For the sustainable development of human utilized energy, and the friendly environment in the future, the renewable energy sources have experienced a constant and rapid growth in recent years, thus the renewable energy based distributed generations (DG) continue to increase in the power system.The connections of the renewable energy sources to the power system are mostly through the power electronic converters. Moreover, for high controllability and flexibility, power electronic devices are gradually acting as the interface between different networks in power systems, promoting conventional power system to a smarter stage. From a generation plant to transmission and distribution networks, the interconnection of power electronic converters will be widely confronted in the near future. Additionally, the combination of distributed generations, local loads, and storage devices promotes the bidirectional power flow in the distribution level of power systems. Therefore direct contact of converters introduces significant uncertainties to power system, especially for the stability and reliability.This dissertation studies the optimization control of the two stages directly connected converters, which is the cascaded power converters, with the objective of improving stability, reliability. Conventional control methods for the power converters are built under different control loops, such as the current loop, voltage loop, power loop, and they are mostly implemented in unidirectional power flow. But the bidirectional application is much different from unidirectional condition, and the stability enhancement is thus critical to the grid interface converter. So this research work will develop control methods to adapt the variation of power flow directions and enhance both the stability and reliability in bidirectional cascaded converter.This research work analyses the control strategies based on the topology of dual active bridges converter cascaded with a three phase inverter. It firstly proposed a dc link voltage and active power coordinative control method for this cascaded topology, and it can reduce dc link voltage fluctuations, enhancing the dc link voltage reliability in case of one sub converter failure. Then the bidirectional power flow effect is analyzed, and an important guide line is proposed for the design of the two stage cascaded converter system. Towards the different stability in different power flow directions, a bidirectional impedance control method is also proposed to unify and improve the system stability. Control system type number is also analyzed in this thesis, then a symmetric proportional control method for the two stage cascaded converter is proposed to reduce the dc link voltage control system type number, which is capable to improve system stability. Afterwards, this dissertation comes up with the concept of front to end impedance control method for the two stage cascaded converter, and it can greatly improve the system stability. At last the thesis concludes the whole research work and outlook the future development trends.
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