Direct Earthed System - ABB Relion REC670 Applications Manual

1MRK 511 358-UEN A
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 79.
IEC07000190 V1 EN-US
Figure 79: Earth fault in Non-effectively earthed systems
8.3.3.5

Direct earthed system

In direct earthed systems, an earth fault on one phase indicates a voltage collapse in that
phase. The two healthy phases will have normal phase-to-earth voltages. The residual sum will
have the same value as the remaining phase-to-earth voltage. See figure 80.
Application manual
Section 8
Voltage protection
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