| Solid State Transformer (SST) is a brand new kind of power transformation device with intelligence. Compared with the conventional power transformer, it integrates power electronic conversion technologies, and adopts the intermediate dc link with high frequency isolation. Under proper control strategy, we can control both the primary side and the secondary side of the SST in a flexible way. The prominent characteristics of SST come to that the input current and the voltage are adjustable and the input factor can be set. Considering the above advantages of SST, once installed in the power system, it can increase system controllability and improve power quality. Furthermore, it will be easy for new energy generation to be connected to grid and propel the smart grid forward with the use of SST.The thesis firstly introduces several typical topologies of SST and their control methods during the process of SST development, and makes comparisons about their respective features. And then we propose a cascaded ternary structure with dc link for SST, which consists of input stage, isolation stage and output stage. We conduct the research with the theory mentioned above. For the core part of the SST, the VSC, we build the analytical model for it. Then we formulate the corresponding control strategies for each stage of the SST, double loops decoupling control strategy for input stage, open loop control for isolation stage and double loops with voltage feed forward control for output stage. Flexible control techniques and modulation methods can be chosen to run SST for different control objectives. In this thesis, we adopt the park transformation concept from electro-mechanics and SVPWM modulation to control the VSC section of SST. Considering the excellent properties of SST, we also dig the applications of SST in the new energy micro-grid, and present the concept of FREEDM intelligent micro-grid and its system constitution. The configuration mode of SST in FREEDM is expounded as well. In the last of this part, we analyze the potential application value of SST in photovoltaic power and wind power generation.According to the designed parameters and given control strategies, the simulation model of each stage is put up using power system toolbox of Matlab/Simulink, in which the optimized parameters are calculated for controller part design. And the control rationality is proved by simulation results. The results show that SST gets the function of reactive power compensating. The input stage control guarantees good follow-up properties of input voltage and current for grid side when the load changes, and keeps the current in phase with the voltage. Meanwhile, the constant output of dc side is implemented. The isolation stage can meet the goal of maintaining stable output with open-loop control in the case of fixed ratio. The output stage control are designed to keep the three-phase voltages constant and balanced under load varing. After emulating the abnormal running state of power system with the overall model of SST, we find that the cascaded SST equipped with dc link can be capable of maintaining the stability of the output voltage and the SST improving power supply conditions, isolating the primary side from secondary side effectively.Based on the theoretical analysis and simulation, we design the SST testing system platform with Digital Signal Processor (DSP) and Insulated Gate Bipolar Translator (IGBT). The hardware system includes main circuit, signal gather and adjustment circuit and driven circuit. For the software part, the main program and the interrupt service routine are accomplished on the DSP(model TMS320F2812) development board. Moreover, double loop decoupling algorithm and SVPWM algorithm are written and debugged. In the end, the superior performance of SST is proved by the experimental results to a certain degree. |
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