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Home / Download Center / Electrical Engineering Books and Technical Guides / (Protection) Relay Guides / 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.

Secondary injection testing for transformer differential protection relay
Secondary injection testing for transformer differential protection relay

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.

Thus, it can be quite tricky to test a numerical transformer differential relay by secondary injection in order to verify that the relay is set properly to protect transformer in a particular application.

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

Two differential protection solutions for wye-delta connected power transformer
Two differential protection solutions for wye-delta connected power transformer (explained later in this paper)

Determining appropriate CT secondary current magnitudes for injection In 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.

Therefore for the transformer differential protection the maximum apparent power among all power transformer windings is typically selected as the base quantity. That was the reason why interposing CTs for the solid-state relays were as well calculated using this maximum power as a base.

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:

SBase = SMax [MVA]

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

Base power is know the base primary current

where:

  • IBasePri_Wi is winding base current in primary amperes
  • SBase is above defined base apparent power for this application in MVA
  • Ur_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.

Title: Secondary injection testing for transformer differential protection relay – Zoran Gajić (Vasteras, Sweden) and Fahrudin Mekic (Allentown, PA)
Format: PDF
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Pages: 25
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Secondary injection testing for transformer differential protection relay
Secondary injection testing for transformer differential protection relay

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