This guide outlines best practices for the repair of dry-type transformers.

Transformers are fundamental to an industrial or utility distribution or transmission system. This article will present basic transformer information that may help the reader appreciate how they operate and their many features.

The intent of this article is to provide an overview of the more common techniques used for dissolved gas analysis (DGA) of mineral oil. Types of gases created during transformer faults. There are several key gases produced during transformer faults. The type of gas and the quantity depends on the materials involved in the fault, the energy dissipated at the fault location and the solubility of the gases in oil.

Topics covered include:

• Types of gases created during transformer faults
• Methods of analysis and interpretation
• Dornenburg ratios
• Rogers ratios
• Key gas method
• Gas limits
• Duval triangle
• Analyzing other gases
• Factors affecting gas concentrations and diagnosis

There are many transformers in use rated up to 10 MVA (10,000 kVA) that were originally wound using aluminum conductors. When damaged or when selected for rewind, the aluminum conductor is often replaced with copper conductor. This is usually fairly routine when the conductor changes are undertaken within the bounds of the original transformer design. It is this type of change that will be reviewed in this article. It is not the intent to provide information here for the complete redesign of the transformer. It is important that all coil dimensions remain as close to the originals as possible. In carrying out the type of change outlined above, there are some design issues and differences in material properties that must be considered before proceeding with the work. Some of these issues are:

• The effect that the difference in thermal characteristics of the materials will have on short term overload capability.
• The effect that the different material properties will have on the original blocking and bracing system.
• The effect that the difference in conductivity will have on conductor size.
• The effect that the difference in conductor size may have on coil resistance and reactance.
• The effect that the different conductor size may have on coil size (axial length) and hence its ability to withstand short circuits.