In order to study the ablation mechanism of buffer layer of corrugation aluminum (Al) sheath high voltage cross-linked polyethylene (XLPE), firstly, we analyzed a ablation failure of 110 kV XLPE insulated cable, and proposed that the concentrated radial current was the cause of ablation failure. Secondly, a simulation model of the faulty cable was established. The current density and its distribution of the contact part of buffer layer and corrugated Al sheath were simulated, and the simulation results were proved by model experiment and calculation. It was found that the size and distribution of radial current were affected by the embedded depth of Al sheath in the buffer layer and the volume resistivity of the buffer layer. Finally, simulation experiments were designed to prove that the radial current concentrated was one of the causes of buffer layer ablation, and the experimental conditions of the simulated ablation experiment were controlled in the thermostat. The surface morphology of the experimental sample and the ablated Al sheath were compared by optical microscope. The results show that the ablation process in the ablation experiment is the same as that in the actual faulty cable, and with the increase of the current density in buffer layer, the ablation start time decreases. This paper reveals the physical mechanism of radial current concentration in the ablation fault of corrugated Al sheath, which provides guideline for relevant fault diagnosis and protection.

Partial discharge inception voltage (PDIV) under repetitive pulse voltage is an important parameter to evaluate the insulation performance of low-voltage random-wound inverter-fed motors. Different from the traditional sinusoidal voltage, the rise time and polarity of the impulse are critical parameters to be considered when PDIV test is carried out on the turn to turn insulation of the winding for inverter-fed motor at repetitive impulsive voltages. In this paper, the influence of repetitive impulsive voltage rise time on the PDIV and energy distribution features of turn to turn insulation for inverter-fed motors with three specification were investigated at repetitive impulse voltages with different impulse rise times (50 ns, 75 ns, 100 ns, and 200 ns), and the mechanism of variation of PDIV with different rise times was discussed. The results show that with the decrease of rise time, the PDIV of three motors decreases, and the maximum decline rate reaches 28.1%. The influence of repetitive pulse polarity on the PDIV test results of low-voltage random-wound inverter-fed motor is limited, indicating that both positive and negative can objectively reflect the PDIV performance of motor. With the increase of impulse rise time, the discharge energy in frequency domain moves from high frequency to low frequency. The research results are expected to provide reference for PDIV test of low voltage inverter-fed motor insulation and the revision of related standards.

It is increasingly urgent to the demand of ultra-high voltage direct current (UHVDC) cable equipment because of the implementation of UHVD transmission project. In order to study the structural design of UHVDC cable joints, we start from the influence of nonlinear conductivity characteristics of polymer insulation materials on the insulation electric field distribution of DC cable joints to analyze the effect of modified silicone rubber on the optimal design of the insulation structure of UHVDC cable joints. The results show that the electric field in the reinforced insulation of DC cable joints will reverse polarity with the increases of the cable load current. The nonlinear conductivity of the silicone rubber is greatly enhanced by doping copper calcium titanate nanofibers inorganic filler, the activation energy and electric field strength coefficient of the silicone rubber are improved, and the threshold electric field strength of conductivity entering the nonlinear region is reduced. The distortion of tangential electric field of double-layer dielectric interface and surface electric field of stress cone and high-voltage shielding tube in the UHVDC cable joint are obviously suppressed by the modified silicone rubber reinforced insulation material. The modified silicone rubber, used as the reinforced insulating material for UHVDC cable joints, can make up for the insufficient that the field distortion in the joint cannot be suppressed effectively by adjusting the structural size of cable joints. The research results can effectively solve the electric field control problem in the design of UHVDC cable joints.

The main negative effect of transformer insulation ageing is the decline of mechanical properties. Once the winding coil is impacted by external short circuit, it will probably produce deformation, which will lead to the loss of insulating ability and eventually cause transformer fault. A mechanical analysis model of insulating paper materials was established according to their structural characteristics on the basis of material mechanics. Combined with the ageing mechanism and influencing factors of insulating paper materials, a test platform for the mechanical properties of insulating paper materials was built. The mechanical properties of insulating paper materials under different ageing degrees were studied to obtain the ageing mechanical properties of insulating paper materials. The results show that the plasticity of the insulating paper material basically disappears under repeated cyclic loading, and the insulating paper under different ageing times can be approximately simplified as an elastic material.

