| HVDC-VSC system is a new HVDC transmission technology based on voltage type self-commutated converter (SCC) 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 HVDC-VSC is a new technology, many technical problems in HVDC-VSC area are still unsolved or solved not well, especially the capability of the HVDC-VSC and the control strategy for the HVDC-VSC. This dissertation will study how to enhance the capability of the HVDC-VSC through reasonable control of three-level VSC. Based on this control strategy, appropriate controllers are designed to control the active and reactive power at each end station.(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 HVDC-VSC connecting two active AC systems, the other suits the HVDC-VSC supplying passive loads. Theoretical analysis on the DC and AC side characteristics of the 3-phase three-level VSC 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. Through mathematical analysis of themodels and combination of the actual systems, the approximately decoupling and linear relationship between the controlled variables and controlling variables is confirmed and the controller is established.(2) The control strategy for HVDC-VSC based on NPC (Neutral-Point Clamped) three-level VSC is proposed. The strategy can increase the capability of the HVDC-VSC and can improve the quality of the output current of HVDC-VSC. The differences of HVDC-VSC control strategy between two-level and three-level are studied. The performance of different pulse width modulation (PWM) is compared and the application of the space vector pulse PWM in the three-level HVDC-VSC is demonstrated in detail.(3) Validity of the proposed control strategy of three-level HVDC-VSC system is testified by digital simulation using PSCAD/EMTDC. Through debugging, the parameters of the controllers are optimized and determined. The simulation results under steady-state conditions show the sufficient response speed and accuracy of the controllers of three-level HVDC-VSC. |
