Hucheng Liang

Surface Layer Fluorination-Modulated Space Charge Behaviors in HVDC Cable Accessory

Space charges tend to accumulate on the surface and at the interface of ethylene–propylene–diene terpolymer (EPDM), serving as high voltage direct current (HVDC) cable accessory insulation, which likely induces electrical field distortion and dielectric breakdown. Direct fluorination is an effective method to modify the surface characteristics of the EPDM without altering the bulk properties too much. In this paper, the surface morphology, hydrophobic properties, relative permittivity, and DC conductivity of the EPDM before and after fluorination treatment were tested. Furthermore, the surface and interface charge behaviors in the HVDC cable accessory were investigated by the pulsed electroacoustic (PEA) method, and explained from the point of view of trap distribution. The results show that fluorination helps the EPDM polymer obtain lower surface energy and relative permittivity, which is beneficial to the interface match in composite insulation systems. The lowest degree of space charge accumulation occurs in EPDM with 30 min of fluorination. After analyzing the results of the 3D potentials and the density of states (DOS) behaviors in EPDM before and after fluorination, it can be found that fluorination treatment introduces shallower electron traps, and the special electrostatic potential after fluorination can significantly suppress the space charge accumulation at the interface in the HVDC cable accessory.

Effects of nonlinear conductivity on interface charge in HVDC cable accessories

Interface charges are easy to accumulate between two different dielectrics with various characteristics, which may cause accelerated degradation of insulation systems. Ethylene-propylene-diene terpolymer (EPDM) is used mainly for HVDC cable joint, which is the most vulnerable part of the cable system because of the interface. Particles with nonlinear conductivity can be doped into the polymer matrix to modify the interface charge behaviors through altering the conductivity under combined stresses. In this paper, silicon carbide (SiC) particles were dispersed into EPDM with 0, 10%, 30% and 50 wt% respectively. The space charge behaviors at the interface between LDPE and EPDM filled with SiC particles was measured under 15 kV/mm. The permittivity increases with the fillgrade and the dc conductivity shows an obvious nonlinear trend under various electrical fields. SiC doping can effectively suppress the interface charge accumulation. The suppression mechanism is attributed to the nonlinear conductivity. As a consequence, the approximate SiC doped EPDM can availably suppress the interface charge accumulation and offer a possible method for the improvement of cable accessory performance.

Ageing estimation of heat-shrinkable material based on analytic hierarchy process

With the continuous expansion of the power system, security and stability of the power distribution network is particularly important. As the power distribution equipments main insulation, heat-shrinkable material has caused great concern of the department of operation and maintenance. Based on plenty of experimental results of the heat-shrinkable material with different operating time, the elongation at break, breaking strength, dielectric constant, dielectric loss and breakdown field strength were selected as the ageing assessment parameters, and an improved evaluation model with assessment parameters and ageing index was established. The analytic hierarchy process method was employed in this paper to reduce single parameter error and improve the accuracy of the prediction model, combined with the sensitivity and correlation of the characteristic parameters. With assessing the ageing index of an actual case, it is shown that the model is feasible and effective to provide a reference for optimizing maintenance cycle of heat-shrinkable material.

Surface charge accumulation and decay of double-layer oil-paper insulation under DC and pulse voltages

An oil-paper insulation system is a key component of converter transformers as well as high voltage direct current (HVDC) bushings. It withstands not only DC voltage under normal conditions, but also impulse voltage when operating overvoltage appears. So this paper studies the surface charge behavior of double-layer oil-paper insulation under both DC and pulse voltages. From the research of different pulse voltage amplitudes, pulse numbers and polarities, conclusions can be drawn that, with the increase in pulse voltage amplitudes, DC and pulse voltages in the same polarity cause the absolute value of the initial surface potential to decrease first and then rise; while in different polarities, the charge with the same polarity as the pulse voltage increases. Considering the pulse number effect, it is obvious that the initial value increases with the increase in the pulse number, and then reaches saturation. Also, with the increase in pulse number, the decay rate decreases.