Transitory Regimes and Their Effects on the Insulation of High Power Transformers

Transformers are the most important pieces of equipment in the electricity transmission and distribution system. Their importance derives both from their high cost and from the fact that they ensure a proper operation of the national power system. The transitory regimes generated during the operation of transformers, such as the overvoltage caused by the lightning impulse and the transient currents when wiring the transformers result in mechanical stress, thermal stress and electrical stress in transformers. These stresses lead to the aging of the insulation system, to avoid possible damage or even the decommissioning of transformers, this paper presents analytical methods for determining the overvoltage transmitted between the windings of transformers due to the lightning impulse and the transient currents that occur when connecting the transformers. The studies were performed on two high power transformers: TTOS-OFAF of 40 MVA, 123 6.3 kV was used to determine the overvoltages that propagate at the windings of the transformer subjected to lightning impulse, and on TTOS-OFAF of 15 MVA, 10.5 / 6.3 kV was performed the study on the effects of the appearance of the connection current on the transformer insulation. The studied phenomena can have a negative impact on the transformer operation.. The results obtained have been validated by laboratory tests, and they can be used to determine the measures to be taken in order to avoid possible damages as of the time of the transformer implementation in the system.