Transient Recovery Voltage mitigation in a Powe...

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Switching Transient Suppressor using LC Snubber Ckt Switching transients or TRV appears across Vacuum or SF6 contacts of a circuit breaker while interrupting an inductive load or an overcompensated transmission line with series capacitors. TRV is the voltage that appears across the contacts of a circuit breaker after arc extinction. Unlike Air or Oil circuit breakers the hard-insulation in a Vacuum or SF6 circuit breaker does not allow the arc to develop during its contact separation process. This abrupt chopping of inductive current raises an excessive voltage (TRV) that escalate a re-strike risk. The power system neutral shifts during a fault condition further results in a high amplitude transient recovery voltage. The Rate of Rise of Recovery Voltage (RRRV) is the peak transient Recovery Voltage divided by total time from Zero voltage to Peak voltage. The magnitudes of TRV and RRRV are crucial specifications of a circuit breaker in determining whether it can clear the fault successfully. While the transient recovery voltage and disturbances come close to Basic Insulation Level (BIL) of MV and HV equipment in industrial and commercial power systems; they do not pose fatal in their first occurrence. But the prolonged exposure to Power Frequency Over Voltage transients (PFOV) weaken insulation and reduce the lifespan of power system equipment. The repetitive transients in medium and high voltage power systems cause transformer HV insulation failure and CT, PT, HT Fuse and Cable blasting. Further, the power system transients percolate through different voltage levels and reach into the low voltage power distribution system. In the LV power system, it causes unwanted tripping and failure in sensitive equipment like AC/DC Drive, Solar Inverter, HMI, SCADA, UPS and Data Center server equipment. While TRV is a voltage sourced surge, a lightning strike is a current sourced or energy sourced surge. The energy released by a lightning stroke must reach the earth irrespective of path resistance. The current magnitude from a lightning bolt does not reduce along with its flow, but the transient voltage rises following the path resistance. Ferroresonance is another type of transient characterized by a sudden jump in voltage or current. It happens when the power system's inductance (XL) equals capacitance (XC) at a harmonic frequency. The resultant resonance appears typically from few second to tens of minute. Depending upon the extent of the power surge, it can cause minor to devastating damage to power system equipment. Power system study could uncover the signature of a Ferroresonance. The recorded graph depicts a harmonic resonance condition at 21st harmonic frequency on a 33kV utility power supply line.

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