Research On Stability Of DC Distribution Power System

In recent years,with the massive connection of distributed photovoltaic and the wide application of DC loads such as electric vehicles,data centers,and LED lighting,the power system shows the characteristics of a high proportion of renewables and a high proportion of power electronic devices.The DC distribution power system with the advantages of high transmission power,high power quality,high efficiency,and simple and flexible control has been developed rapidly.The distributed power sources,energy storage,and load units in the DC distribution system are connected to the DC bus through power electronic converters,which reduces the reliability of the system.The bus voltage control technology among distributed power sources and the oscillation caused by the interaction between power electronic converters restrict the local consumption of renewables such as PV.Distributed power sources such as PV and energy storage have multiple operating modes,and their stability analysis and improvement are relatively difficult.Therefore,the influence of control method,line impedance,and operating modes on the stability and stability criterion in DC distribution power systems are studied in this dissertation.The main works of this dissertation are as follows:1.In order to balance the inherent contradiction between voltage deviation and current distribution accuracy in conventional droop control,a design method of nonlinear droop control is proposed.The coefficient values of the nonlinear droop curve in the heavy load range are designed with a given bus voltage deviation and current sharing accuracy.The influence of droop resistance on stability is analyzed by small signal model,so the linear droop control with negative droop resistance is used in the light load range to achieve a small bus voltage deviation in the full load range.The droop curves in the light load range and the heavy load range are fitted to obtain a nonlinear droop curve to achieve smooth control.The nonlinear droop curve ensures good current sharing accuracy and bus voltage deviation in a wide load range without additional secondary voltage compensation.2.Due to the influence of line impedance,multi-resonance problems are prone to appear in multi-source DC distribution power systems,and a simple resonance point frequency determination and stability improvement method is proposed.The expressions of multiple resonances and corresponding magnitude peaks are derived based on the interaction between line impedance and source converter output impedance.Then,the cut-off frequency of the load converter is designed by the relationship between the resonance peak and the input impedance of the load converter.The voltage and current double-loop control with a large phase margin is used to suppress the amplitude dip at the cut-off frequency.In addition,Series Virtual Impedance(SVI)and Parallel Virtual Impedance(PVI)reshaping methods based on voltage and current double-loop control are designed to improve system stability.3.A Bus Current-Based Series Virtual Impedance(BC-SVI)reshaping method is proposed for the instability problem in multiple modes of distributed sources.Based on the instability problem of multiple modes in the photovoltaic-battery hybrid power system,the design boundary of SVI in multiple modes is derived on the converter with fixed modes to realize amplitude compensation.However,the design boundary does not hold when the resonance peak of the bus voltage-controlled converter(BVCC)is too large,so the SVI reshaping method can only realize the amplitude compensation for a single mode.On this basis,the bus current is fed back to realize BC-SVI compensation,which can realize the amplitude compensation of multiple modes simultaneously and enhance the system stability.4.For multi-source and multi-mode DC distribution power systems,the existing criteria need to predict the number of Right Half-Plane(RHP)poles of all structured subsystems to determine the system stability,are relatively complicated to judge the stability,the multi-mode DC distribution power system has multiple networks structures,which is a complicated judgment.Therefore,a new combined bus port impedance criterion is proposed,which eliminates the RHP poles of the combined impedance by combining the bus port impedance and the output impedance of the source converter.The system stability can be judged directly by the number of crossings of the phase frequency curve of the combined bus port impedance with the phase(2k±1)π.Finally,the three criteria of impedance ratio,bus node impedance,and the proposed combined bus port impedance are compared and analyzed in four typical cases,and the proposed combined bus port impedance judgment method is verified to be simple and accurate.