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442 Preinsertion resistors
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Preinsertion resistors can reduce the capacitor-switching transient considerably The first peak of the transient is usually the most damaging The idea is to insert a resistor into the circuit briefly so that the first peak is damped significantly This is old technology but is still quite effective Figure 425 shows one example of a capacitor switch with preinsertion resistors to reduce transients The preinsertion is accomplished by the movable contacts sliding past the resistor contacts first before mat-
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Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
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Transient Overvoltages Transient Overvoltages 141
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ing with the main contacts This results in a preinsertion time of approximately one-fourth of a cycle at 60 hertz (Hz) The effectiveness of the resistors is dependent on capacitor size and available short-circuit current at the capacitor location Table 41 shows expected maximum transient overvoltages upon energization for various conditions, both with and without the preinsertion resistors These are the maximum values expected; average values are typically 13 to 14 pu without resistors and 11 to 12 pu with resistors Switches with preinsertion reactors have also been developed for this purpose The inductor is helpful in limiting the higher-frequency components of the transient In some designs, the reactors are intentionally built with high resistance so that they appear lossy to the energization transient This helps the transient damp out quickly
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Another popular strategy for reducing transients on capacitor switching is to use a synchronous closing breaker This is a relatively new technology for controlling capacitor-switching transients Synchronous
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Capacitor switch with preinsertion resistors (Courtesy of Cooper Power Systems)
Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
Transient Overvoltages 142 Four
Peak Transient Overvoltages Due to Capacitor Switching with and without Preinsertion Resistor Available short circuit (kA) 4 9 14 4 9 14 4 9 14 Without resistor (pu) 195 197 198 194 197 198 192 196 197 With 64resistor (pu) 155 145 139 150 140 134 142 133 128
TABLE 41
Size (kvar) 900 900 900 1200 1200 1200 1800 1800 1800
SOURCE:
Courtesy of Cooper Power Systems
closing prevents 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 step change in voltage that normally occurs when capacitors are switched, causing the circuit to oscillate Figure 426 shows one example of a circuit breaker designed for this purpose This breaker would normally be applied on the utility subtransmission or transmission system (72- and 145-kV classes) This is a three-phase SF6 breaker that uses a specially designed operating mechanism with three independently controllable drive rods It is capable of closing within 1 ms of voltage zero The electronic control samples variables such as ambient temperature, control voltage, stored energy, and the time since the last operation to compensate the algorithms for the timing forecast The actual performance of the breaker is sampled to adjust the pole timing for future operations to compensate for wear and changes in mechanical characteristics Figure 427 shows a vacuum switch made for this purpose It is applied on 46-kV-class capacitor banks It consists of three independent poles with separate controls The timing for synchronous closing is determined by anticipating an upcoming voltage zero Its success is dependent on the consistent operation of the vacuum switch The switch reduces capacitor inrush currents by an order of magnitude and voltage transients to about 11 pu A similar switch may also be used at distribution voltages Figure 428 shows one phase of a newer type of three-phase synchronous switch used for distribution capacitor banks This particular technology uses a vacuum switch encapsulated in a solid dielectric Each of the switches described here requires a sophisticated microprocessor-based control Understandably, a synchronous closing sys-
Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
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