where:
IL1, IL2 and IL3
are fundamental frequency phasors of three individual phase currents.
The residual current is pre-processed by a discrete Fourier filter. Thus the phasor of
the fundamental frequency component of the residual current is derived. The phasor
magnitude is used within the EF4PTOC protection to compare it with the set operation
current value of the four steps (
IN1>
,
IN2>
,
IN3>
or
IN4>
). If the residual current is
larger than the set operation current and the step is used in non-directional mode a
signal from the comparator for this step is set to true. This signal will, without delay,
activate the output signal STINx (x=step 1-4) for this step and a common START
signal.
8.4.7.2
Internal polarizing
A polarizing quantity is used within the protection in order to determine the direction
to the earth fault (Forward/Reverse).
The function can be set to use voltage polarizing, current polarizing or dual polarizing.
Current polarizing
When current polarizing is selected the function will use an external residual current
(3I
0
) as polarizing quantity IPol. This 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 function input I3PPOL). This dedicated IED CT input is then typically
connected to one single current transformer located between power system star
point and earth (current transformer located in the star point of a star 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 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
0
from the first three inputs
into the pre-processing block by using the following formula:
IPol 3I
IL1 IL2 IL3
0
=
=
+
+
EQUATION2018 V2 EN
(Equation 65)
where:
IL1, IL2 and IL3 are fundamental frequency phasors of three individual phase currents.
1MRK502052-UEN B
Section 8
Current protection
363
Technical manual
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