ABB Relion 670 Series Applications Manual page 271

1MRK 511 401-UEN Rev. K
CompEnable : This is used to either enable or disable the inherent unbalance compensation. If
it is enabled, then the compensation can be triggered. If it is disabled, then no compensation
is performed.
The inherent unbalance error may be in the direction to prevent unbalance protection function
operation or to cause a false operation. So, it is always recommended to enable the
compensation.
If the inherent unbalance error approaches 50% of the warning level, then compensation can
be initiated to correctly operate for the failure of an unit or element as decided by the user.
The setting selection for the neutral voltage unbalance protection can be decided based on the
fusing method used in the capacitor bank configuration. In the externally fused capacitor
banks, several capacitor element breakdowns may occur before the fuse removes the entire
unit. So, selection of the settings for the capacitor element or unit failures are based on the
resulting over-voltage on the remaining healthy capacitor units.
In case of internally fused capacitors, individual capacitor elements within a capacitor unit are
disconnected when an element breakdown occurs. The risk of successive faults is minimized
because the fuse isolates the faulty element within a few cycles. So, selection of the settings
for the capacitor element or unit failures are based on the resulting voltage on the remaining
healthy capacitor elements or units.
In the fuseless or unfused capacitor banks, a failed element is short-circuited by the weld that
naturally occurs at the point of failure. So, selection of the settings for the capacitor element
or unit failures are based on the resulting voltage on the remaining healthy capacitor elements
or units.
The neutral voltage unbalance protection settings should be coordinated with the individual
capacitor unit fuses so that the fuses operate to isolate a defective capacitor unit before the
protection trips and brings the bank out of service. Also, the operation of the function should
have enough time delay to over-ride the external disturbances.
UNUnbalWrn> : Set the warning level in % of UBase . The warning is set above natural errors so
that it operates reliably on the loss of the first or the second fuse. Typically, the warning is set
at 50% or 75% of the unbalance voltage associated with the selected number of operated
fuses.
tDefWrn : Set the definite delay for warning signal in seconds. The typical value is 5 seconds.
UNUnbalAlm> : Set the alarm level in % of UBase .
To allow the effects of inherent unbalance within the bank, the unbalance protection function
alarm is set to operate at one-half the level of the unbalance signal determined by the
calculated alarm condition based on an ideal capacitor bank.
tDefAlm : Set the definite delay for alarm signal in seconds. The typical value is 5 seconds.
The unbalance protection function trip should have enough time delay to avoid false
operations due to inrush, system ground faults, switching of nearby equipment, and
asymmetrical pole operation of the energizing switch. For most applications, 0.1 second is
adequate.
UNUnbal> : Set the trip level in % of UBase . This setting can be used to set the trip level at
which the healthy unit terminal voltage is at the verge of exceeding 110% of its nominal
voltage.
The capacitor units are designed to operate continuously under different operating and
contingency conditions without exceeding the terminal voltage of 110% of the rated RMS
voltage, including the harmonic components and excluding the transients as per IEEE standard
C37.99-2000. If the capacitor unit limits are violated, then the capacitor bank should be
Bay control REC670
Application manual
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Section 10
Unbalance protection
265