Research On Control And Dynamic Characteristics Of VSC HVDC

With the development of power electronics component, especially high power turn-off power electronic components, such as IGBT, IGCT and GTO, now it is feasible to form voltage sourced converters (VSC) for HVDC transmission, it is VSC HVDC. Typical represents of VSC HVDC is HVDC Light. Presently, converter stations are available for powers up 350MW.A 500MW up to 1000MW converters station will be the next generation that is presently under development. HVDC-VSC transmission is a new generation of HVDC system. Now it is less economical than the conventional HVDC-LCC, as the technology matures and rating increase, it is expected that the cost will be reduced. Different from conventional HVDC, PWM control is adopted by VSC HVDC. With PWM it is possible to create any phase or amplitude (up to a certain limit) by changing the PWM pattern, which can be done almost instantaneous. Hereby it is able to regulate the power flow conveniently. On the other hand, with the development of renewable generation and distributed generation, there is a need to feed these renewable generations to network by proper conversion and transmission, and VSC HVDC is the one exactly. VSC HVDC uses the control scheme different from the classical HVDC, so it also brings new features to power system. How to control the VSC HVDC itself and what is the interaction between the AC and DC system are issues now being researched. This thesis focus on the control strategies of VSC HVDC based on established models, and all kinds of control scheme proposed for VSC HVDC is presented in this thesis, in addition, interaction between the VSC HVDC and AC system and dynamics of the both are researched. VSC HVDC models in the power system is established in chapter 2 of the thesis, including AC/DC hybrid power system and multi-machine power system with VSC HVDC, in addition, multi-terminal VSC HVDC model is built by matlab/simulink in chapter 8. A decoupling control strategy base on dq0 axis decomposition is proposed in chapter 3, interaction between AC and DC system is researched, and the simulation results is presented. The simulation results demonstrate that with the proposed control the expected performance can be achieved, and AC and DC system both have a good dynamic performance. In addition, an inverse system nonlinear control strategy is proposed in chapter 3,which realizes the decoupling control of active power and reactive power, with this control strategy, both AC and DC system have good stability features during large and small disturbances In conventional HVDC, DC power modulation method is often used to improve the stability and operation of power system, similarly, power modulation method in VSC HVDC system is also discussed in chapter 4,and a mixed control of active power and reactive power is proposed to improve the system stability, according to research and simulation, significant stability and dynamic performance improvement can be obtained by choosing a correct modulation signals. An optimal coordinated control strategy between the generators and VSC HVDC system is proposed in following chapter 5,this optimal control strategy is applied to AC/DC parallel system and multi-machine power system and perturbations are applied to examine the system dynamics performance. The comparison is made between the optimal coordinated control and independent control for generator excitation and VSC HVDC, the simulation results manifest that optimal coordinated control provides better damping characteristics. In chapter 6,basic control speciality of VSC HVDC and classical HVDC is clarified.the influence of classical HVDC and VSC HVDC on the power system is researched and compared; simulation results demonstrate VSC HVDC with a proper control algorithm is more benefit for stability and operation of system than conventional HVDC. A synthesized stability control for generator and DC system making use of rapid adjustment ability of power flow is proposed in chapter 7. With the proposed control, AC and DC system can achieve a good dynamic performance under variable disturbances, a performance index, which is composed of generator and AC system variables, is formed for the optimization of controller parameters by use of genetic algorithm. Simulation results before and after parameter optimization by GA are provided and compared, and results demonstrate the availability of GA for the control parameters optimization. Because the DC voltage polarity is kept constant when the VSC HVDC flow is reversed, VSC stations can be used to constitute a multi-terminal DC transmission system. In chapter 8,multi-terminal VSC DC models are built, and a "master control + local control"mode and DC voltage control at single terminal is proposed. Various disturbance scenarios are tested in the simulation, and the simulation results show that the proposed control strategy is feasible for operation of VSC multi-terminal HVDC system. Small test system to simulate VSC HVDC system is designed and researched in chapter 9,test results are given.