S&C BankGuard PLUS Setup Instructions page 18

1011-530
×
1.11 10kV
V o = ------------------------------ -
9.96kV
For F = 1, enter at 10 on the horizontal scale. Follow this up to the curve for S = 2 (2
series groups per phase). Here, U
Similarly, for F = 2, enter the graph at 20 on the horizontal scale. Follow this up to the
curve for S = 2. Here, U = 11%.
tp
To determine the lockout level, calculate the midpoint value between U
U for F = 2 (= F ):
tp c
5.2% + 11%
----------------------------- - = 8.1%
2
Thus, the desired lockout level for this capacitor bank is 8.1%.
Graphical Method – Example 2
An installation has the following profile:
•
Highest anticipated continuous system line-to-neutral voltage (V
•
Nameplate rating of the capacitor units (V
•
Number of series groups per phase (S): 7
•
Number of capacitor units in parallel per series group (P): 12
For F = 1, enter at 8.33 on the horizontal scale (1/12 = 8.33% of capacitor units isolated
from a series group). Follow this up to a point corresponding to 7 series groups per
phase (interpolate between the curves for S = 6 and S = 8). Here on the vertical scale,
= 1.076 per unit.
V
o
Similarly, for F = 2, enter the graph at 16.67 on the horizontal scale (2/12 = 16.67% of
capacitor units isolated from the same series group). Follow this up to a point corre-
sponding to 7 series groups per phase. Here, V
for a V of 1.1 or less per unit, our F
o
With V = 139.44 kV and 7 series groups per phase, the capacitor units are normally
L-n
operated at 19.92 kV, their rated voltage. Therefore, no correction factor is needed.
For F = 1, enter at 8.33 on the horizontal scale. Follow this up to a point corresponding
to 7 series groups per phase (interpolate between the curves for S = 6 and S = 8). Here,
U = 1.3%.
tp
Similarly, for F = 2, enter the graph at 16.67 on the horizontal scale. Follow this up to a
point corresponding to 7 series groups per phase. Here, U
Because the number of series groups per phase in this example is odd, you must com-
pute alternate values of U
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,
×
=
U 1.3% 1.33
tp
For F = 2,
×
U tp = 2.8% 1.33
In this example, the desired tap-point voltage imbalance lockout level is the midpoint
= 1.114 per unit
= 5.2%.
tp
): 19.92 kV
r
= 1.17 per unit. Because we are looking
o
in this example is 2.
c
using the adjustment factor for isolation of capacitor units
tp
=
1.7%
= 3.7%
18
for F = 1 and
tp
): 139.44 kV
L-n
= 2.8%.
tp