Failures in transformers can occur due to various reasons and can have significant consequences including service interruptions, equipment damage, and in some cases, safety hazards.

Transformer internal faults occur within the transformer itself, as opposed to external issues like lightning strikes or grid-related problems. These internal faults are often the result of deteriorating insulation, mechanical problems, or issues with the cooling system.

Transformer external faults occur outside of the transformer but have an impact on its operation. These can be faults in the system connected to the transformer, environmental factors, or other external issues.

Reasons of mismatch discusses the causes of mismatches in differential relays of transformers due to non-fault related currents, unbalance caused by CT ratios, magnetizing inrush, and overexcitation, explaining the phenomena that lead to these mismatches, their potential impacts, and recommendations to mitigate these issues.