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IEC06000316 V1 EN-US
Figure 212: Relation between the magnitude of the generator ground fault current
and the fault time
As mentioned earlier, for medium and large generators, the common practice is to have
high impedance grounding of generating units. The most common grounding system is
to use a neutral point resistor, giving an ground fault current in the range 3 – 15 A at a
non-resistive ground-fault at the high voltage side of the generator. One version of this
kind of grounding is a single-phase distribution transformer, the high voltage side of
which is connected between the neutral point and ground, and with an equivalent
resistor on the low voltage side of the transformer. Other types of system grounding of
generator units, such as direct grounding and isolated neutral, are used but are quite
rare.
In normal non-faulted operation of the generating unit the neutral point voltage is close
to zero, and there is no zero sequence current flow in the generator. When a phase-to-
ground fault occurs the fundamental frequency neutral point voltage will increase and
there will be a fundamental frequency current flow through the neutral point resistor.
To detect an ground-fault on the windings of a generating unit one may use a neutral
point overvoltage protection, a neutral point overcurrent protection, a zero sequence
1MRK 502 071-UUS A Section 10
Voltage protection
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062 465
Application manual
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