ABB ret650, Technical Manual

Page: 169 / 670

Current polarizing
When current polarizing is selected the function will use an external residual current
(3I
0
) or the calculated negative sequence current (I
2
) as polarizing quantity IPol. The
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
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 45)
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:
2
I2 = (IL1+alpha IL2+alpha IL3)/3
× ×
IECEQUATION2406 V1 EN
(Equation 46)
where:
IL1, IL2 and IL3 are fundamental frequency phasors of three individual phase currents.
alpha
phasor with an angle of 120 degrees.
The polarizing 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 pre-set equivalent zero-sequence source Impedance in
1MRK 504 135-UEN A
Section 8
Current protection
163
Technical manual