Consulting-Specifying Engineer - March 2009 - (Page 42) Chopped current circuit breaker interrupting process 12 Contact separation 10 8 6 4 2 0 -2 -4 -6 -8 - 10 Figure 1 represents a graph of current interruption. Once interrupted, the resulting TRV has significant high-frequency content. Source: Bala Consulting Engineers Transient recovery voltage Current Time Supply voltage Arc voltage Key: Red: Current Blue: Supply voltage Green: Contact voltage Commissioning projects During a recent maintenance and commissioning project at an international brokerage firm, technicians experienced several incidences of switching-induced transients. A worker was operating the primary circuit breakers when he heard a loud pop and saw a bright flash from the transformer enclosure. All work was stopped and the transformer was replaced, even though factory testing revealed no defects. The breakers had been operated at least 30 times prior to this occurrence. These events occurred during switching of the primary circuits of medium-voltage, dry transformers from 2,500 to 4,000 kVA, with vacuum circuit breakers. This portion of the switchgear is part of a main-tie-main, PLCcontrolled, 4 MVA, 13.2 kV-distribution system. The images below show the 4,000 kVA, vacuum pressure impregnation transformer that was damaged as a result of this event. The vacuum circuit breaker is connected to the transformer with 50 ft of 15 kV cable (Figure 3) and an unloaded 2-MW UPS plant was on the secondary side of the transformer. An investigation of the event revealed that the flash resulted from switching transients. This realization led the engineering team to research possible mitigation solutions. Mitigation methods There are several ways to reduce the effects of switching transients, including transformer-winding design and series inductors. However, the installation of an RC-snubber network will work well in situations similar to those mentioned above. The RC-snubber network lowers the frequency of the transient voltage applied to the transformer primary below the resonance frequency of the circuit. It reduces the development of the oscillatory voltages and provides a low impedance path to ground for the transients. Figure 2 represents an RC-snubber network that was specifically designed for and The 4,000 kVA, vacuum pressure impregnation transformer was damaged as a result of the switching transients. Source: Bala Consulting Engineers 42 Consulting-Specifying Engineer • MARCH 2009
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