S&C BankGuard PLUS, Setup Instructions

Page: 21 / 50

21 1011-530
For F = 1,
For F = 2,
Because the number of series groups per phase in this example is odd, you must com-
pute alternate values of
U
tp
using the adjustment factor for isolation of capacitor units
in a series group between the tap point and ground. In this example, the adjustment
factor is (7 + 1)/(7 - 1) = 1.33.
For F = 1,
For F = 2,
In this example, the desired tap-point voltage imbalance lockout level is the midpoint
between the
higher
value of
U
tp
for
F
= 1 and the
lower
value of
U
tp
for
F
= 2 (=
F
c
):
Thus, the desired lockout level for this capacitor bank is 2.24%.
Gross
Overvoltage
You can calculate the tap-point voltage percent imbalance (
U
tp
) resulting from a fault
within the capacitor bank that would short out an entire series group:
The desired gross overvoltage lockout level is the midpoint between the V
n
for the crit-
ical step and the V
n
resulting from shorting out a series group.
NOTE: If the calculated gross overvoltage lockout level is more than 65,536 volts, set
the setpoint value to its maximum level.
For Example 1 above, the
U
tp
for the critical step (
F
c
= 2) was 11.11%. The
U
tp
resulting
from shorting out a series group is:
Therefore, the desired gross overvoltage lockout level is:
For Example 2 above, the U
tp
for the critical step ( F
c
= 2) was 3.70%. The U
tp
resulting
U
tp
100 1
⋅
7 12 1
–
( )
1
+
-------------------------------- 1.28% = =
U
tp
100 2
⋅
7 12 2 –
( )
2 +
-------------------------------- 2.78% = =
U
tp
1.28% 1.33
×
1.70% = =
U
tp
2.78% 1.33
×
3.70%
= =
1.70% 2.78% +
2
-------------------------------------- 2.24% =
U
tp
100
S 1 –
------------ =
U
tp
100
2 1 –
------------
100%
= =
11.11% 100% +
2
--------------------------------------- 55.56% =