| Converter transformer is the key equipment of HVDC system.It is of great significance for the reliable operation of power grid to make sure safe and stable operation of converter transformer.However,in recent years,the condition of failures of converter transformers has become more serious,which has affected the reliability of HVDC transmission.Converter transformer runs for a long time and valve side winding withstand AC and DC composite voltage.There exists big difference of the complexity of oil-paper insulation partial discharge between converter transformer and AC transformer.Partial discharge is an important criterion for judging the insulation condition of converter transformers,and partial discharge can effectively reflect the operating state of the transformer.Therefore,diagnose of defect development process in converter transformer is also different from that in AC transformer.At the same time,it is of great significance for the diagnosis of transformer oil-paper insulation to the normal operation of HVDC system.At present,there are few researches on the characteristics of single partial discharge waveform under composite voltage.So it is necessary to study the geometric characteristics of a single waveform of partial discharge.There is a positive impact on improving the accuracy of the division of the stage of development of defects.In order to simulate the operating environment of converter transformer,the law of the development of insulation defects of oil-paper in converter transformer has been studied and the correlation with geometric characteristics of single waveform has been studied.In this paper,an experimental platform for the partial discharge of oil-paper insulation has been established,including the needle-plate model and the ball-plate model,the experimental material pretreatment system and the pulse current detection system.The constant voltage method has been used to study the defect development process of partial discharge of oil-paper insulation under compound voltage.And the partial discharge voltage waveform has been collected using non-inductive resistance and oscilloscope.In this paper,the geometric characteristics of a single waveform have been studied during different stages of defect development under the compound voltage which the ratio of AC and DC voltage is 1:1.In this paper,a single partial discharge waveform is obtained by an oscilloscope.The geometric characteristics of the waveform are analyzed to form the maximum amplitude distribution diagram,the total time distribution diagram of the single waveform,and the distribution diagram of the maximum amplitude time.By analyzing the average value and composition of partial discharge in each period,the variation characteristics of each physical quantity in each discharge stage are studied.The average value,variance,skewness and steepness were extracted for each distribution diagram.Then the K-means clustering algorithm was used to cluster the entire defect development process.The whole process of defect development was clustered by K-means clustering algorithm to classify the degree of defect development of oil-paper insulation in the ball plate model and needle plate model.The development of defects is divided into three stages including the initial stage,the middle stage and the late stage of defect development.The mean value of the waveform's geometric characteristics are different in each stage.As can be seen from the three dimensional distribution charts,the dispersion of the geometric characteristics of a single waveform also has a certain regularity within each statistical period.Compared with the ball-plate model,the geometric parameters obtained by the needle-plate model have higher randomness.The degree of the development of the oil-paper insulation defects can be judged by the characteristic quantities combining experimental phenomena.The results provide a new method for evaluating the development of converter transformer defects. |
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