Research On Steady State Characteristic Of VSC-HVDC Based On Linearized Model

In recent years, with the rapid development of the technology of voltage source converter (VSC), VSC-HVDC has gradually become a preferred way for renewable power integration owing to its various merits, including independent control of active and reactive power, no demand for reactive power support and its ability to supply power to island networks. VSC-MTDC, which is constructed by multi-terminal VSC-HVDC, become the embryo of future DC grid. However, few published papers have mentioned about the steady state characteristics of VSC-HVDC based on small signal models. For sake of the steady operation of VSC-HVDC and VSC-MTDC, especially in the situation of interconnection with weak ac systems, this thesis built the small signal model of VSC-HVDC, and focused on the steady state operation of VSC-HVDC.First, this thesis discussed the process of building the small signal model of a mono terminal VSC-HVDC system. Based on the well-known vector current controller, the established small signal model is made up of the differential and alegbraic equations, which include mathematical model of VSC in the dq rotating frame, the phase lock loop (PLL), the outer power control loop and the inner current control loop as. well as the system state equtions. The fesibility of the established model is verified by simulations done in the PSCAD/EMTDC software.Secondly, the steady state characteristics of VSC-HVDC were analyzed utilizing the established small signal modle of VSC-HVDC. This thesis concentrated on the imfluences of each parameters in controller on the steady state characteristics of VSC-HVDC which connects to a weak ac system. The parameters include the gain of PLL, the PI parameters of outer power control loop and the inner current control loop.At the last part, this thesis introduces several dc voltage control strategies of the VSC-MTDC. Based on small signal analysis, the steady state operation of a three terminal VSC-MTDC which adopts droop control is analyzed. Besides, the parameter selection criteria of the droop controller is studied.