Influence of Nonstandard Voltage Stresses on Transformer Insulation Paper

Over the last few years, nonstandard voltage stresses have begun to occur in electrical power systems as a consequence of power electronics usage. Power semiconductors contribute to the advanced management and current quality control of power flow. A mutual interaction between switching pulses, emitted by power electronics, and passive parameters of power system produce high-frequency harmonics and oscillating impulses. These distortions are superimposed on operating voltage of power frequency and stress the insulation system. This additional voltage stress can accelerate degradation processes and subsequently decrease the lifetime of insulation systems. Standard aging models have to be verified to respect these effects. For this purpose, corresponding laboratory measurements are needed. Due to the fact, that the oil filled paper insulation is the most used insulation, the laboratory experiments were focused on this type of combined insulation. Voltage-time characteristics of transformer insulation paper were measured and evaluated for the case of standard sinusoidal power frequency voltage stresses, and the case of sinusoidal power frequency voltage stresses with superimposed high frequency oscillation impulses.