Study On Statistics Of Measured Intrusive Lightning Surge On Transformers And The Impact On Oil-paper Insulating Properties

Transformer is the core equipment of power system, and its safety is important forthe reliability of the grid. The premise of reasonable insulation co-ordination fortransformers is precisely obtaining the actual lightning overvoltage on transformerswhen the lightning impulse insulation level is determined. However, there are fewdetailed studies about the statistical analysis of the intrusive lightning surges and theexperiments on inner insulation for transformers. And the traditional lightning standardwaveform is still used for the insulation design and withstand test, while the intrusivelightning waveform in the field is non-standard. Therefore, the researches on thestatistical regularities of lightning surges and the experiments for oil-paper insulationare so important that they are not only helpful to perfect the theories on lightning, butalso supportive for the insulation design and lightning protection, which possesses theimportant theoretical and practical value.Firstly, based on the intrusive lightning overvoltage surges, monitored at the110kV power transformers for an observation period of6years, the desired signal oflightning surges were obtained by FFT. And the Energy Method and DE Method wereput forward to acquire time parameters, and the time parameters were statisticallyanalyzed. Statistical results show that a bidirectional damped oscillatory type waveformwith a main frequency within10kHz rises up in the measured lightning overvoltages.And the front times and tail times are much greater than the standard ones with alognormal distribution. Especially, a strong correlation described as a belt-likedistribution between front times and tail times has been found.Then, an overvoltage test platform were set up to do the experiment by contrastunder both measured waveform (20/198μs) and the standard one. And the impact to50%breakdown voltage under overvoltages with different time parameters wasanalyzed. The test results show that in the front time range tested(1.2~26μs), the50%breakdown voltage of oil-paper is to do with the front time, and largely insensitive tothe tail time. However, the different thickness, layers, and degree of aging of oil-paperaffect the results.Finally, based on the finite element simulation software, the simulation model ofoil-paper was built, and the potential and electric field distributions as well as thecurrent from the grounding electrode of oil-paper insulating system under overvoltages with different time parameters were calculated.Results show that the current of oil-paper has both the positive and negative partswith extreme values under the positive voltage. The positive current is relative to thevoltage change ratio at wavefront, while the negative current is relative to the voltagechange ratio at wavetail. The maximum of positive current is much greater than thenegative one. Therefore, the positive current is the main factor for affecting theelectrical insulation properties under the positive voltage.