How do you determine the bushing layout on a substation transformer?
The bushing layout on substation transformers isn’t quite as simple as bushings on padmount transformers. The bushings on a padmount are always in the cabinet on the front of the unit with low-voltage bushings on the right and the high-voltage bushings on the left. Substation transformers can have the bushings located almost anywhere on the unit. What’s more, depending on the exact application, the order of substation bushings can vary.
All of this means that when you need a substation transformer, make sure you know the exact bushing layout before you place your order. Keep in mind the phasing between the transformer and the equipment you’re connecting to (breaker, etc.) The bushing layout has to be a mirror image, not identical.
So how do you determine the layout of substation bushings?
There are three factors:
- Bushing Locations
- Terminal Enclosures
Let’s start with the bushing locations:
The American National Standards Institute (ANSI) provides a universal designation for labeling transformer sides: ANSI Side 1 is the “front” of the transformer—the side of the unit that hosts the drain valve and nameplate. The other sides are designated moving clockwise around the unit: Facing the front of the transformer (Side 1), Side 2 is the left side, Side 3 is the back side, and Side 4 is the right side.
Sometimes substation bushings can be on the top of the unit, but in that case, they’ll be lined up along the edge of one side (not in the middle). The transformer’s nameplate will have a full description of its bushing layout.
Now that we’ve cleared up the bushing locations, let’s take a look at phase rotation.
Transformer Phase Rotation
What is transformer phasing
Phasing refers to the order in which the voltages’ waveforms in the transformer reach their respective peaks. The phasing of a particular transformer will dictate the order of the unit’s bushings along each side.
As you can see in the substation pictured above, the low-voltage bushings move from left to right: X0 (the neutral bushing), X1, X2, and X3.
However, if the phasing were the opposite of the previous example, the layout would be reversed: X0, X3, X2, and X1, moving from left to right.
The neutral bushing, pictured here on the left side, can also be located on the right side. The neutral bushing may also be located beneath the other bushings or on the lid of the transformer, but this location is less common.
In any case, each bushing will be clearly labeled on the unit’s tank; the nameplate (pictured above) will also tell you the location of each bushing. In our example, we can see from the nameplate that the high-voltage bushings are on ANSI side 2 (left) and the low-voltage bushings are on ANSI side 4 (right). The phasing of the substation is designated as well.
Substation transformer terminal enclosures
For the safety of anyone who might come into contact with a transformer, regulations require that all terminals be placed out of reach. Additionally, unless the bushings are rated for outdoor use—like top-mounted bushings—they must also be enclosed. Having the substation bushings covered keeps water and debris away from the live components. The three most common types of substation bushing enclosures are flange, throat, and air terminal chamber.
Flanges are typically used as just a mating section to bolt on an air terminal chamber or another transitional section. As pictured below, the transformer can be outfitted with a full-length flange (left) or a partial-length flange (right), which provides an interface on which you can bolt either a transition section or a bus duct.
A throat is basically an extended flange, and as you can see in the image below, it can also connect directly to a bus duct or a piece of switchgear, just like a flange. Throats are usually located on the low-voltage side of a transformer. These are used when you need to connect a hard bus directly to the spades.
Air Terminal Chamber
Air terminal chambers (ATCs) are used for cable connections. They provide more space than throats do, since they need to bring in the cables to attach to the bushings. As illustrated in the image below, ATCs can be either partial-length (left) or full-length (right).
If you are buying or replacing a substation transformer, make sure you run through the above layout requirements to ensure your new unit will work for your project. Time is money, and it’s worth taking the necessary steps to prevent a delayed or impossible installation.
If you need a substation transformer and already know the bushing layout needed for your application, check out the full stock of substation transformers on our website! If you need assistance determining the appropriate layout, contact us and a rep will reach out shortly.