Mechanical breakdown of aged insulating paper around continuously transposed conductors for power transformers under the influence of short-circuit forces - analysis by numerical simulations

The cellulose based oil impregnated insulating system of power transformers is exposed to significant aging due to heat, oxygen, moisture and acids during operation time. This degradation affects the mechanical strength of the cellulose. It is generally accepted that a degree of polymerization (DP) below 200 can be defined as the end of life of an insulation system. In literature however there exist little investigations dealing with the consequences of a reduced mechanical strength. Especially the thin paper insulation wrapped around the conductors is weakened due to aging and might fail under the influence of short-circuit forces on the windings. Thus this contribution presents a mechanical simulation model for such paper insulated power transformer conductors. Investigations concerning the beginning of paper insulation failure are conducted for different aging states, conductor geometries and number of insulating layers. The aging dependent material definition for the insulating paper is based on accelerated aging test. Simulation results show that an increase in radial conductor width leads to higher mechanical stress in the paper layers and for this reason to a higher probability of paper failure. Flat paper wrapped wires generate lower stress inside the insulating layers than large Continuously Transposed Conductors (CTCs). The maximum stress inside the paper is hardly dependent on the number of layers. The maximum paper tensile stress that can be correlated to the DP however is a main indicator for the beginning of failure inside the cellulose insulation system. Furthermore the aging dependent Young's Modulus of paper also affects its stress exposure.