ABB RELION REX640 Technical Manual page 712

Section 4
Protection functions
706
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.
This feature should be used in case of networks where very long time constants are
expected. The temporary sensitivity limit is higher to the set operating characteristics.
In other words, the temporary limit has superposed the unchanged operating
characteristics and temporarily determines the highest sensitivity of the protection.
The temporary sensitivity is less than the sensitivity in section 1 of the operating
characteristic and is supposed to prevent an unwanted trip during the external faults
with lower currents.
Biased low stage
The current differential protection needs to be biased because of the possible
appearance of a differential current which can be due to something else than an actual
fault in the machine. In case of differential protection, a false differential current can
be caused by:
• CT errors
• CT saturation at high currents passing through the machine
The differential current caused by CT errors increases at the same percent ratio as the
load current.
The high currents passing through the protected object can be caused by the through
fault. Therefore, the operation of the differential protection is biased with respect to
the load current. In the biased differential protection, the higher the differential current
required for the protection of operation, the higher the load current.
Based on the conditions checked from the through-fault module, the DC (component)
detection module and the CT saturation-based blocking modules, the biased low-stage
module decides whether the differential current is due to the internal faults or some
false reason. In case of detection of the TF, DC or CT saturation, the internal
differential blocking signal is generated, which in turn blocks the operating signal. In
case of internal faults, the operation of the differential protection is affected by the bias
current .
The Low operate value setting for the stabilized stage of the function block is
determined with the equation:
1MRS759142 C
REX640
Technical Manual