Technical significance of High-Voltage switchgear in electrical unit

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 the 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 times, 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 breakers are 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 circuit breaker and two pressures SF6 circuit breaker. The single pressure system is the state of art for high voltage switchgear system, in present time. Nowadays 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 the 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 categorised as –

• Gas Insulated Indoor Type (GIS),
• Air Insulated Outdoor Type.

Again, outdoor type air insulated circuit breakers are classified as,

• Dead Tank Type Circuit Breaker
• Live Tank Type Circuit Breaker

In dead tank type CB, the switching device (assembly of interrupters) is located, with suitable insulator supports inside a metallic vessel (s) of ground potential, filled with insulating medium. In live tank circuit breaker, the switching device (assembly of interrupters) is located on insulated bushings, 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 Breaker

In this design, a blast of high pressure compressed air is used to quench arc between two detaching contacts, when the arc column ionisation 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 insulating oil requirement can be minimised by placing the interrupting units in an insulating chamber at the live potential on an insulator column.

SF6 Circuit Breaker

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 switchgear in high voltage system.

Vacuum Circuit Breaker

In a vacuum, there is no further ionisation 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 ionisation 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.

Essential Features of High Voltage Circuit Breaker

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 magnetising current
  • Energising long transmission line
  • Charging capacitor bank
  • Switching of out of phase sequence

Terminal Fault

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 –
• 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.

Transient Recovery Voltage

Just after the 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 a combination of many different frequencies due to the complexity of the power network.

Power Frequency Recovery Voltage

This is nothing but open circuit voltage appears across the CB contacts, just after the transient recovery voltage damped out. In three phase system the power frequency recovery voltage differs in different phase. It is highest in the 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 rising of transient recovery voltage can be limited. The dielectric recovery of the arc quenching medium and rate of rising of transient recovery voltage has great influence on the performance of the circuit breaker used in high voltage switchgear system. In an air blast circuit breaker, once ionized air is de-ionised very slowly, thus air takes a long time to recover dielectric strength. That is why it is preferable very slowly, thus air takes a long time to recover dielectric strength. That is why it is preferable to used low-value breaker resistor to slow down the rate of rising 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 pressurised 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

Short line fault in transmission network is defined as the short circuit faults occurred, within 5 km 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.