Research On Control Strategy Of Flexible Multi-state Switch And Energy Storage System Access

The operation structure of the existing distribution network is unreasonable.At the same time,due to the large number of distributed power sources,the direction of the feeder flow of the distribution network is changeable,and voltage fluctuations and over-limit situations occur more frequently.The original tie switch is not enough to solve the above problems.Therefore,the flexible multi-state switch（FMSS）based on power electronic technology has become a research hotspot.Multi-port FMSS is connected to the end of the feeder to regulate the power flow of multiple feeders and at the same time through reactive power compensation.Measures such as voltage fluctuations are suppressed.Research the multi-port FMSS coordinated control strategy to ensure the stable operation of the FMSS system,and design a highly integrated energy control system of"FMSS+energy storage"to complete the space and time scale power flow optimization.This article mainly completed the following work:（1）Research the basic topology and basic principles of FMSS access to the distribution network;established the research object as the three-terminal FMSS based on the MMC topology,analyzed the topology and working principle of the MMC sub-module,and deduced the three-phase MMC.The working principle of FMSS is analyzed,and the mathematical model of FMSS is analyzed to provide a mathematical basis for the derivation of control strategies in subsequent chapters.The relevant valve group-level control strategy is introduced to complete the bottom-level control target in the FMSS port MMC.（2）The FMSS device-level control strategy is analyzed,and the FMSS work mode coordinated control and valve group-level control are analyzed,designed and improved based on model predictive control.First,analyze the traditional PI double closed-loop control.As a multi-port nonlinear system,FMSS has many cascaded PI controllers with many parameters and complex design.The improved three-port FMSS coordinated control strategy is based on the predictive control of the finite control set model,which combines the design of the working mode in the device-level control with the control of the sub-module capacitance voltage equalization and circulation suppression in the valve group-level control,and works on the PQ.The coordinated control of mode,U_dcQ working mode and U_acf working mode is realized.（3）Three-port FMSS is connected to the distribution network as a whole system,and the overall system-level regulation needs to be combined with the operation status of the distribution network.The port device-level control issues upper-level regulation commands.Use the access conditions of the FMSS DC side to connect the energy storage system to the shared DC side to build a highly integrated comprehensive control device of"FMSS+energy storage".The energy storage device absorbs and supplements the DC side power during operation to maintain the overall device power balance,and at the same time improve the utilization rate of the port converter and reduce unnecessary port actions.Through simulation verification,it can be known that the three-level control strategy proposed in this paper can effectively support the work of three-port FMSS,and realize the functions of feeder equalization and distributed power consumption.The overall control strategy of"FMSS+energy storage"has also been verified in simulation.