POST-BREAKDOWN PHENOMENA AND APPLICATIONS
This is the phenomenon which occurs after the actual breakdown has taken place and is of technical importance. Glow and arc discharges are the post-breakdown phenomena, and there are many devices that operate over these regions. In a Townsend discharge see Fig. (1) The current increases gradually as a function of the applied voltage. Further to this point (B) only the current increases and the discharge changes from the Townsend type to Glow type (BC). Further increase in current results in a very small reduction in voltage across the gap (CD) corresponding to the normal glow region. The gap voltage again increases (DE), when the current is increased more, but eventually leads to a considerable drop in the applied voltage. This is the region of the arc discharge (EG). The phenomena that occur in the region CG are the post-breakdown phenomena consisting of glow discharge (CE) and the arc discharge (EG).
A glow discharge is characterized by a diffused luminous glow. The colour of the glow discharge depends on the cathode material and the gas used. The glow discharge covers the cathode partly and the space between the cathode and the anode will have intermediate dark and bright regions. This is called normal glow. If the current in the normal glow is increased such that the discharge covers the entire cathode surface, then it becomes abnormal glow. In a glow discharge, the voltage drop between the electrodes is substantially constant, ranging from 75 to 300 V over a current range of 1 mA to 100 mA depending on the type of the gas. The properties of the glow discharge are used in many practical applications, such as cold cathode gaseous voltage stabilized tubes (voltage regulation tubes or VR tubes), for rectification, as a relaxation oscillator, and as an amplifier.
If the current in the gap is increased to about 1 A or more, the voltage across the gap suddenly reduces to a few volts (20—50 V). The discharge becomes very luminous and noisy (region EG in Fig. 1 This phase is called the arc discharge and the current density over the cathode region increases to very high values of 103 to 107A/cm2. Arcing is associated with high temperatures, ranging from 1000°C to several thousand degrees Celsius. The discharge will contain a very high density of electrons and positive ions, called the arc plasma. The study of arcs is important in circuit breakers and other switch contacts. It is a convenient high temperature high intensity light source. It is used for welding and cutting of metals. It is the light source in lamps such as carbon arc lamp. High temperature plasmas are used for generation of electricity through magneto-hydro dynamic (MHD) or nuclear fusion processes.
Fig. (1) d.c. voltage-current
characteristic of an electrical discharge with
PRACTICAL CONSIDERATIONS IN USING GASES AND
Over the years, considerable amount of work has been done to adopt a specific gas for practical use. Before adopting a particular gas or gas mixture for a practical purpose, it is useful to gain knowledge of what the gas does, what its composition is, and what the factors is that influence its performance. The greater the versatility of the operating performance demanded from an insulating gas or gas mixture, the more rigorous would he the requirements which it should meet. These requirements needed by a good dielectric do not exist in a majority of the gases. Generally, the preferred properties of a gaseous dielectric for high voltage applications are:
(a) high dielectric strength,
(b) thermal stability and chemical inactivity towards materials of construction,
(c) non-flammability and physiological inertness, and environmentally non-hazardous,
(d), low temperature of condensation,
(e) good heat transfer, and
(f) ready availability at moderate cost.
Sulphur hexafluoride (SF6) which has received much study over the years has been found to possess most of the above requirements.
Of the above properties, dielectric strength is the most important property of a gaseous dielectric for practical use. The dielectric strength of gases is comparable with those of solid and liquid dielectrics see Fig. (2.).
It is clear that SF6 has high dielectric strength and low liquefaction temperature, and it can be used over a wide range of operating conditions. SF6 was also found to have excellent arc-quenching properties. Therefore, it is widely used as an insulating as well as arc-quenching medium in high voltage apparatus
Fig (2) d.c. breakdown strength of typical solid, liquid, gas and vacuum insulations in uniform, fields
SF6 and Other Gas Mixtures
SF6 is widely used for applications in power system due to its high dielectric strength and good arc interruption properties. However, SF6 gas has been found to be a green house gas that causes environmental problems. The production and use of SF6 gas has increased steadily and today it is about 10,000 metric tons due to leakages into the atmosphere from the electrical equipment. The concentration of SF6 in the environment has been steadily increasing. The release of SF6 into the atmosphere leads to concentration of large volumes of SF6 gas in the upper atmosphere. SF6 molecules absorb energy from the sun and radiate it into the atmosphere for long duration of time.
There has been a large concern for these environmental effects and therefore the electrical industry has been looking for an alternate gas or gas mixture to be used in electrical equipment which presently use SF6 gas, as an insulating and arc interruption medium. The large amount of experimental data that is presently available suggest that 40% SF6/60% N2 mixtures have all the dielectric characteristics that make it suitable for use as insulation in high voltage equipment. Ideally the gas mixture should be suitable for use in the existing equipment as well as in the equipment that will be designed and manufactured in future.
Extensive research work done in SF6 and its mixtures with N2, air and CO2 has given breakdown values which are 80—90% of the pure SF6 values as shown in Table
Lightning Impulse Breakdown Strength of SF6/Other Gas Mixtures
The industry is looking for a gas mixture that can replace the pure SF6 gas in the existing SF6 insulated apparatus, requiring no change in hardware, test procedures or ratings. SF6/N2 mixture is the one that has been found to be a good replacement for SF6. SF6/N2 mixtures have been used in Gas Insulated Transmission System and were found to perform well. Also, the work done so far has shown that the ability of SF6/N2 mixtures to quench high current arcs is promising.
The cost of such mixtures is low and is less sensitive to field non-uniformities present inside the equipment. In view of the above, the industry is trying to find out the optimum mixture ratio and the total pressure of the SF6/N2 mixture that would be required for a variety of applications. For many applications, such as Gas Insulated Transmission Systems, cables, capacitors, current transformers and voltage transformers, mixtures with different SF6 concentrations varying from 5% to 40%.
SF6/N2 mixtures show promise as a medium in circuit breakers. It has been found that a mixture containing 69% SF6/31 % N2 gave higher recovery rate than pure SF6 at the same partial pressure. It has also been shown that it is possible to further improve the arc interruption properties of SF6 by using SF6/N2 or SF6/He mixtures.
In summary, it may be said that there is an urgent need to significantly reduce the use of SF6 gas and its leakage from power apparatus. Use of gas mixtures appears to be feasible, but it has to be ensured that there is no loss in the performance of the equipment. Wherefore, further research has to be carried out to identify a suitable gas mixture, its pressure and its arc interruption capability to be used in the existing apparatus and the apparatus that will be designed and manufactured in future.