Research On Off-line Fault Detection Of Transformer Bushing And Winding Based On Impulse Frequency Response Method

With the rapid development of China’s economy,the demand for power supply is increasing day by day,which puts forward higher and higher requirements for the stability and reliability of power system.The power transformer is the core of energy transmission in the power system,the importance of its safe operation is self-evident.Insulation aging,short circuit impact current and other internal and external factors make the insulation of transformer bushing and winding vulnerable to damage,and then threaten the stable operation of the power system.At present,frequency response method is widely used by domestic and foreign researchers.It can detect the fault of transformer winding state without the maintenance of the suspension hood.However,this method is limited by the development of portable devices in the process of comprehensive promotion.In addition,the identification of winding faults by frequency response curve still has misjudgment.Therefore,improving the research of transformer fault detection based on frequency response method has important economic and academic significance for building a strong and stable power system.Based on the impulse frequency response method,the pulse coupled injection loop includes a bushing circuit and a winding circuit,and the influence of the bushing circuit fault on the spectrum curve has not been considered.Therefore,a portable pulse generator which can be used in fault detection of transformer windings is developed in this paper.A bushing with a built-in sensor is designed.Based on the bushing,pulse excitation signals can be injected forward for transformer fault detection,and overvoltage signals can be monitored backward.Several kinds of bushing faults are simulated offline,and a bushing fault detection method based on frequency response method is proposed.The experiment of superposition of winding fault and sleeve fault is simulated,and the practicability of the designed portable device is verified.The achievements of this paper are as follows:(1)A portable impulse generator for detecting transformer windings is designed.The control system based on FPGA can output nanosecond pulse control signal and high-voltage DC power program control signal,with high adjustment precision,and the supporting upper computer program and friendly human-computer interaction interface,which realizes the flexible and adjustable pulse parameters.Through the corresponding performance test,it is verified that the working performance of the pulse generator can meet the requirements of subsequent tests.(2)A bushing with a built-in sensor is designed.According to the principle of capacitive coupling,pulse signals can be injected forward and overvoltage signals of the transformer can be monitored in reverse.The correctness of the design is verified by off-line measurement.The simulation analysis of finite element software COMSOL and the pressure test show that the installation of the built-in sensor will not affect the normal operation of the bushing.Compared with Marx and Blumlein transmission lines,the influence of different waveforms on frequency response test was studied.The damp fault,resistance/capacitance fault and short circuit fault test of bushing are simulated,and the influence of different faults on frequency response characteristics is analyzed from the Angle of frequency response method.(3)Tests on the superposition of bushing faults and winding faults were carried out on the model transformer.The two kinds of influencing factors were dominant in the winding part.The bushing faults reflected their influence law on the frequency response curve on the basis of the winding faults.The failure of non-test phase will affect the frequency response curve of test phase to some extent.Two mathematical indexes,RMSE and SSMMRE,are proposed.RMSE amplifies the overall error while SSMRE amplifies the faults in different frequency bands.