Electrical & Electronic Measurements : Instrument Transformers

Instrument Transformers: Instrument transformer used for over current, under voltage earth fault and for relay protection.

It might appear that the extension of range could be conveniently done by the use of shunts for currents and multiplier for voltage measurement, as in DC. But this method is suitable only for small values of current and voltage due to the following reason

• Difficult to achieve accuracy with a shunt on AC.
• The capability of having a shunt of the large range is not possible.
• The power consumed by multipliers become large as the voltage increases.
• The measuring circuit is not isolated electrically from the power circuit.

Circuit Diagram of Instrument Transformer

Advantages of Instrument Transformer

• Their reading does not depend upon circuits constant such as R, L & C as in the case of shunts and multipliers As in the case of shunts and multipliers.
• Possible to standardize the instrument around their ratings this makes the replacement of the instrument transformer very easy.
• The measuring circuit is isolated from the Power circuit.
• Low power consumption in the metering circuit. Several instruments can be operated from a single instrument transformer.

Ratios of Instrument Transformer

Transformation Ratio (K) It is the ratio of the magnitude of the primary phasor to the secondary phasor.

For the current transformer,

For potential transformer,

Nominal Ratio (NR): It is the ratio of rated primary winding current (or voltage) to the rated secondary winding current (or voltage).

Where, CT = Current transformer

PT = Potential transformer

Turns Ratio (N)

For current transformer

For potential transformer

Ratio Correction Factor (RCF)

Different Parameter in Instrument Transformer

The burden of an Instrument Transformer

It is volt-ampere loading.

Secondary winding burden

Or Secondary winding burden

= (Secondary winding current)² × (Impedance of secondary winding circuit)

Current Transformer (CT)

Transformation Ratio

Phase angle  degree :

Where, 

Potential Transformer (PT)

Actual Transformation Ratio (K) :

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