UDC-based Robust Control Of VSC Considering The Instantaneous Power Of AC-and DC-side And Stability Analysis Of DC Distribution Network

Due to the increasing consumption of primary fossil energy worldwide and the increasingly severe environmental problems,the power and energy industries are moving towards the integration of renewable clean energy,energy storage units and modern loads by building flexible and efficient multi-terminal DC distribution networks within traditional ac grid infrastructure.Among them,the DC bus voltage is an important factor to measure the stability of DC distribution network and the reasonable power distribution.In this paper,UDC-based robust control model of DC bus voltage is mainly derived in multi-terminal DC distribution network,considering the instantaneous power of AC-and DC-side.On this basis,a 2-DOF(degree of freedom)robust controller with μ-synthesis is designed to ensure that bus voltage is not affected by the system nonlinear disturbance and parameter variations.1)This paper firstly analyzes the shortcomings of bus voltage deviation,current distribution accuracy and system loss when the traditional droop control is applied to the multi-terminal DC distribution network with common dc bus,and proposes a UDC controller to reduce the bus voltage deviation and ensure that the DC-side output current of each VSC is proportional.Then,on the basis of considering the instantaneous power and internal loss of VSC AC-and DC-side,the small signal model of VSC DC-side voltage output under this control mode is analyzed,and the DC voltage control structure and corresponding disturbance factors are obtained.2)Based on VSC DC voltage control structure,the robust controller is designed.Different from the design methods in existing literatures,this paper conducts modeling analysis of unstructured uncertainty and parametric uncertainty for the control structure which is affected by nonlinear disturbance and parameter variations.Unstructured uncertainty modeling is carried out for the simplified current controller to eliminate the influence of its nonlinear disturbance on the output voltage.The more intuitive controller parameters and system parameter variations such as the value of capacitance and inductance on the DC-side are divided into parametric uncertainties to avoid too conservative modeling results.Furthermore,a 2-DOF robust controller is designed by using the μ-synthesis design method,and the results show that the singular value of the closed-loop system structure with the designed UDC-based robust controller is always less than 1 in the controller bandwidth,which satisfies the requirements of the system’s robust stability and performance tracking.A three-terminal VSC DC distribution network model is built in the PSCAD/EMTDC software.The simulation verification was carried out under the normal operating conditions,the system parameter changes and the converter outage conditions respectively,and the results show that the DC distribution network are less affected by nonlinear disturbances and parameter change with the proposed controller in this paper,accurately control DC bus voltage level,guarantee the system robustness.3)In order to analyze the small-signal stability of DC distribution network interconnection system,VSC output impedance with traditional droop control,UDC control,and UDC-based robust control are deduced respectively.From the corresponding Nyquist curve,the first two control methods are susceptible to parameter changes,when large range could lead to instability of converter system.While the stability margin of Nyquist curve of VSC output impedance with the proposed control method in this paper,compared with the rest of the two kinds of control method is low,but not easily affected by the parameters variations,and the ability to overcome all kinds of nonlinear disturbances and parameter changes are verified from the view point of frequency domain.Finally,the output/input impedance of each port subsystem of the DC distribution network is modeled,and the small-signal stability of the interconnected system is analyzed by using the Middlebrook stability criterion.The results show that traditional droop control is still affected by parameter variations,but in the stable range.The amplitude of the minor loop gain with UDC control tends to infinity,and the system is unstable.For the UDC-based robust control,the Nyquist plots of the minor loop gain are still unaffected by the changes in system parameters,which can achieve accurate control of the DC bus voltage and ensure the stability of the interconnected system.