Risk of oscillations induced collectively by multiple DFIGs and PMSGs in a wind power collecting system

It has been found that same type of multiple wind turbine generators (WTGs), either PMSGs or DFIGs, in a grid-connected wind farm may collectively cause growing oscillations. This paper investigates the risk of oscillations induced collectively by both the DFIGs and PMSGs in a wind power collecting system (WPCS). Analysis is carried out to indicate that if the PMSGs and the DFIGs happen to have components, such as the phase-locked loops (PLLs) or converter control loops, bandwidths of which fall in the same range such that their dynamics are similar, it is possible that multiple DFIGs and PMSGs may jointly induce growing oscillations. Three study cases are presented to validate and demonstrate the analysis and conclusions made in the paper about dynamics interactions between (1) The PLLs equipped by multiple DFIGs and PMSGs; (2) The PLLs of a group of the DFIGs and the DC voltage control loops of another cluster of the PMSGs. The DFIGs and the PMSGs collectively cause growing oscillations, though the dynamics of the DFIGs and PMSGs are completely different. The oscillation risk increases when more DFIGs or/and PMSGs are connected in the WPCS. • A new instability mechanism is identified from collective DFIG and PMSG dynamics. • Similar dynamic components in DFIGs and PMSGs jointly cause oscillations. • Oscillation risk is strongly affected by the number and similarity of dynamic components. • Guidance is provided in WPCS planning to mitigate oscillation risk.