1011-530
24
In the example above, the
V
n
was 120 volts. The
V
n
resulting from a fault which would
open an entire phase winding is:
Therefore, the desired gross overvoltage lockout level is:
Should the calculated gross over voltage lockout level exceed 9,999 volts, the setpoint
should be set at its maximum value.
Set the “Gross Overvoltage Time Delay” to a minimum of 0.5 second, plus the elapsed
time between the energizing of the opening circuit of the reactor switching device and
the opening of the switching device “a” contact.
For example, if the reactor switching device is a 230kV S&C Circuit-Switcher, the mini-
mum “Gross Overvoltage Time Delay” is 0.5 second plus 0.6 second, or 1.1 seconds
total. You can estimate the elapsed time between the energizing of the opening circuit
and the mechanical parting of the disconnect blades as 40% of the maximum operating
time for the particular S&C Circuit-Switcher in use.
Unbalance
Compensation
A certain amount of error voltage is always present between the reactor neutral and
ground. This voltage comes from system voltage imbalances and/or inherent reactor
imbalances (from manufacturing tolerance variations among the phase windings of the
reactor). Because it is impossible to predict how these error voltages will combine vec-
torially, it is important that the magnitude of the error voltage remain low compared to
the magnitude of the reactor neutral-to-ground voltage level for which automatic
switching is desired.
For example, the error voltage may be additive with respect to the reactor neutral-to-
ground voltage resulting from a developing turn-to-turn fault in one phase winding but
subtractive with respect to the reactor neutral-to-ground voltage resulting from a devel-
oping turn-to-turn fault in another phase winding.
In general, you should use the unbalance compensation feature if the magnitude of the
error voltage exceeds 50% of the desired lockout level.
In the installation example above, the lockout level is 120 volts. If the reactor neutral-
to-ground voltage exceeds 60 volts under normal operating conditions, you must either
use the unbalance compensation feature or reduce/eliminate the source of the voltage
imbalance.
Field determination of the need for Unbalance Compensation.
Close the reactor switching device to energize the reactor.
The LCD should display essentially a zero voltage, or at most 50% of the lockout-level
setting. If the displayed voltage is greater than 50% of the lockout-level setting, it will be
necessary to either reduce or eliminate the source of the voltage unbalance, or to uti-
lize the optional Unbalance Compensation feature.
V
n
1
2
---
7200
(
)
volts
3600 volts
=
=
3600 volts
120 volts
+
2
------------------------------------------------------
1860 volts
=
Summary of Contents for BankGuard PLUS
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Page 6: ...1011 530 6 Figure 1 Vo versus F P Ungrounded Wye Capacitor Bank...
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