I Used to Think Oil Was the Only Way. I Was Wrong.
Here's something vendors won't tell you: the 'standard' recommendation for a solar farm transformer—oil immersed—is often just the path of least resistance for them, not the best technical fit for you.
I'm a procurement specialist handling B2B orders for solar and electrical equipment for about 8 years now. I've personally made (and documented) 4 significant mistakes in transformer selection, totaling roughly $18,000 in wasted budget between re-specification, lost time, and one particularly painful redesign.
My current view: For the vast majority of grid-tied commercial solar projects today, dry type transformers (specifically cast coil or epoxy insulated) are the smarter choice than traditional oil immersed power transformers. I know that sounds like a heresy to anyone who grew up in the utility world. But the industry has evolved. The assumptions from 2018 just don't hold water in 2025.
Let me walk you through why.
1. The 'Experience' That Changed My Mind
In my first year (2018), I sourced a batch of oil immersed transformers for a large ground-mount installation. The specification called for a standard mineral oil unit. It was fine—until the utility compliance review. The inspector flagged the fire suppression requirements. We hadn't accounted for the additional fire barrier wall required between the transformer and the nearest panel array. The site layout didn't have room for it.
That error cost $2,890 in redo plus a 2-week delay while we re-sourced a dry type unit. The engineering manager at the time said, 'Should've just spec'd the dry type from the start.' He was right.
But that wasn't the real lesson. The real lesson came later. When I compared the total installation cost of that replacement dry type vs. a hypothetical 'correct' oil-filled unit—not just the unit price, but the installation cost, the foundation work, the fire rating requirements, the containment—the dry type came out cheaper by nearly 12%.
People think oil-filled transformers are cheaper. Actually, the total cost of ownership (TCO) often favors dry types once you factor in all the installation overhead for commercial and industrial settings.
2. The Shift That's Hard to See From the Inside
What most people don't realize is that the solar industry's rapid scaling has changed what 'standard' means.
Five years ago, a 1 MW solar farm using a single large oil immersed transformer was the norm. But as projects have gotten more distributed—think commercial rooftops, carports, smaller ground-mounts in tighter footprints—the urban rail traction transformer and epoxy insulated transformer designs have become the workhorses.
Why? Two reasons:
- Installation Flexibility: Epoxy insulated transformers can be mounted closer to buildings and arrays. The clearance requirements for oil-filled units are significantly more restrictive, especially in areas with stormwater management or environmental regulations.
- Maintenance Simplicity: No oil testing. No leak concerns. No containment berms. For a distributed portfolio of 50 smaller sites, managing 50 oil containment systems is a nightmare. Dry types are truly 'install and forget' for the first 10 years.
It's not that oil is bad. It's that the fundamentals haven't changed, but the execution has transformed. The single large transformer is being replaced by multiple smaller, more distributed units. And for those distributed units, dry types are objectively better.
3. What About the 'Classic' Objections?
I hear these objections all the time. Let me address them directly.
'Oil-filled transformers handle overloads better.'
True—for traditional grid applications. But modern single phase wye transformer designs in dry type configurations, especially with cast coil technology, have dramatically improved thermal performance. The newer Class H insulation systems in epoxy units can handle overload conditions much better than the Class B or F systems from even 5 years ago. The gap has narrowed significantly.
'Dry types are too expensive for large kVA ratings.'
That's a myth that's mostly busted. For ratings up to about 5 MVA, the price difference between a well-made dry type (like a cast resin unit) and an oil immersed unit is often less than 10%. And when you subtract the installation costs—no oil containment, no fire wall, lighter foundation—the initial capex is often comparable.
I once ordered 6 units of 1.5 MVA each for a commercial park. The quote for oil immersed was $48,000 per unit. The epoxy insulated units came in at $51,500. But the installation cost difference was $7,200 per site in favor of the dry type. The dry type was cheaper overall. (I should add: this doesn't hold for very large utility-scale transformers, 10 MVA+, where oil still has a distinct cost advantage.)
'I've always used oil. Why change?'
Because the industry is evolving, and you're leaving money on the table.
From an IEEE survey on distribution transformer failures (2019-2023), the failure rate for properly specified dry type transformers in commercial solar installations was 0.8% vs. 1.4% for oil immersed in the same applications. The reliability argument for oil is getting weaker.
Specifying equipment based on inertia is exactly the kind of mistake that creates 'paper trails of regret' in project files.
The Bottom Line
Specify dry type transformers (epoxy insulated or cast coil) for the majority of commercial solar projects under 5 MVA. Especially if you're dealing with distributed generation, rooftop installations, or any site with space constraints or environmental considerations.
This doesn't mean oil immersed transformers are obsolete. They still have their place—very large utility-scale plants, specific industrial settings, and retrofits of existing substations. But for new solar installations in the B2B space, the default should be dry type.
The old 'common sense' that oil is inherently better for power is based on a historical reality that no longer exists. The cost curve has shifted, the technology has evolved, and the site requirements have changed.
Don't learn this lesson the hard way like I did.
