Stochastic assessment of voltage dips caused by transformer energisation

Abstract

Energisation of large power transformers may cause significant voltage dips, of which the severity largely depends on a number of parameters, including circuit breaker closing time, transformer core residual flux and core saturation characteristic, and network conditions. Since most of the parameters are of stochastic nature, Monte Carlo simulation was conducted in this study to stochastically assess the voltage dips caused by transformer energisation in a 400 kV grid, using a network model developed and validated against field measurements. A dip frequency pattern was identified over 1000 stochastic runs and it was found to be sensitive to residual flux distribution but insensitive to closing offset time distribution. The probability of reaching the worst case dip magnitude (estimated under the commonly agreed worst energisation condition) was found to be lower than 0.5%; about 80% of the dips are likely to be with magnitudes lower than 0.6 pu of the worst case. Nevertheless, there are dips with magnitudes exceeding the worst case dip magnitude, indicating the inadequacy of deterministic assessment approach by using the commonly agreed worst energisation condition.

Assessment on energization sequence for reducing sympathetic interaction between wind turbine transformers

“…Hence, the validation study was conducted based on 1000 residual flux combinations stochastically generated based on the residual flux pattern and magnitude range suggested in Brunke and Frohlich . The method to generate the stochastic residual flux was detailed in Peng et al . and Peng .…”

“…Hence, the validation study was conducted based on 1000 residual flux combinations stochastically generated based on the residual flux pattern and magnitude range suggested in Brunke and Frohlich. 27 The method to generate the stochastic residual flux was detailed in Peng et al 28 and Peng. 29 It was found that, to achieve a no more than 12% deviation to the measured inrush currents, the residual flux pattern should be zero, negative and positive for phases A, B and C, respectively, and the residual flux magnitude should be in the range between 0.275 and 0.311 pu of the peak nominal flux.…”