Interrupting DC current

Several concepts for DC breakers have been presented and a few also has been implemented and tested. Two basic mechanisms can be distinguished:

  • current is extinguished by semiconductors which either are conducting the current continuously or which temporarily take over the main current from a mechanical disconnector
  • current is extinguished by a mechanical breaker assisted by auxiliary circuitry that creates an artificial current zero-crossing in the current through the mechanical breaker

Independent of the method the DC circuit breaker must include an energy absorbing branch, which takes care of the inductive energy in the network at the instant of current extinction. This device normally is a Metal Oxide Varistor (MOV), which also defines the amplitude of the counter-voltage (“protective voltage”) that opposes the source voltage in the network.

The first method requires that the extinguishing semiconductor arrangement can set up a counter voltage that exceeds the voltage source in the network, say by a factor 1.5. In HVDC (High Voltage Direct Current) applications this means hundreds of kV requiring a large number of series-connected semiconductors. The associated losses become significant and for this reason typically an arrangement where a mechanical contact normally conducts the current is used (“hybrid breaker”). In any case the high voltage rating for the semiconductor stack remains.

In the second approach, current extinction is provided by the mechanical breaker itself. The auxiliary circuit serves the purpose of causing a zero-crossing of the current passing through the mechanical breaker, when its contacts are being separated and a sufficient gap has been established. Vacuum interrupters (VI) normally are used due to their very fast (sub-µs) build-up of voltage withstand capability and the short mechanical stroke required (few mm). The auxiliary circuit most often is connected in parallel with the mechanical contacts and it creates a current pulse with amplitude exceeding the main current in the opposite direction relative the main current. The current pulse is achieved by applying a voltage step across a resonant circuit. The impedance in the resonant circuit should be approximately given by the ratio between the protective voltage and the maximum current to be interrupted to survive a reignition during the breaking operation. In the simplest case the capacitor is charged to the protective voltage and a switch initiates the pulse. This switch must be designed to withstand the full protective voltage.