MV DC SwitchgearDC Switchgear

Artificial current zero crossing creating method for DC switchgear

This method is injecting or superimposes a current of opposite direction to the current to be interrupted. The superimposed current can be provided by interaction with a parallel oscillatory circuit or by actively injecting a current. In this way, an artificial current zero-crossing is generated. The interruption is then rather similar to that of an AC circuit.
The figure shows a scheme for actively injecting a current in a direction opposite to the system current and thereby creating a current zero. While the switch in the main circuit is closed, a capacitor is pre-charged from a separate charging unit. The interruption process starts with first opening the contacts of the switch in the main circuit followed by closing the making switch in the parallel circuit. This causes the capacitor discharge current to be superimposed on the current in the main circuit. If the discharge current amplitude exceeds that of the current in the main circuit, an artificial current zero is created, and the current is interrupted.
As the switchgear break time in a few ms, the opening mechanical switch must be very fast. It is possible to use solid-state switching devices in the main path.

Power Switching Components book section 6 by Kaveh NiayeshMagne Runde

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