A Case Study of Moisture and Dust induced Failure of Dry type transformer in Power supply Distribution

Dry type transformers are mostly used as distribution transformers at the end of utility network for ratings up to 3.5 MVA with 415 V rated LV winding or up to 6 MVA with 11 kV or 6.6 kV as LV winding. The ventilation of these transformer enclosures are provided with prime concern to remove the heat during fully loaded or overloaded condition so as to limit the temperature rise within enclosure as per the class of insulation provided and also to enhance the life of insulation system. However a number of recent failures of these types of transformers suggest that a combination of detrimental conditions viz ingress of moisture, ingress of dust into the enclosure and reduction of temperature within enclosure during lightly loaded conditions acting simultaneously can trigger catastrophic failure of the insulation system of dry type transformers. Therefore the ventilation scheme of transformer enclosure and installation layout need to consider these aspects thoroughly before opting out oil filled transformer as an alternative and selecting a dry type transformer for any power supply system. This is specially required to accommodate demands from end users like no oil leakage, indoor installation, reduction of fire hazard, ease of maintainability. This article covers the case studies on failures of three phase 33 kV/0.433 kV, 2 MVA Delta–star (Dyn11), 11 kV/0.433 kV, 300 kVA dry type cast resin transformers. Various probable causes of failures including root cause analysis are included in the paper and a few recommendations are also covered to prevent similar failures.