Pad mounted transformers are used in many different applications. For example, sometimes they need to have tap changers. Taps come in many configurations, although the most common is to have (2) 2.5% ANBN (above nominal below nominal). This means that it will have 4 taps. 2 of them above the standard voltage of the pad mounted transformer, and two of them below the standard voltage of the unit. They will be incremental 2.5% for each step. 11,850, 12,160, 12,470, 12,780, 13,095. That is how the voltages would look for that configuration. Some applications need to have all four taps below nominal or all four taps above. There are a lot of different configurations but those are the most common.
All pad mounted transformers have windings, but they can come in different materials. They can come in aluminum or copper. While copper is a better conductor in general, aluminum windings are held to the same standard as copper windings per the latest DOE regulations. The downfalls of aluminum are made up for when transformers are built. That means that they will have very similar energy losses. When comparing winding material for pad mounted transformers, the biggest problem we are having, is the cost and lead time for copper windings. Copper windings are in high demand, that effects the price and lead time of them. So, pad mounted transformers with copper windings are more expensive and typically have a higher lead time than that of a unit with aluminum.
Another variable to pad mounted transformers is the fluid they are filled with, or the lack thereof. Pad mounted transformers can either be filled with FR3 or mineral oil. Alternatively, they can be made as dry type transformers. Dry type pad mounted transformers are primarily used indoors. They are good for that use case because they are not flammable. Mineral oil being the most common fluid as of now, is the most flammable option. According to cargill.com “FR3 fluid’s flash and fire points (330°C and 360°C, respectively) are more than twice those of mineral oil transformer fluid (155°C and 165°C, respectively). Unlike mineral oil, FR3 fluid is classified as a K-class, “high-fire-point,” “fire-resistant,” and “less-flammable” fluid. Also classified as “non-propagating,” it is self-extinguishing, and will not continuously burn if ignited. Mineral oil, however, will keep burning for hours when ignited, with no way to stop it until all the oil is consumed.”. FR3 is also biodegradable and non-toxic on both soil and water. FR3 is far superior to mineral oil, but it is not fully integrated because it is more expensive.
Pad mounted transformers can be cooled in different ways as well. In more cold environments a transformer may not need a lot of forced cooling. On the other hand, in hotter climates, especially with high voltage transformers, they will benefit from forced cooling. Here are the different types of cooling: ONAN, ONAF, and OFAF. ONAN means Oil Natural Air Natural, what this means is that the cooling is natural. No fans or oil forced cooling. ONAF is Oil Natural Air Forced. ONAF cooled pad mounted transformers have fans on the radiators (forced air). OFAF is when the hot oil is forced from the upper part of the pad mounted transformer tank to a heat exchanger (or radiator), and the air is forced through the heat exchanger by turning on the fan. The cooled oil will then be returned to the bottom of the transformer tank. For dry type pad mounted transformers, they have AF and AN. Dry types have more simple cooling because it can only be cool by air since there is no oil. So, these dry type pad mounted transformers can be cooled with either forced air (fan) or natural air (no fan).
Pad mounted transformers use fuses to disconnect from the primary feeder wire when a transformer fault, or low-impedance secondary-circuit fault happens. Pad mounted transformers don’t necessarily need fusing, but if one of those faults occur it could lead to catastrophic damage to the transformer. Fuses are designed to blow when overcurrent occurs. If a fuse blows for any other there is a problem with the transformer or the power going into it!
Switches are used for pad mounted transformers so that they can easily be powered off or turned back on. However, there are different options. For example, the standard switch is a two position LBOR switch. The two-position switch is easy enough to understand. It allows the pad mounted transformer to be switched on or off. There is also a four-position switch, they are used on loop feed pad mounted transformers. Here is the configuration:
- Position 1 – A-side on and the transformer on (energized)
- Position 2 – B-side on and the transformer on
- Position 3 – A & B-side on and the transformer on
- Position 4 – A & B-side on with the transformer off (not energized)
The tank material of a transformer is very important. Steel being the most common. Although for some more rare cases they need it to be aluminum. Aluminum’s greatest attribute is that it is corrosion resistant without any further treatment. Sometimes that resistance is needed for pad mounted transformers. Aluminum is a lighter material as well, but it is not as strong as steel. So, it may be thicker than a steel tank.
