中文摘要：

电压暂降前母线电压是影响暂降频次评估精度的重要因素之一。根据正常运行情况下母线电压的时变区间特征，用电压幅值区间数矩阵和时间权重因子确定不同电压曲线下的故障前电压，基于区间分析和解析式法提出电压暂降频次区间评估方法，根据电压在区间内的分布规律，以蒙特卡罗法模拟的电压暂降频次的期望值作为评估定量值。对IEEE30节点系统的仿真证明了方法的正确性和完备性。与传统方法相比，区间评估结果量化了不同电压曲线对电压暂降频次评估的影响，随机模拟的暂降频次期望值减少了评估结果误差，为合理安排电压运行曲线和选择最佳用户接入点提供了理论依据，具有一定理论和工程应用价值。

英文摘要：

Prefault voltage is one of the most important factors which affect the voltage sag assessment. However, current stochastic assessment methods usually lack consideration of the prefault voltage fluctuation. Aiming at the interval time-varying characteristic of bus voltage, a mathematical model of the prefault voltage was proposed. The mathematical model consists of voltage interval magnitude matrix and time weighted factor. For the typical characteristics of the prefault voltage, the approaches were researched in detail on the basis of interval algorithm and analytical method. The proposed method has been applied to the IEEE 30-bus reliability test system and compared with the traditional method. The results validate its correctness and practicality. The interval result quantifies the influence of different voltage curve, and the expectations of the Monte-Carlo method agrees with more accuracy, adaptation and application.