7.6.7.1
Operating quantity within the function
If the function is set to measure zero sequence, it uses Residual Current (3I
0
) for its
operating quantity. 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 I3P). This dedicated IED CT input can be for example, connected
to:
•
parallel connection of current instrument transformers in all three phases
(Holm-Green connection).
•
one single core balance, current instrument transformer (cable CT).
•
one single current instrument transformer located between power system WYE
point and ground (that is, current transformer located in the neutral grounding
of a WYE connected transformer winding).
•
one single current instrument transformer located between two parts of a
protected object (that is, current transformer located between two WYE points
of double WYE shunt capacitor bank).
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 I3P 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 (will take I2 from same
SMAI AI3P connected to I3PDIR input (same SMAI AI3P connected to I3P input)):
If zero sequence current is selected,
op
I
3 Io
IA IB IC
= ×
=
+
+
EQUATION2011-ANSI V1 EN
(Equation 47)
where:
IA, IB, IC
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 (51N/67N) protection to compare it with the set
operation current value of the four steps (
Pickup1
,
Pickup2
,
Pickup3
or
Pickup4
). 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 PU_STx (x=step 1-4) for this step and a common
PICKUP signal.
1MRK 506 335-UUS A
Section 7
Current protection
221
Technical manual
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