Research on the Distribution Characteristics and Parameter Estimation of Relay Protection Device Action Time

As a part of the fault clearing time, the selection of the distribution characteristics of the protection device action time is of great significance to the probabilistic stability analysis of power systems. The current research generally assumes that it follows the Normal distribution, but there is no targeted analysis. Through the analysis of the composition and distribution characteristics of protection action time, the difference between Normal distribution and log-normal distribution in the fitting of protection action time is expounded. Taking the action time sample of a power grid protection device as an example, the analysis results show that Log-normal distribution can more accurately reflect the distribution of protection action time than Normal distribution. For the new protection device with a small sample, by treating the action time of the new protection device and the original type of protection device as two populations X and Y, and based on the different sample sizes and variances of the two populations, it is proposed to conduct confidence interval analysis of the difference between the mean values of two populations at 1$-\alpha$ confidence level based on four scenarios. Taking the protection action time of a new type protection device during trial operation as an example, the mean difference and variance ratio of the action time are illustrated by using Normal distribution characteristics and log-normal distribution characteristics respectively, and the quantitative analysis of the difference between the action time of the new protection device with small samples is realized.