Inside Substation Transformers

Electric power substations serve as the critical connection points that keep our entire power system functioning – linking generation to transmission, connecting different voltage levels from 69 kV all the way up to 765 kV, and providing power to major industrial customers like refineries and processing facilities. At the center of these substations are massive power transformers that enable one of the most fundamental functions of our electrical grid: changing voltage levels to efficiently move electricity across vast distances and deliver it safely to end users.

Understanding how these transformers work isn’t typically something non-engineers have been taught, so let’s take a look. At their core, transformers are essentially two or more coils of insulated conductor wrapped around an iron or silicon steel core, with no direct electrical connection between the windings – instead, they’re linked through magnetic flux. This design allows electricity on one side to be stepped down on the other, with current levels changing inversely to maintain power balance. It’s this voltage transformation capability that makes long-distance power transmission economically viable.

In the clip below,  instructor Tim Taylor explains the various components and cooling systems found in substation transformers. This clip is taken from his “Introduction to Electric Transmission” course, which makes complex power system equipment accessible and understandable. This course is included in the “Electric Transmission and Solar PV Inconnection Bundle,” designed specifically for professionals in utility-scale solar who need to navigate transmission interconnection processes.

Transcript below.

 Substation transformers (or power transformers) are the biggest and most expensive piece of equipment in a substation. In a transmission substation, you’d be looking at power transformers that cost $3 million, $5 million, or maybe even a little more than that. 

The size of these are like railway cars, or maybe three quarters of a railway car, or half of a semi-truck trailer, for example. They are big pieces of equipment.

What we see here down in the tank is, we see the windings and the cores. So once again, substations are always three-phase, just like the transmission lines are always three phases: A, B, and C. 

Transformers, they are typically three phase now. You could run into situations in which you actually have three separate transformers, one for each phase. You might see that outside of a large generating station where you’ve got these big step-up transformers. They could be in three separate tanks. But in general, it’s more common to see all three phases in the same tank. 

In this tank you’ve got a lot of mineral oil, which is providing cooling and insulation.

The windings are generally insulated and they have paper insulation usually. Then there’s some cooling mechanisms including radiators and fans. Some transformers actually had pumps on them to pump the oil through the radiator. So keeping a transformer cool is required because of the vast amounts of current that is passing through the conductors creating heat.

Like I said before, heat and moisture are power equipment’s worst enemies. Radiators, fans, pumps, they all serve to help cool the transformer. You see the large bushings and you actually see some surge arresters here as well. They will be frequently mounted to protect the transformer from transient overvoltages and traveling waves.

This oil conservator is just an expansion tank and serves a couple other purposes, but it really provides room for the oil to expand, as because the mineral oil will expand as it heats up. 

Let me point out that these three large bushings that you see here, they would be on the high voltage side. I do see places where the lower voltage bushings would be, and that would be right here. These may be the bushings, but they’re awfully small. They would be right here. 

You got a total of six bushings, three on the high side and three on the low side. Once again, bushings are the transition point, in this case between the tank containing all the oil and the air where the overhead lines are coming in.