Capacitor Switching Research Paper

This section portrays how utilities can deal with troubles related to capacitor-switching transients.
4.7.1. Switching Times
Capacitor-switching transients are very familiar and usually not destructive. However, the timing of switching may be adverse for some sensitive industrial loads. For example, if the load picks up the same time each day, the utility may choose to switch the capacitors coincident with that load increase. There have been several cases where this coincides with the commencement of a work shift and the resultant transient causes several adjustable-speed drives to shut down shortly after the process starts. One simple and economical solution is to establish if there is a switching time that might be more acceptable. For example,

Synchronous Closing Synchronous closing stops transients by timing the contact closure such that the system voltage closely matches the capacitor voltage at the instant the contacts mate. This avoids the There is no step change in voltage that normally occurs during capacitors are energized, causing the circuit to oscillate. A successful synchronous closing needs the switch to maintain a sufficient dielectric strength to withstand the system voltage until its contacts touch. For a grounded-wye capacitor, it is energized at voltage zero. For an ungrounded-wye capacitor, the first phase is energized at voltage zero, the second phase is energized when its voltage is identical to the voltage of the first phase, and finally, the third phase is energized at voltage zero. The peak overvoltage transient is below 1.2 pu. Figure 4.30 shows simulated voltage waveforms due to capacitor energizing. The first phase (top) is energized with switches without closing control, while the second and third phases are with controlled closing. The timing error for the second phase is negligible. The timing error for the third phase is 1 ms resulting in a clear oscillatory transient voltage with a peak of 1.12

This breaker would normally be applied on the utility subtransmission or transmission system (72- and 145-kV classes). This is a three-phase SF6breaker that procedures a specially designed operating mechanism with three independently controllable drive rods. It is talented of closing within 1 ms of voltage zero. The real performance of the breaker is sampled to adjust the pole timing for future operations to compensate for wear and changes in mechanical characteristics. Figure 4.32. Synchronous closing breaker. (Courtesy of ABB, Inc.) Figure 4.33 shows a vacuum switch made for this purpose. It is applied on 46-kV-class capacitor banks. It contains three independent poles with separate controls. The timing for synchronous closing is determined by expecting an upcoming voltage zero. Its achievement is dependent on the reliable operation of the vacuum switch. The switch reduces capacitor inrush currents by an order of magnitude and voltage transients to about 1.1 pu. A similar switch may also be used at distribution voltages. Figure 4.33. Synchronous closing capacitor switch. (Courtesy ofJoslyn Hi-Voltage