ABB RELION 620 Series Technical Manual page 498

Section 4
Protection functions
492
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
GUID-34FA472E-E419-4A0E-94A3-238D7A3CE5ED V1 EN
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.
DC component detection
On detection of a DC component, the function temporarily desensitizes the
differential protection. The functioning of this module depends on the DC restrain
Enable setting. The DC components are continuously extracted from the three
instantaneous differential currents. The highest DC component of all three is taken as
a kind of DC restraint in a sense that the highest effective, temporary sensitivity of the
protection is temporarily decreased as a function of this highest DC offset. The
calculated DC restraint current is not allowed to decay (from its highest ever measured
value) faster than with a time constant of one second. The value of the temporarily
effective sensitivity limit is limited upwards to the rated current of the machine or 3.3
times that of Low operate value, whichever is smaller. The temporary extra limit
decays exponentially from its maximum value with a time constant of one second.
1MRS757644 F
(Equation 58)
620 series
Technical Manual