S&C BankGuard PLUS, Руководство по установке

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S&C Instruction Sheet 1011-510
9
Ungrounded Wye Banks
any secondary of a high-impedance grounding transformer. The voltage rating
of the voltage transformer should match the secondary voltage rating of the
grounding transformer. Be sure that the grounding transformer is connected
to the station bus from which the capacitor bank is tapped. (The grounding
transformer maintains the stability of phase-to-ground voltage relationships for
all but fault conditions.)
C.
To detect and compensate for the inherent capacitor bank imbalance Plus the
system voltage imbalance, in applications where the source is grounded,
con-
nect to one of the following:
• Three S&C 30-Volt-Ampere Potential Devices rated for full system voltage,
each connected to a phase of the station bus from which the capacitor bank
is tapped.
• Three voltage transformers, connected to the station bus, groundedwye/
grounded-wye.
D.
To detect and compensate for the inherent capacitor bank imbalance Plus the
system voltage imbalance, in applications where the source is a delta-connected
tertiary transformer winding,
you must connect to three 1:1 voltage trans-
formers, each of which is then connected to the secondary of a high-impedance
grounding transformer. The voltage ratings of the voltage transformers should
match the secondary voltage rating of the grounding transformer. Be sure that the
grounding transformer is connected to the station bus from which the capacitor
bank is tapped. (The grounding transformer maintains the stability of phase-to-
ground voltage relationships for all but fault conditions.)
The circuits connecting the potential devices/voltage transformers to the
BankGuard Plus Control must be free of variable loads, variable voltage drops,
and ground loops. This insures that the monitored voltages accurately represent
the magnitude and phase angle of the bus voltages. Some possible errors affect-
ing unbalance compensation are:
• Differences in the effective voltage ratios among the three S&C 30-Volt-
Ampere Potential Devices/voltage transformers used to obtain system-
derived voltages.
• Unbalanced or variable loading of the potential devices/voltage transformers
used to obtain system-derived voltages. (Note that this means that station
service transformers are not suitable.)
• Control-wiring voltage drops between the potential devices/voltage trans-
formers and the control. To minimize voltage drops, connect the potential
devices/voltage transformers and the control with dedicated circuits that
are adequately sized.
Example
—A 1 amp current flowing through 1000 feet of #10 AWG wire
results in a 1 V drop in the voltage signal. This may be enough to affect the
performance of the protection scheme.
• Induced voltages in control wiring. Proper shielding is important.
• Ground loops caused by differences in voltage between the grounding
points for the capacitor bank neutral-to-ground potential device/voltage
transformer and those for the potential devices/voltage transformers used
to obtain system-derived voltages. Preferably, the secondaries of all the
potential devices/voltage transformers should be grounded at one point
(per proposed ANSI C57.13.3, “Guide for the Grounding of Instrument
Transformer Secondary Circuits and Cases”).