Uncertainty And Disturbance Estimation-based Control For Robust Controller Of VSC In DC Distribution Networks

The development of distributed energy and DC load brings challenges to the traditional AC power grid,which makes the voltage source converter(VSC)multi terminal DC distribution network become the development direction of power supply in the future.However,as a small inertial system,DC distribution network is vulnerable to various disturbances and instability.The key factor for the stable operation of DC distribution network is to ensure the stability of DC bus voltage,so it is of great theoretical significance and academic value to study its control method and control strategy.In this paper,the control objective is to improve the voltage tracking and disturbance rejection of DC distribution network under various conditions,and realize the reasonable distribution of load current.The robust control method is proposed from the converter valve level and converter station level respectively,and the design of DC distribution network integrated robust controller is completed.The main research works are as follows:(1)Based on the theoretical derivation of VSC double closed-loop proportional-integral(PI)control small signal model under traditional droop control,a three source DC distribution network topology system is built in PSCAD to verify the small signal stability of VSC model and the shortcomings of traditional droop control,that is,it is difficult to take into account the bus voltage deviation and current distribution accuracy,which lays the foundation for the follow-up study of converter valve level and converter station level robust voltage control.(2)The stage control of converter valve is to control the performance of converter.In order to improve the coupling effect and tedious parameter tuning process of the inner current loop of traditional double closed-loop control of VSC,and to better control the output DC voltage of converter,a robust inner current controller of VSC based on uncertainty and disturbance estimator(UDE)control is proposed.The designed inner loop robust current controller uses UDE to estimate and compensate all uncertainties and disturbances in the inner loop current control part of VSC,so as to improve the disturbance rejection of current control and the output performance of converter.The two degree of freedom characteristics of the proposed controller are analyzed from the aspects of tracking and disturbance rejection,and a two degree of freedom bandwidth parameter tuning method is proposed.Based on the VSC small signal model,the stability of the proposed controller is analyzed by simulation under the condition of variable system parameters.The simulation results show that the proposed controller can effectively improve the tracking and disturbance rejection of the inner loop current of the converter compared with the traditional PI control,and can control the DC bus voltage well while ensuring the robustness of the system;at the same time,it verifies that the proposed two degree of freedom bandwidth parameter tuning method can not only realize the decoupling control between the two control bandwidth parameters,but also greatly simplify the parameter tuning process and help to achieve good robust control effect.(3)The converter station level involves the local coordinated control between the converter station and the system.A robust droop control method based on UDE is proposed to solve the problem that the traditional droop control is restricted by DC bus voltage regulation and load current distribution.The controller is divided into load current reference part and UDE control part.In the first part,a reasonable reference value of load current is set according to the droop principle,which is the basis of accurate current distribution;in the second part,a UDE control law is designed to make the actual output load current of each source converter gradually track the reference value of load current obtained in the first part,and provide the reference value required by VSC voltage outer loop control,which is the key to achieve robustness.Taking three source DC distribution network as an example,the output / input impedance Nyquist stability of single VSC unit and the Middlebrook stability of secondary loop of common DC bus source load interface are simulated and analyzed.The results show that VSC and DC distribution network have high stability margin under various disturbances and parameter changes.Simulation results show that the proposed droop control method has better load current distribution performance and stronger bus voltage control robustness than the traditional droop control method.The proposed control from converter valve level to converter station level is applied to the DC distribution network for simulation analysis.The results show that the design of integrated robust control for DC distribution network based on UDE proposed in this paper can ensure the accurate distribution of load current,improve the robustness of DC bus voltage and maintain the stable operation of DC distribution network.