Approximate Model-Free Assessment of Oscillation Stability Determined by the PLLs in a Weakly Grid-Connected Wind Farm with Multiple PMSGs

Abstract

The study investigates the oscillation stability determined by phase locked loops (PLLs) in a weakly grid-connected wind farm with multiple permanent magnet synchronous generators (PMSGs). To this end, an equivalent model of the wind farm is derived. Based on the derived equivalent model, the oscillation stability of the wind farm as determined by the dynamics of the PLLs is analytically examined. This paper explains why the PLLs equipped by the PMSGs in the wind farm can lead to instability risk under the condition of weak grid connection. A simple index is proposed to approximately assess instability risk of the wind farm due to the PLLs. The parametric model of the wind farm with the PMSGs is not necessary to calculate the proposed simple index. Hence, it is model-free and applicable in practice when it is difficult to establish the parametric model. An example of a grid-connected wind farm with 12 PMSGs is presented to evaluate and demonstrate the analysis conducted and the simple index proposed in the paper.