Control Of Interlinking Converter In AC/DC Hybrid Distribution Network

High penetration of renewable energy generation accerlates the process that power electronic equipment is used in all fields of source-network-load-storage.The situation of unidirectional power flow in conventional distribution network has been broken.Active distribution network,which can adjust the power flow direction flexibly and balance the power between source and load,has become a hot spot research.It is a potential shape of future distribution network to take AC distribution system and DC distribution as main axls and achieve the extensive interlinking of grid,renewable energy,load and storage.The thsis is intended to study the topology of AC/DC hybrid distribution system and the control strategies of interlinking converter,aiming at improving the stability and reliability of AC/DC distribution system,hoping to boost the development of AC/DC distribution system.Based on the summary and analysis about the topology of AC/DC hybrid distribution system,An AC/DC distribution system with common DC bus has been proposed according to partition plan of AC distribution network.The interlinking converter in AC/DC hybrid distribution network adopts the transformerless structure with modular multi-level.DC distribution system enhances the capacity of distribution network to consume distributed renewable energy and supply power for DC load.Then a simulation of AC/DC hybrid distribution system is built on Matlab/Simulink,the simulation result Validated the feasibility of common DC bus and transformerless structure.Fault in AC distribution system is a serious threat to stability of AC/DC hybrid distribution network,thus supplementary control strategies with self-adaptive fault recognition based on interlinking converter have been proposed.For three-phase short circurt fault,the load-transfer by DC distribution improves power system reliability,a grid-island switching process has been proposed for interlinking converter to reduce the impulse during load-transfer.For phase short circuit fault,the suppression control of negative sequence current has been adopted to realize the fault ride-through of interlinking converter.For single phase grounding fault,the mechanism that zero sequence current spreads into non-fault AC system through DC system has been analyzed.Zero sequence current controller supported by hybrid submodule topology and novel capacitor charge-discharge control is proposed to isolate zero sequence current.Integral control strategy synthesizes the supplementary control strategies and operation control strategy,ensures the stability of interlinking converter during normal or failed AC system.With the high proportion of power electronic equipment widely used in AC/DC hybrid distribution system,new stability concerns have arisen,the converter-driven stability of both AC and DC system needs Separate analysis.Firstly,the impedance model of DC distribution system has been built.Impedance analysis method is used to anlysis the DC voltage stability of DC distribution system with constant power load.According to the anlysis,virtual resistance control is adopted to improve the damping of system and suppress the voltage oscillation.The RC parallel compensation has been proposed further to modify the impedance characteristic of interlinking converter,and the risk of voltage oscillation is successfully eliminated.Then,the causes of high frequence oscillation in weak AC distribution system has been found by analyzing the principle of phase locked loop in interlinking converter.Supplementary damping control aimed at interlink converter and virtual impedance compensation aimed at grid feedforward voltage have been proposed,and simulation result validated the high frequency oscillation suppression effect of proposed control strategies.