Large internal faults are protected by
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Merz price percentage differential protection
Percentage Differential Protection
This protection is also known as circulating current protection or Merz Price protection. It is used especially for short circuits within the transformer of rating 5 MVA or above. This scheme is employed for the protection of transformers against large internal short circuits. It is not capable of detecting incipient faults. The figure shows the schematic diagram of percentage differential protection for a Y- Δ transformer. The current entering end has been marked as positive. The end at which current is leaving has been marked negative. O and R are the operating and restraining coils of the relay, respectively. The connection is made in such a way that under normal conditions or in case of external faults, the current flowing in the relay operating coil due to CTs of the primary side. Consequently, the relay does not operate under such conditions.
If a fault Earth fault or short circuits occur on the winding the polarity of the induced voltage of the CT of the secondary side is reversed. Now the current in the operating coil from CTs of both sides is in the same direction and causes the operation of the relay. In the star side of the transformer, CTs are connected in delta or vice versa.