Wenhao Niu

Experimental study of partial discharge inception voltage of special fluorocarbon gases and their mixtures in rod-plane gaps under power frequency

Because of the high global warming potential (GWP), SF6 gas was listed as one of the six restricted gases in 1997. Some fluorocarbon gases with low environmental effect and high dielectric performance are studied as the substitution candidates for SF6. This paper presents the partial discharge (PD) capability of some gas mixtures with fluorocarbon at different pressures and ratio of fluorocarbon in rod-plane electrodes under power frequency. The PD inception voltage (PDIV) is measured and analyzed. The experimental results show that the PD capability of some gas mixtures with fluorocarbon is better than that of SF6 and the gas mixtures has a possibility to substitute for SF6.

Study on decomposition gas for faults diagnostic of electrical equipment using fluorocarbons

As the increasing demand for large capacity, high voltage and high reliability power transformers in urban power grid, oil-free, non-flammable and non-explosive equipment is recommended. For this reason, evaporative cooling transformers, gas insulated transformers and other electrical equipment using fluorocarbon as coolant and dielectric have been developed. This paper presents an experimental survey of the main components of decomposition products generated by several faults of electrical equipment using fluorocarbon; here evaporative cooling transformer and gas insulated transformer are taken as example. All experimental studies are carried out by simplified testing setups. The gaseous decomposition products formed during the tests are analyzed and assayed by means of GC/MS (gas chromatography/mass spectrometry) method. The amount of decomposition products varied with temperature, time and discharge capacity are studied. Based on these results, criterions for judging the fault type of this kind equipment is suggested, and it could be helpfulness to develop a diagnostic method for the new electrical equipment using fluorocarbon.

Cooling capability of gas mixtures with fluorocarbon in a gas-insulated transmission Line Model

Because of its high-capacity, low losses and no electromagnetic interference, the gas-insulated transmission line (GIL), which is a replacement of an overhead line in special environments, has been used for over 40 years, mainly as bus ducts of gas-insulated substations or underground connections in power plants. The insulated and cooling gas in GIL is mainly SF6 or its mixtures, which have high global warming potential (GWP) and are listed as restricted gases in the 3rd Conference of the Parties to the United Nations Framework Convention on Climate Change in 1997. Therefore, a lot of researches have been made to find a substitute gas for SF6. In this paper, the cooling capability of gas mixtures with fluorocarbon, which has a lower GWP than SF6 and outstanding insulating performance, is investigated. Heat transfer coefficients of gas mixtures are calculated by measuring the surface temperature of a metal pipe under a certain current. The heat transfer coefficients of gas mixtures with fluorocarbon are analyzed and compared with that of SF6 mixtures. Based on the heat transfer coefficients, the maximum permissible currents of our GIL model with fluorocarbon mixtures or SF6 mixtures are also calculated and compared.

AC breakdown voltage measurement of a fluorocarbon in gaseous state two-phase-flow state and liquid state

The AC breakdown voltages of a fluorocarbon were measured under three states, including in gaseous state, in boiling two phase flow state and in liquid state. Because the insulating medium in two phase flow could be used in some electrical apparatus, the author measured the breakdown voltage of two phase flow under various pressures in the vessel, various water contents in the coolant. The results would be used for designing and manufacturing of electrical apparatus.

Preliminary study of a relay transmitted cooling system with heat pipes and LHP system in large capacity gas insulated transformers

The growing rated capacity of gas insulated transformers (GIT) are confined by the limitation of cooling capability. Hence a new thermal relay transmitted cooling system was applied in GIT and presented in this paper. The whole cooling system consists of conventional heat pipes and loop heat pipe (LHP): Dozens of non-metallic conventional heat pipes are distributed in the windings, absorbing heat and transmitting it to the LHP cooling system which is attached to the upper part of the conventional heat pipes. Then the LHP system brings heat out of the transformer and dissipates it to circumstance by boiling and condensing circulation of the fluorocarbon liquid in the LHP system. This relay transmitted cooling system is self-circulated, needing no additional blowers or fans installed in transformers. An experimental model is designed, fabricated and tested, and the testing results demonstrated that it could improve the cooling capability of GIT greatly, and has the feasibility to develop GIT of larger capacity.

