Techniques for Detecting Paper Degradation in Large Power Transformers

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CHAPTER 1
INTRODUCTION
Power transformers are very important in the electrical power sector; they are among the expensive assets of power generation and distribution systems. . According to Piush Verma [16], transformers are static electromagnetic devices consisting of two or more windings which are linked with a common magnetic field used for the distribution of electrical power from one voltage level to another. Some of the main parts of the transformer are; the windings, tank and insulation (solid or liquid). Transformers normally have an operating temperature range of 60-90oC and an operation lifetime of 40 years [8]. These service temperatures cause gradual degradation of the oil and paper insulation thereby producing degradation by-products such as furans, CO2, CO and water [8]. For reliable operation of transformers, it is therefore important to embark on effective maintenance strategies using various degradation sensing techniques such as the furan analysis and infrared spectroscopy, aiming at identifying the change in properties and concentrations of these by-products associated with the degradation of transformer insulation. Predictions of such problems make the removal of the transformer from service possible before its insulation deteriorates to avoid catastrophic failures or extend the service life of the transformer. This report therefore attempts to examine methods for detecting paper degradation in large power transformers and suggest future trends in this field.

CHAPTER 2
PAPER INSULATION
Insulation is provided by a material that reduces the flow of heat or electricity. The material can be adapted to any form and it is diverse in properties and origin. Example: cloth, paper. Insulating materials have wide applications in electricity because they have high tensile strength, high dielectric strength and good thermal stability. 2.1 PAPER

Paper is made from cellulose fibres found in wood. Solid insulation in transformers is usually provided by cellulose paper which is impregnated with oil. The paper is made by delignification of wood pulp by the kraft processes [5]. It is required to withstand the combined effect of mechanical and electrical stress. Cellulose is a linear polymer consisting of D-anhydro-glucopyranose units joined together by β-1, 4 glycosidic bonds [14]. A single cellulose fibre is formed from a number these chains held together by hydrogen bonds in which the hydroxyl groups are involved [14]. Cellulose is adapted to pulp used to make paper. However when cellulose based materials are operated at high temperatures and surrounding oil, the cellulose chain will cleave [14].

Figure 1: Paper insulated coils of a transformer
Source:http://www.copper.org/application/electrical/pq/casestudy/a6110/Allegheny_6110.html
2.2 IMPORTANCE OF PAPER IN POWER PLANT INSULATION
The electrical winding of transformers contains paper insulation which is immersed in oil, and the oil acts as both coolant and as dielectric element [14]. Therefore the efficient operation of transformers depends on the insulation. Paper is important in power plant insulation for the following reasons: * Their high dielectric strength

* Excellent cooling properties,
* They maintain mechanical stability,
* Easily regenerated.
* Direction of oil flow,
* They are cheap,
* They have low dissipation factor,
* High degree of polymerisation and
* They increase the electrical and physical properties above that exhibited by...
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