Control Of Electronic Power Transformer And Realization Of A 10kV Industrial Prototype

With the progress of technology, new developments have emerged in the modern power system. New developments include the following aspects:the increasing scale of the power grid and the growing transmission capacity; massive renewable energies such as wind power and solar energy have been connected to the power grid, which leads to diversified energy sources for the power grid; high voltage dc transmission and high voltage ac transmission co-existing; new concepts have emerged such as micro-grid, smart distribution networks, etc. All of these new developments have the same need that the power grid needs more control and regulation methods. As the key equipment of the power grid, traditional line-frequency power transformer has limited functions, and cannot meet the future need of the power grid. Electronic Power Transformer (EPT) is a new kind of power transformer, which not only has functions of active power transmission and voltage conversion, but also can achieve the continuous on-load voltage regulation, reactive power compensation, harmonic suppression, short-circuit current blocking, and ac/dc hybrid power grid connection, etc. With a variety of control and regulation methods, EPT can make more contributions for the future power grid.This paper first analyzes the operation of the three-phase EPT with star-configuration and separated dc-links under unbalanced conditions. Then active power adjustment capabilities of zero-sequence voltage and negative-sequence current are compared. Based on the comparison, the maximum input current and operating range of EPT using negative-sequence current under a variety of asymmetric grid voltage sags have been analyzed. A novel inner-loop reference current in the input stage is proposed to improve the dynamic response of the system and reduce the dc-link voltage deviations when unbalanced grid voltage sag happens.The second-order ripples in the high voltage and low voltage dc-links of the separated dc-link EPT are analyzed, and the average state space model of the dc-dc converter is obtained by reducing the state space order. When the frequency in the input stage is the same with that in the output stage, an optimal allocation method for the second-order powers is proposed, which leads to a total minimum value of the second-order powers in the high voltage and low voltage dc-links. Meanwhile, the second-order powers are also evenly distributed on the high voltage and low voltage dc-links. Then, two second-order ripple optimal allocation controllers are proposed, and the parameters of controllers are designed.A new auto-balancing EPT based on modular cascaded multilevel structure is proposed, which does not increase the number of main circuit components, does not increase the cost of hardware, does not increase the difficulty of hardware design and manufacturing, and also can be easily assembled for a high voltage high power device. For this topology, the corresponding control algorithm is proposed. The control algorithm uses the reactive current injection method to expand the maximum operation range of cascade H-bridges when serious active power imbalance happens. In the control algorithm, the reactive current is divided into the leading reactive current and the lagging reactive current. An optimization algorithm is used to obtain the minimum reactive current for different kinds of unbalanced conditions.The design and implementation of a 10kV/400V 500kVA three-phase EPT industrial prototype are presented, which are including main circuit topology, parameters and assembly, control system architecture, control and protection system implementation, laboratory test and field test, etc. The industrial prototype is assembled using the modular design method, which can reduce manufacture and maintenance difficulties, and also can be helpful to the further expansion of the device. The dc-link voltages in the low-voltage side are chosen as feedback values to control the input stage H-bridges, which can decrease the cost of hardware since there is no need for high-voltage dc-link voltage measurement modules. The advantages and disadvantages of several possible methods for individual dc voltage balancing methods are discussed, then the method of adjusting the output power of the EPT is chosen. A novel method of individual dc voltage balancing method is proposed which can effectively eliminate the reactive power circulations brought by the conventional methods.Finally, laboratory test and field test verify the good performance of the EPT industrial prototype, which are including constant frequency constant output voltage regulation, unbalanced load operation, reactive power compensation, harmonic suppression, etc.