by a binary input, a horizontal communication input or an internal signal of the IED
program. The influence of the
BLOCK
signal activation is preselected with the global
setting
Blocking mode.
The
Blocking mode setting has three blocking methods. In the "Freeze timers"
mode, the operation timer is frozen to the prevailing value. In the "Block all"
mode, the whole function is blocked and the timers are reset. In the "Block
OPERATE output" mode, CUBPTOC operates normally but the
OPERATE
output is
not activated.
4.12.2.5
Application
CUBPTOC is designed for the protection against internal faults in double-Y-
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 percent of the nominal
voltage continuously. When an element inside a capacitor bank fails, the remaining
healthier elements experience an increase in voltage across them. If the voltage
exceeds the 110 percent value of the nominal voltage, it can lead to a failure
of the healthier elements of the bank and in turn fail the entire capacitor bank.
Since the capacitor unbalance current is directly proportional to the element
failures, unbalance protection is an effective way of detecting capacitor element
failures. The current unbalance protection function is usually used 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.
Furthermore, CUBPTOC provides a sophisticated method of detecting the number
of faulty elements in each phase by calculating the differential unbalance current.
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 causes unbalance current. With an increasing
number of element failures, the unbalance current increases and CUBPTOC gives
an alarm. The alarm level is normally set to 50 percent of the maximum permitted
level. The capacitor bank needs to be taken out of service to replace the faulty
units. If this is not done, the capacitor bank is tripped when the maximum allowed
unbalance current level is exceeded.
If two simultaneous faults occur in the same phase but in different
branches, there is no change in the unbalance current and CUBPTOC
does not detect this type of faults.
If two simultaneous faults occur in the same branch but in different
phases, it may cause a phase angle equal to a situation where there is
only one fault in the branch. Therefore, the element failure counters show
only one fault instead of two.
1MRS757644 H
Protection functions
620 series
Technical Manual
789
