Time-periodic Finite Element Method And Asymmetrical DC-biased Characteristics Of Series Transformer In UPFC

The series transformer is one of the key devices of Unified Power Flow Controller(UPFC),whose network-side windings are connected in series with the AC transmission line,and the valve-side windings connected with the converter valves of the control system.It provides the electrical insolation between converter valves and AC system,and undertakes the function of power exchanging between the converter valve and AC system.The special connection mode and application determine that the electrical structure and partial electromagnetic characteristics of the series transformer is different with the normal ones.The magnitude and direction of the DC current following into each phase winding of series transformer from the converter valves may be different,which leads to the asymmetric DC-biased phenomenon of series transformer.The research on electromagnetic characteristics and calculation methods of the series transformer under asymmetrical DC bias is helpful to the optimal design,operation and maintenance of series transformer.This dissertation introduces the time-periodic FEM to analyze the transformer DC-biased problems,and proposes a shell-type structure series transformer accordsing to the operating characteristics of the series transformer in UPFC,and establishes an electromagnetic transient model,the characteristics of series transformer under asymmetric DC bias are also studied by using the time-periodic FEM.Some measures are put forward to improve the DC-biased tolerance ability of series transfomer.Some measures about the suppression of asymmetrical DC bias of series transformer are put forward.The work of the dissertation is of theoretical significance and engineering value.The main research works of this dissertation are as follows:(1)Based on the periodic characteristics of the variables in electromagnetic field of transformer under dc bias,the magnetic vector potential and the excitation current are taken as the unknown variables,the field-circuit coupled time-periodic finite element method(TPFEM)was introduced to study the DC bias problems of power transformers.Some corresponding measures are put forward to solve the stability problem in the calculation,and the calculation and experiment of a square laminated core(SLC)under DC magnetic bias are carried out to verify the validity of the algorithm.The sensitivity to DC bias of the SLC with no-load and different loads is analyzed.(2)The local convergence fixed-point method is introduced in the traditional time-periodic FEM,which greatly improves the efficiency of DC bias calculation.The hysteresis characteristics of SLC under DC bias are analyzed and simulated by the improved hysteresis model based on consuming function,the fixed-point time-periodic FEM considering the hysteresis effect is applied to the calculation of transformer under DC bias,and the validity of the algorithm is verified.(3)Based on the special connection mode and operating characteristics of series transformer,a physical structure of series transformer for UPFC system is presented.According to spatial geometries of series transformer,a eletromagnetic transient simulation model of series transformer is set up based on duality theory of electric and magnetic circuits to analyze its electromagnetic characteristics.Taking nonlinear saturation characteristics and eddy current effect of silicon steel sheet into account,in the model,an effective complex permeability is adopted to calculate nonlinear reluctance,the magnetic energy method is used to extract leakage inductance parameters.A low voltage prototype was designed and manufactured for experiment and calculation.And the model is used to analyze current of an actual series transformer with no-load switching.(4)The experiment and calculation of the series transformer u nder asymmetrical DC bias are carried out.The average peak-value factor of excitation currents of the three-phase series transformer under asymmetrical DC bias is defined,and the variation of excitation current of series transformer under different asymmetrical DC bias conditions is analyzed and summarized.And the harmonic characteristics of excitation current and magnetic field are also analyzed.The reactive power of series transformer under asymmetrical DC bias is calculated,and the variation of reactive power is also analyzed and summarized.The suppression measures of series transformer under asymmetrical DC bias are put forward according to its eletromagnetic characteristics.