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 For example, kVar control can be used to maintain a power factor value and voltage control can
be incorporated as an override to remove the bank under abnormal voltage conditions.
3.7.9 Service Conditions. The life of a capacitor unit is shortened by overvoltage,
overheating, chemical change, physical damage, and repeated temperature changes.
3.7.9.1 Operating and Ambient Temperatures. Capacitor units are suitable for continuous
operation with an ambient temperature range of -40C to +46C. When ambient air
temperatures higher than 46C are expected, forced circulation of air may be used. Normally,
the maximum surface temperature of an individual capacitor unit is used to indicate the
operating-temperature condition. The temperature (at any point on the case), generally should
not exceed 55C (131F) under usual operating conditions or 70C (158F) under emergency
operating conditions. Capacitor units may be operated continuously at ambient temperatures as
low as -40C. When energizing capacitors after they have been deenergized for a period of time,
the temperature of the capacitor unit should be considered. There is a risk of damage to capacitor
units if they are energized at temperatures less than -20C.
3.7.9.2 Operating Voltage. Capacitors cause a voltage rise at their application point.
Thus, capacitors are more likely to operate at overvoltages than equipment that causes voltage
drop at its application point. The maximum permissible working voltage of power capacitors is
110 percent of rated voltage. Because the kVAR rating of a capacitor varies as does the square of
the ratio of the applied voltage to the rated voltage, applied voltage must be nominally limited to
rated voltage. Operation of capacitors above rated voltage increases capacitor kVAR, which
further increases the voltage level. Under emergency conditions, however, capacitors may be
operated above 110 percent of rated voltage. The recommended maximum rms overvoltage
without loss of capacitor life is dependent on the duration of each overvoltage. Table 3-2 lists
the recommended limits of overvoltage, expressed as a percent of rated voltage, versus the time
duration of the overvoltage condition. For example, a capacitor could be applied at 2.2 times
rated voltage for a period of one second without incurring any loss of usable life. The
recommended overvoltage limits are listed in Table 3-2.
Large capacitor banks are usually protected by fuses located (or installed) at each capacitor.
Fusing provides each capacitor with proper protection and also allows partial operation of the
bank when an individual capacitor fails. The fuse also serves to indicate unit failure. When a
large shunt capacitor bank is made up of series-connected groups of parallel units, the removal of
one or more units from a group will cause overvoltages on the remaining units. The failure rate
of capacitor units increases rapidly when subjected to overvoltage. Some type of protection is,
therefore, normally installed on large banks. This protection disconnects the bank from the
system or sounds an alarm when a significant number of units have been removed from service.
There are several schemes used to provide this protection. Most schemes require potential or
current transformers on the capacitor bank neutral.
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