential Transformer

The primary and the secondary value is accurately related to the primary voltage in an ideal potential transformer and precisely in phase opposition. But this cannot be obtained actually because of the primary and secondary voltage reduction. Therefore, both the primary and secondary value is introduced in the system.

Voltage Ratio Error

The voltage ratio error can be evaluated regarding the sensed voltage, and it is obtained by the formula as presented below:

where K_t is the nominal ratio, i.e., the ratio of the rated primary value and the rated secondary value. If there is a difference between the complete voltage and practical voltage, then the voltage error happens. Percentage of voltage error can also be obtained by the equation below:

Phase Angle Error

The phase angle error is a problem between the secondary section voltage, which is accurately in phase opposition with the voltage of the primary section. The increases in the number of devices in the relay attached to the secondary side of the potential transformer will rise the errors in the potential transformers.
If there is a basic difference between the phase angle of the primary value ‘V_p’ and the reverse secondary value, the phase angle error happens.