GE C70, Instruction Manual

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GE Multilin C70 Capacitor Bank Protection and Control System 9-3
9 APPLICATION OF SETTINGS 9.2 CONNECTIONS OF SHUNT CAPACITOR BANKS
9
9.2CONNECTIONS OF SHUNT CAPACITOR BANKS 9.2.1 GROUNDED WYE-CONNECTED BANKS
Grounded wye capacitor banks are composed of series and parallel-connected capacitor units per-phase and provide a low
impedance path to ground. This offers some protection from surge overvoltages and transient overcurrents.
When a capacitor bank becomes too large, making the parallel energy of a series group too high for the capacitor units or
fuses (greater than 4650 kvar), the bank may be split into two wye sections. The characteristics of the grounded double
wye are similar to a grounded single wye bank. The two neutrals should be directly connected with a single path to ground.
The double-wye design facilitates better protection methods. Even with inherent unbalances the two banks will respond
similarly to system events, and therefore, methods based on comparing one split-phase versus the other are more sensi-
tive and less prone to system events (phase current balance technique, for example).
9.2.2 UNGROUNDED WYE-CONNECTED BANKS
Ungrounded wye banks do not permit zero sequence currents, third harmonic currents, or large capacitor discharge cur-
rents during system ground faults (phase-to-phase faults may still occur and will result in large discharge currents). Another
advantage is that overvoltages appearing at the CT secondaries are not as high as in the case of grounded banks. How-
ever, the neutral should be insulated for full line voltage because it is momentarily at phase potential when the bank is
switched or when one capacitor unit fails in a bank configured with a single group of units.
9.2.3 DELTA-CONNECTED BANKS
Delta-connected banks are generally used only at distribution voltages and are configured with a single series group of
capacitors rated at line-to-line voltage. With only one series group of units, no overvoltage occurs across the remaining
capacitor units from the isolation of a faulted capacitor unit.
9.2.4 H-CONFIGURATION
Some larger banks use an H-configuration in each phase with a current transformer connected between the two legs to
compare the current down each leg. As long as all capacitors are balanced, no current will flow through the current trans-
former. If a capacitor fuse operates or an element shorts, some current will flow through the current transformer. This bridge
connection facilitates very sensitive protection. The H-arrangement is used on large banks with many capacitor units in par-
allel.
9.2.5 TAPPED CONFIGURATION
Some larger banks use a tapped configuration in each phase with voltage transformers connected from the phase to
ground and from the tap to ground. As long as no fuses have opened and no elements have shorted, the ratio of these two
voltages will remain constant. The ratio changes with any capacitor failure in the phase, providing sensitive protection.