Pad-mounted Transformer is an outdoor transformer utilized as part of an underground distribution system, with enclosed compartment(s) for high-voltage and low-voltage cables entering from below, and mounted on a foundation pad (IEEE std. 100). Pad mounted Transformers are used for utility, commercial or industrial applications. They are designed for bottom entry of cable, however due to some site condition which requires top entry of cable or busduct, some manufacturers design an expansion panel enclosure to act as an extension to house the entry of cables or busduct coming from top.
Standards
- IEC 60076 – Power Transformer
- IEEE C57.12 – IEEE Standard Requirements for Liquid-Immersed Power Transformer
Cooling Methods
- ONAN – Oil Natural Air Natural – Best for transformer rated up to 5MVA. The oil circulates by convection process. Radiator is fitted on the transformer tank to increase the surface area for effective cooling.
- ONAF – Oil Natural Air Forced – For transformers rated more than 5MVA up to 60MVA. Similar to ONAN, the oil circulates by convection process but the external cooling fans are provided for cooling of radiator. These fans are controlled by temperature controller depending on oil temperature to save energy.
- OFAF – Oil Forced Air Forced – For Transformers rated above 60MVA. The circulation of oil is forced by pumping. External cooling fans are also provided for cooling of radiator.
- OFWF – Oil Forced Water Forced
Insulation Oil (KNAN vs. ONAN)
Transformer oil dissipates heat due to copper loss and core loss of Transformer. The oil also provides a medium with high dielectric strength so it can withstand far more voltages across connections inside transformer tank than air.
- ONAN – “O” in ONAN means using mineral oil or synthetic insulating liquid with fire point less than or equal to 300 degrees Celcius. (IEC 60076-2)
- KNAN – “K” in KNAN means using non-mineral oil or insulating liquid with fire point greater than 300 degrees Celcius. (IEC 60076-2). E.g. Silicone oil, Synthetic Ester (Midel brand), Natural Ester (Vegetable oil).
At a certain temperature of oil, a flame momentarily ignites the vapor mixture. This temperature is called the Flash point. When the burning then becomes continuous, the observed temperature at that point is called the Fire Point. This means that KNAN cooling Transformers has lower risk of catching fire than ONAN cooling Transformer (less-flammable).
Commonly used KNAN oil is the Envirotemp FR3 fluid (Natural Ester). It is derived from renewable vegetable oils and therefore environment friendly than mineral oil. It is made up of more than 98% vegetable oil.
Tank design of Transformer using non-mineral oil (KNAN) might be bigger than using mineral oil (ONAN. This is because the heat dissipation factor for mineral oil is higher compared to that of non-mineral oil.
Polychlorinated biphenyls (PCBs) were once widely used as dielectric fluids for Transformers. PCB’s are highly carcinogenic man-made chemical compounds. It can cause cancer in animals and serious non-cancer health effects in humans. PCB production was banned internationally.
Over time, oil degrades from normal operations, due to heat and contaminants. Oil cannot retain high dielectric strength when exposed to air or moisture. Dielectric strength declines with the absorption of moisture and oxygen. These contaminants also deteriorate the paper insulation. (Source: Transformers: Basics, Maintenance, and Diagnostics) Therefore, it is important that insulation oil must be completely dewatered and degassed by vaccuum refiner machine.
Oil-Paper Insulation System
Electrical grade crepe kraft paper tape (densilflex tape). Crepe paper is used as an insulation in manufacture of oil cooled Transformer. Crepe paper has greater surface area which helps to retain more oil and hence lower the working temperature of Transformer.
Electrical Systems Design 