In this paper, a photoelectric joint detection system composed of a photomultiplier tube and an electric field sensor was used to carry out the discharge tests of ball-plate electrode gap with burr of different gap distances under positive switching impulse voltage. Compared with the discharge tests of the rod-plate and that of ball plate, the 50% discharge voltage(U_50%) and the physical characteristic parameters under the initial streamer were obtained and compared. The results show that in the ball-plate electrode gap with burr defects, the initial delay, the field strength jump value, and the light power jump value of the initial streamer are positively correlated; in the initial stage of streamer, the burr can shorten the initial delay of discharge, especially for short gap. After the initial streamer developing, the development law of initial streamer of ball-plate electrode with burr is similar to that of rod-plate electrode gap, and the burr aggravates the development of discharge. For the whole discharge process, the burr defect will greatly reduce the discharge voltage of the ball-plate electrode, so that it is close to the discharge voltage of the rod-plate electrode gap under the same clearance.

The interface pressure between the cable accessories and the body is the key to the stable operation of the cable. In order to study the influence of long-term cooling and heating cycles on the interface pressure of cable accessories, we designed a test system to simulate the operation state of cable using the temperature control mode of cooling and heating cycles. The dynamic thermo-mechanical analysis of silicone rubber materials before and after the experiment was carried out. The results show that the elastic modulus of silicone rubber material under long-term cooling and heating cycles increases significantly at the test temperature. In addition, with the increase of ageing time and ageing temperature, cable accessories deform irreversibly, resulting in the decrease of interface pressure and the sealing performance. The experimental results are verified by finite element simulation of interface pressure based on cable accessory parameters.

Using bipolar repetitive square wave voltage, with the help of ultra-high frequency testing method, the influence of the voltage amplitude, frequency, duty ratio and rise time of repetitive square wave on the frequency domain energy distribution of inverter-fed motor inter-turn insulation discharge was experimentally studied. The results show that that the frequency domain energy of discharge above 0.50 GHz increases significantly with the increase of duty ratio; the energy of each frequency band increases with the increase of the pulse voltage amplitude, and the high-frequency energy above 1.50 GHz continues to attenuate with the increase of frequency; the rising rate of pulse voltage has a significant impact on the energy of the discharge frequency domain. The high-frequency energy of the discharge pulse at 1.00-2.00 GHz increases with the increase of the voltage rising rate. When designing the UHF sensor for PDIV test of inverter-fed motor insulation under high-frequency and steep pulse voltage, the influence of the above-mentioned pulse parameters on the energy distribution in frequency domain should be fully considered to improve the sensitivity and signal-to-noise ratio of the PDIV test.

In this paper, the theoretical basis for ageing diagnosis of XLPE cables based on the electromagnetic wave velocity in cables was elaborated, and the calculating method of average wave velocity according to input impedance spectrum was introduced. Then the effect of uniform ageing and segmented ageing of cables on average wave velocity was studied by simulation, and a method to diagnose cable ageing on the basis of average wave velocity was proposed. Finally, cable models including uniform thermal ageing and segmented thermal ageing were built in the laboratory, and the effectiveness of the above diagnosing method was verified. The results show that the average wave velocity of the accelerated uniform ageing cable and the segmented ageing cable decreases from 166 m/μs to 139 m/μs and 152 m/μs, respectively. The more serious the ageing of cables, the lower the wave velocity in cables. The proposed method can provide more information for ageing diagnosis of XLPE cables.

In order to study the failure mechanism of water-blocking buffer layer in high voltage power cable, especially the electrochemical corrosion characteristics, we analyzed the composition of the white powders from failed cables, and confirmed the production mechanism of different composition in the white powders combining with theoretical analysis and simulation experiments. The moisture absorption characteristics of the buffer layer was tested, and the influence of the water content and current density of the buffer layer on its electrochemical corrosion characteristics was studied. Finally, based on electrochemical impedance spectroscopy, the influence of electrochemical corrosion on the electrical characteristics of the buffer layer was identified, and the evolution rule of the buffer layer from electrochemical corrosion to final ablation was determined. The results show that moisture ingress due to dampness or mechanical damage of the cable sheath causes electrochemical corrosion of the buffer layer and leads to the generation of white powder (consisting of NaHCO₃ and Al(OH)₃) and results in the final failure of the cable. The corrosion rate is closely related to the current density, while the final corrosion degree mainly depends on the moisture content of the buffer layer.

