Project snapshot
| Sector | Utility-scale solar PV — 50 MW grid-connected plant |
| Region | Middle East |
| Application | Inverter duty step-up (field arrays) + inverter station building |
| Product | S20 sealed oil-immersed / SCB13 cast resin dry-type |
| Plant rating | 50 MW |
| Inverter duty transformers | 8 × 6.25 MVA |
| Substation transformer | 63 MVA（(50 + 0.5) / 0.99 × 1.20 = 61.2 MVA） |
| Voltage chain | 0.8 kV → 33 kV → [132] kV |
| Vector group | Dyn11/ YNd11 |
| Impedance voltage | 6–8 % |
| Cooling | ONAN / ONAF |
| Ambient design | 40–55 °C |
| Expected annual yield | 1,700–2,200 kWh/kWp → 约 85–110 GWh/Y |
| Design life | 25 |
| Standards | IEC 60076； |
| Delivered | [Q · 20 ] |
The challenge
A 50 MW desert PV plant is a hard duty for a transformer, and almost none of the difficulty shows up in the rating.
Heat, and the swing that comes with it. Design conditions for this class of site run 40–55 °C ambient, with measured extremes in the region reaching 48.7 °C air temperature and close to 70 °C at module surface. A transformer rated at the IEC reference ambient of 40 °C does not simply carry its nameplate into that environment — it has to be de-rated or designed with thermal margin for the worst month, not the average one. Add large day-night swings and the insulation sees a daily thermal cycle for 25 years.
Dust. Soiling is a genuine generation issue in this region, not a housekeeping footnote: measured desert installations have shown roughly 10 % reduction in energy production when modules are left uncleaned, with regional estimates ranging 5–15 %. Sealed construction was therefore a requirement for the field units, not a preference.
Harmonics from the inverters. PV inverters are switching devices. They inject characteristic harmonics — predominantly 5th, 7th and 11th — into the low-voltage winding. Those harmonics raise eddy-current loss in proportion to the square of the harmonic order, so a transformer designed only for sinusoidal load can run hot while carrying well below its rating.
The duty cycle is mostly not at rated output. This is the one that decides the economics. A utility-scale solar plant operates at rated output only about 20–25 % of the year. The remaining 75–80 % of the time, output is below rated or zero — but the transformer stays energised, drawing no-load loss every hour of every day regardless of how much the plant is generating. For solar duty, no-load loss dominates lifetime energy cost, not load loss.
And the load itself is not stable: generation follows irradiance, so the plant connects and rides through variable output continuously rather than holding a steady profile.
Our solution
Sealed oil-immersed units for the field arrays
For the outdoor field positions we supplied S20 series sealed oil-immersed transformers. Two reasons drove this. First, thermal performance: oil-immersed construction with ONAN / ONAF cooling handles desert ambient and the daily thermal cycle with margin, and remains the dominant choice for utility-scale PV above roughly 5 MVA where dry-type becomes cost-prohibitive. Second, sealing: a fully sealed tank keeps windblown dust and moisture out of the insulation system, which matters on a site where soiling is measurable in lost megawatt-hours.
The full trade-off between the two technologies is set out in our guide to oil-immersed vs dry-type transformers.
Cast resin dry-type for the inverter station building
Inside the inverter station building the calculus changes: the constraint is fire safety in an enclosed space, not weather. There we supplied SCB13 cast resin dry-type units — epoxy vacuum-cast windings, self-extinguishing, no flammable liquid to contain or monitor. Cast resin also tolerates indoor humidity and runs without re-drying, which suits a building that is cooled intermittently.
Harmonic tolerance for inverter output
Inverter duty transformers were built with K-factor rated windings and electrostatic shields to absorb the harmonic spectrum the inverters produce. The vector group does part of the work too: with a delta-connected low-voltage winding (Dyn11), triplen harmonics circulate in the delta rather than propagating to the grid side.
Where measured THD exceeds 5 %, filtering is added at the station rather than expecting the transformer to absorb it — because a K-rating, as we explain in our data centre case, is a heat-survival rating, not a filter. Power quality compliance was specified against IEEE 519, and harmonic capability assessed per IEEE C57.110.
Low no-load loss for a 24/7 energised duty
Because the plant spends three quarters of the year below rated output while the transformers remain energised, we optimised for no-load loss rather than for full-load efficiency. The arithmetic is worth stating: a 1,000 kVA unit with a 1.5 kW no-load loss consumes 13,140 kWh a year whether the plant produces at full capacity or not at all.
