Small-signal Stability Analysis Of Power Grid Integrated With Doubly Fed Induction Generators

Utilizing eigenvalue analysis method, the doubly-fed induction wind turbine oscillation mode and the effect on small signal stability of the system after the doubly-fed induction generator accesses power system are analyzed in DIgSILENT simulation software in this paper. The analysis uses QR algorithm for solving eigenvalues. The main study in this paper is as follows:(1) Calculation of eigenvalues and eigenvectors for shaft oscillation mode is made for the analysis of the shaft oscillation mode’s participation factors, controllability and observability about the state variables with method of modal analysis in the single machine infinite bus system of DFIG. In the two cases, respectively connecting synchronous generator and DFIG to the infinite system, the influence of external synchronization unit and DFIG for the shaft oscillation modes of original double-fed unit is analyzed. The analysis draws the conclusion that synchronization unit do not participate in the DFIG shaft oscillation modes and double-fed wind turbines will be involved in each other’s shaft oscillation modes and form oscillations between each unit. The conclusion is drawn from the analysis on impact of changes in wind speed on the shaft oscillation mode that shaft oscillation mode shows a gradually weakened small signal stability when wind speed gradually increases to the rated wind speed and shaft oscillation mode shows a gradually increased small signal stability after the wind speed exceeds the rated wind speed and pitch angle starts to increase.(2) By comparing changes in critical eigenvalues and damping ratio of system in Nine Bus System, it is concluded that Double-fed wind turbine integrated into power system in the way instead of synchronous generators will weaken the small signal stability of system.(3) Small signal stability analysis of effect of DFIG’s accessing capacity to the system is made in Nine Bus System. The analysis comes to the conclusion that impact of DFIG’s accessing capacity on small signal stability of system is similar to the case of the static power injection by comparison with effect of same output power of static power injection. The conclusion is drawn that unity conclusions illustrate that DFIG is more favorable or more unfavorable to the system’s small signal stability than synchronous unit cannot be drew from the comparison of modal calculation results of same capacity and output of the synchronization units and double-fed wind turbines accessing to power system.