Research On Converter Non-disturbance Grid-connected Strategy In Medium And Low Voltage DC Distribution Network

Building the flexible DC distribution network is an important means to absorb new energy power generation,improve power quality,and establish an urban energy Internet.It is also an important measure to achieve carbon peaks and carbon neutrality.As the key equipment in the DC distribution network,the inverter is also one of the weakest links in the DC distribution network.In the scenario of converter station’s failure recovery,expansion or micro-grid’s islanding to the grid,when the offline converters were re-connected to the DC distribution network without an effective grid-connection strategy,it more prone to being disturbed and thus lead to secondary fault.Therefore,restraining the inrush current and voltage fluctuation caused by offline converters were re-connected to the grid is of great significance to promote the construction of DC distribution networks.Taking the medium and low-voltage DC distribution network as the research object,this paper focuseed on the non-disturbance grid-connected control strategy of modular multilevel converters and the non-disturbed grid-connected control strategy of DC transformers.At the medium-voltage level,the main research was the non-disturbance grid-connected control strategy of modular multilevel converters.F irstly,the grid-connected equivalent model of the AC side and DC side of the modular multilevel converters were established.The grid-connected disturbance mechanism was analyzed from the aspects of converter start-up and operating mode switching.On this basis,a non-disturbance grid-connected strategy for modular multilevel converters based on smooth switching of control modes was proposesed in this paper.The starting inrush current was suppressed by the soft-start strategy.Through the smooth switching strategy,the control signal jump caused by mode switching was avoided,and the voltage disturbance was reduced.Then,Matlab/simulink was used to establish a non-disturbance grid-connected simulation model of modular multilevel converters to simulate the DC-side grid-connection and AC-side grid-connection of the converter.The results showed that the grid-connected disturbances were in compliance with the non-disturbance grid-connected evaluation index defined in this paper.Finally,the effectiveness and versatility of this strategy were verified by experiments on the semi-physical real-time simulation platform Star Sim.At the low-voltage level,the main research was the non-disturbance grid-connected control strategy of DC transformers.F irstly,the disturbance mechanism of DC transformer grid-connected was analyzed from three aspects: DC transformer starting,DC transformer control mode switching,and photovoltaic-energy storage system control mode switching.The main cause is the capacitor charging and the controller ou tput signal jump.Then,for problems in the controller signal jump,the smooth switching strategy of DCT operation mode and the smooth switching strategy of PV operation mode were respectively proposed.The control of the DCT is coordinated with the contro l of the system through the switching of the control mode,ensuring that the control rights of the middle/low voltage bus voltage under the master-slave control are owned by one.Combined with the soft-start method of duty cycle and voltage control coordin ation,a non-disturbance grid-connected strategy for DC transformers based on multi-converter coordination was proposed.Finally,Matlab/simulink was used to establish a simulation model for DC microgrid grid-connected via DCT and PV and storage system gri d-connected via DCT.By comparing with the traditional grid-connection strategy,the whole process of DC transformer grid-connection without disturbance was analyzed.It was verified that the strategy can effectively suppress the voltage fluctuation and in rush current at the grid-connection point,and ensured the stable operation of the system.The non-disturbance grid-connected strategies of the converters studied in this paper considered different voltage levels and different grid-connected scenarios.It has certain reference value for the online connection of converters in medium and low-voltage DC distribution networks,the smooth connection for new energy power generation,and the suppression of black start disturbances.And it can improve the power supply reliability of the medium and low-voltage DC distribution network system.