ABB RELION REX640 Technical Manual page 1046

Section 4
Protection functions
1040
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.
GUID-D92473D0-A8A8-4193-84DC-11F1B3F0A4E6 V1 EN
Figure 561: Application example of capacitor bank unbalance protection
The phase current analog input I_A and at least one unbalance current
must be connected to the protection relay for the HCUBPTOC
function to start working.
1MRS759142 C
REX640
Technical Manual