ABB RED650, Applications Manual

Page: 87 / 454

close to zero). Note that all small pre-fault differential currents are subtracted, no
matter what their origin. This action is made separately for each phase.
When a disturbance occurs, values of the pre-fault differential currents are not
updated, and the updating process is only resumed 100 ms after normal conditions
have been restored. Normal conditions are then considered when there are no start
signals, neither internal nor external fault is detected, the power system is
symmetrical and so on. If an Open CT condition is detected, the compensation of
charging currents is stopped immediately and the charging currents are temporarily
memorized by the function. When Open CT signal resets, the process of
compensation is resumed with the same charging current as before. The
consequence of freezing the pre-fault values during fault conditions in this way will
actually introduce a small error in the resulting calculated differential current under
fault conditions. However, this will not have any practical negative consequences,
while the positive effect of maintaining high sensitivity even with high charging
currents will be achieved. To demonstrate this, two cases can be studied, one with a
low resistive short circuit, and one with a high resistive short circuit.
The charging current is generated because there is a voltage applied over the line
capacitance as seen in figure
32 . If an external short circuit with negligible fault
resistance occurs close to the line, the voltage in the fault location will be
approximately zero. Consequently, zero voltage will also be applied over part of
the line capacitance, which in turn will decrease the charging current compared to
the pre-fault value. As mentioned above, the value of the pre-fault “false”
differential current will be frozen when a fault is detected, and, as a consequence,
the value of the subtracted charging current will be too high in this case. However,
as it is a low resistive fault, the bias current will be comparatively high, while the
charging current and any errors in the approximation of this will be comparatively
low. Thus, the overestimated charging current will not jeopardize stability as can be
seen from figure
33 , showing the characteristic of line differential protection. In
this figure, the considered fault will appear in the section well in the restrain area.
1MRK 505 393-UEN B
Section 6
Differential protection
Line differential protection RED650 2.2 IEC 81
Application manual