Polyimide (PI) has been widely used as a kind of special engineering plastic. However, the further industrialization development of PI is limited due to its poor melting property. Thermoplastic polyimide (TPI) is developed by modifying the thermosetting PI, and its melting property and processability are improved qualitatively compared to thermosetting PI, so that it can be applied to flexible copper clad laminate, 3D printing and other fields. In this paper, the synthetic method, thermal, mechanical, and processing properties of TPI were summarized, and the application of TPI in flexible printed circuit and other important industrial fields were concluded. The future research directions of TPI were proposed.

Natural ester insulating oil has higher ignition point and flash point, excellent natural degradation ability and good insulation performance, and it is a green substitute for mineral insulating oil. Due to the physical and chemical characteristics of natural ester insulating oil, it is still facing many challenges to widely used in transformer equipment. In this paper, the preparation process of natural ester insulating oil was reviewed combining with its shortcomings. The research results of natural ester insulating oil were expounded from the aspects of additive modification, mixed modification, nano modification, and chemical modification. Finally, the future research direction was prospected.

The effect of nano-Al₂O₃ on the microscopic properties of vegetable insulating oil was studied by molecular simulation, and its result was verified by tests. The mechanism of surface interaction between nano-Al₂O₃ and vegetable insulating oil molecules was analyzed, models of vegetable insulating oil before and after nano-Al₂O₃ modification were established. The hydrogen bond, radial distribution function (RDF) of oil molecule, and diffusion coefficient of water molecule were studied, and the thermal ageing experiment was conducted on vegetable insulating oils with different concentration of nano-Al₂O₃. The results show that compared with the vegetable insulating oil without modification, there are more hydrogen bonds in the modified vegetable insulating model, the peak value of RDF is greater, and the diffusion coefficient of water molecules is smaller. In the process of thermal ageing, the dielectric loss of the modified oil is smaller than that of the unmodified oil, which shows that the nano-Al₂O₃ modified vegetable insulating oil has excellent stability, and its thermal stability and insulation performance are enhanced.

In order to discover the bonding mechanism of self-adhesive silicone rubber tape, we prepared a self-adhesive silicone rubber tape through the reaction of boric acid and methyl-vinyl silicone rubber. The variation of bonding force at interface of self-adhesive silicone rubber tape with temperature and soaking time was traced by tensile tests, and the self-fusion effect was verified. A physical model of self-fusion effect was proposed with the help of infrared spectrum test. The results show that when the content of boric acid in the reaction system is 2.5%, the self-adhesive tape has the highest initial bonding force. When silicone rubber has the molecular structure of short chain boron-containing siloxanes imbedded in long chain siloxane network, the well contacted silicone rubber interfaces can bond through complexation between boron and oxygen atoms in the boron-containing siloxanes, and the bonding force is stronger than the cohesion among polysiloxane molecules. The complexation can not complete instantly, and the higher the temperature of interface, the shorter the time of complexation. Under 60℃, the complexation strength can reach the optimum value within 12 hours.

An aramid 1313 fibrid was prepared by precipitation method, and its morphology structure and properties were characterized by fiber length test, thermogravimetric analysis (TGA), X-Ray diffraction (XRD), scanning electrical microscopy (SEM). The dewaterability of aramid 1313 fibrids and chopped fibers in the papermaking process was evaluated by dewatering time, and the mechanical properties and electrical insulation properties of the paper were studied. The results show that with the increase of molecular weight of aramid 1313 fibrid, the average length of fibrids increases, and the tensile strength, elongation at break, tearability, and insulating properties of the paper increase. The fibrid is non-crystalline structure. With the increase of molecular weight, it becomes to film structure, and the thermostability increases. However, the thermostability of chopped meta-aramid fiber is still higher than that of aramid 1313 fibrids.

