High Voltage Switchgear
The power system deals with voltage above 36KV, is referred as high voltage switchgear. As the voltage level is high the arcing produced during switching operation is also very high. So, special care is to be taken during designing of high voltage switchgear. High voltage circuit breaker, is the main component of HV switchgear, hence high voltage circuit breaker (CB) should have special features for safe and reliable operation. Faulty tripping and switching operation of high voltage circuit are very rare. Most of the time these circuit breakers remain, at ON condition, and may be operated after a long period of time. So CBs must be reliable enough to ensure safe operation, as when required. High voltage circuit breaker technology has changed radically in the last 15 years. Minimum oil circuit breaker (MOCB), air blast circuit breaker and SF6 circuit breaker are mostly used for high voltage switchgear.
Vacuum circuit breaker is rarely used for this purpose as till date vacuum technology is not adequate for interrupting very high voltage short circuit current. There are two types of SF6 circuit breaker, single pressure SF6 breaker and two pressures circuit breaker. The single pressure system is the state of art for high voltage switchgear system, in present time. Now days SF6 gas as arc quenching medium, has become most popular for high and extra high voltage electrical power system. Although, SF6 gas contributes to the greenhouse effect. It has 23 times stronger impact on the greenhouse effect, than that of CO2. Hence, leakage of SF6 gas during the service life of circuit breaker must be prevented. In order to minimize the emission of SF6 gas, the N2 - SF6 and CF4 - SF6 gas mixture, may be used in circuit breaker in future, as substitute of pure SF6. It must always be taken care of that, no SF6 gas comes out in atmosphere during maintenance of the CB.
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On the other hand, SF6 circuit breaker has the major advantage of low maintenance. High voltage switchgears are categorized as,
- Gas insulated indoor type (GIS),
- Air insulated outdoor type.
- Dead tank type circuit breaker
- Live tank type circuit breaker
In dead tank type CB, the switching device is located, with suitable insulator supports inside a metallic vessel(s) at ground potential filled with insulating medium. In live tank circuit breaker, the interrupts are located in an insulated bushing, at the system potential. The live tank circuit breakers are cheaper and required less mounting space. There are mainly three types of circuit breaker, as we said earlier, used in high voltage switchgear system i.e. air blast circuit breaker, SF6 circuit breaker, oil circuit breaker and vacuum circuit breaker is rarely used.
Air Blast Circuit BreakerIn this design, a blast of high pressure compressed air is used to quench arc between two detaching contacts, when the arc column ionization is least at currents zero.
Oil Circuit BreakerThis is further classified as bulk oil circuit breaker (BOCB) and minimum oil circuit breaker (MOCB). In BOCB, the interrupting unit is placed inside an oil tank of earth potential. Here oil is used as both insulating and interrupting medium. In MOCB on the other hand, the oil requirement can be minimized by placing the interrupting units in an insulating chamber at live potential in an insulator column.
SF6 Circuit BreakerSF6 gas is widely used as arc quenching medium in HV applications today. Sulfur hexafluoride gas is a high electronegative gas having excellent dielectric and arc quenching properties. High dielectric and insulating properties of SF6, make it possible to design high voltage circuit breaker with smaller overall dimension, shorter contact gap. Excellent insulating property helps to design and construct indoor type high voltage switchgear.
Vacuum Circuit BreakerIn vacuum, there is no further ionization between two separated current carrying contacts, after current zero. The initial arc is caused by it will die as soon as next zero crossing but as there is no provision of further ionization once the current is crossed its first zero, the arc quenching is completed. Although the arc quenching method is very fast in VCB, but till it is not a suitable solution for high voltage switchgear, as VCB made for very high voltage level is not economical at all. The essential features to be provided in high voltage circuit breaker, to ensure safe and reliable operation the breakers used in high voltage switchgear, must be capable of being operated safely for,
- Terminal faults.
- Short line faults.
- Transformer or reactors magnetizing current.
- Energizing long transmission line.
- Charging capacitor bank.
- Switching of out of phase sequence.
Terminal FaultGenerally the load connected to the power system is inductive in nature. Due to this inductance, when short circuit current is just interrupted by a circuit breaker, there is a chance of high restriking voltage of high frequency oscillation in order of few hundred Hz. This voltage has two parts
- Transient recovery voltage with high frequency oscillation immediately after the arc extinction.
- After die down this high frequency oscillation, power frequency recover voltage appears across the CB contacts.