Unfused Capacitors; Connections Of Shunt Capacitor Banks; Grounded Wye-Connected Banks; Ungrounded Wye-Connected Banks - GE C70 Instruction Manual

CHAPTER 8: APPLICATION OF SETTINGS

CONNECTIONS OF SHUNT CAPACITOR BANKS

The expected failure of the capacitor unit element is a short circuit, where the remaining capacitor elements absorb the
additional voltage. For example, if there are six capacitor units in series and each unit has eight element groups in series,
then there is a total of 48 element groups in the string. If one capacitor element fails, this element is shorted and the
voltage across the remaining elements is 48 / 47 of the previous value, or about 2% higher. The capacitor bank remains in
service; however, successive failures of elements aggravate the problem and eventually lead to the removal of the bank.
The fuseless design is usually applied for applications at or above 34.5 kV where each string has more than 10 elements in
series to ensure the remaining elements do not exceed 110% rating if an element in the string shorts.
Figure 8-3: Fuseless shunt capacitor bank and series string

8.1.5 Unfused capacitors

Contrary to the fuseless configuration, where the units are connected in series, the unfused shunt capacitor bank uses a
series or parallel connection of capacitor units. The unfused approach is normally used on banks below 34.5 kV, where
series strings of capacitor units are not practical, or on higher voltage banks with modest parallel energy. This design does
not require as many capacitor units in parallel as an externally fused bank.
8.2 Connections of shunt capacitor banks

8.2.1 Grounded wye-connected banks

Grounded wye capacitor banks are a 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 can be split into two wye sections. The characteristics of the grounded double wye
are similar to a grounded single wye bank. Connect the two neutrals directly with a single path to ground.
The double-wye design facilitates better protection methods. Even with inherent unbalances, the two banks respond
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similarly to system events, and therefore, methods based on comparing one split-phase versus the other are more
sensitive and less prone to system events (phase current balance technique, for example).

8.2.2 Ungrounded wye-connected banks

Ungrounded wye banks do not permit zero sequence currents, third harmonic currents, or large capacitor discharge
currents during system ground faults (phase-to-phase faults can still occur and results 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.
However, 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.
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL 8-3