ABB REG316 4 Series Operating Instructions Manual page 682

The protection was checked with regard to security to block. All testing was made
with and without remanence flux in the current transformer core. It is difficult to
give general recommendations for additional margins for remanence flux. It
depends on the demands of reliability and economy.
When current transformers of type TPY are used, practically no additional margin
is needed due to the anti remanence air gap. For current transformer of Type
TPX, the small probability of a fully asymmetrical fault together with maximum
remanence flux in the same direction as the flux generated by the fault has to be
kept in mind at the decision of an additional margin. Fully asymmetrical fault
current will be achieved when the fault occurs at zero voltage (0°). Investigations
have proved that 95% of the faults in the network will occur when the voltage is
between 40° and 90°.
Fault current
The current transformer requirements are based on the maximum fault current for
faults in different positions. Maximum fault current will occur for three phase faults
or single phase to ground faults. The current for a single phase to ground fault will
exceed the current for a three phase fault when the zero sequence impedance in
the total fault loop is less than the positive sequence impedance.
When calculating the current transformer requirements, the maximum fault current
should be used and therefore both fault types have to be considered.
Cable resistance and additional load
The current transformer saturation is directly affected by the voltage at the current
transformer secondary terminals. This voltage, for a ground fault, is developed in a
loop containing the phase and neutral conductor and additional load in this loop.
For three phase faults, the neutral current is zero, and only the phase conductor and
additional phase load have to be considered.
In the calculation, the loop resistance should be used for phase to ground faults
and the phase resistance for three phase faults.
RET 316 / RET 316*4 current transformer requirements
The current transformer effective overcurrent factor should meet the two
requirement below. The requirement assume 40 to 300 msec maximum dc time
constant for the network.
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