De-energizing motors can produce surges that may adversely affect
motors under certain conditions, much 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.
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.
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
highfrequency 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 re ignitions can build up with each
succeeding re ignition.
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.
Because some vacuum circuit breakers and interrupters may create high
voltages by chopping, prestrikes, or restrikes, some manufacturers have
equipped vacuum switchgear with surge limiting arresters on the switchgear bus
or on the load side of each outgoing interrupter. For protection of switched
machines, the arresters should be on the machine circuit rather than on the
bus.
1 comment:
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