Presence of ground-fault detection
This detection of ground fault is performed in two levels, first by evaluation of the
magnitude of zero sequence current, and secondly by the evaluation of the zero and
negative sequence voltage. It is a complement to the ground-fault signal built-in in the
Symmetrical component based phase selector.
The complementary based zero-sequence current function evaluates the presence of
ground fault by calculating the 3I
0
and comparing the result with the setting parameter
INRelPE. The output signal is used to release the ground-fault loop. It is a complement
to the ground-fault signal built-in in the sequence based phase selector. The condition
for releasing the phase-to-ground loop is as follows:
The output from this detection is used to release the ground-fault loop.
|3I
0
|>maxIph: INRelPE
where:
|3I
0
| is the magnitude of the zero sequence current 3I
0
maxIph is the maximum magnitude of the phase currents
INRelPE
is a setting parameter for the relation between the magnitude of 3I
0
and the
maximum phase current
The ground-fault loop is also released if the evaluation of the zero sequence current by
the main sequence function meets the following conditions:
|3I
0
|>IBase · 0.5
|3I
0
|>maxIph ·INRelPG
where:
maxIph is the maximal current magnitude found in any of the three phases
INRelPG
is the setting of 3I0 limit for release of phase-to-ground measuring loop in % of
IBase
IBase
is the global setting of the base current (A)
In systems where the source impedance for zero sequence is high the change of zero
sequence current may not be significant and the above detection may fail. In those
cases the detection enters the second level, with evaluation of zero and negative
sequence voltage. The release of the ground-fault loops can then be achieved if all of
the following conditions are fulfilled:
|3V
0
|>|V
2
| · 0.5
Section 6 1MRK505222-UUS C
Impedance protection
292
Technical reference manual
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