Through adding 2-methyimidazolium cobalt (ZIF-67) nano-fillers into 3,3′,4,4′-biphenyltetracarboxylic anhydride (BPDA) - 4,4′-diaminodiphenyl ether (ODA) type polyimide (PI) matrix, we prepared a PI/ZIF-67 three-layer composite films with sandwich structure. The structure of ZIF-67 and PI/ZIF-67 three-layer composite films were characterized by FTIR, XRD, and SEM, and the effect of ZIF-67 content on the thermal stability and dielectric property of the composite films was studied. The results show that when the mass fraction of ZIF-67 is less than 10%, the initial decomposition temperature of composite film is higher than 500℃, which shows good thermal stability. The dielectric constant of PI/ZIF-67 composite films is obviously lower than that of PI. When the mass fraction of ZIF-67 is 10%, the dielectric constant of PI/ZIF-67 composite film decreases by 50%. When the mass fraction of ZIF-67 is 5%, the dielectric constant of PI/ZIF-67 composite film decreases by 71%. When the mass fraction of ZIF-67 is less than 10.0%, the dielectric loss of PI/ZIF-67 composite film is slightly higher than that of pure PI.

Natural contamination tests were conducted on insulators from ultra-high voltage (UHV) converter stations under four typical climate conditions, including extreme arid, semi-arid, semi-humid, humid. The inhomogeneous coefficient characteristics of contamination of the insulators were obtained, and the effects of diameter, layout, and AC/DC voltage on the contamination performance of insulators for converter stations were analyzed. The results show that the equivalent salt density (ESDD) ratio of the top and bottom surface of the insulator from Tianshan station, Lingzhou station, Zhongzhou station and Suzhou station is 1.56, 0.41, 1.26 and 1.45, respectively. Compared with the vertical post insulator, the horizontal arrangement of the wall bushing insulator has a larger area exposed to the rain, and there is obvious washing effect by the rain on the pollution of the insulator surface. The ESDD of insulators arranged horizontally at Lingzhou Station and Suzhou Station is 0.64 and 0.63 times smaller than that of insulators arranged vertically. The difference of AC/DC insulator pollution accumulation is mainly determined by electric field force, pollutant particle size, and wind speed. The AC/DC pollution accumulation ratio of post insulators from Tianshan Station, Zhongzhou Station, and Suzhou Station is 1.12, 0.84 and 0.52, respectively. With the increase of the diameter of post insulator, the ESDD increases gradually. The research results can provide data support for the external insulation design of UHV converter stations in China.

In order to investigate the influences of dead time in power electronic pulse width modulation (PWM) algorithm on the properties of insulation for inverter-fed motor, we built a repetitive pulse generator with 2-10 μs of dead time using full-bridge solid-state switch and high speed and real time control technique. Under the bipolar repetitive pulse voltage with 4 kV of peak to peak voltage and 5 kHz of frequency, the effect of repetitive square wave voltage dead time on the partial discharge (PD) statistical characteristics and corona-resistant lifetime of inverter-fed motor insulation were investigated through turn to turn insulation tests and ultra-high frequency test. The results show that the increase of dead time may significantly increase the attenuation effect of surface discharge at the rising edge and falling edge of repetitive square wave, weakens the inhibition of surface charge reverse electric field on discharge in discharge process. The number of discharge increases, and the electrical ageing of insulation accelerates, which leads to the decrease of corona-resistant lifetime of turn to turn insulation for inverter-fed motor. It is concluded that when adding dead time to protect the power device in designing power electric system, we should pay attention to the effect of dead time on the accelerated ageing of insulation system in inverter-fed motor, increase the insulation margin properly.

According to the actual situation of outgoing section at Changping station of Zhangnan-Changping 500 kV round Ⅲ line (Beijing section), a trampled composite insulation cross arm narrow base linear tower for 500 kV line was designed and developed. The cross arm structure used the form of “two pull and two press”, and suspension insulator string was not required, which can reduce the fault caused by the monsoon discharge at tower head. Alicyclic epoxy umbrella skirts were arranged at intervals in the middle of compression bar, which makes the structure can be trampled. According to the original line form, considering the tower structure, air clearance requirements, and deflection deformation calculated by simulation under the conditions of reserved safety factor, we confirmed the structure height and mandrel diameter of the product. The mechanical instability conditions of the structure were analyzed under three severe working conditions, including break line, vertical strong wind, and ice covering. The results show that the overall first-order buckling coefficient is greater than 2.5, the axial stress of each unit is far less than the mechanical strength of metal and composite materials, and the structure has high safety. The mechanical properties of 1∶1 true type test tower and the electrical properties of upper phase composite insulation cross arm can meet the test standard and design requirements.

