As the decarbonization policy has been reinforced by suggesting carbon neutrality and reduction targets for each country in response to climate change in the international community, the amount of power generation and power generation facilities using renewable energy is increasing. With the proliferation of new and renewable energy sources, DC-based power conversion devices and direct current technology application methods to power distribution systems have been increased and becoming popular. Furthermore, the medium-voltage direct-current(MVDC) and low-voltage direct-current(LVDC) have been replacing the existing AC- based distribution systems. The structures for the DC distribution system, which provide higher efficiency and flexibility compared to existing AC distribution systems, can be classified into unipolar and bipolar structures. In the unipolar structure, it provides a single common DC voltage. In the bipolar structure, on the other hand, the neutral point is shared between the positive and negative DC poles, which allows two DC-link voltage to be independently provided. The bipolar structure enables stable operation and flexible response through continuous half-power supply with a healthy pole, even if a DC-side fault occurs at one of two poles. However, the bipolar structure applied to the existing bipolar DC distribution system has a disadvantage that an additional power conversion circuit such as a voltage balancer is required to cope with different loads of each pole of the DC-side. In addition, there is a disadvantage in that the DC-link response characteristic is limited by the DC-link capacitor of each converter.
This thesis proposes a new single-stage power conversion circuit using a zigzag transformer without additional power conversion circuits(e.g. voltage balancers or DAB). The proposed circuit provides a decoupled independent DC-link control of two DC-link voltage in bipolar DC distribution and a fast dynamic response for controlling DC-link voltages. The feasibility of the proposed circuit method was verified through both simulations and experiments.