AC breakdown voltage of thermal bubble induced two-phase flow condition of inflammable fluorocarbon by self-heating electrode

Although it has been found that the insulating characteristics of thermal bubble induced two-phase flow changes with pressure and heat power, its breakdown mechanism has not been known clearly so far. In order to obtain actual working conditions of two-phase flow state of fluorocarbon, a special apparatus with self-heating plane-plane electrodes is fabricated to study the relation of AC breakdown voltage (BDV) and thermal parameters, such as heat density, working pressure, electrode surface temperature, and so on. The pressure in the test apparatus could be adjusted individually by a pressure regulator. The test results under atmosphere pressure (0.1MPa abs.) show the BDV of the fluorocarbon liquids remains relatively flat with the heat density less than 7000W/m2, and drops sharply when the heat density increases until 20000 W/m2. Then the BDV remains the breakdown value of vapor at higher heat density. The thermal parameters and the flow style are also studied with the BDV decreasing line. The test data can provide design constraints for the inflammable fluorocarbon liquid immersed transformers.

The experimental study of a novel cooling system of a power transformer in an urban underground substation

Decades of experience in underground substations indicated that the traditional cooling technologies have their own shortcomings. Therefore, a novel Oil-Evaporative liquid cooling technology is presented. This new cooling technology could be considered as a two-step cooling system, which could effectively transport heat energy from thermal sources underground to radiators on the ground by phase changing of evaporative liquid in an oil-evaporative cooler. Thanks to the special characteristics of the evaporative liquid, the novel Oil-Evaporative liquid cooling system has been proven its effectiveness owing to high reliability, excellent cooling effect, smaller volume and less installation area. Based on the experimental data obtained in this paper, the novel Oil-Evaporative liquid cooling system could be precisely designed and widely used in future.

Puffer characteristic calculation of a circuit breaker using fluorocarbon gas mixtures

Because of its high dielectric strength, good partial discharge capability, low global warming potential (GWP), none ozone depletion potential (ODP) and nontoxicity, fluorocarbon gas is possible substitute for SF6, and can be used hopefully in electrical equipment, such as circuit breaker. In this paper, we calculate the pressure characteristic of circuit breaker using gas mixture containing fluorocarbon gas as arc quenching gas based-on the first law of thermodynamics and the law of hydrokinetics. The pressure characteristic curves of the gas chamber and the gas mass flow pass the nozzle are obtained through computation. In addition, the influences of mixtures ratio and nozzle throat diameter to the pressure and gas mass flow rate are achieved. It shows that the bigger ratio of fluorocarbon is corresponding to the bigger chamber pressure and relative bigger mass flow as to the same structural factor with no energy input; while considering the arc energy bigger ratio corresponding to smaller chamber pressure. And the nozzle throat diameter has a large impact on the pressure and gas mass flow rate. Besides, we also compared the gas pressure and gas mass flow rate of mixture gas with that of SF6 gas. These results provide a helpful reference for the design of the new type circuit breaker.

Partial discharge performances under non- uniform electric field in gas mixtures with small fluorocarbons

This paper describes the partial discharge (PD) performances of gas mixtures with small fluorocarbons because their global warming potential (GWP) is smaller than that of SF6 gas, which was regulated as one of six restricted gases. It is difficult to apply pure fluorocarbon gases as insulating medium directly in gas insulated switchgear (GIS) and gas circuit breakers (GCB), because their liquefying temperature is high. Thus, the mixtures have various contents and ratios. The PD inception voltages of gas mixtures containing fluorocarbon and N2 are measured and analyzed as a function of gas pressure, electrode distance and the ratio of the fluorocarbon. The PD performances of fluorocarbon gas and its mixtures are compared with those of SF6 gas, which is now widely used as the insulating medium in GIS and GCB. The results show that gas mixtures containing special fluorocarbon have a possibility to substitute for SF6 gas.