Research On Control Strategies For Flexible Interconnected Low-Voltage Distribution Networks

With the increasing penetration of distributed energy and the widespread adoption of fast charging stations for electric vehicles,the distribution network faces various power quality issues such as high harmonic current content,unbalanced three-phase current,and node voltage exceeding limits.Traditional power quality control solutions have been difficult to meet the high energy quality requirements of distribution users under the new scenario of widespread access to new sources and loads due to the problems of the radial network architecture,one-way power supply mode,and insufficient flexibility of primary regulation equipment in the distribution network.By using intelligent soft normally open points(SNOP)based on power electronics technology,traditional contact switches can be replaced,and different feeders can be flexibly connected to fully utilize the local reactive compensation capability of device ports and the active control capability between multiple ports.This approach allows for flexible regulation and optimized operation of distribution network flow.In this thesis,the flow control potential of SNOP in distribution networks is explored,and the flexible interconnected low voltage distribution network control strategy based on SNOP is studied to achieve comprehensive power quality control of low-voltage distribution networks under the new scenario of widespread access to new sources and loads.The main content of this article includes:Firstly,a local power quality control strategy for distribution networks based on SNOP is proposed.The three-phase four-bridge arm SNOP mathematical model and power transfer characteristics are deduced by comparing three common topologies of SNOP and taking the back-to-back structure voltage source converter(VSC)as the research object.To address the problems of large harmonic current in the distribution feeder and three-phase current imbalance,a single-ended power quality control scheme of SNOP is studied.This scheme is based on instantaneous reactive power theory,adopts the"i_p-i_q"current extraction method to extract the load-side fundamental positive sequence current component,and uses a hysteresis controller to adjust SNOP to form compensating current,thus improving the power quality of the single-ended feeder.The paper also conducts simulation verification of power quality control under nonlinear and unbalanced load conditions.Secondly,a flexible interconnection distribution network terminal voltage limit violation control strategy based on SNOP"active-reactive"collaboration is proposed.In response to the problem of voltage limit caused by the excessive output of distributed energy on the distribution network feeder line,a SNOP-based collaborative voltage regulation scheme is proposed.Compared with the existing regulation scheme,the proposed scheme has a larger voltage regulation range under the same capacity,which can effectively improve the acceptance capacity of the distribution network feeder line to distributed energy.In addition,a multi-state collaborative voltage regulation control strategy based on SNOP is proposed according to the initial voltage of each feeder node and combined with energy storage devices.This strategy can effectively control the voltage limit of the feeder node while reasonably allocating the regulation power,thereby ensuring that the voltage of the remaining feeder lines is within a safe range.The effectiveness and applicability of the proposed control strategy are verified by simulations.Finally,a flexible interconnect distribution network voltage coordinated control technology for weak communication conditions is proposed.In the distribution network environment with poor communication conditions,"VSC-ESS"integrated and"VSC-ESS"separate distribution network voltage coordinated control strategies are designed,respectively.The proposed control technology uses the DC bus as the communication channel to achieve voltage limit control in a weak communication distribution network environment while ensuring the voltage levels of the remaining feeder lines.The proposed control technology is validated through MATLAB/Simulink simulation.