ABB Relion 670 Series Applications Manual page 229

1MRK 511 401-UUS F
9.3.3.4 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 99.
ANSI07000190 V1 EN-US
Figure 99: Ground fault in Non-effectively grounded systems
9.3.3.5 Direct grounded system
In direct grounded systems, an ground fault on one phase is indicated by voltage collapse in that
phase. The other healthy phase will still have normal phase-to-ground voltage. The residual sum
will have the same value as the remaining phase-to-ground voltage, which is shown in Figure 100.
Bay control REC670
Application manual
ANSI07000190-1-en.vsd
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Section 9
Voltage protection
M13853-18 v10
GUID-EA622F55-7978-4D1C-9AF7-2BAB5628070A v8
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