P54x/EN OP/La4 Operation
(OP) 5-86
MiCOM P543, P544, P545 & P546
OP
CTS Block
IN> Protection
Inhibit
IN Derived/Measured
> Setting
VN Polarizing
Voltage VN Pol>
Slow VTS
Block
EF Timer Block
Directional
Check
IDMT/DT
EF Start
EF Trip
P1633ENb
&
&
&
Figure 57 Directional EF with neutral voltage polarization (single stage)
1.30.1 Negative sequence polarization (Not for SEF)
In certai
n applications, the use of residual voltage polarization of DEF may either be not
possible to achieve, or problematic. An example of the former case would be where a
suitable type of VT was unavailable, for example if only a three limb VT was fitted. An
example of the latter case would be an HV/EHV parallel line application where problems with
zero sequence mutual coupling may exist.
In either of these situations, the problem may be solved by the use of negative phase
sequence (nps) quantities for polarization. This method determines the fault direction by
comparison of nps voltage with nps current. The operate quantity, however, is still residual
current.
It requires a suitable voltage and current threshold to be set in cells N>V2pol set and
N>2pol set, respectively.
Negative sequence polarizing is not recommended for impedance earthed systems
regardless of the type of VT feeding the relay. This is due to the reduced earth fault current
limiting the voltage drop across the negative sequence source impedance (V2pol) to
negligible levels. If this voltage is less than 0.5 volts the relay will cease to provide DEF.
The logic diagram for directional earth fault overcurrent with negative sequence polarization
is shown in Figure 58.
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