Multivariate Analysis of Transformer Resonant Overvoltages in Power Stations

In this paper, cable-transformer resonant overvoltages are studied on the low-voltage (LV) side of a 410-MVA generator step-up (main) transformer, due to ground fault and energization. The transformer is placed in a network configuration often found in Norwegian hydropower stations. The overvoltages are calculated when systematically varying the length of the high-voltage (HV) side cable and that of the cable between the main transformer LV side and the station transformer. This multivariate analysis is performed efficiently, using frequency-domain calculations and the inverse Numerical Laplace Transform. Time-domain simulations are used for analyzing the critical cases in detail. Both types of calculations make use of a blackbox, wideband model of the transformer, and novel procedures are introduced for initializing computations from correct 50-Hz initial conditions. Among the findings are that extreme overvoltages can occur on the station transformer if current-limiting reactors are placed in series with the cable between the main transformer LV side and the station transformer.