De-energizing motors can produce surges that may adversely affect motors under certain conditions, such as when the motor is de-energized before it has come up to speed (aborted starts) or when a vacuum breaker is used to do the switching. When air-magnetic switchgear is used to de-energize a motor that is running under normal operating conditions, no significant surges are produced [B54].
This is true because the back emf of the motor after it is disconnected does not immediately go out-of-phase with the source side voltage. Therefore, the recovery voltage across the breaker is not severe and restrikes do not usually occur.
However, when a breaker is tripped before the motor comes up to speed, the back emf is low and the recovery voltage can be severe. Thus, restrikes are likely to occur causing severe surges. These surges can have magnitudes above 2.7 pu with front times of 1 μs or less [B54].
When vacuum breakers are used to switch motors, several problems can occur such as current chopping, virtual current chopping, and multiple reignitions. Current chopping is the forcing of a premature current zero by the switching device.
When the load being switched is inductive, the energy stored in the magnetic field at the time of the forcing of a current zero is converted to energy stored in an electric field. If the capacitance of the system is low, this can result in quite high surge voltages.
The surge voltage magnitude is proportional to the amount of current chopped and to the surge impedance of the circuit on the load side of the vacuum breaker. The current chopping performance of vacuum breakers has been improved by the development of new contact materials, such as chromium copper, which chop and interrupt at lower currents.
Published data indicate that current chopping overvoltages to ground during switching of normal motor load current do not exceed 3 pu when the vacuum breaker has chromium-copper contacts.
Virtual current chopping occurs because of the ability of vacuum breakers to interrupt at high frequency current zeroes. When the first phase of a vacuum breaker opens, it is possible to get a reignition or restrike of the current in that phase.
This can produce a high-frequency current oscillation in the other two phases that have yet to interrupt. If the superposition of the high-frequency current onto the 60 Hz current results in the total current passing through zero in one of these other phases, then that phase may interrupt.
This is very much like chopping the fundamental frequency component of current. The amount of current that can be virtually chopped is larger than the amount of current that is chopped during
normal load current chopping.
The surges produced can be severe if a number of parallel load feeders are on-line on the same supply bus. However, if the surge impedance upstream of the breaker is high or equal compared to the load side, the virtually chopped surges are comparable to or somewhat larger than those produced by normal current chopping. This phenomenon generally has a low probability of occurrence.
Multiple reignitions in a vacuum breaker are a relatively infrequent occurrence that may result from certain combinations of inductance and capacitance in the circuit. A vacuum breaker can interrupt on the first high frequency oscillation current zero after the contacts have parted.
Therefore, the contacts may still be close together and the dielectric withstand of this small gap may be relatively low. If the system recovery voltage builds up fast, a reignition may occur.
When the arc is reestablished, it will not extinguish until the next current zero. This may happen quite quickly because the vacuum breaker is a good interrupter at high frequency current zeroes.
When the arc is extinguished, the contact gap may still be small. Another reignition of the opposite high-frequency polarity may occur. This can be repeated. The surge voltages due to reignitions can build up with each succeeding reignition.
The front times of such surges can be short (steep fronts) and can damage motor turn insulation. It is the steep fronts caused by the high frequency with reignition amplification that are likely to puncture turn insulation, rather than just high voltage.