Low-loss core construction typically carries a 15–25 % price premium, with payback commonly inside four to seven years — comfortably within a 25-year plant life. That is why the result on this project is stated as reduced no-load losses across the plant lifecycle rather than as a peak efficiency figure.
Technical specification
| Parameter | Value |
|---|---|
| Plant rating | 50 MW |
| Inverter duty transformer | 6.25 MVA × [ ] units |
| Substation transformer | 63 MVA |
| Voltage ratio (IDT) | 0.8 / 33 kV |
| Voltage ratio (substation) | 33 / [132] kV |
| Frequency | 50 Hz |
| Vector group | Dyn11 / YNd11 |
| Impedance voltage | 6–8 %（IDT）； 10–14 % |
| Cooling | ONAN / ONAF |
| Ambient design | 40–55 °C |
| Altitude | 1,000 m； |
| Harmonic provision | K-factor rated windings + electrostatic shield；IEEE 519 |
| Measured THD | [20 ] % |
| Design life | 25 |
| Standards | IEC 60076；IEEE C57.110；IEEE 519 |
Testing and verification
Every unit passed routine tests per IEC 60076 before release: winding resistance, voltage ratio and vector group, no-load loss and current, load loss and impedance voltage, applied and induced voltage withstand, and insulation resistance. Full FAT documentation was issued for the EPC’s review.
No-load and load loss were verified against the guaranteed values, since those figures are what the plant’s loss capitalisation is based on. Temperature rise was assessed at the site design ambient rather than at the IEC 40 °C reference, with any required de-rating documented..
See our guide to transformer routine testing for what to expect before shipment, and cooling classes (ONAN, ONAF, AN, AF) for how the ratings differ.
Result
The plant connects and rides through variable generation on transformers specified for the desert environment rather than for a temperate catalogue: sealed tanks keeping dust out of the insulation, thermal margin sized to the worst month, harmonic-tolerant windings on the inverter side, and fire-safe cast resin inside the station building.
Across the plant lifecycle, the decision that pays back is the low no-load loss design — because on a 50 MW plant that operates at rated output only a fifth to a quarter of the year, the loss that matters is the one incurred every hour the transformer is energised.
Planning a utility-scale PV, wind or storage project? Review our renewable energy transformer solutions, or send us your plant rating, ambient data and grid voltage for a proposal.
Frequently asked questions
How do I size the step-up transformer for a 50 MW solar plant?
Work from plant output plus auxiliaries, divided by power factor, times a future margin. For a 50 MW plant with 0.5 MW of auxiliary load at PF 0.99 and a 1.20 expansion margin: (50 + 0.5) / 0.99 × 1.20 ≈ 61.2 MVA, rounded up to the next standard rating — 63 MVA, or 75 MVA depending on utility requirements. Inverter duty transformers are sized to the block they serve; a common 50 MW arrangement is eight 6.25 MVA units, one per 6.25 MW block, stepping 0.8 kV up to 33 kV for the collector network.
Why doesn’t solar get the diversity benefit that industrial loads do?
Because all inverter blocks peak together. A cloud-edge transient can drive every block to peak simultaneously for several minutes, so sizing the transformer to the exact sum of inverter nameplates with no margin is a known commissioning failure mode. Solar inverters do not behave like diversified industrial loads — add margin.
Oil-immersed or dry-type for a solar plant?
For utility-scale field duty, oil-immersed dominates: better thermal performance under continuous load, lower cost per kVA at larger ratings, and sealed tanks keep dust and moisture out. Dry-type is the right answer indoors — inverter station buildings and other enclosed spaces where fire safety and the absence of flammable liquid matter more than cost. Dry-type becomes cost-prohibitive above roughly 5 MVA.
Why is no-load loss more important than load loss on a solar plant?
Because of the duty cycle. A utility-scale solar plant runs at rated output only about 20–25 % of the year; the rest of the time output is below rated or zero. Load loss falls with the square of current, so at part load it is small — but the transformer stays energised and draws no-load loss continuously. A 1,000 kVA unit with 1.5 kW of no-load loss consumes 13,140 kWh a year regardless of generation. Low-loss core construction typically costs 15–25 % more and pays back in four to seven years, well inside a 25-year plant life.
What does desert heat do to the transformer specification?
Design conditions for desert sites run 40–55 °C ambient, against the IEC reference maximum of 40 °C. Above that reference the transformer must either be de-rated or built with additional thermal margin, and the calculation should be done for the site’s worst month rather than the annual average. Dust is a separate issue — measured desert installations have lost around 10 % of energy production to soiling alone, which is one argument for sealed tank construction in the field.