Research On Stability Improvement Of Islanded DC Microgrid With Constant Power Load

Microgrid is an effective way to promote clean energy consumption and improve power supply reliability.Compared with AC microgrid,DC microgrid has the advantage of low construction and operating cost,simple control and so on.The negative damping of the tightly control load converter is one of the important factors that threaten the stable operation of the DC microgrid.So it is the key technical difficulties in the development of DC Microgrid to suppressing LC resonance and improving stability margin.In this paper,the dissertation focuses on the stability improving of off-grid DC microgrid with constant power loads.The main work is summarized as following:(1)The topologies and control modes of converters in single master dual CPL DC microgrid are introduced in detail.Then,the small disturbance impedance model of DC microgrid is established by the small perturbation analysis of energy storage,photovoltaic and load converters.Finally,the correctness of each converter model are verified by simulation.(2)Based on a single master single CPL DC microgrid,the dominant poles of the DC bus closed-loop transfer function are analyzed,from which the effects of load type and power,PV switching and power,droop coefficient on the stability of DC microgrid are revealed.After explaining the mechanism of DC bus voltage instability by the reduced order circuit,a DC voltage feedback control based on high-pass filtering is proposed.Finally,Matlab / Simulink simulation shows the proposed control is effective and has good robustness to the parameter perturbation such as the increasing of CPL power,PV power,droop coefficient and filter inductance,the decreasing of filter capacitor and so on.(3)Based on a single master dual CPL DC microgrid,the mechanism of load resonance is analyzed by the reduced order circuit.After analyzing the effect of changes of filter parameters and CPL small disturbance impedance on load resonance,a virtual RC control on the load converter is proposed,in which keeping the load converter stability,suppressing load resonance,minimizing filter output impedance are considered as the principle of control parameter selection.Finally,the correctness of parameter analysis and effectiveness of virtual RC control are verified in the Matlab / Simulink simulation.