Stability Analysis And Control Of DC Distribution System With Parallel Connection Of Multiple Converters

The DC distribution network can absorb DC load and distributed generation more flexibly,and will become an important part of urban distribution system in the future.With the increase of DC load types and quantity,single converter is difficult to meet the demand of power supply capacity.Parallel operation of multiple converters can improve the power transmission capacity,but,it is easy to cause interaction coupling to make the system oscillate and even cause instability.Based on this problem,this paper analyzes the instability nature of the multi-converter parallel DC distribution network and the factors affecting system stability,and designs effective converter control measures to improve the stability margin.The research content of this article is summarized in the following three points:（1）According to the dynamic small disturbance theory,the admittance model of the DC distribution system with multiple converters in parallel based on P-U_dc droop control is derived,and then the RLC parallel resonant circuit with converter as excitation source is obtained from the simplified circuit model of the system,which is used to obtain the analytical expression of DC bus voltage oscillation frequency to characterize its instability mechanism.The oscillation frequency obtained by frequency domain analysis and time domain simulation is consistent with the analytical expression,which verifies the correctness of the theoretical analysis.Finally,the influence of control parameters and physical parameters on the system stability is studied by combining Nyquist criterion and time domain simulation.It is found that when the droop coefficient increases and the AC line inductance or filter inductance decreases,the system stability is improved.（2）When the distance between the power grid and the AC bus is long,its small disturbance admittance can not be ignored.It is inevitable that multiple converters will interact with each other when they are connected at the same time.Based on Norton’s theorem,the interaction between multiple converters and grid admittance is equivalent to the interaction between single converter’s dynamic admittance and equivalent grid admittance,and the interaction strength is expressed in the form of small disturbance value of converter’s output current.By this method,the critical stability values of filter inductance L_f and scale coefficient K_ip of inner loop current controller are determined.Finally,the accuracy of theoretical analysis is verified by time domain simulation.（3）In order to improve the stability margin of the multi-converter parallel DC distribution system,the virtual inertia control based on first-order filter is added to the voltage control outer loop of the original P-U_dc droop control.The simulation result shows that the anti-interference ability of the system is effectively improved.At the same time,the relationship between the virtual inertia capacitor C_vir and the voltage damping coefficient D_r and the system stability is obtained.