Study On Electrical Treeing Characteristics In Cross-linked Polyethylene

Since the XLPE cables are widely used in power system, the dielectric breakdown characteristics of XLPE cables are of great concern for the security and stability of power system. Electrical treeing is one of the major breakdown mechanisms for solid dielectrics subjected to high electrical stresses, so it's of great importance to study the electrical treeing characteristics and mechanisms in XLPE cable insulation for the improvement of safety and reliability of power system. In this paper, based on the current research works, the electrical treeing characteristics in XLPE cable insulation are experimentally and theoretically studied.Treeing process in XLPE samples subjected to various voltages, frequencies and pin-plane spacings are uninterrupted recorded by an online microphotograph system. The radial extent length and spatial fractal dimension of electrical tree structures, and the time series fractal dimension of tree growth rate are computed from the experimental results. The investigations of treeing characteristics are divided into three parts:electrical treeing phenomenon, spatial characteristics of electrical treeing and time characteristics of electrical treeing.The phenomenon in the three steps of electrical treeing:initiation, propagation and breaking down are separately described and analyzed. Tree growth rate and fractal dimension under different voltages, frequencies and distances between electroces are found out. It can be concluded that the influences of voltage, frequency and pin-plane spacing on treeing characteristics are full of regularity. Trees in different structures have the similar initiation structure. With the larger voltage and shorter pin-plane spacing, the time for initiation was longer and less dispersed. The influence of frequency on initiation is relatively small. Two types of breakdown were observed in experiments:the breakdown happened when the tree was closely to the plane and the breakdown happened right after the voltage was applied. The latter type of breakdown was observed when the voltage was higher, and the trees have the similar lateral propagation structure before breakdown. The influences of voltage, frequency and pin-plane spacing on tree propagation are full of regularity. Under the same frequency, with the increasing voltage, the tree structures changed from sparse branch-type to dense branch-type and then to bush-type, and the corresponding fractal dimension and growth rate increased. Under the same voltage, with the increasing frequency, the tree structures changed from sparse branch-type to dense branch-type and then to vine-type, and the corresponding fractal dimension and growth rate increased..Based on FDTG (Field Driven Tree Growth) model, the structure characteristics and spatial growth rate characteristics of electrical trees are investigated. It can be found in the structure analysis that the local electric field and frequency regularly affect the tree structure. Bush-like trees only formed under higher voltages, and the electric field in a very small zone near the needle tip is an important influencing factor for the formation of bush-like trees. Pure vine-like trees formed only under high frequencies, and the lowest frequencies that pure-vine-like trees appear increased with voltage. For double-structure trees, the local electric field at the transition location almost stayed constant under different voltages and pin-plane spacings, but obviously increased with frequency. During the spatial growth rate analysis, the growth rate curves were divided into three regions according to the values, and the local electric field at the dividing line were computed, which can be seen as the critical electric field EC.It can be concluded that the change tendency of dividing lines is coincident with the change tendency of local electric field. Higher voltage, higher frequency and smaller pin-plane spacing all could result in the narrower region of slowly growth. The critical electric fields Ec increased slightly with voltage, and stay almost the same with frequency and pin-plane spacing.Based on electrical treeing kinetic model, the time characteristics of electrical trees are investigated. According to the influencing mechanics of frequency on electrical treeing process and the existing electrical treeing kinetic model, an improved electrical tree growing time model has been built, in which tree growth time depends on electric field and frequency. A new parameter:energy threshold, which is derived from the improved electrical tree growing time model, was introduced and computed from the experimental results. Calculating results of energy threshold regularly change with different conditions and structures. The change trend of energy threshold under different frequencies according to the model are similar to the calculating results from the experiments, which means the consideration of frequency in the improved model is correct. According to the experimental results of all time points in all of the electrical trees, using the dynamic electric field calculating equation, the energy threshold and the corresponding local electrical field are calculated, and two important parameter of the growth time calculating model are calculated by curve fitting. Finally according to the improved growth time calculating model, electrical tree growth time can be calculated from frequency, voltage, pin-plane spacing, tree growth length and fractal dimension.