Thermal Modelling of Electrical Insulation System in Power Transformers

Abstract

Temperature is one of the limiting factors in the application of power transformers. According to IEC 60076-7 standard, a temperature increase of 6 C doubles the insulation ageing rate, reducing the expected lifetime of the device. Power losses of the transformer behave as a heating source, and the insulating liquids act as a coolant circulating through the windings and dissipating heat. For these reasons, thermal modelling becomes an important fact of transformer design, and both manufacturers and utilities consider it. Different techniques for thermal modelling have been developed and used for determining the hot-spot temperature, which is the highest temperature in the winding, and it is related with the degradation rate of the solid insulation. First models were developed as a first estimation for modelling the hot-spot temperature and the top-oil temperature. These models were based on thermal-electric analogy and are known as dynamic models. Other two different kinds of models are widely used for thermal modelling, known as Computational Fluid Dynamics (CFD) and Thermal Hydraulic Network Models (THNMs). These two techniques determine the temperature and velocity fields in the winding and in the insulating fluid. In this chapter, the different techniques for transformer thermal modelling will be introduced and described.

Estimating Wind Farm Transformers Rating through Lifetime Characterization Based on Stochastic Modeling of Wind Power

“…Regarding the top-oil temperature, the values it assumes depend on the heat generated by the Joule losses in the windings (i.e., load losses) and no-load losses, the ambient temperature, and the thermal characteristics of the oil. Regarding the hot-spot temperature, the values it assumes depends on the heat generated by the load losses, the temperature of the oil, and the thermal characteristics of both winding and oil [25][26][27][28][29]. In the case of a significant presence of non-linear loads or converter-based generators, the loads due to non-sinusoidal voltages can have a role in the thermal behavior of the transformer and consequently on its lifetime [30][31][32].…”

“…where ∆t = T n . In (26) the only complication lies in the generation of the increments (W ti − W ti−1 ). By keeping in mind the properties of the standard Wiener process, also named standard Brownian motion, it is intuitive to assume (W ti − W ti−1 ) is an independent random variable distributed according to the law √ ∆tN(0, 1).…”

“…By keeping in mind the properties of the standard Wiener process, also named standard Brownian motion, it is intuitive to assume (W ti − W ti−1 ) is an independent random variable distributed according to the law √ ∆tN(0, 1). The Euler-Maruyama method is substantially summarized by Formula (26), where only the additional burden to generate random vector from the N(0, 1) distribution is required.…”