When detecting the oil-paper insulation status of transformer based on frequency domain dielectric response (FDS), the nonlinear changes in FDS of oil-paper insulation will affect the accuracy of insulation assessment. Therefore, it is necessary to study the nonlinear changes in the FDS of oil-paper insulation with different ageing degree. The multiple sets of oil-paper insulation models with different ageing degree were prepared in laboratory, and the nonlinear FDS of oil-paper insulation were tested at different temperatures. The results show that the larger the test excitation amplitude, the more significant decrease in the low-frequency range of FDS curve. As the testing temperature and ageing degree increase, the characteristic frequency range of the dielectric loss factor curve showing non-linear changes moves towards the high-frequency direction. On the basis of the modified Davidson-Cole model, the shape parameters in the model were obtained by curve fitting calculation, and then the equivalent circuit model based on the transfer function was established. The nonlinear variation of the impedance modulus of oil-paper insulation was simulated and calculated. According to the simulation results, the circuit parameters of various impedance models are closely related to the nonlinear changes of the dielectric properties of oil-paper insulation, while the change of impedance makes the curve change in the corresponding frequency range.

In this paper, a simulation and analysis model was established by finite element software to study the physical characteristics of interturn insulation faults in dry-type air-core shunt reactor windings, and explore the sensitive state variables for easy fault monitoring. Firstly, the "field-circuit" coupling model based on actual reactor was established in the finite element simulation software, and its correctness was verified. Then, interturn insulation faults were set at different layers and heights of its windings, and the variation laws of the total loop current, equivalent impedance, active power loss, and other electrical characteristic quantities with fault positions were studied, the changes of spatial magnetic induction intensity and winding force were analyzed. The results show that the change rate of the studied electrical characteristic quantity varies with the short circuit position between single turns of the winding, which increases from the inner layer to the outer layer in the radial direction, and decreases in the outermost layer, and gradually decreases from the middle to the end of the winding in the axial direction. The spatial magnetic induction intensity and the stress on the faulty winding under the condition of single interturn short circuit of the winding increase obviously, and the change rate of equivalent resistance, power factor, and active power loss are all above 500%, which change obviously and are convenient to obtain, they can be used as online monitoring quantities of interturn insulation fault in dry-type air-core shunt reactor.

In this paper, the rotating disc electrode atomic emission spectrometry (RDE-AES) was used to determine the content of 21 mental and pollution elements such as silver, aluminum, and barium and other pollutants in transformer insulating oil. The influences of the immersion depth of disk electrode and the homogenization method of oil sample on the test results were investigated, and the test method was verified with the standard oil and insulating oil samples. The results show that when the immersion depth of disk electrode is 1/3 of its diameter, the relative error of the detection results is small after 3 min of ultrasound and vigorous shaking. The detection limits of each elements is 0-1.07 μg/g, the relative standard deviations (RSD, n=6) are 0.7%-2.9%, and except for element V, the recoveries of the other 20 elements are 96.64%-110.01%. The element P content in insulating oil was used to evaluate the class A and class B uncertainties, and the uncertainty of each component was synthesized and expanded to obtain an uncertainty of (8.34±1.06)μg/g, (k=2). The uncertainty mainly comes from the standard oil, measurement repeatability, and daily standardization processes.

In order to study the effects of compression molding and extrusion molding on the dielectric properties of nanocomposites, these two molding methods were used to prepared the crosslinked polyethylene/organic montmorillonite (XLPE/OMMT) nanocomposites, respectively. The effects of the interlayer spacing change of OMMT in different composites on the resistance-temperature characteristics, dielectric constant, dielectric loss, and electric strength of the composites were discussed. The results show that the force field effect during the molding process can affect the intercalation dispersion effect of OMMT. The tensile stress during extrusion molding makes the OMMT sheets in the sample orient along the tensile direction, and form a regular arrangement unit, which hinders the migration of carriers, thereby improving the resistance-temperature characteristics of the sample. The movement of polymer molecular segments is lim-ited between the oriented lamella, which makes the dipole in the sample decrease, and the dielectric constant and dielectric loss factor decrease. At the same time, the effective intercalation of OMMT and the hybrid structure formed by the polymer cause diffuse reflection of electrons, extending the electron motion path, and improving the electric strength of the extruded sample.

The voltage endurance coefficient n of XLPE insulation can characterize the electrical life characteristics of the insulation, and the n value can be obtained by analyzing the failure data obtained from the accelerated life test. However, the accuracy of the n value is affected by the applied field strength in the accelerated life test and the analysis method of the failure data obtained in the test. In this paper, the evaluation method of voltage endurance coefficient n was studied. The constant stress test and the step stress test were conducted on XLPE insulation samples, respectively, and a test data analysis method based on damage eigenvalues was proposed. This method established the mapping relationship between accumulated damage threshold (D_c) and n in the test through the damage matrix, and used the change of matrix rank as the selection criteria of n value. The results show that according to the proposed method, the XLPE insulation voltage endurance coefficient n₂ obtained in constant stress test is 12.43 and n₃ obtained in step stress test is 12.04, which compare with the XLPE insulation voltage endurance coefficient n₁ of 12.83 obtained by linear fitting, the difference is only 3% and 6%. Moreover, the proposed method can save more than 75% of the test time, which verifies the effectiveness of the method.

