ABB RELION REX640 Technical Manual page 1127

1MRS759142 F
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.
Figure 631: Example of double-Y-connected shunt capacitor bank unbalance
protection
REX640
Technical Manual
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.
Protection functions
1127