S&C BankGuard PLUS, Setup Instructions

Page: 23 / 50

23 1011-530
You must establish the settings for each control device installation. The method
described in this section is
for ungrounded wye-connected shunt reactors only . It also
describes how to calculate the gross overvoltage level and when to use unbalance com-
pensation.
Before you begin, you will need on the following information:
•
The highest anticipated continuous system line-to-neutral voltage (in volts)
•
The estimated inherent system voltage imbalance, expressed as the maximum per-
unit system line-to-neutral voltage variation of any one phase from the average of
the line-to-neutral voltages of the three phases (typically less than 0.01 per unit)
•
The anticipated inherent reactor imbalance, expressed as the maximum per-unit
reactance variation (due to manufacturing tolerance) of any one phase from the
average of the reactances of the three phases
This section uses the following variable names:
V
L-n
= Highest anticipated continuous system line-to-neutral voltage
Δ
V
L-n
= Estimated inherent system voltage imbalance
Δ
X = Anticipated inherent reactor imbalance
V
n
= Reactor neutral-to-ground voltage
To calculate the lockout level for the reactor:
A. Calculate the V
n
due to inherent source voltage imbalance and inherent reactor imbalance:
B. To determine the desired lockout level, multiply V
n
by 2.
Formula Method
An installation has the following profile:
•
Highest anticipated continuous system line-to-neutral voltage ( V
L-n
): 7200 volts
•
Estimated inherent system voltage imbalance (
Δ
V
L-n
): 0.01
•
Anticipated inherent system voltage imbalance (
Δ
X ): 0.015
Calculate V
n
:
Thus, the lockout level is 2 (60 volts) = 120 volts.
Gross
Overvoltage
You can calculate the reactor neutral-to-ground voltage (
V
n
) resulting from a fault
which would open an entire phase winding:
NOTE: If the calculated gross overvoltage lockout level is more than 5000 volts, set the
detector to its maximum level.
V
n
(volts)
Δ
X
Δ
V
+
L n
–
3
----------------------------------
⎝ ⎠
⎛ ⎞
V
L n –
=
V
n
(volts)
0.015 0.01
+
3
------------------------------
⎝ ⎠
⎛ ⎞
7200 volts 60 volts = =
V
n
1
2
---
V
L n –
=
Ungrounded Wye Shunt Reactors