Analysis On Operation Characteristics Of Converter Transformer In HVDC

Converter transformer is the core equipment of UHV DC transmission system,plays a connecting role of AC grid and DC side of the grid, but the operating environment is very poor and the failure rate is always high which seriously delayed the pace of construction of power grid in China. Transformer accident is mainly due to the partial discharge caused by the defect of the main insulation and the excitation current distortion caused by relay protection device malfunction.No matter what kind of situation, it is a serious threat to the safe and stable operation of HVDC transmission system.Systematically analyzes of the operating characteristics of the converter transformer and find its improvement program is abnormal critical.The electrical conductivity of converter transformer insulation material is nonlinear affected by the intensity of electric field and temperature. To study how the material nonlinearity changes converter transformer polarity reversal electric field distribution, this paper presented a numerical fitting of the nonlinear variation rules of conductivity of insulation.Then calculated and analysis the electric field of oil paper insulation in the polarity reversal process distribution using RC equivalent circuit from the point of view of analytic function.With the help of MATLAB four order Runge Kutta algorithm calculus the nonlinear variation law.Equivalent surface charge method to analysis the amount of bound charge and time constant on the insulation board.The ELECNET finite element software is used to simulate the electric field of the side and the valve side winding of a ± 400 k V converter transformer and the influence of temperature, field strength and oil channel temperature gradient on the electric field distribution of the actual converter transformer is obtained finding out the position of the main insulation defects.At last,according to the calculation and simulation results, the improvement direction of insulation is proposed.In order to investigate the influences of DC magnetic bias and inrush current of converter transformer no-load characteristics, this paper explained the operation mechanism from the angle of the basic magnetization curve of the iron core.PSCADfinite element software is used to model a ± 400 k V converter transformer. By loading different magnitude of the DC current bias and different trigger angle to simulate the DC bias, and the phenomenon of superposition of the two the waveform and variation law of exciting current are obtained by simulation.Based on the principle of Fu Liye transform, the harmonic content and the increasing trend of the excitation current are proposed by using MATLAB software.In addition, the threedimensional finite element model of no-load transformer under DC bias condition is established by using the MAGNET finite element software based on the field circuit coupling principle.Verify its accuracy by comparing the rated no-load condition of the excitation current.Three dimensional magnetic field of the no-load converter transformer under different DC bias conditions is simulated and the distribution of the magnetic field is obtained and the corresponding no-load loss is obtained by the half wave average method.The research of this paper can provide the theoretical basis and the improvement direction for the design of the main insulation of converter transformer and relay protection.