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Conventionally, the 2-parameter Weibull model, Arrhenius-Weibull model, has been used vastly for transformer aging-dependent unavailability modeling. However, this model only uses the lifetime feature to describe the transformer's degradation process and to calibrate the Weibull parameters, which harms the accuracy of aging-dependent unavailability forecasting. In response, this paper develops a 3-calibratable-parameter Weibull model for evaluating the transformer aging-dependent unavailability. In the proposed model, both the individualized aging threshold and lifetime are taken into the calibration of the Weibull parameters to accurately characterize the heterogeneity in transformer populations. First, a degree of polymerization (DP) analysis and Monte Carlo Simulation (MCS) based approach is proposed for estimating the transformers’ uncertain aging thresholds and lifetimes. Then, the Maximum Likelihood Estimate and Particle Swarm Optimization are jointly adopted to model the relationship among the calibratable Weibull parameters, aging threshold, and lifetime. Finally, the analytical formula of aging-dependent unavailability is derived from the established 3-calibratable-parameter Weibull model using an integral-discretization method. A real utility application example in China's Chongqing power system has been presented to validate and demonstrate the practicality and usefulness of this method. [ABSTRACT FROM AUTHOR]

During the process in which a vacuum circuit breaker (VCB) switches off the shunt reactor (SR), an overvoltage with high amplitude and high frequency (HF) will occur because of current chopping and multireignition. To reproduce this switching overvoltage faithfully, the voltage and current characteristics of the VCB were analyzed in this work, based on the field test data from a 220-kV substation in Chongqing, China. Then, typical parameters of three basic factors mentioned in Helmer's VCB model, chopping current value, dielectric dynamic insulation strength, and HF current quenching capability, were selected after statistical analysis and an arc reignition model of VCB was established. Based on this presented model, the influences of the surge arrester, the resistor–capacitor (RC) snubber, and the installation positions and quantities of these protective devices on overvoltage suppression performance were studied in simulation. In addition, the field test using the surge arrester and the RC snubber as the suppression measures was performed again in the same substation. Finally, we derived the conclusion that the most suitable suppression scheme for overvoltage caused by VCB switching off 10-kV SR is installing two 12/32.4 kV surge arresters at the terminals of the cables for the SR and the substation transformer (ST). [ABSTRACT FROM AUTHOR]