VSC-HVDC Control Strategy Based On Static Synchronous Series Compensator

VSC-HVDC system is a new HVDC transmission technology based on voltage type self-commutated converter named voltage sourced converters(VSC) and series-connected insulated gate bipolar transistors(IGBT) or gate turn off thyristors(GTO) controlled by pulse width modulation(PWM). Because VSC-HVDC is a new technology, many technical problems in VSC-HVDC area are still unsolved or solved not well, especially the VSC-HVDC protection strategy when the some fault happens to the system. The main work of this dissertation is how to improve the stability of VSC-HVDC system through the control strategy.(1) The steady-state interaction between VSC and its connecting AC systems are analyzed and two steady-state mathematical models are established, one of which suits the VSC-HVDC connecting two active AC systems, the other suits the VSC-HVDC supplying passive loads. Theoretical analysis on the DC and Acside characteristics is conducted, and describe the relationship between the controlled variables such as the active power, reactive power, dc voltage, dc current, ac voltage and the controlling variables such as modulation index, modulation angle.(2) Based on the analysis of the SSSC principle, a comparison between the two modes of operation of the SSSC, constant reactance mode and constant quadrature voltage mode, is made from the synchronizing power, steady state stability limits.(3) According to the characteristic analysis of both VSC-HVDC and SSSC, a novel strategy is proposed to solve the rotor speed problem in VSC-HVDC. On the basis of establishing Matlab simulation model, comparison before and after merging SSSC in VSC-HVDC is made. Simulation results show the validity and the effectiveness of the proposed control strategy.