The statistical property of wind power fluctuation, which does not fit to the normal distribution because of its high probability of the extremal outlier, is regarded as a source of severe damage to power systems. In view of this, the authors have proposed an evaluation method for the impact of wind power fluctuation on power system quality, assuming that this heavy-tailed uncertainty obeys a power-law. In this paper, we first examine the validity of this assumption based on real data of frequency deviation under wind power interconnection. Then, the evaluation method is improved by extending theoretical results, and is applied to analysis of load frequency control model to verify its advantage over Monte Carlo methods.