When we switch off an online circuit breaker to cut an inductive load, it is ideally desired to interrupt the current of the system during its zero crossing of the current waveform. But practically it is somewhat impossible to maintain the condition. In normal circuit breaker the current interruption may occur at an instant near to the zero crossing point but not exactly at zero crossing point of the current waveform. As the load is inductive in nature, this sudden interruption of current, causes high di/dt which results in high transient voltage in the system.
In low or medium voltage power system, this transient voltage during circuit breaker operation may not much affect the system performance, but in extra and ultra-high voltage systems, this is quite effective. If the separation of contacts in circuit breaker is not sufficient at the current interruption instant, there may be re-ionization between the contacts due to transient over voltage, hence the arcing may be re-established.
When we switch on an inductive load like transformer or reactor, and if the circuit breaker closes the circuit near to the voltage zero crossing, there will be high DC component of current. This may saturate the core of transformer or reactor. This leads to a high inrush current in the transformer or reactor.
When we switch on a circuit breaker to connect a capacitive load to the system, such as capacitor bank, it is desirable to connect the current path at zero crossing of system voltage waveform.
Otherwise due to sudden change in voltage during switching, high inrush current is created in the system. This may be followed by over voltage in the system too.
The inrush current along with over voltage stress mechanically and electrically, the capacitor bank and other equipment in line.
Generally, in circuit breaker all three phases open or close nearly at same instant. But there is 6.6 ms time gap between the zero crossings of two adjacent phases of three phase system.
Sometimes it is also referred as controlled switching device or CSD.
This device takes voltage waveform from potential transformer of bus or load, current waveform from current transformers of the load, auxiliary contact signal and reference contact signal from circuit breaker, closing and opening command from control switch of the circuit breaker installed in control panel. Voltage and current signal from each phase are required to identify exact instant of zero crossing of the waveform of individual phase. Breaker contact signals are required to calculate the operational delay of the circuit breaker, so that the opening or closing pulse to the breaker can be send accordingly, to match the interruption and zero crossing of either current or voltage wave, as per requirement.
This device is dedicated for manual operation of the circuit breaker. During faulty tripping, the trip signal to the circuit breaker is directly sent from the protection relay assembly, bypassing the device. The Phase Synchronizing Device or PSD may also be associated with a bypass switch which can bypass the device from the system if required in any situation.