High Voltage Switchgear
The power system deals with voltage above 36KV, is referred as high voltage. As the voltage level is high the arcing produced during switching operation is also very high. So, special care 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 should have special features for safe and reliable operation. Faulty tripping and switching operation of high voltage circuit are very rear. 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, air blast circuit breaker and SF 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 SF circuit breaker, single pressure SF 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 SF gas as arc quenching medium, has become most popular for high and extra high voltage electrical power system. Although, SF gas has strong impact on the greenhouse effect. It has 23 times stronger impact on the greenhouse effect, than that of CO. Hence, leakage of SF gas during the service life of circuit breaker must be prevented. In order to minimize the emission of SF gas, the N – SF and CF – SF gas mixture, may be used in circuit breaker in future, as substitute of pure SF. It must always be taken care of that, no SF gas comes out in atmosphere during maintenance of the CB.On the other hand, SF circuit breaker has the major advantage of low maintenance.
High voltage switchgears are categorized as,
1.Gas insulated indoor type,
2.Air insulated outdoor type.
Again, outdoor type air insulated circuit breakers are classified as,
1.Dead tank type circuit breaker
2.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 busing, 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, SF circuit breaker, oil circuit breaker. And vacuum circuit breaker is rarely used.
Air Blast Circuit Breaker
In 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 Breaker
This 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 on an insulator column.
SF Circuit Breaker
SF 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 SF, 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 Breaker
In 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 provide 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,
2. Short Line Faults
3.Transformer or reactors magnetizing current.
4.Energizing long transmission line.
5.Charging capacitor bank.
6.Switching of out of phase sequence.
Generally 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
1.Transient recovery voltage with high frequency oscillation immediately after the arc extinction.
2.After die down this high frequency oscillation, power frequency recover voltage appears across the CB contacts.