S&C Instruction Sheet 1011-540 9
Control Operation
Ungrounded and Grounded Wye Shunt
Capacitor Banks
The BankGuard Plus Control provides protection of
ungrounded (Figure 3) and grounded (Figure 4) wye-
connected shunt capacitor banks. As individual capacitor
units in a series group are isolated from the bank by their
respective fuses, the control automatically protects the
surviving capacitor units in the group from cascading
voltage overstress. The control locks out the entire bank
when the neutral-to-ground voltage exceeds the calculated
level. See Instruction Sheet 1011-530, “BankGuard Plus®
Control:
Setup
,” for more details
In general, the
Unbalance Compensation
feature
should be enabled if the magnitude of the error voltage
caused by system voltage imbalances and/or inherent
capacitor bank imbalances approaches 50% of the value of
the neutral-to-ground voltage calculated for one isolated
capacitor unit.
For ungrounded wye banks, the control uses an ana-
log input and digital voltmeter to detect the capacitor
bank neutral-to-ground voltage, as monitored by an S&C
15-Volt-Ampere Potential Device. A digital filter attenuates
harmonics and noise. When the
Unbalance Compensa-
tion
feature is disabled, the magnitude of the real-time
neutral-to-ground voltage is compared with the
Gross
Over Voltage Level
,
Lockout Level
, and
Alarm Level
setpoint values, if enabled. Capacitor unit failures will
cause the real-time neutral-to-ground voltage magnitude
to change. If the real-time neutral-to-ground voltage mag-
nitude exceeds a setpoint value continuously for a time
greater than the applicable
Time Delay
setpoint value,
the appropriate control action is taken. The control will
lock the bank out if the
Gross Over Voltage Level
or the
Lockout Level
setpoint value is exceeded. The control
will issue an alarm if the
Alarm Level
setpoint value is
exceeded.
Figure 3. A system diagram of a BankGuard Plus Control for ungrounded wye-connected capacitor banks.
Figure 4. A system diagram of a BankGuard Plus Control for grounded wye-connected capacitor banks.
