The effect of thermal oxygen ageing on the properties of irradiated ethylene propylene diene rubber (EPDM) materials were analyzed, and its theoretical service life was evaluated by analyzing the change rule of compression permanent deformation rate with time. The results show that with the increase of ageing time and ageing temperature, the irradiated EPDM is aged more and more serious, the carbonyl content becomes higher and higher, and the compression set and compression stress relaxation become greater and greater. The SEM results show that the EPDM degrade seriously during ageing process. According to Arrhenius equation, the theoretical life of the irradiated EPDM is 7.58 years at 50℃.

The key power equipment with the oil-pressboard insulation winding structure is prone to appear the problem of acetylene content exceeding the standard when putting into operation, and then partial discharge and arcing phenomena are produced. Therefore, the point discharge and arcing characteristics of oil-pressboard insulation were studied under switching impulse voltage and superimposing power frequency electrical stress. The discharge and arcing characteristics of different point defects were compared and analyzed, and the influence of the superimposed phase of impulse voltage and power frequency voltage was discussed. The results show that the arcing process of the defect model containing pressboard after breakdown can divide into high-frequency re-arcing stage, stable arcing stage, and re-arcing stage before the voltage restored. Among them, the high-frequency re-arcing stage has higher arcing frequency and lower recovery voltage. Under the switching impulse voltage, the pure oil gap defect model shows multiple breakdown, and the oil-pressboard composite insulation defect model shows that the pressboard breakdown directly and the oil pressboard breakdown successively, and the defect model type affects the discharge characteristics and arcing rule. The superposition phase affects the discharge characteristics and arcing law by affecting the electrical stress magnitude.

To research the strain characteristic and its measurement method of encapsulation insulation for dry-type air-core reactor, the methods of making test model, embedding sensor and strain gauge, and calculating strain were studied. A test model was made and a test system was built, and the internal strain of encapsulation was studied during the process of reducing temperature and applying current at different low temperatures. The results show that the circumferential thermal expansion coefficient of the encapsulation insulation is less than the axial thermal expansion coefficient and there is anisotropy, which makes the circumferential strain larger than the axial strain. During the process of reducing temperature, negative strain is produced by circumferential shrinkage of the encapsulation insulation, which decreases gradually with the temperature decreasing, the decline rate gradually slows down in the same cabin temperature, and the value increases significantly with the increase of temperature decline amplitude in different cabin temperatures. The axial strain is positive when the cabin temperature is higher, which gradually changes to a negative value with the temperature decreasing, and the change law also changes from gradually increasing to gradually decreasing with the temperature decreasing. During the process of reducing temperature, the circumferential and axial strains have negative jumps many times, resulting in shrinkage micro-cracks. The strain change during process of applying current is contrast to the process of reducing temperature, with the increase of temperature, the circumferential strain increases gradually, the axial strain decreases rapidly, and neither strain jumps. By calculating the maximum thermal stress, it is further verified that the circumferential compression stress is larger than the compression strength of the epoxy glass fiber, resulting in the damage of encapsulated insulation.

In order to analyze the influence of natural ester insulating oil on the insulating properties of power transformers, at first, the winding wave processes of a 110 kV natural ester insulating oil and a mineral insulating oil power transformers with the same insulation structure were calculated, and the longitudinal insulation margins of the two transformers were obtained. And then, the winding potential with linear distribution under power frequency and the nonlinear winding potential calculated by wave process were used as excitation sources respectively, the composite electric field of the two insulating oil transformers main insulation were simulated, and their distribution characteristics were compared and analyzed. At last, the main insulation margins of the two insulating oil transformers were calculated by using the whole-region electric flux line scanning method, and in view of the weak main insulation strength of natural ester insulating oil transformer, the structure was improved and the improvement effect was analyzed. The results show that through adding insulation paper boards and adjusting the size of oil ducts, the minimum insulation margin of the improved natural ester insulating oil transformer increases by 15.8%, which meets the requirements of main insulation strength, which provides a reference for developing large natural ester insulating oil transformer and optimizing its structure.

Taking 220 kV rod-shaped suspension composite insulator as an example, we conducted simulation and calculation on a 1/4 tower model for 220 kV line composite insulator by the COMSOL multiphysics software, and the influences of the grading rings with different types and structures on the electric field distribution of composite insulator were studied. The results show that the electric field uniformity effect of semicircular grading ring is slightly weaker than that of the tubular grading ring, and the voltage equalizing effect of bird-proof grading ring is basically the same as that of the tubular grading ring installed at low voltage end. The surface electric field intensity of composite insulator decreases with the increase of pipe diameter of high-voltage end or double-end grading ring, and the decline decreases gradually. When the diameter of high-voltage end grading ring increases, the surface electric field intensity of composite insulator decreases at first and then increases. When the diameter of double-end grading ring increases, the surface electric field intensity at high-voltage end decreases at first, then increases and finally decreases, while the surface electric field intensity at low-voltage end keeps increasing. Increasing the covering depth of grading ring at high voltage end can improve the surface electric field at the middle of the composite insulator, and lead to the increase of electric field at the connecting end, which is not conducive to the safe operation of composite insulator.

