What is a Phase Synchronizing Device?

What is a Phase Synchronizing Device?

Phase Synchronizing Device Definition

A Phase Synchronizing Device (PSD) is defined as a device that synchronizes the switching of circuit breaker poles to the zero crossing of the phase voltage or current waveform.

Controlled Switching Device

Also known as a Controlled Switching Device (CSD), it ensures accurate timing during circuit breaker operations.

Voltage and Current Synchronization

The PSD uses voltage and current waveforms to identify zero crossings and synchronize breaker operations accordingly.

When switching off a circuit breaker to cut an inductive load, it’s best to interrupt the current at the zero crossing of the current waveform. However, this is difficult to achieve exactly. In normal circuit breakers, current interruption occurs near, but not exactly at, the zero crossing point. Because the load is inductive, this sudden interruption causes a high rate of current change (di/dt), resulting in high transient voltage in the system.

In low or medium voltage power systems, transient voltage during circuit breaker operation may not significantly affect performance. However, in extra and ultra-high voltage systems, it is more impactful. If the circuit breaker contacts are not sufficiently separated at the moment of interruption, re-ionization can occur due to transient overvoltage, leading to re-established arcing.

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 connecting a capacitive load, like a capacitor bank, it’s best to switch on the circuit breaker at the zero crossing of the 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.

In circuit breaker, all three phases usually open or close almost simultaneously. However, there is a 6.6 ms time gap between the zero crossings of adjacent phases in a three-phase system.

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.

Inductive Load Management

Switching on inductive loads at the right moment prevents high inrush currents that can damage equipment.

Capacitive Load Switching

Proper timing when switching capacitive loads reduces the risk of high inrush currents and overvoltage.