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SERIES AND SHUNT CAPACITOR BANK APPLICATIONS

SERIES CAPACITOR BANK APPLICATIONS

Installation of a series capacitor bank in a transmission line (standard or thyristor controlled) has the potential for increasing the magnitude of phase-to-ground and phase-to-phase switching surge over-voltages due to the trapped charges that can be present on the bank at the instant of line reclosing. In general, surge arresters limit the phase-to-ground and phase-to-phase over-voltages to acceptable levels; however, one problem that can be serious is the recovery voltage experienced by circuit breakers when clearing faults on a series compensated line. Depending the bank’s characteristics and on fault location with respect to the bank’s location, a charge can be trapped on the bank, and this trapped charge can add to the surges already being generated during the fault clearing operation. The first circuit breaker to clear is sometimes exposed to excessive recovery voltages under such conditions.

FIGURE 10.25 Voltage magnification circuit.

SHUNT CAPACITOR BANK APPLICATIONS

Energizing a shunt capacitor bank typically results in maximum over-voltages of about 2 pu or less. However, there are two conditions where significant over-voltages can be generated. One involves a configuration (shown on Fig. 10.25) where two banks are separated by a significant inductance (e.g., a transformer). When one bank is switched, if the system inductance and bank 1 capacitance has the same natural frequency as that of the transformer leakage inductance and the bank 2 capacitance, then a voltage magnification can take place.

Another configuration that can result in damaging over-voltages involves energizing a capacitor bank with a transformer terminated transmission line radially fed from the substation at which the capacitor bank is located. During bank switching, phase-to-phase surges are imposed on the transformer, and because these are not very well suppressed by the usual phase-to-ground application of surge arresters, transformer failures have been known to result. Various methods to reduce the surge magnitude have included the application of controlled circuit breaker closing techniques (closing near voltage zero), and resistors or reactors pre-inserted in the closing sequence of the switching devices.

Re-striking of the switching device during bank de-energizing can result in severe line-to-ground over-voltages of 3 pu to 5 pu or more (rarely). Surge arresters are used to limit the voltages to acceptable levels, but at higher system voltages, the energy discharged from the bank into the arrester can exceed the arrester’s capability.

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