Control And Applications Of Electronic Power Transformer

Power system is the most important energy carrier in modern society. It presents a variety of features in the development process. On the one hand, power system’s size and voltage level are increasing dramatically and the type of electrical load is more complex. On the other hand, as one of the driving forces of economic recovery, power system will be more intelligent and become a "Smart Grid". As one of the most basic electrical equipment in the power system, power transformer is not sufficient to meet the challenges of modern power systems. The traditional bulky power transformer lacks the intelligent control ability and dc conversion interfaces. Scholars from various countries focus on the new type of transformer.Electronic Power transformer (EPT) is an entirely new class of transformer based on power electronics technology that retains the basic functions of conventional transformer and extends additional functions, such as highly flexible and controllable ability and a variety of DC&AC interfaces. Due to such properties, EPT can meet many new requirements of future smart grid. This dissertation devotes itself mainly to the research on the control strategy and applications of EPT.The first part of the dissertation focuses on the design and control of the presented three-stage EPT topology. In chapter2, middle frequency transformer’s design issue is analyzed. The Middle frequency transformer design involves the appreciate choice of magnetic material, winding topology, and conductor size. The design process should take into account many criteria such as isolation, temperatures rise and leakage inductance and optimize the size, weight and efficiency. The winding process should select the appropriate winding method and winding arrangement and compromise between the leakage inductance and stray capacitance. The thermal model is given and the heat sink of middle frequency transformer is designed. Finally, a1500V/385V/28kW middle frequency transformer is taken as an example and the brief design process and the real middle frequency transformer is presented. In chapter3, winding power loss and magnetic core loss expressions for a MFT are studied. In order to evaluate the winding current’s harmonics, a clear expression for instantaneous winding current is given based on the proposed circuit separation method. Based on the winding current harmonics, the winding losses expression considers the current harmonics’ skin and proximity effects is given. In order to evaluate the magnetic core loss, a novel magnetic core losses expression are proposed to consider the additional power loss resulting from the effect of large double frequency sinusoidal component in excitation voltage waveform and magnetic induction waveform. The calculated,simulated and experiment results demonstrate that the proposed method is accurate to estimate the total power loss of MFT with non-sinusoidal excitation waveforms.In chapter4, the unbalanced grid voltage control of EPT is studied. The normal unbalanced control strategy’s drawbacks are analyzed in three phase cascaded rectifier. A novel unbalanced control strategy is proposed to improve low voltage ride-through capability of EPT. The simulations results demonstrate that it is effective to balanced the dc voltages in each phase and restrain the double frequency ripple power in dc capacitor by injecting appropriate negative sequence current.In chapter5, the harmonic minimization problem of the output stage of EPT is studied. A simple and generalized formula expression for the line-voltage THD and Weighted THD of cascaded multilevel converter is first derived and validated. Based on this formula, the line-voltage THD optimized problem is established and solved by the PSO algorithm. The optimal switching angles produced by above optimized problem are used to train an artificial neural network (ANN). The well-designed ANN is used to replace the traditional look-up tables. The calculation and simulation and experiment results show that the proposed generalized formula expression for line-voltage THD and Weighted THD are accrate and simple. PSO algorithm is effective to minimze the line voltage THD and ANN is effective to produce optimal switching angles and control control the output voltage in a real-time and closed loop manner.The seond part of the dissertation focuses on the typical application of EPT. In chapter6, the traction system based on single phase EPT is studied. This new traction system’s input stage is a single phase cascaded multilevel rectifier, a new control strategy is proposed by injecting the minimal reactive current to balance the dc voltages among the series H-bridges when the loads are seriously unbalanced. The completely integrated control system includes the motor torque control is also presented. Numerous simulations verify that the proposed control strategy extends the load’s unbalanced operating range and and the completely integrated control system has a good dynamic behavior.In chapter7, the DC micro-grid based on EPT is studied. In order to optimize the operation of EPT-DC mircogrid, a new nonlinear controller based on feedback-linearization control theory is proposed for direct-driven permanent magnet synchronous generator wind turbine to realize maximum power point tracking algorithlm. Compared with the traditional MPPT algorithm, the proposed one has better performance. The control strategy for different interface dc-dc converters in the new DC micro-gird is also given. Simulation results show that, in islanding mode and grid-connected mode, EPT-DC microgrid has good dynamic behavior and the power management is effective to work.