In order to study the influence of ageing status of oil-paper insulation of EMU on-board traction transformer on its residual life, the dissolved gas content in oil was used as the characteristic parameter to reflect the ageing of oil-paper insulation in this paper. Since the ageing process of oil-paper insulation has dynamic characteristic, a hidden Markov model was established according to the corresponding hidden state and observation state between the ageing status of oil-paper insulation and the characteristic parameters. Firstly, the number of hidden states is determined using Bayesian information criteria, and then the observation sequence is clustered by using the k-Means algorithm. On this basis, the state of the observation sequence is identified by the Viterbi algorithm to determine the ageing state of oil-paper insulation. Finally, the reliability function is determined by combining the proportional risk model. The accuracy of the model is verified according to the online monitoring data of the dissolved gases in the oil of on-board traction transformer of CRH2 EMU, and it is obtained that the remaining life of the oil-paper insulation is basically consistent with the actual life, which can provide theoretical reference for the subsequent development of traction transformer maintenance strategy.

In this study, 4,4ʹ-diaminobenzanilide (DABA), 2,2ʹ-dimethyl-[1,1ʹ-biphenyl]-4,4ʹ-diamine (m-TB), dianhydrides pyromellitic dianhydride (PMDA), and 4,4ʹ-oxybisphthalic anhydride (ODPA) were used as raw materials, and a polyimide (PI) film for oganic light emitting diodes (OLED) flexible substrates was successfully synthesized. The results show that when the molar ratio of diamine to dianhydride is 0.990, the feeding time is 120 min, the reaction temperature is 0-30℃, the stirring speed is 200-250 r/min, and the reaction time is 240 min, the gel amount during the synthesis of polyamide acid is small, and viscosity can meet the requirements of industrial synthesis. After thermal imimization at 400℃, the glass transition temperature of the polyimide film is 450℃, the 1% weight loss temperature is 554℃, the thermal expansion coefficient is 4.1×10^-6 K^-1, the tensile strength is 326.9 MPa, the tensile modulus is 9 572.8 MPa, the electric strength is 623 kV/mm, and the dielectric constant is 3.251, which meet the industrial application requirements of OLED flexible substrate.

Space/surface charge accumulation is a potential cause for the decrease of surface flashover voltage of DC GIL insulators. Applying nonlinear conductive coating is an effective method to improve the surface insulation performance. In this paper, a mathematical model was established for the regulation of space/surface charge and surface electric field of insulator by electric field dependent nonlinear con-ductive coating. The nonlinear relationships between the current density of insulating gas and the solid conductivity of insulator and the electric field intensity were comprehensively considered. The internal charge distribution law of insulator under temperature gradient distribution and the influence mechanism of nonlinear conductive coating on the surface charge accumulation of insulator were studied through this model. The results show that the nonlinear conductive coating promotes the dissipation of space charge obviously, and the homopolar charge near the high voltage electrode dominates the surface charge distribution. Due to the improvements of surface charge distribution and tangential electric field, the surface flashover performance improves. The positive charge may accumulate between the insulator and the coating interface, and decrease gradually from the high voltage electrode to the ground electrode.

With the large-scale construction of UHV long-distance power transmission, vegetable insulating oil instead of mineral insulating oil may become the future development trend. In order to analyze the differences and reasons of DC breakdown characteristics of LDPE (low density Polyethylene) under different environments, the DC breakdown experiments of LDPE were conducted in vegetable insulating oil and mineral insulating oil, and the impacts of insulating oil discharge process on the breakdown of LDPE samples were analyzed through simulation. The results show that the electric strength of LDPE in vegetable insulating oil is significantly higher than that in mineral insulating oil. The surface flashover characteristics and breakdown points of the two insulating oils are also different. The field intensity generated by discharge of vegetable insulating oil is small, so the breakdown field of LDPE in vegetable insulating oil is higher. The difference of discharge process of insulating oil is also the reason for the difference of flashover characteristics and breakdown point.

In order to quickly detect the overheating defects of wire insulation materials by using the key parameters of gas derivatives, the relationship curves between the concentration of gas derivatives and the temperature wire insulation materials were obtained through experiments. A thermoelectric coupling simulation model was established to simulate the effects of wire parameters, wire laying mode and ambient temperature on wire insulation temperature. A heat-flow coupling simulation model was established to simulate the effects of overheating defect location, threading tube horizontal length, and gas class on the sensor response time of escaped gas derivatives in the fluid field. The weight of influence degree was determined by orthogonal test. The results show that CH₄, C₂H₂, and CO are the main gases escaping from overheating defects of PVC insulation material. The selection of wire parameters has the greatest influence on insulation material temperature. The type of detected gas has the greatest influence on the response time of sensors.