Electrical tree is a common phenomenon in XLPE cable fault, and there may be voltage fluctuation during cable operation. However, there are few reports on the influence of voltage amplitude on the growth morphology and partial discharge characteristics after electrical tree is initiated. In this paper, the difference of growth morphology and partial discharge characteristics under different voltage amplitudes after electrical tree initiated was studied. The experimental results show that when the applied voltage is higher, the electrical tree morphology tends to develop into a cluster tree, and when the applied voltage is lower, the electrical tree morphology tends to develop into a dendritic tree or a single branched tree. The maximum partial discharge intensity test at higher voltage can be used to predict breakdown fault, but it is difficult to predict the growth of electrical tree at lower voltage through partial discharge intensity. The maximum discharge interval of electric tree becomes longer with the increase of voltage applied time, and the intermittent discharge is more obvious. The discharge interval of electric tree is closely related to the growth of electrical tree. The intermittent partial discharge occurs only when the growth of electrical tree is stagnant, and the partial discharge hardly stagnates when the electrical tree is growing.

In order to study the influence of typical defects at the silicone rubber/cross-linked polyethylene (SR/XLPE) composite interface of cable terminal on the electric field, temperature field, and stress field distribution, a 10 kV cable termination simulation model was established by using COMSOL Multiphysics simulation software. The electric-thermal-force field distribution with metal particles, semiconducting particles, and scratch defects on the composite interface was simulated. The results show that the electric field, temperature field, and stress field on the interface defect area are distorted to different degrees when metal particles or semi-conductive particles exist on the composite interface of cable terminal, and the influence of metal particles on the physical field distribution at the interface is more obvious. For the air gap defect caused by the scratch of XLPE at the truncation of outer semiconductive layer, it is found that the electric field at the interface air gap defects is distorted and local hot spots are generated. The coating of silicone grease on the interface can obviously improve the distortion of electric field and temperature field at the scratch, but the stress distribution in the scratch area is not uniform due to the filling of silicone grease, and the stress is high at both ends and is low in the middle.

Partial discharge test has been widely used as an effective means of defect detection in AC system, but the partial discharge detection test method of DC cable is still in its infancy. In order to determine the field test voltage of DC partial discharge detection, the relationship between DC test voltage amplitude and air gap defect detection rate of ±320 kV cable prefabricated terminal was studied in this paper. Taking the air gap defect commonly existing on the composite interface of ±320 kV cable prefabricated terminal as the research object, we calculated the maximum electric field intensity in the air gap through multi physical field simulation, and whether the defect may produce partial discharge was judged by the electric field intensity exceeding the threshold. Based on the probability and statistical analysis, the partial discharge probability density function of cable joints under the condition of considering random factors such as cable operating temperature and air gap position was established, and the function equation between defect detection rate and ambient temperature and DC test voltage was presented. The results show that the function equation between the detection rate of air gap defect at the cable interface and the ambient temperature and DC partial discharge test voltage conforms to the fourth power function, and the detection rate increases with the increase of ambient temperature. If the DC test voltage is 1.85U₀, the detection rate is 99.25% at 20℃, and the detection rate can reach 99.85% at 30℃. The DC test voltag amplitude can be determined by the function equation.

In order to investigate the effects of glassfiber architecture on the mechanical and fire-resistant performance of insulating slot wedge, unidirectional glassfiber, laminated glassfiber fabric, and rolled glassfiber fabric reinforced slot wedge were prepared using fire-resistant resin, glass fiber, glassfiber fabric as raw material, and the 3 types of slot wedge samples were conducted longitudinal impact strength test, width-directional bending strength test, and combustion tests. The results show that with the same sectional dimension and the same content of glassfiber for slot wedges, the order of longitudinal impact strength from strong to weak is glassfiber fabric reinforced slot wedge, glassfiber fabric laminated slot wedge, unidirectional glassfiber reinforced slot wedge; the order of elastic deformation during width-directional bending from big to small is glassfiber fabric reinforced slot wedge, glassfiber fabric laminated slot wedge, unidirectional glassfiber reinforced slot wedge; the order of fire resistance from strong to weak is glassfiber fabric reinforced slot wedge, glassfiber fabric laminated slot wedge, unidirectional glassfiber reinforced slot wedge. When the glassfiber content increases from 50% to 70%, the total afterflame time of the unidirectional glassfiber reinforced slot wedge, glassfiber fabric laminated slot wedge, and rolled glassfiber fabric reinforced slot wedge increases by 51.7%, 31.8%, and 46.3%, respectively.

A new method based on S-transform and time-frequency domain reflection (TFDR) was proposed to locate the local defect of power cable. The principle of traditional cable defect location method was analyzed firstly. The time-frequency distribution of the TFDR test waveform was obtained by S-transform, and the time-frequency cross-correlation function of time-frequency distribution was calculated. Then the location of cable local defect was estimated by the local peak of the time-frequency cross-correlation function curves. The results show that compared with the traditional TFDR methods, there is no cross-term interference problem in the time-frequency distribution for the proposed method, so the local peak of interference in the time-frequency cross-correlation function curve is eliminated, and the reliability of cable defect location results is high and the detection blind area of local defect is small. Finally, the simulation and measurement analysis were carried out on the 10 kV cross-linked polyethylene (XLPE) cable, and the results show that the proposed method can effectively locate the local defect of cable and the absolute error of defect location is small.

