Research On The Partial Discharge Characteristics Of Oil-immersed Transformers Under Impulse Voltage

With the strategy that "National Connection and Global Connection" is carried on by SGCC,the oil immersed power transformer is required to be high voltage and large power capacity.Meanwhile,the insulation properties and condition tolerance of power transfer needed higher requirements.Impulse withstand voltage test is an important means to evaluate the over-voltage withstand ability for the power transformer.Moreover,the partial discharge study under impulse voltage condition can effectively detect the potential insulation defect,and improve the accuracy and precision for the power transformer insulation evaluation.Currently,the partial discharge study under impulse voltage is still at the starting stage compared with under AC and DC voltage,and cannot provide the technical support for the power transformer partial discharge evaluation and diagnosis.Therefore,the research that the oil immersed power transformer insulation structure and typical defects,the mechanism and characteristics of partial discharge under impulse voltage condition is carried out in this thesis.Firstly,the free partial discharge impulse test platform and the partial discharge measurement system are set up.The typical oil immersed power transformer insulation model is made and processed with the relative standard requirements.Meanwhile,the test condition is verified and proposed the relative test method,which guarantee the preciseness and effectiveness for the experimental research.Secondly,the oil and air gap model of oil-paper insulation is developed according to the oil immersed power transformer typical structure.The mechanism and characteristics of partial discharge under standard impulse voltage condition is studied through the modeling,simulation and experiment.With the theoretical analysis of the simulation and experiment results,the theory of dielectric barrier discharge and micro bubble is used to explain the discharge process,and establish the corresponding evaluation model for the discharge intensity.Moreover,the relative parameters variation of partial discharge is represented,such as volt-second relationship,starting characteristics,pulse repetition rates and amplitude etc.,which can improve the improve the accuracy and precision for the power transformer insulation evaluation.Finally,the four defect models of air gap,suspended metal,needle plate and surface is developed according to the oil immersed power transformer typical structure.With the simulation and experiment method,the partial discharge process and characteristics of typical insulation defect is studied under the standard,oscillation,lightning and switching four kinds of impulse voltage.Then,the time domain characteristic map of partial discharge in transformer insulation defect is established,and the partial discharge signal is identified and recognized,which improve the accuracy and precision for the power transformer insulation evaluation.