Study On Three-phase Four-line Power Electronic Transformer And Its Interaction Control With Distributed Generation

Due to the increasing demand for electricity causing the gradual depletion of fossil fuels and environmental pollution, wind energy and photovoltaic cells, as the representative of distributed generation (DG), power generation technology of which is continuously developing, particularly grid-connected technology of DG through the interface inverter connecting grid growing concern. Grid-integration of DG can achieve effective complementarity between large grid and DG, thus improving the power supply reliability of users to a certain extent, but there are some problems in traditional grid-connected way such as power flow not controlled, not effective fault isolation between sides.To solve the above problem, power electronic transformer (Power Electronic Transformer, PET) can be applied to DG grid structure. It has some functions such as power conversion, adjustable power factor, controllable two-way power flow and its structure with AC and DC bus is easily accessed to all kinds of DGs. Since most loads of the distribution network are unbalanced, and when a large number of DGs access to the distribution network in the single phase form, the output power fluctuation of DG easily leads to each phase power flow unbalance of the distribution network and low utilization rate of DG energy.Therefore, this paper proposes DG grid-connected structure based on three phase and proposes the corresponding coordination control strategy to achieve maximum use of DG energy. First, model and control strategies at all levels of the three-phase four-wire PET topology are analyzed and the effectiveness of control strategy at all levels is verified by simulation. Focusing on the three-phase four-leg output stage decoupling control with decoupling into three independent single-phase inverters, which is the theoretical foundation for the interphase flow control. Secondly, the operating characteristics of single phase DG is analyzed separately from the perspective of power distribution control under DG grid-connected and grid-off mode, and the ettectiveness of the control methods in two modes is verified by the corresponding simulation analysis. In the off-grid mode, in view of power distribution inaccuracies of traditional droop control, improved droop control based on voltage and frequency recovery is proposed to achieve the accuracy of power sharing, and small signal model is established to verify the proposed method improving system stability. In the grid-connected mode, for controlling DG as voltage source and making DG output power according the scheduling command, PQ droop control is adopted. Then, power coordinated control between PET and DG is studied from two aspects including power flow distribution between PET and overall three phase DG, distribution network interphase power flow distribution considering single phase DG connected into distribution network. The part focuses on coordinated control with the distribution network interphase power flow unbalanced distribution. In order to ensure maximum utilization of DG energy, PET uses improved phase droop control to achieve power flow from earnings phase to shortfall phase while DG adopts PQ control. At the same time, the voltage amplitude and frequency deviation caused by power fluctuation are quickly adjusted through the voltage amplitude and frequency correction link, which improves the stability of distribution network operation. And through simulation the effectiveness of the proposed control strategy is verified. Finally, in the paper, a three-phase four-wire PET system platform is also built and experiments verify the effectiveness of the proposed control strategy.