Generator models for overvoltage simulations

Generator winding models were developed for simulating dielectric stresses arising from 5-50 kHz oscillatory transients and from steep-fronted surges. For oscillatory transients, field tests were used to build a distributed parameter line model, which was then simplified to a second-order R, L, C network. For steep-fronted surges, a new empirical formula for the winding surge impedance was developed and validated. Simulations based on these models show that the inductive transfer of lightning may be larger than previously reported. Also, power frequency bias is larger on an adjacent phase than transfer within one phase. Higher stresses may arise for distant lightning surges having an initial phase-to-phase component and for fault-generated transients resonating within the generator winding. Reflections at the end of the first slot can amplify steep-fronted surges. A novel model for capacitive transfer in the step-up transformer may replace expensive factory surge tests