Co-simulation And Analysis Of "Three Transformers" For UHV Transformer Under DC-bias

In order to solve the problem of the reverse distribution of energy centers and power centers in China,UHV transmission technology has developed rapidly.Due to the high voltage level of the UHV transformer,in order to meet the insulation performance during voltage regulation,the neutral point voltage regulation method is adopted,and the compensation voltage is used to ensure the voltage level of the low voltage side during voltage regulation.Therefore,the UHV transformer is often three during normal operation.The transformer is put into operation at the same time.Therefore,when analyzing the DC bias of the UHV transformer,it is necessary to consider the mutual influence relationship between the three transformers.In this thesis,the finite element model of three transformers is established by finite element software.The fourth-order Runge-Kutta method is used to solve the circuit differential equations of three transformers.The DC bias results of three transformers at the same time are obtained.There is a direct parallel connection between the main body,the voltage regulation and the compensation of the UHV transformer.The winding resistance is small and the inductance is large.A small error in the simulation calculation will also form a circulation in the parallel circuit.The large-inductance and small-resistance circuit has large DC current error,and the time constant is large,and the attenuation is slow.The long-term effect on the winding changes the DC offset of the transformer,which will cause a large error in the DC bias simulation calculation result.In this paper,in the circuit where the windings are directly connected in parallel,according to the ratio of the internal resistance of the winding and the equivalent resistance of the series,the calculation error of DC current is shortened and the effect of the transient process is accelerated,so that the simulation results of the DC bias of the UHV transformer are more accurate.Since the main body of the UHV transformer is connected to the earth through the voltage regulation and compensation,when the voltage regulation is changed to different taps,the DC current in the main body is shunted in the voltage regulation and compensation,the voltage regulation and the compensation variation.The obtained DC current is different.This paper analyzes the influence of the voltage regulating transformer on the DC bias of the UHV transformer by establishing the finite element model and circuit model of the three transformers under different pressure regulating taps.By establishing the load circuit model with different characteristics on the low-voltage side,the effects of DC bias on the voltage and current harmonics,voltage and current waveform and amplitude of the UHV transformer are analyzed when the low-voltage side capacitive load,resistive load and inductive load are analyzed.In this thesis,through the joint simulation of the main transformer,the voltage regulation and the compensation transformer of the UHV transformer,considering the mutual influence relationship between the three transformers,the more accurate UHV transformer DC bias magnetic simulation results are obtained,and the simulation is simulated.The calculation results provide a basis for the subsequent analysis of the temperature rise and vibration of the UHV transformer.