Small signal model and DC side low frequency oscillation analysis of AC/DC distribution network

Abstract

With the rapid development and wide range application of AC/DC distribution network,its stability has also become the focus. The low-frequency oscillation at the DC side in the commissioning stage of an AC/DC distribution network project is analyzed. Based on the small signal modeling,the eigenvalue analysis method is used to analyze the influence laws of short circuit ratio (SCR),power level and control parameters on low-frequency oscillation. Next,the measures of changing controller parameters and adding additional controllers to suppress oscillation are proposed. The theoretical analysis and simulation test show that weak grid affects the stability of voltage source converter (VSC). Low frequency oscillation occurs when the 375 V DC side power increases to exceed the rated power. The integral coefficient of the Buck converter current inner loop proportional integral (PI) controller does not affect the stability of the system,but it affects the oscillation frequency after the oscillation occurs. If the proportional coefficient of the voltage outer loop PI controller is too small or the integral coefficient is too large,the system is unstable. Changing voltage outer loop PI controller parameters of Buck converter and adding additional controller can effectively suppress low-frequency oscillations. The theory and simulation analysis of the actual AC/DC distribution network oscillation influence factors are of reference significance for the commissioning and operation analysis of other AC/DC distribution network projects.

Keywords

• ac/dc distribution network
• small signal model
• eigenvalue analysis
• low frequency oscillation
• additional controller
• oscillation suppression