| Solid state transformer(Solid State Transformer,SST)based on power electronics technology and high-frequency transformer technology has been widely studied by experts and scholars due to its advantages of optimal power quality control,fault isolation and AC/DC energy management.The existing SST usually adopts the threelevel structure of rectifier,isolation and inverter,among which the isolation stage is the key part for SST to realize the energy conversion and isolation between power grids.Generally,the isolation stage adopts the modular dual-active-bridge(Dual-active Bridge,DAB)converter.With the increase of grid voltage level and output power,the number of DAB converter modules required by the middle stage also increases,which also increases the design and control difficulty of the middle stage.To solve the above problems,this paper proposes a solid state transformer based on multi-winding high-frequency transformer(Solid State Transformer based on Multi-Winding High-frequency Transformer,MHT-SST).Compared with the traditional SST,the MHT-SST intermediate stage adopts the multi-active-bridge(Multi-active Bridge,MAB)converter based on the multi-winding high-frequency transformer,which is more integrated in structure and more flexible in control.It can greatly reduce the device volume and avoid the errors and un-synchronization caused by the independent control of each module in the traditional SST intermediate stage.This paper focuses on the optimal control and the secondary ripple voltage suppression of MHT-SST.The main work is as follows:(1)An average power control method of MHT-SST based on low voltage side power feedforward is proposed.Firstly,the power characteristic of MHT-SST is analyzed,and the average mathematical model of MHT-SST is established.In view of the existing SST control strategy lacking of considering the actual parameters,the average power control method of MHT-SST based on low voltage side power feedforward is proposed,which can simplify the overall control and keep the balance of the transmission power between ports.Finally the simulation verify the effectiveness of the proposed control method.(2)MHT-SST secondary ripple power decoupling strategy is proposed.First,combined with MHT-SST middle class structure,the mechanism of the secondary ripple power of MHT-SST is analyzed and the MHT-SST secondary wave power coupling model is established.Then,this paper proposes the secondary wave power decoupling control strategy,which can reduce the impact of the impulse voltage in the transient process and lift the response speed of the device.At last,the proposed decoupling strategy is verified by the simulation.(3)The second ripple voltage suppression method of single-phase and three-phase MHT-SST is proposed respectively.According to the generating mechanism of secondary ripple power of MHT-SST and the decoupling strategy of secondary ripple power,the secondary ripple voltage suppression method is proposed for single-phase and three-phase MHT-SST respectively.Finally,the simulation and experimental models of single-phase and three-phase MHT-SST are built to verify the effectiveness of the proposed strategy. |