Aimed at the early fault of 10 kV high power traction motor end winding insulation layer, the failure mechanism of insulation layer crack damage under complex electromagnetic and mechanical forces was analyzed on the basis of fracture mechanics theory. A three-dimensional analysis model was established by using the finite element method, and the stress-strain states of the insulating layer crack at different positions were calculated numerically by piecewise loading electromagnetic force. At the same time, the stress intensity factor was introduced to evaluate the degree of crack damage expansion. The results show that compared with the insulating layer crack damage at the nose end, the crack expansion speed at the rod joint is faster and the fracture degree is greater. The initial damage state of the crack also has an important influence on the fracture and expansion degree, and the transverse crack is easier to expand.

Firstly, a corresponding simulation model was built on the basis of terminal structure of 220 kV high-voltage dry cable, and combined with the nonlinear conductivity equation of insulating materials, the electric field distribution and temperature distribution in the terminal with different reinforced insulating materials, ambient temperature, and applied voltage were studied. And then, the electric field distributions of the terminal with defects such as misplacement of stress cone installation, surface bulge, and bubbles in the reinforced insulation were compared and analyzed. Finally, the corner shape of stress cone and the distance between stress cone edge and reinforced insulation were optimized, the electric field distribution of the terminal after optimization was analyzed, and the optimal distance between stress cone edge and reinforced insulation was proposed. The results show that the nonlinear silicone rubber insulating material can well homogenize the electric field. The change of external ambient temperature will decrease the temperature difference between the internal core and the external umbrella skirt, and the maximum field strength in terminal increases significantly with the increase of ambient temperature. The stress cone installation dislocation makes the field strength at the three-phase junction increase sharply. The bulge on stress cone surface makes the local field strength in terminal increase sharply. When there are bubbles in reinforced insulation, the bubble size has little effect on the maximum field strength in the cable terminal. By changing the corner shape of stress cone into a circular arc shape, the field strength at the corner decreases by 75.26%. By increasing the distance between stress cone edge and reinforced insulation to 5 mm appropriately, the field strength at the stress cone corner decreases.

On the basis of laser ultrasonic technology, a peak reconstruction algorithm of surface wave was studied, and an experimental platform of two-dimensional laser excitation scanning laser ultrasonic detection for basin insulators was built. Then the artificial bubble, impurity, and crack defects were detected and studied, and the basin insulator with crack fault in practical field application was carried out off-line detection. The results show that the reconstructed peak image based on laser ultrasonic peak reconstruction algorithm can better reflect the defect information of basin insulator in scanning interval, which proves that this method can realize the off-line detection of surface crack defect on the basin insulator.

Aimed at a new type of three-element mixed insulating oil, we developed a mixed insulating oil transformer with high overload capacity. On the basis of the analysis on the heat resistance grade and temperature rise limit of the mixed insulating oil transformer and mineral insulating oil transformer, the temperature rise of insulating oil distribution transformer with different overload capacity was compared combined with the relevant calculation and test detection of the model machine. Some structural optimization suggestions of mixed insulating oil distribution transformer were proposed and the cost was compared. The results show that the overload capacity of oil-immersed distribution transformer can increase from 1.5 times to 1.75 times by using the hybrid insulation system composed of a new type of high temperature resistant ternary mixed insulating oil and DPE insulating paper or Nomex T910 insulating paper, and the corresponding cost increases less than 8%, which has strong practicability and economy.

Due to the low thermal conductivity of XLPE main insulation and low temperature of seawater, it is easy to form a large temperature gradient in submarine cable insulation. The formation of temperature gradient will cause a radial difference in the aggregation and dielectric properties of XLPE, so as to influence the electrical treeing deterioration process. In order to study the electrical tree characteristics of XLPE under temperature gradient, a temperature gradient electrical tree experimental platform in the range of 10-90℃ was built, and the initiation and growth characteristics of electrical tree under different temperature gradient were measured. The results show that the changes of needle tip electric field and XLPE aggregation under different temperature gradient will affect the initial voltage of electrical tree. With the increase of temperature gradient, the main structure of electrical trees presents gradual change characteristics of rattan-branch tree, bush-rattan tree, and bush tree.