Resonance Investigation of Grid Connected DFIG System

Abstract

Grid connected DFIG system may suffer stability issues of subsynchronous resonance (SSR) and high hrequency resonance (HFR) when connected with series or shunt compensated weak network. The negative effective resistance of the system and the inappropriate phase difference margin between DFIG system and weak network at the magnitude frequency intersection point causes resonance instability in the system. This study discussed the detail analysis of SSR as well as HFR based on complete impedance model of grid connected DFIG system. The SSR/HFR analysis has been done based on 7.5 KW (small scale) and 2 MW (large scale) grid connected DFIG system. The impedance interaction at phase difference of ≥ 180° between DFIG system and weak network is a direct cause of SSR/HFR instability in the system. For SSR/HFR analysis, the size of the single DFIG system is more important than size of aggregated DFIG system. As the capacity of single DFIG system increases, the system will be more prone to SSR/HFR instability and the analysis of aggregated DFIG system can be estimated using single DFIG system. During the analysis, influencing factors such as L/LCL filter, transformer configuration, power rating, wind speed, compensation level, and PI controller parameters have shown significant impact on SSR where as L/LCL filter, transformer configuration and compensation level on HFR. Wind speed and PI controller parameters have no significant impact on HFR. Virtual impedance based HFR mitigating strategy has been implemented in grid/rotor/stator part of the DFIG system to eliminate HFR instability from the system. The mitigating strategy is more effective when it is incorporated in the grid part than stator/rotor part of the DFIG system. The effectiveness of the proposed technique in the stator/rotor can be further improved by including resonant controller in the virual impedance. SSR mitigating strategy will not be discussed in this paper.