ABB RELION REX640 Technical Manual page 1137

1MRS759142 F
Figure 636: Bank, unit and element of a capacitor bank
HCUBPTOC is designed for the protection against internal faults in H-bridge-
connected capacitor banks. This unbalance protection detects an asymmetry in
the capacitor bank, caused by blown fuses or short-circuits across bushings, or
between capacitor units and the racks in which they are mounted.
Normally, the capacitor units are designed to withstand 110% of the nominal
voltage continuously. When an element inside a capacitor bank fails, the remaining
healthy elements experience an increase in voltage across them. If the voltage
exceeds 110% of the nominal voltage, it can lead to a failure of the healthy
elements of the bank and in turn fail the entire capacitor bank. Since the capacitor
unbalance current is directly proportional to element failures, unbalance protection
is an effective way of detecting capacitor element failures. The current unbalance
protection function is usually used along with the three-phase capacitor bank
overload protection function to increase the sensitivity of protection for capacitor
banks.
Due to the two-stage (operation and alarm stage) unbalance protection and the
natural unbalance compensation facility, the protection of capacitor banks with
internal fuses can be implemented with a very high degree of sensitivity.
The unbalance protection function can be used for internally fused, externally fused
and fuseless shunt capacitor banks. Since a fuseless capacitor bank lacks the
individual capacitor unit fuses, current unbalance protection becomes even more
critical for fuseless applications.
When an individual element fails, it creates current unbalance. With an increasing
number of element failures, the unbalance current increases and HCUBPTOC gives
an alarm. The alarm level is normally set to 50% of the maximum permitted level.
The capacitor bank then should be taken out of service to replace the faulty units.
If not, the capacitor bank is tripped when the maximum allowed unbalance current
level is exceeded.
REX640
Technical Manual
Bank Unit
Protection functions
Element
1137