CHAPTER 8: APPLICATION OF SETTINGS CONNECTIONS OF SHUNT CAPACITOR BANKSC70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL 8-38The expected failure of the capacitor unit element is a short circuit, where the remaining capacitor elements absorb theadditional 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 thevoltage across the remaining elements is 48 / 47 of the previous value, or about 2% higher. The capacitor bank remains inservice; 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 inseries 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 string8.1.5 Unfused capacitorsContrary to the fuseless configuration, where the units are connected in series, the unfused shunt capacitor bank uses aseries or parallel connection of capacitor units. The unfused approach is normally used on banks below 34.5 kV, whereseries strings of capacitor units are not practical, or on higher voltage banks with modest parallel energy. This design doesnot require as many capacitor units in parallel as an externally fused bank.8.2 Connections of shunt capacitor banks8.2.1 Grounded wye-connected banksGrounded wye capacitor banks are a series and parallel-connected capacitor units per-phase and provide a lowimpedance 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 orfuses (greater than 4650 kvar), the bank can be split into two wye sections. The characteristics of the grounded double wyeare 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 respondsimilarly to system events, and therefore, methods based on comparing one split-phase versus the other are moresensitive and less prone to system events (phase current balance technique, for example).8.2.2 Ungrounded wye-connected banksUngrounded wye banks do not permit zero sequence currents, third harmonic currents, or large capacitor dischargecurrents 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 isswitched or when one capacitor unit fails in a bank configured with a single group of units.834755A1.CDR