Three typical stator defect models were fabricated using the generator stators in actual operation, and then conducted accelerated thermal ageing tests. The difference on partial discharge characteristics of the three typical defects before and after ageing was studied using pulse current method. The results show that after ageing, the partial discharge inception voltage (PDIV) of the model with internal defect increases, while the PDIV of the model with slot discharge and end discharge decrease, this is because there are large number of cracks on the epoxy mica tape insulation surface due to thermal ageing. The phase resolved partial discharge (PRPD) pattern of internal discharge model is symmetrical, and the width of discharge phase becomes narrow after thermal ageing; the PRPD pattern of slot discharge model changes from hill shape to semi ellipse shape after thermal ageing, and the discharge density increases significantly; the PRPD pattern of end discharge model shows the characteristics of corona discharge, and the discharge characteristics at the positive and negative half-cycle of alternating current is obvious asymmetry.

Basin insulator is the core component of gas insulated metal transmission line (GIL) equipment. A large amount of charge will accumulate on the surface of insulators under long-term effect of unipolar DC electric field, resulting in the electric field distortion at the solid-gas interface, which leads to surface flashover of insulators easily and reduces the insulation performance of equipment. On the basis of relevant research results of DC insulators, this study restrains the accumulation of surface charges by reasonably controlling the surface field strength of insulators and increasing the resistivity of insulating materials. The structure design was optimized by simulation methods, and the insulator was prepared by casting, then dielectric tests and mechanical performance tests under AC and DC stress was carried out and passed, which verified the correctness of design.

Partial discharge (PD) caused by interface defects is one of the main causes of interface breakdown, but the initiation and evolution of interface PD of cable accessories is not clear now. In this paper, the electric field distribution of actual cable accessory interface was simulated and analyzed. According to the calculation results of electric field, an interface test unit was designed and manufactured using the built-in electrode method, and then the test unit were conducted partial discharge initiation experiments. The results show that the electric field distribution of the interface test unit is similar to that of the actual cable accessory interface, and the PD development law obtained by the interface test unit is the same to that of the actual cable accessory interface. The interface test unit can effectively simulate the electric field of actual cable accessory interface, and at the same time, the introduction of metal electrodes were avoided into the test interface. It is concluded that the test unit can be used to study the initiation and evolution of PD at the cable accessory interface.

In order to improve the accuracy of relative permittivity measurement by means of non-contact method, we built a corresponding model by electric field finite element analysis and obtained the distribution of electric field and potential of the electrode and around. The edge effect of electrode was analyzed and its influences on the measuring result of relative permittivity were investigated. In addition, the influence of potential factors on the measuring error of non-contact method was analyzed, and then the factors which should be taken into consideration in the correction was confirmed, which were the electrode gap distance, the ratio of sample thickness to electrode gap distance, and the relative permittivity. The results show that the measuring error of relative permittivity increases with the increase of electrode gap distance and relative permittivity of the sample, while decreases with the increase of the ratio of sample thickness to electrode gap distance. Under the model with the electrode gap distance of 0.2-5 mm, the ratio of sample thickness to electrode gap distance of 0.5-1, the relative permittivity of 1-20, 2 380 groups of actual and calculated values of relative permittivity are obtained, and the relative error is -0.37%--41.72%. According to the above relationship among the electrode gap distance, the ratio of sample thickness to electrode gap distance, the measured value and real value of relative permittivity, the measured relative permittivity is then corrected by the 3D interpolation method and matching method. The correction method is verified by simulation and is used to correct the measured relative permittivity of high temperature vulcanized silicone rubber. The results validate the proposed method.

