ABB 615 series Technical Manual page 577

1MAC059074-MB A
615 series ANSI
Technical Manual
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 trip. This module blocks the operation of 87G, 87M 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 trip 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
motor. In case of differential protection, a false differential current can be caused by:
• CT errors
• CT saturation at high currents passing through the motor
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.
Section 4
Protection functions
571