Testing numerical transformer differential relays

Numerical transformer differential relays require careful consideration regarding how to test them properly. These relays provide different types of protection such as restrained phase differential, high set phase differential, restrained ground differential and overcurrent protection. All protection elements that are enabled should be adequately tested. A common commissioning practice is to test all the numerical relay settings to verify they were properly entered. Automated testing using computer software to run the test set has made this possible since the overall commissioning for a numerical relay could consist of several hundred tests. While this is a good check, it is still important to ensure that the transformer is thoroughly protected for the particular application.