Fog chamber is an indispensable part of artificial simulation for insulator pollution test. In this paper, the influence of pollution degree and humidity on the discharge characteristics of porcelain insulator in fog chamber environment was studied in combination with simulation and test. At first, the electric field distribution of clean and dirty insulators in 110 kV transmission line with and without fog chamber was simulated by the software Comsol Multiphysics. And then a scheme to make the electric field near the high voltage end of insulator with and without fog chamber approximate by reducing the test voltage with fog chamber was proposed. At last, tests were carried out with the voltage obtained by simulation. The results show that compared with the condition without fog chamber, the electric field around the clean insulator is larger at the high voltage end and smaller at the low voltage end under the condition with a fog chamber. Moreover, the electric field around the insulator in the fog chamber increases with the increase of pollution degree. The simulation results are verified by the test results, with the increase of the insulator pollution degree and the humidity, the number of discharge photons increases. This paper proposes a scheme to re-duce the test voltage in the artificial fog chamber to optimize the insulator pollution discharge test, and the test effect is more consistent with the actual field application status.

In view of the burning loss and fire risk of T-type cable plug used in high voltage devices when the conductor is overheating, we built an experimental platform for the pyrolysis of rubber material for T-type cable plug , and the pyrolysis behavior of T-type cable plug was studied based on FTIR method. The results show that in the temperature range of 80-260℃, the pyrolysis characteristic components of T-type cable plug are silicone grease, CO₂, and H₂O. The escape temperature of CO₂ and H₂O is 80℃, and their concentration reaches the maximum value at 200℃, while the escape temperature of silicone grease is above 200℃, and the escape temperature increases with the increase of temperature. The results of non-isothermal thermogravimetric analysis show that the weight loss rate at 305℃ is 2.50%.

SF₆ is often used in GIE as an insulating medium, it will decompose when overheating or appearing partial discharge inside GIE, the decomposition products, such as SO₂, SOF_2, SO₂F₂, H₂S, and CO will produce after further reaction. In this paper, the components H₂S and CO were quantitatively measured by photoacoustic spectroscopy detection technology, and the factors affecting photoacoustic signal were discussed theoretically. A photoacoustic spectrum experiment platform was built, and the gas was measured quantitatively based on the photoacoustic effect. Appropriate gas absorption lines were chosen as characteristic spectrum line to avoid the potential cross-interference of other gas components. According to the results of HITRAN simulation, the characteristic spectrum line of H₂S was chosen as 6 336.6 cm^-1, and the characteristic spectrum line of CO was chosen as 6 380.3 cm^-1. The results show that the linearity between the gas concentration of CO and H₂S and the amplitude of pure photoacoustic signal is extremely high, which suggests that the gas concentration can be accurately calculated through the measurement of photoacoustic signal value of gas. With the background gas of SF₆, the lower limit of detection for CO is 9.88×10^-6, and the lower limit of detection for H₂S is 1.75×10^-6.

In the actual manufacturing and field installation, defects such as air, metal impurities, and uneven distribution are easily introduced into the silicone rubber insulation layer of cable joint, which endangers the safe operation of power grid system. At present, the detection of power equipment defects are mostly destructive tests, and the experimental results can only reflect the overall situation of equipment. As a non-destructive testing technology, ultrasonic testing can accurately locate and image defects inside the material, and reflect the changes of local performance inside the device. In this study, a set of ultrasonic testing platform were independently designed and built, and ultrasonic testing was conducted on the artificial silicone rubber samples with different defects. The results show that for flexible materials such as silicone rubber, ultrasonic testing can better image the bubbles, air gaps, and steel needle defects in the sample. Compared with similar X-ray testing, ultrasonic testing has an obvious amplification on the tiny bubbles defects. In addition, the ultrasonic testing technology can reflect the change of stress and density inside the material, and can accurately locate the specific depth of the defect according to the ultrasonic echo. This technology has broad application prospects in the location identification and online detection of internal defects in the silicone rubber insulation layer of cable joints.