Abstract
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.
Highlights
This paper deals with two of the transitory regimes encountered during the operation of transformers: the overvoltage propagating in the windings of transformers when subjected to the lightning impulse, and the transient current that occurs when wiring the transformers [1,2,3,4,5,6,7,8]
The paper includes 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
To determine the overvoltages transmitted between the windings of the transformer, a study was conducted on a 40 MVA, 123/6.3kV TTOS-OFAF transformer
Summary
The impact of transient phenomena on the operation of high-power transformers is a current concern starting with their design stage to ensure proper insulation.This paper deals with two of the transitory regimes encountered during the operation of transformers: the overvoltage propagating in the windings of transformers when subjected to the lightning impulse, and the transient current that occurs when wiring the transformers [1,2,3,4,5,6,7,8].The paper includes 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.Both transitory regimes presented in this paper generate mechanical stress, thermal stress and electrical stress in transformers.To benefit from an optimal maintenance program, the active part of the transformer (magnetic core and windings) must be assessed as follows [1, 8,9,10,11,12,13,14,15]: 1. the thermal assessment is aimed at the faster-than-anticipated aging due to the excess temperatures caused by the hot spot of the winding, the hot spot in the core and the defects of paper-insulated hot metals; 2. dielectric evaluation follows the process of degradation of solid / liquid insulation due to high thermal or electrical voltage, moisture in the paper / dielectric liquid, manufacturing defects and contamination with conductive particles; 3. the mechanical assessment is aimed at the disintegration of the structure in the core, the windings and the solid insulation, due to one of the many reasons, such as the strong electromagnetic forces resulting from the flow of Marian Duta et al.: Transitory Regimes and Their Effects on the Insulation of High Power Transformers short-circuit current through the windings; the mechanical deformation of the windings due to the shocks during transportation, installation or during system failures, etc.; and the degradation of solid insulating materials [16,17,18,19,20,21,22,23,24,25]. The impact of transient phenomena on the operation of high-power transformers is a current concern starting with their design stage to ensure proper insulation. The paper includes 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 Both transitory regimes presented in this paper generate mechanical stress, thermal stress and electrical stress in transformers. Some of the operations concerning the maintenance of power transformers can be established following the modeling of the transient phenomena, starting with the design stage For these reasons, this paper assists the users of transformers in using proper protections for their implementation in the system and in avoiding possible failures, e.g. the controlled connection of transformer phases, to avoid high currents when wiring the transformer
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