The residual voltage is pre-processed by a discrete fourier filter. Thus, the phasor
of the fundamental frequency component of the residual voltage is derived.
This 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 (-3U
0
) or negative sequence voltage (-3U
2
) is
used to determine the location of the earth fault. This ensures 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
0
) as the 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 a 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-US
(Equation 11)
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 polarizing current is derived. This
phasor is then multiplied with the pre-set equivalent zero-sequence source
impedance in order to calculate the equivalent polarizing voltage UIPol in
accordance with the following formula:
0s
UIPol Z IPol (RNPol j XNPol) IPol= × = + × ×
EQUATION1877 V2 EN-US
(Equation 12)
1MRK 505 384-UEN A Section 6
Current protection
Breaker protection REQ650 2.2 IEC 115
Technical manual
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