Primary-side protection of distribution substation transformers

Locations of faults affecting the transformer

Primary-side protective devices are widely applied at load-tap transformers where the available primary fault current exceeds the device’s primary-fault interrupting rating. Overlapping protection from statistically rare high-current primary faults is afforded by the line-terminal circuit breakers and first-zone phase- and ground-fault line- protective relays.

On systems using line-terminal circuit breakers, circuit interruption following a high-current primary fault is typically accomplished by the line breakers in 3 cycles.

A properly applied transformer protective system will overlap some of the protection provided by the line-terminal circuit breakers and supplement the protection afforded by the secondary-side protective device(s).

To understand how a primary-side transformer protective device is applied, let’s look at Figure 1. It shows a one-line diagram of the relay protection scheme used for a typical radially tapped transformer protective scheme – including four locations where faults may occur.

Location 1: Faults on Bus Between Transformer Protective Device and Transformer, and Transformer Primary Bushing-to-Ground Faults

This location is typically not part of the relaying for a primary-side protective device due to the location of the primary- side current transformers. It is important to remember that the current transformers for the transformer differential protection are typically located on the bushings of the transformer.

Therefore, the primary-side protective device will not receive a trip signal for events on the primary bus, and this device will not get called on to clear full primary-side fault currents on the primary side. For example, primary arrester failures will be cleared by the line terminal breakers, as shown in Figure 1 above.