engleski

Sensitive turn fault protection for power transformer

Three most typical weaknesses of the electro- mechanical and solid-state power transformer differential relays have been: 1. Long operating time in case of heavy internal faults followed by main CT saturation 2. Unwanted operations for external faults and transformer inrush 3. Bad sensitivity for low-level internal faults, such as winding turn-to-turn faults With the introduction of the numerical technology the first two problems could be handled in much better way [4]. However sensitivity for internal winding turn-to-turn faults did not improve much. This paper introduces the new protection principle, which will improve differential relay sensitivity for minor internal turn-to-turn faults. However, the first two challenges are as well efficiently removed by applying the solution described in the paper. A short circuit of a few turns of the winding will give rise to a heavy fault current in the short- circuited turns, but changes in the transformer terminal currents will be very small, because of the high ratio of transformation between the whole winding and the short-circuited turns. For that reason, the traditional transformer differential protection was typically not sensitive enough to detect such winding turn-to-turn faults before they developed into more serious and costly to repair ground-faults. Alternatively, such faults can be as well detected by Sudden Pressure Relay relays. However, these relays detect such low- level faults with a delay typically 50 ms – 100 ms that often allows the fault to evolve into a more serious one. The new protection principle is based on the theory of symmetrical components [1] and [2], or more exact, on the negative-sequence currents. The very existence of relatively high negative-sequence currents is in itself an indication of a disturbance, as the negative sequence currents are superimposed, pure-fault quantities. The negative-sequence quantities are particularly suitable for different kinds of directional tests. For specific applications they seem to be superior to the zero-sequence quantities, which so far have been more extensively used, mostly due to the fact that they were easy to measure.

transformer protection; turn to turn protection

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