Mathematical Modeling And Simulation Analysis For Voltage Source Converter Based HVDC

With the increasing rating and improved performance of self-commutated semiconductors, Voltage Source Converter (VSC) based HVDC (VSC-HVDC) has been realized. VSC-HVDC has been attracted more and more attention for its favorable advantages. Domestic study on VSC-HVDC is just at the beginning. In this dissertation, the mathematical modeling and transient simulation for VSC-HVDC, as well as the interaction between VSC-HVDC and ac system are studied.Firstly, the steady-state power characteristics and the control patterns of VSC-HVDC are analyzed detailedly. Based on the developed steady-state mathematical model of VSC-HVDC for newton method power flow calculation, a power flow calculation algorithm, which calculates ac system and dc system alternately, is proposed by considering that most control objects of VSC-HVDC are ac qualities. In this algorithm, the capacity limit of VSC is also taken into account. The initial values for transient simulation of system equipped with VSC-HVDC can be got from this algorithm.Based on the method of injecting current at nodes, a detailed model of VSC-HVDC is built up for transient stability analysis. This model is general for including three main parts of VSC-HVDC, .i.e. quasi-model of VSC ac side, dynamic model of inner dc system and model of control system. With the help of User Defined (UD) function provided by Power System Analysis Software Package (PSASP), transient stability simulation for system, in which VSC-HVDC is paralleled with ac lines, is realized.Small disturbance stability analysis model, which includes linear equations for VSC ac side, for inner dc system and for control system respectively, is established for VSC-HVDC. By using frequency-domain response identification and time-domain response prony identification, the opened-loop transfer function between VSC-HVDC active power reference and the active power of its paralleled ac line is got for single-machine and multi-machine system, respectively. Based on the identified opened-loop transfer function, VSC-HVDC damping controller is designed in terms of pole assignment technology. Nonlinear simulation results show that VSC-HVDC equipped with the designed damping controller can suppress low-frequency oscillation effectively.Considering that the topological changes of VSC-HVDC are complicated, a transient mathematical model capable of reflecting its interior characteristics and physical course is developed by introducing the concept of single-pole two-valued logic switching function. Moreover, by omitting the high-frequency component in the switching function, the transient model of VSC in dqO frame in established and its decoupling controller is also designed. In dqO frame and with the...