Switching in DC-networks, it is not self-evident. Turning of a switch in an AC network, will cause the current to get interrupted during the first zero crossing. A possible spark will occur in the switch, but only for a very short moment, thanks to the natural zero-crossing of an AC current. For a DC network this is totally different. There is no natural zero-crossing in a DC grid! Mechanical switching gives sparks, as can be seen in the red trace in scope 3 in the figure below.
By using power electronics, you can switch off the DC current, but also here a high over-voltage appears. A surge-arrester or MOV can be applied here.
Using simulations in Caspoc you can simulate what might happen during tun-off. Voltages and currents can be calculated, and also the switching losses during the switching off is calculated using the simulation. In this simulation study, a DC switchboard is simulated. A large current is turned off with the aid of the IGBT. Models of the DC bus bar, and the overvoltage protection MOV are also included in the model. The model of the IGBT is a thermodynamic model. The transition from conducting to blocking state is internally described by algebraic-differential equations.
During the simulation a DC current flows through the inductive load and IGBT. At the moment of turning-off, across the IGBT arises an over-voltage, which is absorbed by the MOV. The IGBT slowly turns-off and the current returns to zero.
Switching in DC grids causes over-voltages, as can be seen in the figure below. A large voltage spike occurs after switching of the DC current through the switch and inductive load.
There are various types of surge arrestors in the simulation. The ESD [Electro Static Discharge] models the actual spark. It will start conducting as soon as the voltage is higher than a threshold voltage and will conduct the current until the current drops below its holding current. As long as the ESd is conducting a predefined voltage is across the ESD device.
The Zener diode and Metal Oxide Varistor MOS model a clamping device, used to clamp a voltage and to prevent over-voltage during a turn-off.
In the simulation below a MOV is inserted to catch the over-voltage across the switch. The over-voltage across the switch is now limited, since the voltage clamped by the MOV.
