ABB RELION 670 Series Applications Manual page 215

1MRK 505 337-UUS A
8.3.3.4 High impedance grounded systems
In high impedance grounded systems, ground faults cause a neutral voltage in the feeding
transformer neutral. Two step residual overvoltage protection ROV2PTOV (59N) is used to trip the
transformer, as a backup protection for the feeder ground fault protection, and as a backup for
the transformer primary ground fault protection. The setting must be above the highest occurring
"normal" residual voltage, and below the lowest occurring residual voltage during the faults under
consideration. A metallic single-phase ground fault causes a transformer neutral to reach a
voltage equal to the nominal phase-to-ground voltage.
The voltage transformers measuring the phase-to-ground voltages measure zero voltage in the
faulty phase. The two healthy phases will measure full phase-to-phase voltage, as the faulty phase
will be connected to ground. The residual overvoltage will be three times the phase-to-ground
voltage. See figure 100.
Application manual
Section 8
Voltage protection
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