S&C BankGuard PLUS Setup Instructions page 13

Error Voltage
from System Voltage
Imbalance
Unbalance Compensation – Example 1
Assume a per-unit capacitor bank imbalance of 0.01 and a per-unit system voltage
imbalance of 0.005.
Error Voltage
from Capacitor Bank
Imbalance
Error Voltage
from System Voltage
Imbalance
If these error voltages are additive, the total error voltage could be as high as
100 volts – 28% of the neutral-to-ground voltage resulting from the isolation of one
capacitor unit (357 volts in this example). Therefore, you do not need to use unbalance
compensation (and the required additional potential devices) unless field experience
indicates otherwise.
Unbalance Compensation – Example 2
Assume a per-unit capacitor bank imbalance of 0.01 and a per-unit system voltage
imbalance of 0.005.
Error Voltage
from Capacitor Bank
Imbalance
Error Voltage
from System Voltage
Imbalance
If these error voltages are additive, the total error voltage could be as high as
697.2 volts. This does require unbalance compensation, since the neutral-to-ground
voltage resulting from the isolation of one capacitor unit in this example is 598 volts.
⎛ ⎞
Per-unit
⎜ ⎟
⎜ ⎟
System Voltage
⎜ ⎟
⎝ Imbalance ⎠
= ----------------------------------------------------------------------------------------------- -
3
( ) 20000 volts ( )
0.01
= ------------------------------------------------ -
3
( ) 20000 volts (
0.005
= --------------------------------------------------- -
3
( ) 139440 volts (
0.01
= --------------------------------------------------- -
3
( ) 139440 volts (
0.005
= ------------------------------------------------------ -
3
13
⎛ ⎞
System
⎜ ⎟
⎜ ⎟
Line-to-Neutral
⎜ ⎟
⎝ Voltage ⎠
= 66.7 volts
)
=
33.3 volts
)
= 464.8 volts
)
= 232.4 volts
1011-530