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, 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 gascontributes 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 - SF6and 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.On the other hand, SF6 circuit breaker has the major advantage of low maintenance.
High voltage switchgears are categorized as,
1. Gas insulated indoor type (GIS),
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 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.
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.
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 in an insulator column.
SF6 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.
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 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,
1. Terminal faults.
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.
Just after extinction of arc transient recovery voltage appears across the CB contacts, with high frequency. This transient recovery voltage ultimately approaches to open circuit voltage. This recovery voltage can be represented as
The frequency of oscillation is governed by the circuit parameter L and C. The resistance present in the power circuit damps out this transient voltage. The transient recovery voltage has not a single frequency, it is combination of many different frequencies due to complexity of the power network.
This is nothing but open circuit voltage appears across the CB contacts, just after the transient recovery voltage damped out. Inthree phase system the power frequency recovery voltage differs in different phase. It is highest in first phase. If the network neutral is not earthed, the voltage across the first pole to be cleared is 1.5U where U is the phase voltage. In an earthed neutral system it will be 1.3U. By using damping resistor, the magnitude and rate of rise of transient recovery voltage can be limited. The dielectric recovery of the arc quenching medium and rate of rise of transient recovery voltage has great influence on the performance of the circuit breaker used in high voltage switchgear system.
In air blast circuit breaker, once ionized air is de-ionized very slowly, thus air takes long time to recover dielectric strength. That is why it is preferable very slowly, thus air takes long time to recover dielectric strength. That is why it is preferable to used low value breaker resistor to slow down rate of rise of recovery voltage. On the other hand ABCB is less sensitive to the initial recovery voltage because of high arc voltage in SF6 circuit breaker, the interrupting medium (SF6) has faster rate of recovery of dielectric strength, than air. Lower arc voltage, makes SF6 CB more sensitive to the initial recovery voltage.
In oil circuit breaker, during arc having pressurized hydrogen gas (produced during recombination of oil due to arc temperature) provides quick recovery of dielectric strength immediate after current zero. Hence OCB is more sensitive to rate of rise of recovery voltage. It is also more sensitive to initial transient recovery voltage.
Short line fault in transmission network is defined as the short circuit faults occurred, within 5km of the line length. Double frequency being impressed on the circuit breaker and the difference of source and line side transient recovery voltage, both voltages start from instantaneous values at the opposition of the circuit breakers prior to the interruption. On the supply side the voltage will oscillate at supply frequency and ultimately approaches to open circuit voltage. On the line side, after interruption, trapped charges initial traveling waves through the transmission line, since there is no driving voltage on the driving side, the voltage ultimately becomes zero because of the line losses. The network diagram and the line side and supply side voltage wave forms are shown in the figure below.