Research On Modeling Methodology And Control Technology Of Power Electronic Transformers

With continuous economic and social development, the demands for grid stability, security and economic operation are increasing significantly. As an important element of smart grid much preferable to the traditional transformers, power electronic transformer (PET) renders good prospects for future development, which consists of power electronic conversion topology and a high-frequency transformer used to achieve magnetic coupling. A PET can be used to achieve voltage transformation and energy transfer in intelligent power systems, with unique advantages including flexible control of the voltage magnitude and phase shift, high system stability, convenient integration of distributed renewable power generation, real-time control of power flow and so on.In the first stance, fundamentals of power electronic transformer as well as some typical implementations of the physical topology were elucidated in the thesis. Then an equivalent model for dynamic analysis of PET was established, further, dynamic characteristics of PET were studied under different loading conditions. The results show that, a three-phase PET based on PWM and double closed-loop control scheme can achieve unity power factor and produces little harmonic currents for the inductive load. Under capacitive loading, A PET with PWM rectifier's double-loop controlling in the primary side can also approximately realize unit power factor.Parallel operation of multiple PETs can provide even high reliability, flexible power supply and reduced spare capacity. Further studies were carried out, including parallel operation of PETs as well as PETs with conventional transformers. In addition, different control strategies regarding host machine and slave machine control were also analyzed in details. The host control can effectively maintain the DC side voltage and the input current waveform, which makes the systems equivalent to a linear load. While for the slave machine control, it makes the systems equivalent to a linear load, and the secondary side equivalent to a controlled current source.Simulations were also done for applications of PETs in the power systems, regarding various dynamic behaviors including system stability, suppression of voltage collapse, power flow control, fault current limiting and power quality improvement. The research results indicate that. PET-based applications can control the power factor close to unity while voltage fluctuation emerges in the distribution network, while for the transmission system. Application of PETs can potentially improve damping performance to disturbances, rapidly eliminate system failures and effectively maintain stable operation of the systems.The proposed research provides fundamentals and useful reference for further developing the key technologies of PETs.