Secondary injection testing for transformer differential protection relay

The basics of differential protection

Differential protection for power transformers has been used for a very long time. It is based on ampereturn-balance of all windings mounted on the same magnetic core lag. In order to correctly apply transformer differential protection three different compensations are provided.

In order to correctly apply transformer differential protection the following compensations shall be provided:

1. Current magnitude compensation for measured current magnitude difference on different sides of the protected transformer;
2. Power transformer phase angle shift compensation; and
3. Zero sequence current compensation (i.e. zero sequence current elimination).

With modern numerical transformer differential relays all above compensations are provided in the relay software.

This paper will address these topics as well as provide standardized solutions for secondary injection testing for transformer differential protection relay from any manufacturer.

Determining appropriate CT secondary current magnitudes for injection I_n order to perform secondary injection the appropriate magnitude of the secondary currents shall be determined.

It is well known fact that two power transformer windings are galvanically separated and have different voltage and current levels on the primary side. Thus the only common electrical quantity for two windings is electrical power which flows through them.

Note that the maximum value among all windings, as stated on the protected power transformer rating plate, is typically selected. This can be simply written as following equation:

S_Base = S_Max [MVA]

When the base power is know the base primary current on each power transformer side can be calculated by using the following equation:

where:

• I_{BasePri_Wi} is winding base current in primary amperes
• S_Base is above defined base apparent power for this application in MVA
• U_{r_Wi} is winding rated phase-to-phase, no-load voltage in kV. Its value for every winding is typically stated on the protected power transformer rating plate.

Note that when a power transformer incorporates an on-load tap-changer (i.e. LTC) different rated phaseto-phase, no-load voltages are given, one for each tap for at least one of the windings. Than it is necessary to select a rated voltage for a one given tap for which it is required to perform the testing.

Typically the mid-tap value is used. Alternatively relay can be tested for more than one LTC position.