Special methods for the calculation of temporary overvoltages due to transformer saturation

Summary form only given as follows. During power restoration, the energization of a no load transformer can lead to high overvoltages, the amplitude being sensible to initial conditions. This document describes an approach based on the AUTO program, which is a complementary tool to EMTP, and may help the understanding of the behaviour of the electrical circuit. The case of a progressive energization of a transformer is described, with a progressive energization from the hydraulic power plant of Pied de Borne towards the thermal power plant of Tricastin. Tests have been performed by the Exploitation Department, with a progressive voltage rise, the network having a resonant frequency at 164 Hz, with a distortion of the voltage above 0.6 Un, and the presence of the third harmonic, the maximum being obtained after ten seconds. A single phase of the network has been used and equations of this network have been solved with the AUTO program, showing that the network can generate the third harmonic, depending also on the value of the flux in the network, the third harmonic being reached for a value of 0.65 kWh; it has also been validated by EMTP calculations. A time domain approach, with programs like EMTP is however necessary to calculate the maximum overvoltage in a three phase network, especially when the phase to phase insulation of the equipment is concerned. In the case of high overvoltages, palliative solutions can be suggested like synchronizing devices in circuit-breakers, circuit-breakers with resistances and zinc-oxide arresters.