The lightning impulse 50% breakdown voltage (U_50%) of the insulators used on the traditional iron cross-arm is relatively low, which leads to lightning breakage of the insulated conductor. Therefore, the composite insulated cross-arm, with good insulation performance, high strength, and light weight, has received significant attention and can help to improve the lightning resistance level of distribution network. In order to further study the lightning protection performance of the composite insulated cross-arm, lightning impulse flashover tests were conducted at different altitudes to obtain the U_50% and volt-second characteristic curves of the 10 kV composite insulated cross-arm. On the basis of the tests, a lightning strike simulation model of 10 kV lines was established by using the ATP-EMTP software to calculate the lightning resistance level of the composite insulated cross-arm. The results show that at an altitude of 22 m, the positive and negative polarities of U_50% of the composite insulated cross-arm are 415.43 kV and 589.14 kV, respectively, while those of the pin insulator are 151.05 kV and 200.77 kV, respectively. At an altitude of 2 100 m, the lightning resistance level of the composite insulated cross-arm decreases by about 20%, but it is still much higher than that of the pin insulator. Therefore, the composite insulated cross-arm shows excellent lightning protection performance and has broad application prospects.

The problem of nanoparticle agglomeration in wire enamel can be solved by modification and gradual dispersion of nanoparticles. Five modifiers, emulsifier OP-10, silane coupling agent AP1231, silane coupling agent A112, γ-propyl trimethoxysilane (KH560), and N-[3-(trimethoxysilyl) propyl] ethylenediamine (KH-792), were used to modify the surface of nano silica particles. The effects of modifier type, modifier dosage, modification temperature and time on the modification effect of nano-SiO₂ particles were explored. The effects of nano-SiO₂ particles before and after modification on the performance of corona resistant wire enamel were compared by corona resistance test, and the dispersion of nano-SiO₂ particles before and after modification was observed by transmission electron microscopy (TEM). The results indicate that the nanoparticles modified by KH-560 have the best dispersion in wire enamel, and the best process conditions are as follows: the dosage of KH-560 is 1.5wt%, the modification temperature is 90℃, and the modification time is 4 h. The wire enamel with high stability and higher than 50 h of corona resistance time can be obtained by gradually dispersing modified nanoparticles into the wire enamel, and the appropriate addition amount of nanoparticles is 13.5wt%.

With the improvement of ecological environment, transmission line faults caused by birds are also increasing. Among them, the tripping accident of transmission lines caused by bird has become an important factor threatening the safe operation of power grid. Based on this, we explored the influence of the thickness and length of insulating sheath on the breakdown voltage and surface flashover voltage respectively through tests. Then the configuration method of the length and thickness of insulating sheath was put forward. Finally, a real test platform simualting the the flashover of bird droppings was established to verify the feasibility of the configuration method. The results show that when the bird droppings penetrate the entire air gap, the breakdown voltage of the insulating sheath is a quadratic function of its thickness. The surface flashover voltage is proportional to the flashover distance. The configuration method proposed in this paper can provide guidance for the application of insulating sheath in practical engineering and has engineering practical value.

The accumulation of space charge is easy to cause the internal electric field distortion of cross-linked polyethylene insulation, resulting in insulation ageing and even failure. Nano-doping or modification can inhibit the accumulation of space charge, so it has received widespread attention. In this paper, the space charge properties and electrical conductivity properties of cross-linked polyethylene composite doped by silica and magnesium oxide nano filler were measured, and the action mechanism of nanofillers was analyzed combined with the theoretical band gap model. The results show that the MgO-doped cross-linked polyethylene composites have higher dielectric constant, lower space charge accumulation, and smaller electrical conductivity compared to silica-doped cross-linked polyethylene composites.

The environment-friendly insulating fluid for transformer has developed rapidly in recent years, insulating liquids such as natural ester, synthetic ester, and modified ester have become the research focus of new insulating material for transformer. With their applications in 110 kV and above transformers, the dissolved gas analysis (DGA) fault diagnosis method of mineral oil filled transformer is no longer suitable for the ester insulating liquid filled transformer. In this paper, a thermal fault simulation system with temperature difference between copper and liquid was designed to simulate the low temperature thermal fault. The soybean, rapeseed, camellia based natural ester, synthetic ester, and palm based modified ester insulating liquids were used as research object, the experimental results of dissolved gas in pure liquid and liquid-paper insulating system under 200℃ and 300℃ low temperature thermal fault were analyzed, and the applicability of the IEC ratio method and Duval pentagon method were discussed. The results show that the IEC ratio method is still applicable to 200℃ and 300℃ low temperature thermal fault diagnosis of natural esters and synthetic ester. The Duval pentagon method can be used to recognize the low temperature thermal fault of natural esters and synthetic ester after adjustment. However, the DGA results of modified ester are inconsistent to those of mineral oil and natural esters, and the Duval pentagon method based on mineral oil is not suitable for the low temperature thermal fault diagnosis of palm based modified ester.