ABB RELION 650 Series Applications Manual page 82

Section 6
Differential protection
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Figure 34: Charging currents
The magnitude of the charging current is dependent of the line capacitance and the system
voltage. For earth cables and long overhead lines, the magnitude can be such that it affects
the possibility to achieve the wanted sensitivity of the differential protection. To overcome
this, a charging current compensation is available in line differential protection. When enabled,
this algorithm will measure the fundamental frequency differential current under steady state
undisturbed conditions and then subtract it, making the resulting differential current zero (or
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 34. 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 35, showing the characteristic of line
differential protection. In this figure, the considered fault will appear in the section well in the
restrain area.
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Application manual