Research On Control Strategy Of Electronic Power Transformer

Since the past 100 years, transformers have been widely used as an extremely important electrical component in electric power system, and its working principle, primary functions and disadvantages such as volume large, weight heavy, etc., have not been changed. Meanwhile, the electric power system has changed significantly and deeply: the increase of nonlinear load has deteriorated the power quality, and the customers required the high quality power energy simultaneously. So, the power quality problem becomes an increasingly important issue.It is a valuable issue to reform the functions of transformer and overcome its deficiencies, and make it meet different requirements of modern and future electric power system. Therefore, since twenty century, some domestic and foreign scholars have explored the new type power transformer actively.Electronic power transformer (EPT) is an entirely new class of transformer which retains the basic functions of conventional transformer and extends additional functions. A significant additional advantage of EPT is that the magnitude and phase angle of voltage in both the primary side and the secondary side of EPT are controllable in real-time. Due to such properties, EPT can meet many new requirements of future electric power system. This dissertation devotes itself mainly to the research on the relative control strategy of EPT for distribution system (DEPT).In Chapter 1 and Chapter 2, the basic theory of EPT and state of the art in this field are introduced firstly. An EPT with supercapacitors energy storage system is proposed in Chapter 3, and the control strategy of the proposed system is given. Also, an experimental research system is built. Simulation and experimental results have verified that the proposed system can compensate voltage momentary interruptions, thereby improve power system reliability effectively.In Chapter 4, parallel operation and its control scheme of EPTs are explored, and six parallel system configurations are given. And a wireless control based on the well-known droop method is realized by simulation, and the simulation results show excellent power sharing and voltage regulation characteristic, a proper dynamic response. Also, the parallel operation of EPT and conventional transformer based on the idea that the secondary voltage of conventional transformer is selected as the reference voltage of EPT's output voltage is realized by simulation and experiment. Results show that the control strategy has good current-sharing regulation characteristic, and achieves reasonable load distribution in the proposed parallel system with different ratings.In view of the four objectives of DEPT, in order to enhance the dynamic performance and the disturbance rejecting capability of DEPT, a linear optimal control strategy of DEPT is presented in Chapter 5. And the mathematical model including the input stage, the isolation stage and the output stage of DEPT is built, and the optimal controller of DEPT based on the optimal control theory is designed.Considering that EPT is a special nonlinear system with some switch devices essentially, the nonlinear control of DEPT is studied in Chapter 6. And the basic principle and design method of multi-input multi-output nonlinear system state feedback exact linearization is proposed, and a nonlinear controller of DEPT based on the differential geometry and the optimal control theory is designed. Research results show that the proposed nonlinear controller can solve the nonlinearity problem of the model system completely in the whole operating range, and realize the relative control objectives of DEPT perfectly.