Study On Influence Of Oil Flow To The Breakdown Characteristics Of Oil-Paper Insulation Under Compound Electric Field In Converter Transformer

Converter transformer is an important equipment of UHVDC (ultro high-voltage direct current). It is the hub between AC and DC grids. Its stability directly affects to the safe operation of the system. The operating converter transformer not only withstands AC voltage, switching over-voltage and lightning impulse voltage but also DC voltage, superimposed AC-DC voltage and polarity reversal voltage. Under influence of the above voltages, there are considerable differences in the characteristics of inner electric field distribution, breakdown characteristics of insulation material, space charge accumulation and distribution, oil-flow electrification between a converter transformer and a normal power transformer, which can cause abnormal discharge and breakdown of the insulation system in a converter transformer. According to the need of the industry, the breakdown characteristics of typical oil-paper insulation under compound electric field in a converter transformer was studied as follows:Firstly, the test model suitable for oil flow condition is designed. The old oil-flow electrification test system is rebuilt,which makes the test system can be joined with the new breakdown test chamber. And the oil-flow electrification test and breakdown test can be carried out under the new test system with the tunable oil flow velocity and adjustable oil temperature..Secondly, the study on oil flow electrification characteristics of oil-paper insulation under applied electric field and without applied electric field was carried out. The research result shows that the oil flow current goes up with the velocity increasing, and the current amplitude under applied electric field is higher than that without applied electric field.Thirdly, the breakdown characteristics test with different velocity is carried out under different superimposed AC-DC voltage. At room temperature, the influence of voltage waveform and velocity to the oil-paper insulation is anlysed and concluded. The research result shows that the breakdown voltage goes down with the velocity increasing and the breakdown voltage goes up at first, then goes down with the AC content increasing in compound voltage.At last, the breakdown characteristics test with different velocity is carried out under polarity reversed voltage. The research result shows that the breakdown voltage goes down at first, then rises up a little with the velocity increasing.The researching results can be referred to the operation and the insulation design of converter transformer.