ABB ret650, Technical Manual

Page: 167 / 670

• 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
star point and earth (that is, current transformer located in the star point of a
star connected transformer winding).
• one single current instrument transformer located between two parts of a
protected object (that is, current transformer located between two star
points of double star 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 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,
Iop 3I IL1 IL2 IL3
0
= = + +
EQUATION1874 V2 EN
(Equation 41)
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 residual current is derived. The phasor
magnitude is used within the EF4PTOC protection to compare it with the set operation
current value of the four steps (
IN1> , IN2> , IN3> or IN4> ). 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 STx (x=step 1-4) for this step and a common START signal.
8.4.7.2 Internal polarizing
A polarizing quantity is used within the protection in order to determine the direction
to the earth 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 either the residual voltage
3U
0
or the negative sequence voltage U
2
as polarizing quantity U3P.
The residual voltage can be:
1MRK 504 135-UEN A
Section 8
Current protection
161
Technical manual