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Section 7
Current protection
96
where:
UL1, UL2 and UL3
alpha
The polarizing phasor is used together with the phasor of the operating directional
current, in order to determine the direction to the earth 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 UpolMin.
It shall be noted that residual voltage (U
to determine the location of the earth fault. This insures the required inversion of the
polarizing voltage within the earth-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
0
user can 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 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
into the pre-processing block by using the following formula:
=
=
+
+
IPol 3I
IL1 IL2 IL3
0
EQUATION2018 V2 EN
where:
IL1, IL2 and IL3 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:
are fundamental frequency phasors of three individual phase voltages.
unit phasor with an angle of 120 degrees.
) or negative sequence voltage (U
n
1MRK 505 288-UEN A
) as polarizing quantity IPol. The
2
from the first three inputs
0
Technical manual
) is used
2
(Equation 11)
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