According to the structure of cable buffer layer, a partial discharge test platform with controllable humidity environment for buffer layer was established. Three typical defect models were designed and conducted partial discharge tests by pulse current method. The results show that the variation of relative humidity has little effect on the geometric shape of partial discharge pattern, while has large effect on the characteristic parameters of partial discharge. With the increase of humidity, the discharge amplitude of three models decreases at first and then increases. Under constant humidity and voltage, the partial discharge activity in the needle-plate system is higher than that in the other models. Under 180 V of voltage and 90% of humidity, the maximum pulse amplitude reaches 770 mV.

In order to better use the repetitive surge oscillograph (RSO) method to quantitatively evaluate the turn-to-turn insulation defects of generator rotor winding, we conducted turn-to-turn short circuit simulation tests under different positions, different short circuit coils, and different short-circuit resistance on a real rotor when the generator was being repaired. The response waveform and characteristic waveform of the two poles were obtained by the RSO device. Three characteristic parameters that is voltage deviation rate, time deviation rate, and fault severity factor were proposed to study the characteristic waveform. The results show that RSO can be used to quantitatively evaluate the severity of turn-to-turn insulation defects of rotor, the fault type of rotor winding can be quickly determined according to the amplitude and deviation rate of voltage of characteristic waveform, and the fault location can be basically realized according to the time deviation rate. The voltage amplitude of the RSO characteristic waveform has a linear positive correlation with the number of short-circuit coils, and has an exponential relationship with the short-circuit resistance and the center time, respectively. The fault severity factor has a linear positive correlation with the number of short-circuit coils. According to the analysis results, two methods to quantitatively evaluate the severity of rotor turn to turn insulation defects are proposed, which can be used to accurately evaluate the turn-to-turn insulation status of rotors.

A detection platform of insulation partial discharge (PD) under pulse voltage was constructed by using electromagnetic detection method with the help of a high-voltage pulse power source with adjustable voltage parameters of repetitive pulse. The influence law of frequency variation of repetitive square wave voltage on the PD statistical characteristics of single point and multipoint interturn insulation discharge model for inverter-fed motor was studied. The results show that with the increase of frequency, the amplitude of both insulation single point and multipoint discharge decrease significantly. This may be caused by composite factors such as the change of residual charge distribution on insulation surface, the increase of the excitation probability of initial electron, and the increase of surface temperature after discharge. According to the research results, the influence of square wave pulse voltage frequency on the PD pulse should be carefully considered in the PD detection of motor insulation, and the appropriate voltage frequency should be selected to improve the sensitivity and test accuracy.

The influence of sampling period, voltage rising speed, and overcurrent breaking time on the breakdown voltage of insulating materials in the process of breakdown voltage test was analyzed. Proper improving technology and method was select to eliminate or reduce the error. The results show that the dispersion of test results is reduced by digital filtering, weighted average, and other methods, and the accuracy of dynamic measurement is improved at the same time.

The fault type recognition and condition assessment play the crucial roles in fault diagnosis and maintenance. Different defects will produce partial discharge signals with difference, and the partial discharge signal also changes with the defect severity and the evolution of partial discharge. This situation can be regarded as pattern recognition of different severity levels and evolution stages, and the pattern recognition is a typical classification problem. In this paper, classification problems such as pattern recognition and state assessment were reviewed. Compared to classification results based on mathematical statistics, artificial intelligence has achieved nearly 100% of recognition accuracy. However, there are still some shortcomings in current research, this paper gives some solution strategies and prospects future research direction.

High voltage power cables demand stable and reliable connections with the power system through cable accessories, and silicone rubber (SIR) and ethylene-propylene-diene monomer (EPDM) are the main reinforced insulation materials for cable accessory due to their excellent dielectric properties and mechanical properties. However, with the continuous improvement of high voltage transmission level, the electrical properties of cable accessories also need further development. In this paper, the research status inside and out of China was summarized, then the factors affecting the electrical properties of SIR and EPDM for high voltage cable accessories were discussed. The research progress in improving method of electrical properties were analyzed, and the research progress in the interfacial electrical properties of double-layer dielectric was introduced. At last, the research difficulties of electrical properties modification of rubber reinforced insulation for high voltage cable accessories were summarized, and the research trends of the rubber insulating materials for high voltage cable accessories were prospected.