Coordinated Control Method Of DC Interconnected Distribution Network Based On Power Electronic Transformer

With the intensification of energy shortage and environmental pollution,the upgrading and transformation of the energy structure is imminent.Large-scale using of renewable energy to replace traditional fossil energy power generation for power generation has become one of the inevitable choices to solve this problem.At present,the urban AC distribution network did not consider the functions of distributed generation(DG)connection and regional interconnection based on renewable energy in the initial planning stage.Therefore,building an energy interconnection network with flexible,interconnected,controllable,environmentally friendly,and intelligent characteristics has become an inevitable requirement.Medium-voltage DC(MVDC)power distribution technology has become an important direction for the development of the future distribution network due to its advantages of long power supply radius,high operating efficiency,and closed-loop operation.Power electronic transformer(PET)with multiple types of AC and DC ports can achieve flexible access and efficient matching of DC sources and loads,and provide the possibility of realization for flexible DC interconnected distribution networks based on MVDC-link.It is regarded as an effective carrier for energy routing in the future distribution network.This paper proposes a multiport PET based on the isolated modular multilevel converter(I-M~2C),which is used as a flexible interconnection device(FID)to transform the traditional distribution network and build a flexible DC interconnected distribution network.Relying on the multiple ports of PET,inter-regional MVDC feeders and low-voltage DC(LVDC)microgrids can be constructed without additional equipment.At the same time,it realizes functions such as interconnection of power supply at all levels and regions,and"plug and play"of DG.Because of the single-stage power conversion topology of PET,it can realize independent decoupling control of each port based on AC/DC double modulation freedoms,which improves the flexibility of power flow distribution in the distribution network while ensuring the stability of the distribution network.Firstly,the topology and modulation strategy of I-M~2C based PET were introduced.This new PET totally eliminates the individual capacitors in the traditional two-stage PET,which significantly reduces the number of the power electronic devices and the complexity of control strategy.The flexible DC interconnected distribution network system architecture based on the PET is studied.Through the power supply mode of dual-terminal parallel and the scheme of MVDC feeder interconnection,the anti-risk ability of the distribution network is improved,and the flexible networking of DG is realized by constructing the LVDC microgrid.Furthermore,established a mathematical model of three ports in I-M~2C power electronic transformer based on AC/DC double modulation freedoms,according to the mathematical model of ports and the requirements of different steady-state operating modes,a coordinated control method based on double modulation freedoms for each port is proposed.At the same time,the master-slave control between regions under the cross-regional interconnection mode was introduced.The various operating modes of flexible DC interconnected distribution network were summarized.Finally,a PET prototype based on I-M~2C with 800V/10k W was built to show that the proposed circuit topology is feasible and effective.And a 10k V simulation model was built by Matlab/Simulink to verify the effectiveness and feasibility of the flexible DC interconnected distribution network coordinated control strategy.