Analysis on the Behavior of Undamped and Unstable High Frequency Resonance in DFIG System

As the wind power generation develops, the Doubly Fed Induction Generator (DFIG) based wind power system may suffer Sub Synchronous Resonance (SSR) and High Frequency Resonance (HFR) in the series and parallel compensated weak network. The principle and frequency of HFR have been discussed using the Bode diagram as an analysis tool. However, the HFR can be categorized into two different types: undamped HFR (which exists in steady state) and unstable HFR (which eventually results in complete instability and divergence), both of them are not investigated before. Since both the undamped HFR and unstable HFR are critical to the output wind power quality as well as the safe and reliable operation of the DFIG system, it is meaningful to discuss them using the Nyquist Criterion from two perspectives, 1) determining either the undamped HFR or the unstable HFR happens; 2) estimating the amplitude of the undamped HFR. The influence factors, including the weak network shunt capacitance, the current PI controller parameters are discussed when estimating the amplitude of the undamped HFR. The experimental and simulation results of a 7.5 kW down-scaled DFIG setup are provided to validate the analysis on the undamped HFR and unstable HFR. Index Terms — DFIG system impedance modeling; high frequency resonance (HFR); Nyquist Criterion; undamped HFR; unstable HFR.