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As explained in the TOUR section, the line current differential protection does not much in its operating characteristics from other differential protections working on the Merz Price current differential principle. Essentially, the currents measured at the relay points sampled at the ends of the protected line are summed vectorially on phase basis to find the residual current. If this residual current (or better known as operating current) is higher than the compensated operating current calculated from the per unit values, the differential protection will operate.
To test the restraint characteristic of the line current differential relay, enter the settings for the line current differential protection. With these settings, APRICT_87L calculates pickup values of the secondary currents based on the restraint and operating per unit values.
APRICT_87L recommends to use the standard method of testing differential protection relays. This is to start reducing or increasing the “holding” current on LOCAL/REMOTE while maintaining injection of current at the REMOTE/LOCAL. However, this test method is difficult as the line current differential relays are installed at two different locations. One way to overcome this problem is to temporarily use the protection relay of adjacent circuit to replicate the relay at REMOTE. This could be handy as transmission lines are always on double circuit towers with identical protection scheme. Alternatively, if the numerical protection relay has a built in function that can simulate current at the remote end, the restraint characteristic can also be checked using this method.
If the line current differential has more than two terminals (transmission line is tapped), the restraint characteristic is checked by injection of current of two relays in turns. (Click image to enlarge)
Try online to see how the settings change the protection characteristic.
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