When is transformer oil filtration required during operation?

Filtration is required when condition monitoring shows elevated moisture, reduced breakdown voltage, rising acidity, increasing particle count, or poor dielectric dissipation factor. It is also applied after repairs, after oil handling or topping up, following alarms related to Buchholz or pressure relief, or when leaks introduce air and moisture. Filtration is common during planned maintenance outages, and in critical assets it may be performed online using dedicated systems that treat oil without de energizing the transformer.