Deepening Research On Analytical Model Of HVDC Transmission System Based On State Space Method

High voltage direct current(HVDC)transmission system has the advantages of large transmission capacity,long transmission distance and low loss.It is easy to realize the interconnection of AC systems in different regions,which is becoming more and more important in the process of power system modernization in China.The formation of AC-DC hybrid power grid has a significant influence on the stable and safe operation of China’s power system,especially the transient response when a fault occurs.Accurate modeling and rapid solution of the DC system are particularly important for fast and accurate stability analysis.The existing state space model can quickly and accurately reflect the transient characteristics of HVDC transmission system.On this basis,aiming at the problems of the existing state space model,such as the imprecise DC current and the current sudden change in the snubber circuit,this paper proposes an in-depth study of the analytical model of HVDC system considering the thyristor snubber circuit.In this paper,the state space analytical models of converter,snubber circuit,DC line and control system of HVDC system are established.The different operating conditions of the converter are expressed by a set of differential equations.Based on the theory of power electronics,the dynamic response of snubber circuit and the relationship between voltage and current in commutation process are analyzed.The snubber current is introduced into the state equation of the converter as a state variable,and the initial value of the snubber current under different working conditions is expressed by the variable coefficient matrix,which solves the problem of discontinuity of state variables.The control principle of HVDC control system and phase-locked loop is introduced in detail,and its time domain response model is established according to the control logic block diagram.The above analysis lays the foundation for the algorithm.This paper further deduces the analytic algorithm of state space model.The state equation of primary equipment is solved by implicit trapezoidal method,and the whole system equation of primary equipment is obtained by current relation.In order to accurately simulate the dynamic response of the converter valve on the inverter side,the end time of the converter’s conduction condition and commutation condition are solved respectively,which effectively improves the calculation accuracy of the differential equation of the primary equipment.For the control system,the backward Euler method is used to simplify the state equation into the difference equation,and then the data interaction between the control system and the primary equipment simulation is realized.Finally.a complete algorithm flow chart is formed.Based on the theoretical analysis and calculation of state space establishment process,the model and algorithm proposed in this paper are programmed in MATLAB,and the simulation calculation of steady-state operation,symmetric fault and asymmetric fault is carried out.The simulation results are compared with the CIGRE HVDC standard example in PSCAD/EMTDC.The results show that the model have effectively improved the calculation accuracy of the state equation after considering the snubber circuit,and the description of the thyristor component is closer to the PSCAD thyristor component model.The accuracy and effectiveness of the proposed model and algorithm are verified,and can provide a reference for modeling research on analytical calculations in HVDC transmission systems.