Your first step in forming an effective transformer maintenance plan (or evaluating the one you have) is to determine how critical each transformer is (e.g., on a scale of 1 to 5). This allows you to allocate your limited maintenance resources.
Transformer failure risk factors vary by operating environment, so your next step is to carefully assess the environment of each transformer for issues such as heat, contaminants, power quality, load distribution, and access for maintainability. For each of these, assign a severity rating.
Your budget is limited, so you must make decisions that balance cost against loss. This means you gear your program to control the most severe risk factors for the most critical transformers. What’s left over can be allocated to the next level of criticality.