The protection logic of 87T relay is shown in the figure below: The same current base is used for differential current settings. The rated current can be considered as the current seen at the secondary side of the CT at winding 1 during normal operating condition. Based on the rated current specified in the relay control panel, the per unit values of the currents are calculated. It has to be noted that the input CT current polarity should be either “looking towards the transformer” for all inputs or “looking away the transformer” for all inputs and all the CTs are considered to be Y connected. Then, the magnitudes and angles of the currents are corrected from Y side to delta side according to the transformer winding connections in order to ensure the currents used by the differential element are in phase. ![]() ![]() First, the current magnitudes are adapted to the primary side based on the winding voltage levels of the transformer. The differential current is calculated as a phasor summation of the current inputs from the CTs. This model implements two types of differential functions – unrestraint (87U) and restraint (87R) differential elements. It is sensitive to faults within the differential zone, and it should be set to make it insensitive to external faults. ![]() The differential relay trips whenever the difference between two (or three) input currents exceeds a predefined value.
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