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Of everything on a solar site, the pad-mount transformer is the one the utility looks at hardest. It is the boundary between your equipment and their network, and it is specified against their standard rather than yours. Getting it wrong does not cause a small problem — it causes an energisation delay, and energisation delays are the most expensive kind.
This page is for EPCs and developers specifying the interconnection transformer for commercial, industrial and utility projects, and for anyone who has been handed a utility specification they need to meet. It covers what the utility actually decides and what you can still choose.
The utility decides more than you do
On most projects the rating is yours to choose but the voltage, impedance, tap arrangement and enclosure details are dictated by the utility standard. Their specification exists so that everything connected to their network behaves predictably for protection and voltage regulation, and it is not negotiable in the way a commercial choice is.
The practical consequence: send the utility specification with the enquiry. Without it, any quotation is a guess about insulation levels, impedance and terminal arrangements, and those are exactly the fields that cause a redesign.
How the rating is actually chosen
Select transformer kVA from maximum inverter AC output, required reactive-power duty, the loading profile, ambient conditions and utility requirements. Do not assume solar inverters will avoid simultaneous full output. Any reduced rating or overload allowance requires a documented thermal and network assessment.
Two things people get wrong: sizing to the DC array rather than to the AC output, and ignoring the loading profile. A plant that peaks for two hours a day can be rated differently from one with a flat profile, and the difference is worth having properly calculated rather than rounded up.
Compartmental or conventional
Compartmental construction separates terminal areas with barriers according to the selected design and utility requirements. It does not by itself make maintenance safe while other parts remain energised; access and isolation follow the manufacturer procedure and approved site rules.
Where the standard is silent, compartmental is the safer choice, and it is also the one that makes energisation and later maintenance easier to sign off.
Fluid choice: mineral oil or ester
Mineral oil is the default and the cheapest. A biodegradable ester fluid costs more and buys you two things: a much higher fire point, which can reduce fire separation requirements, and an environmental profile that matters near watercourses or on sites with a strict environmental permit.
Where a site has either constraint, the ester is usually cheaper than the civil works needed to satisfy a mineral-oil separation requirement. Worth pricing both before deciding.
Losses are a tariff question
Specify the efficiency requirements applicable to the selected transformer type, market and delivery date. Evaluate both no-load and load losses using the actual operating profile and energy cost; neither loss component is always dominant.
Ask for both loss figures, load and no-load. A quote that gives you only total losses hides the one that dominates over the life of the project.
The physical items that delay energisation
Most delays at this stage are not electrical. They are:
- The pad — dimensioned to the utility standard, level, and with the conduit stubs in the right place
- Clearances — front clearance for door swing and for the utility’s own access, plus whatever the local code requires from buildings and property lines
- Security — penta-head bolts, locks, and often a fence
- Grounding — to the utility standard, which is usually more prescriptive than the general electrical code
We issue the installation and clearance drawing with the unit so these are checked before the pad is poured rather than at the walk-through.
Specifications at a glance
Configuration reference
Swipe horizontally to view all columns →
| Parameter | Typical value | What it depends on |
|---|---|---|
| Rating | 75 to 5000 kVA examples; confirm the selected design | Maximum apparent-power duty, operating conditions and utility requirements |
| Primary voltage | Up to 34.5 kV | Utility distribution voltage at the point of connection |
| Secondary voltage | 480 V, 600 V, or to specification | Inverter or collection voltage |
| Type | Oil-immersed, sealed, compartmental or conventional | Utility standard and maintenance approach |
| Fluid | Mineral oil, or biodegradable ester where specified | Environmental requirement and fire separation |
| Cooling | ONAN, with KNAN or KFAF options | Loading profile and ambient temperature |
| Impedance | Typically 5.75 percent, to utility specification | Fault level and voltage regulation |
| Tap changer | Off-circuit, plus or minus 2 x 2.5 percent, or to specification | Utility voltage range |
| Losses | Specify applicable efficiency requirements and measured load/no-load losses | Transformer scope, destination market and delivery requirements |
| Enclosure | Tamper-resistant, locked, penta-head hardware | Utility security requirement |
| Compliance documents | Confirm applicable transformer standards and required third-party documents | Exact design, utility specification and inspection requirements |
| Documentation | Test report, nameplate data, installation and clearance drawing | Issued with every unit |
Figures are selection references. Confirm the final ratings, dimensions, available certifications and warranty against the current datasheet and written proposal for the selected model or system.
Questions buyers ask about pad-mount transformers
Do we need one, or will a substation transformer do?
It depends on the interconnection voltage and the utility’s standard. Pad-mount is typical up to a few megawatts at distribution voltage; above that, or where the utility wants a metered substation, the design changes. The utility specification answers this.
How long is the lead time?
Stocked ratings ship from US warehouse inventory with the date confirmed in writing. Units built to a specific utility specification are scheduled and quoted with a firm date, because impedance and terminal arrangements are not interchangeable between utilities.
Can we use one we already have?
Sometimes, if the rating, voltage, impedance and fluid all match what the utility has specified. Most second-hand units fail on one of those, and the re-test and recertification cost is often more than the difference.
What comes with it?
Confirm the unit test report, nameplate data and installation drawing, plus any third-party documents required for the selected design and market. Test standards, acceptance criteria and witnessed testing should be agreed with the utility before shipment.
Who is responsible for the pad?
Usually the site contractor, to the utility’s dimension drawing. We supply the drawing with the unit so the pad is right first time — a wrong pad is the most common cause of an energisation delay.
What to send us
Send the utility specification if you have it, otherwise the interconnection voltage, the inverter AC output, and the site location. We confirm the rating, impedance, fluid, terminal arrangement and build date in writing.
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Send the rating, the voltage and the utility standard
With the kVA rating, primary and secondary voltage, and the utility standard that applies, we confirm the efficiency level, the enclosure and the test documentation that ships with the unit.
Send your requirementsOr reach us directly: +86 136 7367 3267 · sales@gtpsolar.com