ABB RELION REX640 Technical Manual page 711

1MRS759142 C
REX640
Technical Manual
I I I
= 1 + 2
d
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During normal conditions, there is no fault in the area protected by the function block,
so the currents and
I I
1 2
practice some differential current exists due to inaccuracies in the current transformer
on the phase and neutral sides, but it is very small during normal conditions.
The module calculates the differential current for all three phases.
The low-stage differential protection is stabilized with a bias current. The bias current
is also known as the stabilizing current. Stabilization means that the differential
current required for tripping increases according to the bias current and the operation
characteristics. When an internal fault occurs, the currents on both sides of the
protected object are flowing into it. This causes the biasing current to be considerably
smaller, which makes the operation more sensitive during internal faults.
The traditional way for calculating the stabilized current is:
I I
1 − 2
I
=
b
2
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The module calculates the bias current for all three phases.
Through-fault detection
Through-fault (TF) detection module is for detecting whether the fault is external, that
is, going through, or internal. This information is essential for ensuring the correct
operation of the protection in case of the CT saturation.
• In a through-fault situation, CTs can saturate because of a high fault current
magnitude. Such AC saturation does not happen immediately when the fault
begins. Thus, the TF module sees the fault as external because the bias current is
high but the differential current remains low. If the AC saturation then occurs, a
CT saturation-based blocking is allowed to work to prevent tripping.
• Normally, the phase angle between the machine neutral and line side CTs is 180
degrees. If an internal fault occurs during a through fault, an angle less than 50
degrees clearly indicates an internal fault and the TF module overrules, that is,
deblocks the presence of any blocking due to CT saturation.
CT saturation-based blocking
Higher currents during the motor startup or abnormally high magnetizing currents at
an overvoltage (transformer-fed motor) or an external fault may saturate the current
transformers. The uneven saturation of the star and line side CTs (for example, due to
burden differences) may lead to a differential current which can cause a differential
protection to operate. This module blocks the operation of MPDIF biased low stage
internally in case of the CT saturation. Once the blocking is activated, it is held for a
certain time after the blocking conditions have ceased to be fulfilled.
are equal and the differential current I
Section 4
Protection functions
(Equation 153)
= 0. However, in
d
(Equation 154)
705