Electronic Power Transformer Of Distribution System

Transformers have been used widely as an extremely important electrical component in electric power system since they were invented in the late 19th century. During the past 100 years, the primary functions of transformer have not changed as voltage transformation and magnetic isolation. Yet, the electric power system has changed significantly and deeply: the stability problem becomes more serious and power quality is an increasingly important issue. These problems can't be resolved only using traditional transmission and transformation electrical device and technology.In answer to these deficiencies, this dissertation sets forth a power electronics based transformer named as Electronic Power Transformer (EPT). EPT is an entirely new class of transformer which retains the basic functions of conventional transformer and extends additional function. 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. So, this dissertation devotes itself mainly to the research on the application of EPT in distribution system.The basic theory of EPT and state of the art in this field are introduced firstly, and a general mathematical model is established for EPT. Based on the model, a control strategy is developed to achieve power quality regulation with the input side of EPT as a current-controlled source and the output side as a voltage-controlled source. The simulation results show that EPT has excellent input/output characteristics and can prevent harmonics from propagation in either direction.In succession, parallel operation of EPTs is explored in the dissertation. The modes of parallel system configuration are summarized. Based on the analysis of the parallel modes, several kinds of control schemes, including master and slave control, power deviation control, distributed logic control and output droop characteristics based wireless control, are proposed. And the master and slave control strategy is discussed in detailed, in which one EPT is controlled as a voltage source and the others are controlled as the current sources. The voltage source is developed as a master to establish the voltage of the output common bus and provide constant voltage and constant frequency output. And the current sources are operated as slave units to perform current sharing control. The validity of the proposed control strategy is investigated through simulations with two EPTs parallel system.Afterward, a novel Auto-balancing EPT (A-EPT) is proposed. In the proposed A-EPT, the cascaded- multilevel converters are used for the high voltage stage and the interleaving parallel technology is adopted for the low voltage stage. Subsequently, a simple effective control method is designed for A-EPT. In the proposed control scheme, the triangle carrier phase-shifted technique is used to obtain the high equivalent switching frequency and good input characteristic, and the corrected phase-shifted signals are introduced to achieve dc capacitor voltage balance. Computer simulations are performed to verify the performance of the proposed A-EPT. The simulation results show that the proposed A-EPT can keep the system auto-balancing with fast regulation, and has much better performances than conventional power transformer. And then, an experimentation system is established to verify the performance of A-EPT. The experimentation results are satisfied.The input and output reactive power of EPT can be regulated flexibly. This feature meets the requirement of the reactive power optimization in the distribution system. So, the dissertation discusses the problem about reactive power optimization of EPT based distribution system. Analysis of the 42 nodes test example indicates that EPT in place of traditional OLTC (On-Load Tap Changer) can reduce power loss effectively, improve voltage quality of the grid and reduce the expense of the reactive-load compensation equipment.Finally, the dissertation discusses the application of EPT in transmission system. An optimal coordinated control strategy between the generator and EPT is proposed. The coordinated control strategy is designed on the basis of the single machine infinite bus power system with double feeder lines, where the one transmission line is equipped with conventional transformer and the other is equipped with EPT. The simulation results show that EPT can effectively improve the stability and performances of power system.