Nominal Ratio of Instrument Transformers

This article explains the concept of the Nominal Ratio, a crucial parameter for understanding instrument transformers, specifically current transformers (CTs) and potential transformers (PTs).

Definition of Nominal Ratio

The Nominal Ratio is defined as the ratio of the rated primary winding current (or voltage) to the rated secondary winding current (or voltage). This term is fundamental in the world of instrument transformers, which are used to step down high currents or voltages to measurable levels for metering and protection purposes.

Nominal Ratio for Current Transformers (CTs)

For a current transformer, the nominal ratio is the relationship between the rated current on the primary side and the rated current on the secondary side. In simpler terms, it tells you how many amps on the primary side correspond to one amp on the secondary side under ideal conditions.

The formula for the nominal ratio of a current transformer is:

➤ Kn = Rated primary winding current / Rated secondary winding current

Where:

• Kn represents the Nominal Ratio
• Rated primary winding current is the current the CT is designed to measure on the primary side.
• Rated secondary winding current is the current produced by the CT on the secondary side when the rated primary current is flowing.

Nominal Ratio for Potential Transformers (PTs)

Similarly, for potential transformers, the nominal ratio expresses the relationship between the rated voltage on the primary side and the rated voltage on the secondary side. It indicates how much the voltage is stepped down from the primary to the secondary.

The formula for the nominal ratio of a potential transformer is:

➤ Kn = Rated primary winding voltage / Rated secondary winding voltage

Where:

• Kn represents the Nominal Ratio
• Rated primary winding voltage is the voltage the PT is designed to measure on the primary side.
• Rated secondary winding voltage is the voltage produced by the PT on the secondary side when the rated primary voltage is present.

In summary, the Nominal Ratio provides a straightforward way to understand the transformation capabilities of instrument transformers, whether they are stepping down current or voltage. This ratio is essential for accurately interpreting measurements taken using these devices.