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Oil-filled power transformers were first conceived and manufactured in the mid-to-late-1800s (1) (2). They were (and to this day are) an essential component of the electrical power transmission and distribution infrastructure.

Power transformers perform a power conversion function. A transformer takes electrical power of certain voltage and current characteristics through its primary terminals and delivers this power on its secondary terminals with changed voltage and current levels. By increasing voltage, and thus reducing the current level, power transformers minimise losses and enable the economical transmission of electrical power over long distances.

While performing this primary function, the core and coils (or active part) of a power transformer produce a certain amount of losses which generate heat. The function of the cooling system is to remove this heat at a steady and controlled rate in order to maintain an acceptable temperature differential between the active part and its surrounding medium.

Additionally, the active part and its components are subject to a wide range of voltage stresses. The function of the insulation system is to keep these stresses below the desired maximum stresses, both under normal and abnormal operating conditions. This is accomplished by the careful selection and arrangement of the materials that comprise all active components of the transformer.

The engineers that first designed and manufactured transformers in the latter part of the 19th century realised that they could improve the cooling and electrical insulating performance of those early power transformers by immersing their active parts in mineral oil. This gave rise to the liquid-filled transformers of our days.

Mineral oil, as well as other types of liquids, have excellent dielectric insulating properties. Mineral oil provides a great insulation medium when used to impregnate the Kraft paper typically used to wrap winding conductors and other elements of the windings.

The capacity to remove heat from the active part is also improved by using a liquid instead of a gas (such as air) as the medium that surrounds the active part.

A secondary benefit of using a liquid insulating medium can be realised by the fact that this liquid is in direct contact with all active elements inside the transformer. A great deal of knowledge about the condition of the transformer can be inferred by analysing certain properties of the insulating liquid.

In this primer we will discuss why this is the case and how we can use this behaviour to the advantage of the person or team in charge of operating and maintaining liquid-filled power transformers.

Assessing the condition of power transformers has the main objectives of determining whether a particular transformer displays an abnormal behaviour and, if so, identifying what type of failure mode is the cause of this behaviour.

For the asset owner of a power transformer fleet, the main value of having a Dissolved Gas Analysis (DGA) program rests in gaining the capability to detect failure modes which, if gone undetected, would otherwise increase the operational risk of that fleet to unacceptable levels. By detecting issues and opportunely reacting to them, an organisation can mitigate the probability of facing the consequences (financial, safety, environmental, reputational, etc.) of major transformer failures.

The field of study created by power transformers is vast and rich. There is an abundance of generalised behaviours as well as individual cases and experiences. In writing this chapter, we have tried to substantiate all statements with adequate references. However, it is inevitable that some of our own opinions, shaped by +25 years working in this field, have made it through the various sections in this chapter. We hope the reader does not take this as a lack of rigour, but rather as a humble attempt to provide him or her with a valuable collection of knowledge in this field, both past and present.

Note: Parts of this document were first published on Chapter 1 of the Power Transformer Condition Monitoring and Diagnosis released by The Institution of Engineering and Technology