In order to relieve the conflict between tight land supply and land acquisition for transmission lines in tight land supply area, we developed a new structural type of composite material insulated tower, which looked like a rocket, and the wires passed through the insulated composite material rocket tower to compress the line corridor to the greatest extent. Besides, the electrical performance of the 110 kV composite rocket tower was systematically studied. The results indicate that the dry lightning impulse voltage of the 110 kV composite rocket tower is 979 kV, the impulse voltage under wet condition is 576 kV, the average value of 1 min wet power frequency flashover voltage is 282 kV, and the power frequency pollution flashover voltage at 0.20 mg/cm² of salt density and 1.00 mg/cm² of gray density is 106.2 kV. Compared with the traditional 110 kV suspension composite insulator iron tower, the lightning impulse voltage value of 110 kV composite rocket tower increases by 78%, the impulse voltage value under wet condition increases by 34%, the 1 min withstand wet power frequency flashover voltage increases by 22.6%, and the pollution flashover voltage value is equivalent. In addition, the line corridor is greatly compressed, and the minimum line corridor width is 1.8 m, which is much smaller than the corridor width value (5.5-7.5 m) of 110 kV iron tower.

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 equivalent extended Debye model of oil-paper insulation in the damp state no longer has linear characteristics, which makes the evaluation method of oil-paper insulation based on time-frequency conversion have errors. In this paper, the advantages of time domain and frequency domain dielectric spectroscopy were integrated, and the grey correlation algorithm was used to construct the evaluation model of oil-paper insulation damp condition on the basis of the time and frequency domain dielectric response parameters. Firstly, oil-paper insulation samples with different moisture content were prepared, and the influence of moisture and temperature on the polarization and depolarization current (PDC) and frequency domain spectroscopy (FDS) curves was analyzed. Furthermore, the characteristic parameters of PDC and FDS curves under different damp conditions were extracted. On this basis, the grey correlation algorithm was used to integrate the time and frequency domain dielectric characteristic parameters of oil-paper insulation to obtain its standardized reference vector. Finally, the accuracy of evaluation model was verified by laboratory sample data and field transformer data, which provides a new method for warning and diagnosis of transformer early fault.

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.

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.

Effect of swelling effect of silicone rubber (SiR) caused by silicone grease coated on the interface between high voltage cable accessories and cable body on the electrical tree characteristics of SiR under monopolar impulse voltage (250/2 500 μs) was studied. The isothermal surface potential decay (ISPD) and equilibrium solvent swelling were carried out to characterize the influence of swelling effect on the trap characteristics and crosslinking structures of SiR. The results show that with the increase of swelling time, both the initiation probability and fractal dimension of electrical tree in SiR increase. After 340 h of swelling, the tree initiation voltage of 50% probability under positive and negative impulse voltage decrease from about 37 kV and about -41 kV to 32 kV and about -39 kV, respectively. In addition, the polarity effect can be observed in the electrical tree process of SiR under impulse voltage. The initiation and growth of electrical tree is easier under positive impulse voltage than negative impulse voltage. With the increase of swelling time, the shallow trap density increases, and the deep trap and physical crosslinking density decrease. The destruction of physical crosslinking structure leads to the expansion of free volume, which intensifies the collision ionization processes. Meanwhile, the decrease of deep trap density enhances the charge transport processes. Multiple physical processes intensify the fracture of molecular chains, which furtherly weakens the resistance to electrical tree of SiR. The polarity effect of electrical tree in SiR under impulse voltage is mainly caused by the different distribution of hole-type and electron-type traps.

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.

Compared with the traditional cable joint, the cable fusion joint has the advantages of no stress cone and no movable interface, which has more promising in the new cable operation project and the old cable reconstruction and maintenance project. In order to evaluate the performance of fusion joint after long-term operation in the power grid, the fusion joint was conducted electro-thermal-mechanical coupling simulation under the operation of power grid. The stress distribution and temperature distribution of cable joint were studied through changing the effective value of operating current and considering the short-time overload and material parameter difference between new and old insulation layer. The simulation results show that the stress distribution at the interface between the old and new insulation layer is uneven when the material parameter difference between the new and old insulation layer increases. The increase of operating current will not only increase the overall temperature of fusion joint, but also increase the stress of insulating layer. The radial stress of short-time overload operation for 1 hour is about 4 times bigger than that of steady-state operation, the axial stress is about 3 times bigger than that of steady-state operation, and the core temperature is about 80 K higher.

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.

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.