5
1011-530
Ungrounded Wye Banks
Applicable
Software
This instruction sheet was prepared for use with software
UPPD106S
. You can find the
release date on the Setup disk label. For questions regarding the applicability of infor-
mation in this instruction sheet to future software releases, please contact S&C.
Ungrounded Wye
Banks
You must establish the settings for each control device installation, using either graphs
or formulas. Both methods are described in this section
for ungrounded wye-con-
nected shunt capacitor banks only
. It also describes how to calculate the gross over-
voltage level and when to use unbalance compensation.
Before you begin, you will need on the following information:
•
The number of series groups per phase
•
The number of capacitor units in parallel per series group
•
The highest anticipated continuous system line-to-neutral voltage (in kV)
•
The nameplate rating of the capacitor units (in kV)
This section uses the following variable names:
S
=Number of series groups per phase
P
=Number of capacitor units in parallel per series group
V
L-n
= Highest anticipated continuous system line-to-neutral voltage
V
r
= Nameplate rating of the capacitor units
V
o
= Voltage applied to surviving capacitor units, per unit of capacitor-unit normal volt-
age
V
n
= Capacitor bank neutral-to-ground voltage, per unit of line-to-neutral voltage
F
= Number of isolated capacitor units
F
c
=The “critical step” – the number of isolated capacitor units at which
V
o
exceeds the
capacitor manufacturer’s recommended maximum working voltage (which is generally
1.1 per unit)
Graphical Method
Figure 1 shows the per-unit voltage applied to surviving capacitor units in a series
group (
V
o
), versus the percentage of capacitor units isolated from the same series
group (
F/P
). shows the per-unit capacitor bank neutral-to-ground voltage (
V
n
), versus
the percentage of capacitor units isolated from the same series group (
F/P
).
A.
Using Figure 1, find the Vo for a series of steps corresponding to increasing values of F, up
to and including Fc.
NOTE: If the capacitor units are operating at a voltage other than the rated voltage,
multiply these F/P values by VL-n/Vr.
B.
Using , find the Vn for the same series of steps corresponding to increasing values of F, up
to and including Fc.
C.
Convert Vn to an actual voltage by multiplying the values found in Step B by VL-n.
D.
Determine the desired lockout level: the midpoint between the Vn (actual voltage) for the
critical step Fc and the Vn (actual voltage) for Fc-1.
Summary of Contents for BankGuard PLUS
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