Digital signal pulse control board PFSK152

Digital signal pulse control board PFSK152

Digital signal pulse control board PFSK152

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