ABB Relion Technical Manual page 218

Section 8
Current protection
212
The polarizing phasor is used together with the phasor of the operating directional current,
in order to determine the direction to the ground fault (Forward/Reverse). In order to
enable voltage polarizing the magnitude of polarizing voltage shall be bigger than a
minimum level defined by setting parameter VpolMin.
It shall be noted that residual voltage (V
determine the location of the ground fault. This insures the required inversion of the
polarizing voltage within the ground-fault function.
Current polarizing
When current polarizing is selected the function will use an external residual current (3I
or the calculated negative sequence current (I
select the required current.
The residual current can be:
1. directly measured (when a dedicated CT input of the IED is connected in PCM600 to
the fourth analog input of the pre-processing block connected to EF4PTOC (51N/
67N) function input I3PPOL). This dedicated IED CT input is then typically
connected to one single current transformer located between power system WYE
point and ground (current transformer located in the WYE point of a WYE connected
transformer winding).
• For some special line protection applications this dedicated IED CT input can
be connected to parallel connection of current transformers in all three phases
(Holm-Green connection).
2. calculated from three phase current input within the IED (when the fourth analog
input into the pre-processing block connected to EF4PTOC (51N/67N) function
analog input I3PPOL is NOT connected to a dedicated CT input of the IED in
PCM600). In such case the pre-processing block will calculate 3I
inputs into the pre-processing block by using the following formula:
= × = + +
I 3 Io IA IB IC
Pol
EQUATION2019-ANSI V1 EN
where:
IA, IB and IC are fundamental frequency phasors of three individual phase currents.
The negative sequence current can be calculated from the three-phase current input within
the IED by using the pre-processing block. The pre-processing block will calculate the
negative sequence current from the three inputs into the pre-processing block by using the
following formula:
) or negative sequence voltage (V
n
) as polarizing quantity IPol. The user can
2
1MRK 502 048-UUS A
) is used to
2
)
0
from the first three
0
(Equation 51)
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