Load DC Bias Calculation Of UHV Transformer Based On Low Voltage Winding For Reactive Compensation

The DC bias problem caused by HVDC transmission and geomagnetic storms threatens the safe and stable operation of transformers.UHV transformer is one of the most important equipments in UHV transmission system.The research of DC bias in UHV transformer is of great significance for the reliable operation of power system.Based on the field-circuit coupling algorithm,a magnetic field model of a UHV transformer with a third winding,i.e,a low-voltage winding,is established.The inductance parameters are calculated by the method of finite element analysis and the DC bias calculation is performed when the load on the low-voltage side is a resistor.In order to obtain the accurate DC bias characteristic and improve the calculation efficiency,need to cascade a resistance in the high voltage side of the transformer,the influence of series resistance on the high,middle and low voltage winding current and excitation current of the transformer was analyzed.The load DC bias characteristics of UHV transformer in different DC bias were obtained.The results show that the difference current pulses of high,medium and low voltage windings and excitation current pulses occur in the first half cycle of a complete cycle,the difference current of the second half cycle close to zero,DC bias only affects half a cycle of the winding current waveform,the other half of the current waveform is basically unchanged.The field coupling model provides an effective basis for the calculation of the DC bias of the actual load on the low-voltage side of the UHV transformer.When the low-voltage side load is reactor,a field-circuit coupling model of the low-voltage side of the UHV transformer with inductance is established.The time constant of the load is large,which seriously affects the calculation efficiency,a method that artificially series resistance and iterative compensation voltage on the low-voltage side is proposed,used to solve the problem of long transition time.By calculating the voltage and current waveforms of the inductance,the DC bias characteristics of the inductor at the low voltage side can be achieved..When the low-voltage side load is capacitance,a field-circuit coupling model of the low-voltage side of the UHV transformer is established,and the DC bias analysis of the low-voltage side load is carried out.The DC bias characteristics of the capacitive load with different DC offset are analyzed and the voltage characteristics of the load at the low-voltage side are obtained.It is found that when the DC bias current reaches a certain level,the load voltage at the low voltage side is obviously higher than the rated voltage.Pointed out that in actual operation,this situation may threaten the safe operation of the transformer.