ABB Relion REG670, Руководство по применению

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IEC06000316 V1 EN
Figure 212: Relation between the amplitude of the generator earth fault current
and the fault time
As mentioned earlier, for medium and large generators, the common practice is to
have high impedance earthing of generating units. The most common earthing system
is to use a neutral point resistor, giving an earth fault current in the range 3 – 15 A at
a non-resistive earth-fault at the high voltage side of the generator. One version of this
kind of earthing is a single-phase distribution transformer, the high voltage side of
which is connected between the neutral point and earth, and with an equivalent resistor
on the low voltage side of the transformer. Other types of system earthing of generator
units, such as direct earthing 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-earth 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 earth-fault on the windings of a generating unit one may use a neutral
point overvoltage protection, a neutral point overcurrent protection, a zero sequence
overvoltage protection or a residual differential protection. These protection schemes
are simple and have served well during many years. However, at best these schemes
protect only 95% of the stator winding. They leave 5% at the neutral end unprotected.
Section 10
1MRK 502 071-UEN -
Voltage protection
430 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual