Complementary Optimization Of Hybrid AC/DC Distribution Networks With Power Electronic Transformer

Large-scale grid-connected renewable sources bring new challenges and higher requirements for flexible access and effective regulation of power system.In addition,decentralized geographical distribution characteristics are not conducive to centralized optimization and coordinated control of networks.The AC/DC hybrid system based on power electronic transformer(PET)can realize interconnection and mutual benefit of distributed generations in a wide range,which has become a new development trend.Therefore,how to exert the flexible regulation capability of power electronic transformer,and how to coordinately control distributed generations interfaced the AC/DC hybrid distribution network to achieve complementary operation are urgent problems to be solved.The topologies and functional characteristics of PET are analyzed,and the multi-port PET steady-state model considering the loss characteristics is established.Based on the system perspective,the deterministic analytical expressions are used to establish the VSC power injection model of PET,considering the converter loss,which can elaborate multi-port characteristics and loss characteristics of PET.The topologies of PET are diverse,and the loss characteristic curve is difficult to describe by analytical method.Thus,a kriging interpolation model is proposed for optimization analysis of PET,which isn't affected by the loss characteristic curve and has better flexible properties.In order to coordinate active power of distributed generations in different networks and realize the complementary operation of AC/DC networks,a complementary optimization model of AC-DC hybrid distribution network with PET is proposed in this paper.The low-voltage AC side is a three-phase asymmetric system.The three-phase unbalance inhibition mechanism of PET is analyzed.Then,the optimization model with the objective function of the minimum active power loss,considering the three-phase unbalance constraint,is established to improve economy and reliability of the system.Futhermore,the controllable distributed generations in the AC/DC network are coordinated by PET.The two-layer control strategy is adopted to improve the autonomous operation capability of the AC/DC distribution networks and the utilization efficiency of the distributed generations.On this basis,the target cascade analysis method is adopted to build up the complementary optimization model of AC/DC networks with PET,and the coupling variables and local variables are decoupled and coordinated to realize the mutual coordination and complementary optimization of AC/DC networks.In this paper,an example of AC-DC hybrid distribution network with PET is constructed,and the proposed model is verified by simulation analysis.The results of comparing PET and traditional transformer show that independence and adjustability among three phase of the port is beneficial to the comprehensive suppression of network voltage unbalance factor and the reduction of the system's active power loss.By comparing the optimization results of the centralized coupling optimization model and the decoupling complementary optimization model,it is verified that the kriging interpolation model proposed in this paper can adapt to the flexible and diverse PET loss characteristics,and the complementary optimization of AC/DC networks can be realized by the decoupling complementary optimization model.