Sha Yanchao

Effect of interface on space charge behavior in multi-layer oil-paper insulation

Oil-paper is used as the main insulation materials in the convert transformer, one of the vital equipment in HVDC power system. The characteristics of space charge in oil-paper insulation are complex, so the research on space charge phenomenon in converter transformers is instructive on insulation design. This paper reports an investigation into the space charge accumulation and dissipation in a two-layer oil-paper insulation system under negative and positive voltages. Planar samples have been subjected to DC electric stresses for extended periods of time and the space charge distribution has been measured by the pulsed electroacoustic (PEA) technique. The influence of the interface between oil-paper layers on space charge dynamics has been analyzed, so does the influence of the polarities of the applied voltage. The test results showed that both the interface and the polarity affected the behavior of space charge. The interface between two oil-paper layers acted as a barrier to both negative and positive charge, as a result charge accumulated adjacent to the interface. The polarity of applied voltage played an important role in determining the interface charge accumulation, since the charge accumulated at the interface was in accordance with applied voltage, namely negative voltage for negative charge and positive voltage for positive charge. Because of the interface, the space charge dynamics in two-layer samples were slower than single layer paper and the interface trapped charge made charge around it dissipated slowly.

Electrical tree initiation and propagation in silicone rubber under long-term AC aging

Electrical tree break-down is the major failure problem in cable accessories, however, mechanisms of the electrical tree initiation and propagation are still lack of deeply research, especially in silicone rubber (SIR) which is widely used as insulation material for cable accessories. Long-term aging (0- 1000 h) experiments under the different voltages were conducted to observe electrical tree initiation and propagation processes. In trees initiation stage, with increasing of applied voltage, electrical tree initiation probability becomes larger, while tree initiation time dramatically decreases. The experiment observations demonstrated four shapes of trees which are dominated. As applied voltage increasing, the numbers of electrical trees life-cycles increased and exponential relationship was found between tree length and magnitude of applied voltage. It is noted that charges, whose energy exceeding a certain critical value, destroy microstructure of dielectric accumulatively via injection and extraction, and cause electrical trees initiate when it accumulates to a visible extent.