• 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 SMAI AI3P connected to I3PDIR input which was connected
to I3P input also):
If zero sequence current is selected,
op
I 3 Io IA IB IC= × = + +
EQUATION2011-ANSI V1 EN-US (Equation 125)
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 PUSTx (x=step
1-4) for this step and a common PICKUP signal.
8.4.7.2 Internal polarizing
M13941-82 v10
A polarizing quantity is used within the protection in order to determine the direction to the
ground fault (Forward/Reverse).
The function can be set to use voltage polarizing, current polarizing or dual polarizing.
Voltage polarizing
When voltage polarizing is selected the protection will use the residual voltage -3V
0
as polarizing
quantity V3P.
This voltage can be:
1. directly measured (when a dedicated VT 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
V3P). This dedicated IED VT input shall be then connected to open delta winding of a three
phase main VT.
2. calculated from three phase voltage input within the IED (when the fourth analog input into
the pre-processing block connected to EF4PTOC (51N/67N) analog function input V3P is NOT
Section 8 1MRK 502 066-UUS B
Current protection
480
Technical manual
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