SF6 gas circuit breakers: a review of live tank and dead tank technology

SF6 gas circuit breakers: a review of live tank and dead tank technologyIn today’s power sector, long and bulk power transmissions are important as the generating stations and load centres are located very farElectrical energy is an essential requirement for the economic development of any country. Particularly for developing countries, the energy sector assumes a critical importance in view of the ever-increasing energy needs. This is the reason electrical power system network is expanding day by day to serve the increasing energy demand of the nation.The modern power system deals with huge power network and huge numbers of associated electrical equipment. During short-circuit fault or any other types of electrical fault, these equipment and power network suffer a high stress of fault current which may damage the equipment and networks permanently. For saving these equipment and the power networks, the fault current should be cleared from the system as quickly as possible. Again after the fault is cleared, the system must come to its normal working condition as soon as possible for supplying reliable quality power to receiving ends. In addition to that for proper controlling of power system, different switching operations are required to be performed. So for timely disconnecting and reconnecting different parts of power system network for protection and control, there must be some special type of switching devices which can be operated safely under huge current carrying condition. During interruption of huge current, there would be large arcing in between switching contacts, so care should be taken to quench the arc in safe manner. Circuit breakers are deployed to perform this operation. The circuit breaker has got the service life of around 30 years. During this tenure, it needs to work very reliably whenever an abnormal condition occurs in the power system.Circuit breakers are classified in following ways:Circuit breakers types by voltage class. Low voltage . Medium voltage . High voltage . Extra high voltage. Ultra high voltageCircuit breaker types by installation. Indoor Circuit breaker. Outdoor circuit breaker.Circuit breaker types by external design. Dead tank circuit breaker. Live tank circuit breaker.Circuit breaker types by interrupting medium. Air circuit breaker. Oil circuit breaker. Vacuum circuit breaker. SF6 gas circuit breaker.In today’s power sector, there is a need of long and bulk power transmission as the generating stations and load centres are located very far. There is a need to improve the transmission efficiency by increasing the voltage, and thereby proportionately reduce the current in the line conductors .This allows power to be transmitted within acceptable loss limit. The reduced current flowing through the line reduces the heating losses in the conductors. SF6 gas circuit breakers are used at EHV and UHV level. The external designs available in EHV and UHV circuit breaker are mainly of live tank type and dead tank type as mentioned. The live tank circuit breaker is a switching device in which the vessel housing the interrupter is at a potential above the ground level.The dead tank circuit breaker is in which a vessel at ground potential surrounds and contains the interrupter and the insulating medium. The live tank and dead tank circuit breakers have their own advantages as well as disadvantages. The proper selection of circuit breaker requires a detailed review of all the aspects discussed in subsequent paragraphs.Nowadays, global warming is becoming the concern for survival of our planet. Global warming is the increase of earth’s average surface temperature due to effect of greenhouse gases. SF6 gas is one of the greenhouse gases and has got the highest global warming potential (32,000 times more than CO2). SF6 gas in the circuit breakers and its possible contribution to the greenhouse effect should be noted and its release to the atmosphere should be minimised.It is advantageous to decrease gas volumes and leakage rates as much as possible. At UHV level, the live tank circuit breaker requires 15 per cent of the SF6 gas as compared to the dead tank breaker of the same voltage class. That means for same leak rate, the DTB has 600 per cent more impact on the environment. The dead tank breakers are manufactured as large castings which are bound to have porosity in it. This porosity is a culprit for the more leakage rates than that of the live tank breakers.  Therefore, from an environmental point of view, which is tomorrow’s survival need, a live tank circuit breaker with its relatively small volume of SF6 gas is a much preferred solution compared to dead tank circuit breakers worldwide.Population growth followed by an increasing demand for power and the crunch of space are today’s issues for India. To tackle the space constraint issue, if one gives a thought for selecting a circuit breaker for air-insulated substation, the dead tank breaker could be the choice. It has got an advantage of grounded tank with bushings where current transformers can be mounted easily which is not the case for live tank breakers. If the deeper thought is given on this selection, and the overall substation area is considered, space saving would not be much attractive with dead tank circuit breakers. It will be around 5-6 per cent as compared to live tank breaker substation. When these two technologies are reviewed from the failure mode aspect, one will realise following consequences with dead tank circuit breakers like line to ground failures, particle sensitivity issues, failure due to grading capacitor oil leakage, etc.Line to ground failureAs the tank is grounded, the insulation is always under stress between line to ground and prone to failure in dead tank breaker. Short length drive rods, insulating tubes, support insulators, bushings throat shields and short striking distances are highly stressed. Solid insulation degradation is a continuing process in case of dead tank breakers. This failure mode is almost absent in case of live tank circuit breakers. Even in case of loss of SF6, live tank circuit breakers can withstand line to ground voltage where as DTB can’t withstand the voltage.Particle sensitivityConducting and non-conducting foreign particles in gaseous dielectric creates stress enhancement/floating potentials/partial discharges, leading to insulation failure. This failure mode is very predominant at UHV level in case of dead tank circuit breakers as the insulation is always stressed between live conductor and the grounded tank.  In case of live tank breakers, this failure mode is quite rare.Grading capacitor oil leakageUse of oil-filled grading capacitor inside DTB tank is biggest source of contamination, often leading to oil leakage and unmanageable messy maintenances. It is a predominant failure mode. In case of DTB, the capacitors are enclosed in the tanks and hence exposed to vacuum and gas pressure cycles makes grading capacitor more venerable to leakage. In case of live tank breakers as grading capacitors are external, surrounded by air, oil leak is not an issue with circuit breaker integrity and visually detectable unlike DTB.Dead tank breakers are shipped with large size heavily cantilevered interrupter assembled into tank which causes high-shipping stresses. This leads to unnoticeable micro cracks in solid insulating material like drive rod, insulating supports and insulating