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Protection Riddle No.70 - Transformer differential protection commissioning
How can be verified the transformer differential relay connections to CTs and be sure correct CT polarity in commissioning stage?
Author : Sara - From: Iran
Sun, February 21st, 2010 - 19:40
Commissioning transformer differential protection schemes involves several levels of testing:
- Hardware tests verify transformer turns ratio, CT turns ratios, and CT polarity.
- Functional tests validate the performance of the relay elements with the installed settings and test the dc control circuits. Trip tests verify that the relay operates the correct lockout relays and breakers.
- In-service or commissioning tests verify the primary and secondary ac current circuits. We must take into account the transformer ratio and connection; the CT ratio, wiring, and connections; and the relay settings.
The last item is, by far, the most challenging aspect of assuring certainty in commissioning. Modern transformer differential relays have settings that compensate for the difference in the secondary currents, adjusting for the transformer connection (e.g., delta-wye) and removing zero-sequence current.
In order to perform commissioning tests, we must apply balanced three-phase currents to the primary system. Some users energize the transformer to the system and begin to apply load. Ideally, it is prefered to perform this test without connecting to the power system. For example, use a portable generator or a station service transformer to supply a reduced voltage three-phase power supply to one of the windings of the transformer and apply a short circuit to the remaining winding. An example test setup is shown in Figure below.

Through this procedure, we can check the following:
- The phase rotation and angle of the currents
- Secondary current magnitudes
- The relationship of the high-side currents to the low side currents
- The operate or differential current (should be nearly zero)
For the transformer shown in Figure, given a 240 Vac source, we can calculate expected relay currents (magnitude and angle). Using relay metering data, we then observe the measured currents. If the measured (actual) currents do not match the calculated (expected) currents and/or we observe differential current, we must perform troubleshooting to systematically check CT wiring, connections, and relay settings to correct the discrepancy.

For REF protection test the following diagram can be used.

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