Study On Accumulative Effect Characteristics Of Oil-paper Insulation In Transformer Under Different Impulse Waveforms

Oil-paper insulation in power transformers subjects to repeated lightning impulse voltages within the lifespan, which threatens the safety operation of transformers. Due to the attenuation, reflection, and part winding resonance in the transformers, the actual lightning impulse waveform in transformers is usually bidirectional and oscillating, which is quite different from the standard lightning impulse waveform(SLIW) used in the traditional withstand tests. It is essential to identify the breakdown characteristics of oil-paper insulation under non-SLIW and to quantitatively compare them with those under SLIW. On that basis, the impulse insulation levels of power transformers can be rationally determined and then the insulation specifications can be evaluated to maintain high equipment reliability. In view of this, typical impulse waveforms are defined according to the measurement results of on-line monitoring system in substations. The breakdown characteristics, accumulative breakdown characteristics and accumulative damage characteristics of oil-paper insulation are experimentally studied, which not only can improve the theory research about lightning impulse waveform, but also provide the technical support for insulation design in transformers and lightning protection in substation.Firstly, the double-exponential decay impulse waveform and damped alternating oscillation waveform are selected as two representative impulses based on the actual lightning invasion voltages. Breakdown characteristics of oil-paper insulation under different impulse waveforms are obtained by changing different wave parameters like wave front/tail time, oscillation frequency, damping rate and so on. Also, a feasibility study on the prediction of 50% breakdown voltage was performed irrespective of wave shape. As a result, breakdown characteristics vary much between two kinds of waveforms, and the steepness of the wave peak and the duration of high voltage around wave crests are considered to be two important factors affecting the breakdown voltage.Then, researches on accumulative breakdown characteristics are taken and the V-N characteristics under both kinds of waveforms are obtained. The failure probability of oil-paper insulation are analyzed using Weibull distribution model, and the influential regularity of different wave parameters are determined respectively. As a result, an evident accumulative effect exists under both impulse waveforms and the statistics of V-N characteristics satisfy the Weibull distribution. However, compared with double-exponential decay impulse waveform, the wave parameters of damped alternating oscillation waveforms have a greater impact on the experimental results.Finally, researches on the accumulative damage characteristics are taken from three aspects of mechanical properties, surface morphology and electrical properties. The insulation performance and parameter variation of oil-paper samples are comparatively studied under both kinds of waveforms before/after accumulation. As a result, after accumulation, the mechanical properties of samples show a certain decline, the surface roughness increases obviously and the electrical properties decline a lot. It can be inferred that the accumulative damage effect is mainly shown as the generation of polar material and the deformation of the fiber structure during accumulation.