What is the effect of N2 and sulfur hexafluoride mixture on insulation properties?- SF6在线监测系统_SF6检漏仪_SF6传感器_SF6密度继电器

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What is the effect of N2 and sulfur hexafluoride mixture on insulation properties?

发布时间：2014-11-09 来源：ydlcb 浏览次数：38次

The International Conference on Power Grids (CIORE) formed a special Working Group (TASK FORCE D1.03.10) to study the insulation properties of N2/ sulfur hexafluoride gas mixture and its use, especially in the use of gas insulated transmission lines (GIL). On the one hand, the purpose of the research is to reduce the impact on the greenhouse effect; Second, the use of mixed gas can reduce costs, which is especially important for large gas consumption GIL

the results of the research were compiled into the Technical Brochure No.260. The research results are summarized below.

N2/ sulfur hexafluoride gas mixture has good insulating properties, even at low sulfur hexafluoride content. With sulfur hexafluoride gas content of 10% to 20%, the appropriate insulation performance can be achieved, and 10% to 20% sulfur hexafluoride gas content is suitable for GIL from the technical, ecological and environmental aspects. In order to achieve the insulation strength of pure sulfur hexafluoride gas, it is only necessary to appropriately increase the pressure by about 45% to 70%, and the amount of sulfur hexafluoride and its leakage rate will be reduced by about 70% to 85%. The increase in field strength due to electrode curvature and roughness is also easy to consider in device design.

In this mixture, the breakdown voltage in the presence of impurities is slightly lower than that of pure sulfur hexafluoride gas with the same insulation strength. However, existing diagnostic devices can be used for this gas mixture, which has equal or higher detection sensitivity than in pure sulfur hexafluoride gas. The discharge current and signal emission of fixed impurities at the live parts are similar to that of pure sulfur hexafluoride gas. The signal emission of active impurities is independent of the gas type and mixing ratio.

The current zero extinguishing ability and current breaking performance of the mixed gas are poor, and even the breaking ability of the isolation switch to the small charging current of the bus will be greatly degraded. The lead discharge channel changes direction more often. There is a greater risk of flashover to the ground in this mixture than in pure sulfur hexafluoride gas when the lead discharge branch and the contact arc are ignited. The flashover to ground is caused by the branching of the lead discharge as it propagates in the contact. This phenomenon occurs when the longitudinal electric field between the contacts suddenly becomes a transverse electric field, and the main leader of the continuous contact head generates a lateral branch to form a transverse electric field to the ground. The poor breaking ability of N2/ sulfur hexafluoride gas is due to the fact that the number of lead steps is more than that in pure sulfur hexafluoride gas, and the probability of the lead step changing its direction is greater.

The International Electrotechnical Commission IEC6125 standard specifies the test procedure for disconnecting the bus charging current in GIS under harsh conditions. A sulfur hexafluoride insulated isolation switch passed the required 50 breaking tests at two voltage levels of 550KV and 420KV. When N2/ sulfur hexafluoride gas with the same insulation strength was tested, two flashover to ground occurred after only 17 times of closing at 420KV. Therefore, this gas mixture is not suitable for any breaking task. The total amount of sulfur hexafluoride gas can be minimized in some N2/ sulfur hexafluoride insulated GIS, but all rooms performing the breaking task should be filled with pure sulfur hexafluoride gas. Therefore, replacing sulfur hexafluoride gas with N2/ sulfur hexafluoride mixture in GIS will be an uneconomical technical solution, and there will be no ecological advantage. However, the N2/ sulfur hexafluoride mixture is suitable for high pressure equipment without a breaking task and has proved particularly suitable for GIL.

In short, for a long time, people have carried out a lot of research to find the alternative gas of sulfur hexafluoride, but it has not been successful. Studies have shown that from the insulation point of view, only nitrogen (N2) and air can replace sulfur hexafluoride gas. Their insulating capacity is only one-third that of sulfur hexafluoride gas. But with these gases, the equipment needs to be redesigned and a lot of materials are used.

N2/ sulfur hexafluoride is a good alternative gas from an ecological and economic point of view. The breakdown strength of N2/ sulfur hexafluoride gas mixture is related to the concentration and pressure of sulfur hexafluoride in nitrogen. Technically speaking, the composition of nitrogen to 40%, there is little change in electrical strength. Even a mixture of 80%N2 20% sulfur hexafluoride has more than twice the electrical strength of pure N2 or air.

Sulfur hexafluoride gas mixture can only be used as an insulating medium, but not as an arc extinguishing medium in circuit breakers.

9. Sulfur hexafluoride mixed gas with GIS

The sulfur hexafluoride /N2 mixture can be used in all parts of the GIS that undertake the insulation task, but not in the compartment where arc suppression is required. In GIS, the gas mixture can be 20% (minimum) to 52% (maximum) of the total volume, depending on the structure of the equipment. When the pure insulation compartment is filled with 15% sulfur hexafluoride 85%N2 gas mixture, and the pressure is increased from 0.4MPa to 0.8MPa, the saving amount of sulfur hexafluoride gas is between 14% and 36.4% under the condition of ensuring the insulation strength.

10. Sulfur hexafluoride mixed gas with GIL

The gas-filled transmission line (GIL) consists of a coaxial aluminum alloy tube body. The pipeline is laid directly on the ground like a pipe. GIL aluminum tube is elastic, bending radius can be more than 400m, and can be arbitrarily changed direction, using corner components. The porosity free connection of each pipe section is made by rail welding. When the laying length is 1200m, the air chamber is separated. At this point, each segment is monitored with a pressure sensor. The GIL ohmic resistance is small, so the heat loss and heat discharged to the ground is small. The capacitance of GIL is also small, and no compensation device is required for conveying large capacity.