Transformation Ratio in Instrument Transformers

The Transformation Ratio is a fundamental concept when dealing with instrument transformers, specifically current transformers (CTs) and potential transformers (PTs). It essentially describes how the transformer scales down the primary quantity (current or voltage) to a more manageable secondary level for measurement purposes.

Definition of Transformation Ratio

In general terms, the Transformation Ratio is defined as:

The ratio of the magnitude of the primary phasor to the magnitude of the secondary phasor.

This can be expressed mathematically as:

R = |Primary Phasor| / |Secondary Phasor|

Where:

• R represents the Transformation Ratio
• |Primary Phasor| is the magnitude of the primary electrical quantity (current or voltage).
• |Secondary Phasor| is the magnitude of the secondary electrical quantity (current or voltage).

Transformation Ratio for Current Transformers (CTs)

For a current transformer, the Transformation Ratio is the ratio of the primary winding current to the secondary winding current. In other words, it tells you how many times larger the current in the primary winding is compared to the current in the secondary winding.

R = Primary Winding Current / Secondary Winding Current

For example, if a CT has a transformation ratio of 100:1, it means that for every 100 amps flowing in the primary winding, there will be 1 amp flowing in the secondary winding.

Transformation Ratio for Potential Transformers (PTs)

For a potential transformer, the Transformation Ratio is the ratio of the primary winding voltage to the secondary winding voltage. This ratio indicates how many times the primary voltage is stepped down (or up) to get the secondary voltage.

R = Primary Winding Voltage / Secondary Winding Voltage

For instance, a PT with a transformation ratio of 1000:100 would step down 1000 volts on the primary side to 100 volts on the secondary side.

Understanding the transformation ratio is crucial when making accurate measurements using instrument transformers. It allows you to calculate the actual primary value from the secondary reading obtained from measuring devices.