Application of Electric Heating Method in SF6 Density Monitor

Due to the partial liquefaction of SF6 gas at ultra-low temperature, the SF6 gas density in the SF6 gas density monitoring equipment decreases, which reduces the breaking capacity and insulation level of SF6 switchgear, and cannot guarantee the reliable operation of SF6 switchgear. For this reason, this paper proposes a solution to heat the SF6 gas in the SF6 switchgear with an electric heating device, so that the temperature of the SF6 gas can always be higher than the liquefaction temperature of the SF6 gas.

The electric heating device is used to heat the SF6 gas in the SF6 pressure gauge, which can effectively solve the problem of low-temperature liquefaction of SF6 gas at ultra-low temperature. The technical key of the electric heating design scheme is: the SF6 gas electric heating device is added to the SF6 switchgear. When the ambient temperature drops to a certain value (such as: -25℃), the heater can be automatically put into operation. The internal arc extinguishing and insulating medium are heated; no matter how low the ambient temperature is (for example: -50°C), the gas temperature in the SF6 switchgear is always higher than the temperature required for SF6 gas liquefaction, which solves the problem of SF6 low-temperature gas liquefaction in severe cold areas.

In the electric heating design scheme of SF6 switchgear, the heat transfer method of the heating device includes heat conduction, heat radiation and heat convection. For the different structural forms of SF6 switchgear, the electric heating method adopted is also slightly different. Commonly used design structures are:
 ① The heating method of winding an electric heating tape on the outer wall of the equipment tank;
② Insert the heater into the inside of the SF6 gas chamber and heat it directly.

In order to solve the problem of reliable operation of high-voltage SF6 circuit breakers at ultra-low temperatures, electric heating devices can be used to heat SF6 gas in SF6 density monitor. SF6/N2 mixed gas can also be used as the arc extinguishing and insulating medium of switchgear. In addition, it is also possible to reduce the rated pressure of the SF6 gas in the SF6 density monitor to reduce the SF6 gas liquefaction point. However, no matter which method is adopted, there are advantages and disadvantages.

The method of reducing the pressure of SF6 gas or using SF6/N2 mixed gas will cause the breaking performance of the equipment to decrease, which often cannot meet the requirements of the short-circuit breaking capacity of the equipment in the bidding document. At the same time, relevant type tests are required according to the standard GB1984. The test period is long and the cost is high. The electric heating method requires changes to the existing structure of the equipment and increases the cost of materials.

It is believed that electric heating is the most economical and effective solution through comprehensive analysis. In addition, it is also important to pay attention to the design of low-temperature products: choose low-temperature-resistant seals, low-temperature-resistant grease. And heat in the operating mechanism box to ensure that the opening and closing characteristics remain unchanged and can span a large temperature range of porcelain sleeve used etc.

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