Applications Of Voltage Source Converter In Power Systems

The interconnection of large area power grids and the appearance of long-distance power transfer lines with heavy loads contribute to the best use of power resources and improvements of economy and reliability of power generation and transmissions. But at the same time they also bring new issues into power system stability and control, especially low-frequency oscillations. The traditional thyristor excitation systems can only provide damping through field windings and it is difficult to tune the controller parameters to suppress the large range of power oscillation frequency. Therefore, the research on how to improve the stability and transfer capacity of modern power systems becomes more and more important.High-efficiency utilization of electrical power and improving power supply quality and reliability are important requirements for smart grids. The increasement of non-linear loads and high-capacity impact loads and the presence of distributed generation (DG) have heightened power quality depravation. However, the requirement to the electric energy quality of users, especially industry enterprises, has been more and more rigorous. It is necessary to provide high quality power supply for special users through effective approaches. On the other hand, most of high voltage motors are not equipped with variable frequency speed systems at present, which increases power lose and is not consistent with the guiding ideology for energy-saving and emission reduction in Chinese electric power industry.Due to the good control characteristics and flexible structure, voltage source converter (VSC) has widespread application in power systems. This dissertation mainly studies on the new applications of VSC, including power generation, distribution, consumption and power quality regulation as follows:First is the research on the basic theory of VSC excitation system and its advantages as compared to traditional thyristor excitation systems, and the Philips-Heffron model of a single-machine infinite bus system with VSC excitation system is established. The damping torque analysis is applied to confirm the damping contribution of reactive current injection, so the mechanism of improvements of power system damping characteristics is revealed. The transient enhancements by excitation control and reactive current control of VSC excitation system are analyzed separately. The simulation and physical model results show that, VSC excitation system can improve damping significantly and improve power system static-state and transient stability when compared with thyristor excitation systems; and it also has the ability to improve the maximum transfer capability of long-distance transmission systems.Then, a VSC-based transformerless variable frequency speed system for high voltage multi-phase motor is proposed in this dissertation, which decreases requirements on voltage and current stress of power electronic devices; so the size and cost can be reduced. The corresponding control strategy is developed and implemented in DSP. The experimental results in a nine-phase motor laboratory prototype show the correctness and feasibility of the proposed system.Then, this dissertation introduces the development of a VSC-modular-based10kV/400V/500kVA electronic power transformer (EPT), and discusses the control system design process in detail. A new DC voltage balance method has been proposed for cascaded H-bridge converters through regulating the active power of inverters. The positive and negative sequence current separate control strategy is utilized for operation of unbalanced three-phase loads. The start-up process of EPT is also analyzed and designed. The simulation and low voltage prototype results show that EPT with the developed control methods could work normally, maintain the DC voltage balance, and adapt to unbalanced operation.At last, a fast and continuous voltage regulator is proposed in this dissertation. EPT serves as an auxiliary voltage regulator for the line frequency transformer, so that the size and cost of EPT can be reduced significantly. Different connections between EPT and line frequency transformer are compared. Two sample schemes are given and corresponding control method is developed. The simulation and experimental results demonstrate that the proposed voltage regulator is able to maintain load voltages at the desired value effectively under operation conditions, such as reference value change, voltage flicker, voltage rise, voltage dip, load changes and EPT fails.