Transient phenomena in power transformers theoretical and practical approach
In recent decades the theoretical and experimental study of transient phenomena regarding power transformers has experienced a great development. Results of these studies allowed the elucidation of the main theoretical and practical aspects of the transient phenomena due to propagation of impulse voltages inside the transformers. These studies increased the safe operation of power transformer and led to the introduction of rational insulation usage and also helped to develop efficient protection measures.
Calculation methods for lightning impulse voltage distribution in power transformers
This paper aims to develop and update the methodology for calculating the stress to which the internal insulation of power transformer is subjected during lightning impulse test. Power transformer internal isolation consists of insolating intervals of oil impregnated paper, transformer board and oil. In whole, the transformer's insulation must withstand the voltage testing levels prescribed by the standards. One of the methods for determining the internal stress of various parts of the power transformer's insulation is by using mathematical models.
Concepts for Research of Combined Electric Field at Lightning Impulse Test for HV Windings
The goal of this research is to qualify and to quantify the influence of radial stress on axial stress of transformer coil insulation at lightning impulse test. Combined electric field is investigated experimentally using complex models that simulate both radial and axial electric field. The models are made of flat wire and continuous transposed conductor (CTC), with different insulation increases and different spacer thickness. They will be vacuum dried and oil impregnated and will be tested in high voltage laboratory using lighting impulses in appropriate incremental steps. Because of statistical evaluation, 12 identical models of the same type (geometry) will be tested. Total number of model types including flat wire and CTC is 48. Partial discharges during lighting impulse testing will be checked too. Various potential design criteria, such as simple combined field stress, maximal electric field on the insulated conductor, safety factor along electric field line, stressed space and streamer criterion will be checked on the basis of the electric field calculated for the actual models geometry and obtained breakdown voltages. Paper describes all the basic steps in experimental investigation of combined stresses in transformer HV winding including models design and LI testing procedure.