One zone of distance relaying with a mho characteristic is commonly used for system phase-fault backup. These relays are usually connected to receive currents from current transformers in the neutral ends of the generator phase windings and potential from the terminals of the generator.
If the generator is connected to the system using some means other than a delta-wye step-up transformer (i.e., direct connection, wye-wye transformer, etc.), then standard ct and vt connections made to a standard Mho distance relay will provide accurate measurement of impedances for system faults (neglecting infeed).
However, if there is a delta grounded-wye step-up transformer between the generator and the system, special care must be taken in selecting the distance relay and in applying the proper currents and potentials so that these relays see correct impedances for system faults. With some relay designs, the phase angle of the voltages applied to the relay have to be shifted so that they are in phase with the system voltages in order for the relay to see system faults correctly.
If required, this phase shift is accomplished by using auxiliary voltage transformers connected in delta-wye as shown in figure 1.
Application of system back-up relays— Unit generator-transformer arrangement
NOTE—This is a phase shifting transformer only. The turns ratio is chosen so that the line-to-line voltages on either side of the auxiliary vts are 1:1.
When a generator is connected directly to a system, the connections to the relay are shown in figure 2.
Application of system back-up relays—Generator connected directly to the system
In both cases, for the connections shown, the relay will not only provide backup for system faults but it will also provide some backup protection for phase faults in the generator and generator zone before and after the generator is synchronized to the system.
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