Approach for an integral power transformer reliability model

Abstract

SUMMARYIn electrical power transmission and distribution networks power transformers represent a crucial group of assets both in terms of reliability and investments. In order to safeguard the required quality at acceptable costs, decisions must be based on a reliable forecast of future behaviour. The aim of the present study is to develop an integral transformer lifetime model which involves degradation mechanisms for most relevant subsystems, applicable to individual power transformers and transformer populations. In this paper, we present a predictive model for power transformer reliability which involves three essential ingredients: failure statistics, physical understanding of the degradation process, and actual knowledge of the present condition. The model is based on evaluation of existing literature and past experience on degradation mechanisms, failure modes and diagnostic techniques. The model is illustrated for integral reliability of three transformer failure modes, related to the degradation of the transformer winding insulation, of bushings and of tap-changers.

Calculation of the remaining lifetime of power transformers paper insulation

“…This rate of cellulose macromolecule scission is directly correlated with the hotspot temperature, which is the temperature at critical locations influenced by transformer loading. Due to the above described mechanism and the fact that transformer loading is expected to increase in future, winding paper insulation failure may become the dominant cause of transformer failure if the loading approaches the rated load [1,12].…”

“…Nowadays, society has become more and more dependent on the availability of power, putting pressure on the reliability, availability and cost efficiency of power supply [1][2]. These assets are components that operate in a high voltage, high current and consequently high power environment.…”

“…Although transformers are extremely energy efficient, the dissipated heat is a limiting factor for the maximum loading of power transformers. The insulating medium must be capable of dealing with large electric stresses, strong electromechanical forces and high temperatures [1].…”

“…According to present statistics, tap-changers and bushings can also contribute to transformer failure [2,6]. However, they can be more easily repaired or replaced than transformer windings and their insulation.…”

“…Thus, the paper is a critical factor in paper-oil insulation. A bad paper quality leads to premature insulation degradation, which in turn can lead to transformer failure after, for example, a winding short circuit [1]. Manufacturers often define the expected life of power transformers to be between 25 and 40 years.…”