April 5, 2018

Transformer insulation regeneration and drying in online mode

Abstract Transformer insulation consists of liquid (mineral oil) and solid (cellulose) parts. The influence of temperature, oxidation and moisture degrades the insulation, creating acids and...

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Abstract

Transformer insulation consists of liquid (mineral oil) and solid (cellulose) parts. The influence of temperature, oxidation and moisture degrades the insulation, creating acids and sediments that negatively affect its characteristics. While there are different methods of insulation renovation, this article will present regeneration and drying in online mode, which is one of the best methods that siginificantly extends the life of the transformer.

Keywords: transformer, insulation, regeneration, drying, revitalization

1. Introduction

Mineral oils in combination with cellulose are the most common form of insulation in transformers, distinguished by very good characteristics such as resistance to oxidation and other chemical degradation, stability at high temperatures, viscosity and high dielectric strength. However, a high load, electromagnetic field and water penetration after manufacturing and during the transformer operation lead to the initial degradation of insulation. Also, the products of aging accelerate this process and over time become a dominant factor degrading the insulation system. If the degradation process is not stopped or mitigated, the insulation will lose properties and jeopardize the reliability and safety of the transformer operation. The damage caused as a result of breakdown of insulation can be extensive leaving long-term consequences and posing significant financial losses.

2. Degradation of transformer insulation

Over time, under the influence of temperature, electromagnetic fields, catalytic impact and oxidation the transformer oil starts to lose its characteristics [1]. Oil oxidation leads to the formation of acids which attack cellulose fibers and metals, forming metallic soaps, lacquers, aldehydes, alcohol and ketones which are deposited as acid precipitate. The precipitate increases the oil viscosity and therefore reduces its heat dissipation. In the electric field it can also cause partial discharges, and in severe cases lead to breakdown of transformer insulation.

A problem that often occurs during the lifetime of a transformer is the appearance of water in the insulation system. Water as a polar molecule negatively affects the insulating capability of the material, deteriorating its electrical and mechanical characteristics. Water can penetrate from the atmosphere through the places where the oil is leaking, during inspections of transformers, but also as a product of cellulose degradation. Paper insulation has a higher affinity to water than oil, which means that the water is not evenly distributed in the transformer insulation. It is considered that approximately 99 % of water in the transformer is contained in the solid insulation [2]. Water and oxygen together accelerate aging of the oil, and with the aging of the transformer oil, interfacial tension decreases while the level of acids increases, as shown in Figure 1.

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