Serious problem for failure probability evaluation of equipment due to voltage sag is the uncertainties and mathematical expression of influencing factors. The uncertainties contained in voltage sag, equipment voltage tolerance and possible operation state are researched. The intension and extension uncertainties of influencing factors are used to distinguish their property. Stochastic and fuzzy variables are introduced to express intension and extension uncertainties.The mathematical models of influencing factors are established using stochastic and fuzzy models and a multi-uncertainty evaluation model is proposed also. In this method, maximum entropy principle is used to extract the probability distribution of voltage sag intensity and the determination principle of membership function of fuzzy safety event is used to determine the multi-uncertainty evaluation model. As a case study, personal computer is simulated. The simulation results show that this method is correct and viable and it can be easily used in other fields. Keywordsvoltage sag; sensitive equipment; failure probability; multiuncertainty; intension and extension uncertainty; mathematical model; maximum entropy principle; fuzzy safety eventI.

Since energy information in nodes can comprehensively reflect the reactive injection and voltage level of node, this paper proposes a new partitioning method based on static energy function and multi-threshold search algorithm for decentralized voltage control. With the energy information of node, the node energy correlation degree index (NECDI) is proposed to evaluate reactive relevance between different nodes. Based on the NECDI, the energy sensitivity matrix (ESM) is then established. After that, the ESM is decomposed to get voltage control area (VCR) by multi-threshold search method. Finally, plenty of simulation results about IEEE 30-and IEEE 118bus systems, and some comparisons with the traditional partitioning method demonstrate the validity of our proposed algorithm, which is effective to solve some questions of the practical engineering projects. Key word: static energy function; energy information; node energy correlation degree index; energy sensitivity matrix; multi-threshold search I.