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Vegetable insulating oils, with higher fire point, lower biological hazard and better availability compared to traditional mineral insulating oils, have enormous potential to become the substitute of the latter, given the trend of green development of the power equipment, for example, the transformers. However, the obviously different constitution of the vegetable oils makes the circumstance hard when considering their application on the insulation for actual transformers. One of the significant deficiencies of the vegetable oils in working condition is their insufficient lightning impulse endurance under partial high electric fields and large discharge gaps. In this article, three kinds of transformer insulating oils, rapeseed oil, FR3 and 25# mineral oil were chosen to investigate the performances of these insulation materials under long-gap lightning impulses with positive and negative polarity respectively. The variation rules of breakdown voltage and velocity with the gap length were obtained. And the difference of the lightning impulse voltage endurance between the vegetable and mineral insulating oils as well as its causes based on theories of streamer and ionization potential (IP) were analyzed, which led to the conclusion that the gap increase causes more threat to the positive lightning impulse breakdown of vegetable insulating oils, compared to its counterpart.

In recent years, C 4 F 7 N (fluorinated nitrile) is considered to be the most potential candidate gas for SF 6 in gas insulation equipment because of its excellent electrical performance and environmental protection advantages. However, there are few studies on the effect of O 2 on decomposition characteristics of C 4 F 7 N-CO 2 mixture. In this paper, we set up an experimental platform to simulate the thermal decomposition. We studied the effect of O 2 on the decomposition path of C 4 F 7 N-CO 2 mixture. The influence of pressure on the decomposition products of C 4 F 7 N-CO 2 mixture has also been studied. Gas chromatography-mass spectrometry (GC-MS) was used to detect the decomposition products. The experimental results show that the mixture of C 4 F 7 N-CO 2 decomposes into C 3 F 8 , CF 3 CN, C 3 F 6 , C 4 F 10 , C 2 F 5 CN and C 4 F 8 at an overheated temperature of 500 °C, and the mixture of C 4 F 7 N-CO 2 decomposes into C 3 F 8 , CF 3 CN, C 3 F 6 after adding O 2 . The addition of O 2 reduced the type of decomposition products and accelerated the decomposition rate of C 4 F 7 N-CO 2 mixture. The addition of O 2 should be avoided in the engineering application of C 4 F 7 N-CO 2 mixture. Increasing the pressure of C 4 F 7 N-CO 2 mixture in gas insulated equipment can effectively reduce the effect of O 2 on the decomposition of overheat.

C 4 F 7 N gas mixture has excellent insulating property, which is a gas insulating medium widely concerned by scholars at home and abroad in recent years. In order to reduce the harmful solid by-products produced during the discharge process, a certain amount of O 2 should be added to the C 4 F 7 N/N 2 gas mixture. This paper investigated the potential of C 4 F 7 N/N 2 /O 2 gas mixture in high-voltage gas insulating electrical equipment, tested the breakdown characteristics of C 4 F 7 N/N 2 /O 2 gas mixture containing oxygen of different volume fractions under 0.6MPa by gas insulation test platform, and analyzed the decomposition products and contents of the gas mixture after breakdown by GC-MS. It is found that adding a certain amount of O 2 to the C 4 F 7 N/N 2 gas mixture can improve the insulation self-recovery performance under high pressure, the main decomposition products of C 4 F 7 N/N 2 /O 2 gas mixture after multiple breakdowns are CF 4 , C 2 F 6 , C 3 F 6 , C 3 F 8 , CF 3 CN, C 2 F 5 CN, CO, COF 2 , C 2 N 2 . The content of these products increased first and then decreased with the further increase of oxygen content. Comprehensively considering the influence of O 2 addition on the insulation properties and discharge decomposition characteristics, as for HV engineering application, it is recommended to add 6% O 2 in the C 4 F 7 N/N 2 gas mixture.

The movable contact of conventional air circuit breaker (ACB) includes multiple parallel contacts. The arc motion process directly affects the breaker capacity of ACB. Therefore, it is important to study the arc motion process of multiple parallel contacts model. A three-dimensional (3D) magneto-hydro-dynamic (MHD) model of air arc plasma is built to investigate the arc motion process. The anode of this model consists of two parallel contacts. The potential vector approach is adopted to calculate the electromagnetic field. The thin sheaths surrounding the electrodes are also employed to represent the voltage drop of electrodes. Based on this model, the evolutions of arc voltage, temperature, and current density are obtained. It is noted that the anode arc root form on two parallel contacts with the arc evolution, and the cathode root is the only one. The arc column is merged together due to their attraction.

Accurate temperature-rise prediction in gas insulated switchgears (GIS) is necessary to reduce the number of heat-run tests during the product development process. An electromagnetic-thermal-fluid coupled model using computational fluid dynamics (CFD) is developed to predict the temperature-rise of a 110-kV three phase GIS in common tank type. The GIS includes the three-position isolation/ground switch and the circuit breaker insulated with SF 6 gas at 5 bar. In this model, the contact resistance, eddy current, skin effect, thermal flow, and thermal radiation are considered. The ohmic losses generated in the conducting components are calculated through the harmonic electromagnetic analysis with MAXWELL 3D software. Then, the predicted losses are imported to the commercial CFD software ANSYS Fluent to fulfill the thermal simulation. Based on this model, the distributions of heat generation, gas velocity, and temperature-rise are obtained. The results show that the maximum power density and temperature-rise both occur at the contact zones of the three-position isolation/ground switch. To verify the developed model, the temperature-rise tests of the GIS are carried out at 2200 A. The simulation and measurement results are close with a maximum error of 3.99 K.

Dissolved gas analysis (DGA) is attracting greater and greater interest from researchers as a fault diagnostic tool for power transformers filled with vegetable insulating oils. This paper presents experimental results of dissolved gases in insulating oils under typical electrical and thermal faults in transformers. The tests covered three types of insulating oils, including two types of vegetable oil, which are camellia insulating oil, Envirotemp FR3, and a type of mineral insulating oil, to simulate thermal faults in oils from 90 °C to 800 °C and electrical faults including breakdown and partial discharges in oils. The experimental results reveal that the content and proportion of dissolved gases in different types of insulating oils under the same fault condition are different, especially under thermal faults due to the obvious differences of their chemical compositions. Four different classic diagnosis methods were applied: ratio method, graphic method, and Duval’s triangle and Duval’s pentagon method. These confirmed that the diagnosis methods developed for mineral oil were not fully appropriate for diagnosis of electrical and thermal faults in vegetable insulating oils and needs some modification. Therefore, some modification aiming at different types of vegetable oils based on Duval Triangle 3 were proposed in this paper and obtained a good diagnostic result. Furthermore, gas formation mechanisms of different types of vegetable insulating oils under thermal stress are interpreted by means of unimolecular pyrolysis simulation and reaction enthalpies calculation.