S&C BankGuard PLUS, Installation Manual

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S&C Instruction Sheet 1011-510
Ungrounded Wye Shunt Reactors
• Three S&C 30-Volt-Ampere Potential Devices rated for full system volt-
age, each connected to a phase of the station bus from which the reactor
is tapped. Connect to TB1-9, TB1-10, TB1-11, and TB2-12 (Phase voltage
return).
• Three voltage transformers, connected to the station bus, grounded-wye/
grounded-wye. Connect to TB1-9, TB1-10, TB1-11 and TB2-12 (Phase voltage
return).
D. To detect and compensate for the inherent reactor imbalance Plus the system
voltage imbalance, in applications where the source is a delta-connected ter-
tiary transformer winding,
you must connect to three 1:1 voltage transformers,
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 reactor 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 affecting 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 transform-
ers 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 reactor neutral-to-ground potential device/voltage transformer and those for
the potential devices/voltage transformers used to obtain systemderived volt-
ages. 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”).