ABB RELION REX640 - Page 790

I I I
d
= +
1 2
(Equation 201)
During normal conditions, there is no fault in the area protected by the function
block, so the currents
I
1
and
I
2
are equal and the differential current I
d
= 0.
However, in 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 I
b
=
−
1 2
2
(Equation 202)
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.
Protection functions
1MRS759142 F
790 REX640
Technical Manual