Research On Voltage Control Strategy Of Flexible DC Distribution Network

The flexible DC(Direct Current)distribution network has the advantages of small line loss,large transmission capacity,and easy access to distributed power sources,and has received extensive attention and research by relevant scholars.In the flexible DC distribution network,the voltage is the only standard to characterize the power balance of the distribution network.Due to the existence of multiple ports and multiple modes of operation in the DC distribution network,the coordination control strategies of the converter stations at each end of the multi-terminal flexible DC distribution network are studied in order to maintain voltage stability in the distribution network.The existing control strategy has the problem of excessive dependence on inter-station communication and large voltage fluctuation during control mode switching.To solve this problem,this thesis proposes a new composite control strategy.In addition,the inherent inertia of the DC distribution network is small,and due to the barrier of the power electronic converter,the wind turbine cannot provide inertial support for the DC distribution network when the power fluctuations in the network.For this problem,this thesis proposes to use the rotational kinetic energy stored by the wind turbine to provide inertial support for DC voltage.The main research contents are:(1)Based on the structure of the flexible DC distribution network,a mathematical model of a three-phase two-level voltage source converter is established.The vector control model of three-phase two-level voltage source converter is given by Parker transformation and d and q axis decoupling.The mathematical model of permanent magnet synchronous wind turbine is established and the wind turbine grid-connected converter i_d=0 vector control model.is given.(2)Aiming at the problem that the existing control strategy has high communication requirements and the voltage fluctuation of the mode switching transient process is large,combined with the master-slave control,margin control and droop control,a new composite voltage control strategy is proposed.Under the new composite voltage control strategy,each terminal converter station can spontaneously switch the operating mode according to the set voltage threshold.During mode switching,the master and slave converter stations can simultaneously share unbalanced power,thereby reducing voltage fluctuations.When the main station is working at a constant voltage,the DC voltage can be maintained at the rated voltage level.After the main station loses the voltage regulation capability,each slave station spontaneously compensates the unbalanced power according to the droop curve,and keeps the DC voltage running within the normal interval.Finally,the two-terminal DC distribution network model is built in PSIM(Power Simulation).The simulation results show that the proposed control strategy has better voltage control capability.(3)Due to the existence of a large number of synchronous generators;the inertial control strategy of the wind turbine is proposed.The calculation method and adjustment method of virtual capacitor of DC distribution network are obtained.The coupling relationship between DC bus voltage and wind turbine speed is established.The variable of wind turbine speed is calculated by the fluctuation of DC voltage,and the maximum power point tracking curve coefficient is adjusted to control the release of rotating kinetic energy to improve the inertia of DC distribution network.The MATLAB(Matrix Laboratory)was used to establish a DC distribution network model with wind power generation.The simulation results show that after the inertia control of the wind turbine is added,the DC voltage change rate caused by the load fluctuation is significantly reduced,and the change of voltage tends to be gentle.