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What is CSD (Controlled Switching Device) in Electrical System

Definitions
CSD (Controlled Switching Device):
A device that is installed at the relay and control panel to overcome the transient behaviour of voltage and current during switching. This device synchronizes the switching of each pole of the circuit breaker according to the zero crossing of the corresponding phase. This device is also known as a phase synchronizing device, or PSD for short.

Requirement
When we switch off an online circuit breaker to disconnect an inductive load, we ideally want to interrupt the current of the system at the zero crossing of the current waveform. However, this is practically impossible to achieve. In a normal circuit breaker, the current interruption may occur near the zero crossing point, but not exactly at it. Since the load is inductive, this sudden interruption of current causes a high rate of change of current (di/dt), which results in a high transient voltage in the system.
In low or medium voltage power systems, this transient voltage during circuit breaker operation may not affect the system performance much, but in extra and ultra-high voltage systems, this can have a significant impact. If the gap between the contacts in the circuit breaker is not enough at the current interruption instant, there may be re-ionization between the contacts due to the transient overvoltage, which may cause the arcing to resume.
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