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Section 8
Voltage protection
8.3.3.4
High impedance earthed systems
In high impedance earthed systems, earth faults cause a neutral voltage in the feeding
transformer neutral. Two step residual overvoltage protection ROV2PTOV is used to trip the
transformer, as a backup protection for the feeder earth-fault protection, and as a backup for
the transformer primary earth-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 earth fault causes a transformer neutral
to reach a voltage equal to the nominal phase-to-earth voltage.
The voltage transformers measuring the phase-to-earth 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 earth. The residual overvoltage will be three times the phase-to-
earth voltage. See figure 115.
IEC07000190 V2 EN-US
Figure 115: Earth fault in Non-effectively earthed systems
8.3.3.5
Direct earthed system
In direct earthed systems, an earth fault on one phase is indicated by voltage collapse in that
phase. The other healthy phase will still have normal phase-to-earth voltage. The residual sum
will have the same value as the remaining phase-to-earth voltage, which is shown in Figure 116.
214
IEC07000190-2-en.vsd
1MRK 505 370-UEN D
M13853-18 v10
GUID-EA622F55-7978-4D1C-9AF7-2BAB5628070A v8
Busbar protection REB670
Application manual
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