1MAC050144-MB C
Section 4
Protection functions
615 series ANSI
323
Technical Manual
The traditional way for calculating the restrained current is:
Equation 26
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, which
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 trip. This module blocks the operation of 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
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