Research And Realization On The Key Technologies Of Electronic Power Transformer

The power system is one of the most important parts in the modern society, and power transformers are widely used in that as the extremely important electrical components, which realize the voltage level change and the galvanic isolation. During the past several decades, power system is larger in the scale, more complicated in the network structure and more complex in the operation mode, so that the stability problem becomes more serious. And larger unbalanced non-linear loads are connected to the grid which would generate non-sinusoidal currents and deteriorate the grid voltage, so that it affects the power quality seriously and even affects the reliability of power supply for those loads which are sensitive to power quality. Recent year, the concept of "Smart-Grid" is expounded in some papers, and the network structures and the smart operation modes of "AC&DC Micro-grids" have been a hot topic. However, the traditional power transformers cannot resolve the stability problem, cannot improve power quality, and cannot provide flexible operation modes and power grid interfaces for "Micro-grids".Electronic power transformer (EPT) is an entirely new type of power transformer. Based on power electronics technology, EPT retains the basic functions of traditional power transformer and extends additional functions, such as the fully controllable of the active/reactive power and the flexible DC&AC power grid interfaces. Hence, EPT can meet the demands of "Smart-Grid".In the dissertation, several topologies of EPT are introduced firstly, and the modularized cascaded multilevel topology of EPT is well suited for the high-voltage and high-power applications. Based on the conventional topology, the internal model is established for EPT, and the formulas of calculating the electrical parameters are derived. Finally, the experiment is carried out and the experimental results could verify the rationality of the model analysis. In succession, the dissertation provides guidance on the design and selection of the key elements of EPT for the designers. According to the model analysis, the optimal control strategy on the phase angles of AC voltage/current is developed to restrain the electrical parameters in order to reduce the size and cost of EPT. EPT provides a new harmonic suppression scheme for power system. Firstly, the harmonic voltage/current from primary/secondary side cannot flow through EPT. Secondary, EPT itself do not produce excess harmonic current by Cascaded Multilevel circuit, by parallel operation and by optimal control strategy. Based on the optimal control strategy, the optimal regulation of DC-bus voltage can better suppress harmonic current of rectifier of EPT.There are several shortcomings of the Cascaded Multilevel topology of EPT, so that the Modular Multilevel topology of EPT (MMC-AEPT) as a novel auto-balancing topology shall be found, and it has many benefits. It can make one side balance if the other side is not. In addition, MMC-AEPT can provide DC-voltage power grid interfaces for other grids, and becomes the key component of "Smart-Grid". The simulation results show that MMC-AEPT can keep the system auto-balancing.Finally, the hardware of10kV/400V0.5MVA EPT is proposed in detail. The design methods and the design principles of each component are proposed in the dissertation, and they can also be used in the other similar devices.