Research On Control Strategy Of Flexible HVDC Transmission Based On Grid-connected New Energy

With the rising proportion of new energy generation,the proportion of new energy power connected to the power grid is getting higher and higher.However,new energy power generation is greatly affected by external environmental factors,showing the characteristics of volatility and randomness.If it is directly connected to the grid,it will seriously affect the power quality of the power grid.Voltage source converter based high voltage direct current transmission(VSC-HVDC)is a new transmission mode.On the one hand,VSC-HVDC can realize active and reactive decoupling control and supply power to isolated islands;on the other hand,VSC-HVDC can improve the transmission capacity and stability of the system and meet the requirements of distributed generation transmission.High-voltage flexible DC transmission plays a significant role in grid connection of new energy sources such as photovoltaic and wind power,and can be used as an ideal distributed generation transmission scheme.The establishment of large-scale new energy electric fields puts forward higher requirements for grid-connected technology and long-distance large-capacity power transmission technology.In order to improve the reliability and stability of flexible HVDC system transmission,this thesis has carried out the research on the VSC-HVDC low voltage ride-through strategy and system control strategy of the grid-connected photovoltaic power station.First,the current status of photovoltaic power generation grid-connected technology and DC transmission control strategy research is described,and the working principle and mathematical model of the converter station of the DC transmission system are analyzed;the basic control strategy of flexible DC transmission and the common controllers at the converter station level are summarized.And the overall control structure of flexible DC transmission at both ends.Then,it briefly summarizes the operating characteristics of the photovoltaic power generation system,and analyzes the principles,advantages and disadvantages of the current low-voltage ride-through strategy of photovoltaic power stations.It is proposed to add a load unloading device near the DC end of the inverter side of the flexible DC transmission system,and at the same time add static Static Synchronous Compensator(STATCOM),when the grid connection point fails,the unloading device consumes the "surplus electric energy" generated by the photovoltaic power station,and STATCOM releases necessary reactive power to the grid to support the grid connection point voltage to avoid further expansion of the fault range;In order to verify the stability of the strategy,a 1MW photovoltaic power station was built with a flexible DC transmission system grid-connected model using Matlab/Simulink simulation software,and the unloading device was not added,the static synchronous compensator was not added,the additional unloading device and the static The comparative analysis of the three situations of the synchronous compensator shows that the scheme improves the low-voltage ride-through capability of the system and avoids the photovoltaic system from offline operation due to the transient fault of the grid.Finally,the two photovoltaic power plants are connected to the grid via VSC-HVDC,and at the same time,the control strategy for the island power supply system is studied,and the coordinated control strategy based on the micro-synchronous phasor measurement unit(μPMU)is proposed.The outer loop is designed on the idea of margin control.Voltage controller,built a four-port DC transmission model in Simulink simulation software,including two photovoltaic power plants,one-port AC power grid and one-port passive network,which simulates changes in light,photovoltaic power plant withdrawal and grid failure,etc.System operating conditions and simulation results show the reliability and applicability of the strategy.The thesis has 55 pictures,3 tables,and 77 references.