In order to solve the problem that the cable capacity much lower than the design standard due to the high thermal resistant environment in the running process of duct cable laying, this paper developed the cable pipe materials that graphene microchip and graphite flake modified HDPE (PG-GNPs-HDPE) with high thermal conductivity, then established a simulation model and verified the improving effect of using the high thermal conductive pipeline of PG-GNPs-HDPE under the common duct laying condition of the current urban distribution network. The results show that compared with the ordinary HDPE pipe, the carrying capacity of 110kV 500mm² cable group with 2×4 laying is increased by 9.32% after using PG-GNPs-HDPE high thermal conductivity pipe; for 10 kV 3×400 mm² cable group with 3×4 laying, the carrying capacity increases by 7.42%.

In order to explore the influence of high voltage cable oil filled terminal grounding system defects such as lack of copper net and lead seal defects of on the voltage and current of oil filled terminal, we built an RLC equivalent model of oil filled terminal. On the basis of capacitance parameters and test parameters computed by COMSOL, the change of terminal voltage and current characteristic under the defects was analyzed, and then it was verified by actual fault cases. The results show that the loss of copper net leads to the grounding failure of the outer semi-conductive layer, and the voltage of the outer semi-conductive layer increases significantly, results in the discharge. The lead seal fracture leads to the unreliability of the connection between the aluminum sheath and the tail pipe, results in the voltage suspension of terminal aluminum sheath, and causes the discharge between the aluminum sheath and the buffer layer under long-term operation. Defects between the lead seals reduce the contact area between the aluminum sheath and the lead seal, the minimum allowable contact area at the lead seal is about 10%.

Natural ester insulating oil is a kind of environmentally-friendly liquid dielectrics with good insulation properties, and it has been generally applied in oil-filled electrical equipment. Investigating the types and contents of fatty acids in different natural ester insulating oils will be beneficial to grasp the physical and chemical properties of different insulating oils. In this paper, a method of determining the fatty acid composition in natural ester by ¹H-NMR was presented, and the results were compared with that of GC-MS. The results show that this simple and rapid determination of fatty acid content in natural ester insulating oil by ¹H-NMR is feasible and reliable. The maximum error absolute value of fatty acids content measured by ¹H-NMR spectroscopy and GC-MS is 2.95%, which is in the tolerance interval.

During the operation of high-voltage DC cables, space charge generates in the insulation layer, it leads to electric field distortion and even insulation breakdown. In order to study the effect of space charge in the actual operation of cable, we obtained the temperature field of high-voltage direct current submarine-land composite cable under the conditions of rated current, maximum steady-state current, and short-term overload current, respectively. According to the simulation results, the temperature difference in the space charge test was set as 10℃, 20℃, and 40℃, respectively, and then the temperature field of 400 kV high-voltage DC cable under different condition was simulated. The results show that the temperature difference between the two sides of XLPE insulation layer is 8.1℃ and 18.7℃under rated current and the maximum steady-state current, respectively; while the temperature difference is 61.1℃ under the short-term overload for 1 hour; and that is 41.1℃ under short-term overload for 36 hours. The experimental results show that the temperature gradient enhances the accumulation of heteropolar charge at the low temperature side of positive pole of XLPE, which leads to the distortion of electric field. Through the correction computation of the electric field distortion rate, it is found that the maximum electric field distortion rate has a linear relationship with the average electric field strength. With the increase of temperature gradient, the electric field distortion rate also increases. The maximum electric field distortion rate is 1.68 under the temperature gradient of 40℃ with the applied electric field of 50 kV/mm.

In this paper, the current research status of partial discharge (PD) measurement using non-sinusoidal voltage waveforms at home and abroad were summarized, the PD characteristics under non-sinusoidal voltage waveforms such as continuous pulse square-wave voltage, triangular-wave voltage, and trapezoidal-wave voltage were reviewed, especially the effects of voltage rise time, frequency, and duty cycle of square-wave voltage on the PD activity were discussed deeply. Finally, the great significance and existing problems of PD measurements using non-sinusoidal voltage waveforms were discussed. Future development of PD measurements using non-sinusoidal voltage waveforms was proposed, and how to use the non-sinusoidal voltage waveforms to enhance the understanding of discharge physical process and insulation ageing mechanism were prospected.