The step-up transformer is a critical but often underspecified component in solar plant design. It steps up the inverter's low-voltage AC output (typically 270V–800V) to the medium-voltage (MV) grid connection level (10 kV, 20 kV, 33 kV, or higher) and must handle the unique load profile of a solar plant: high harmonics from inverter switching, highly variable output from zero to full load thousands of times per year, and potential DC injection. Getting the transformer specification wrong results in higher losses, premature failure, or utility rejection of the interconnection application. This guide covers sizing, type selection, loss optimization, standards compliance, and procurement considerations for EPC engineers.
In a typical utility-scale or large C&I solar plant, the electrical architecture from panel to grid is:
Solar panels → DC combiner → String/central inverter (270V–800V AC) → LV/MV step-up transformer → MV collection network → Main substation power transformer → Grid POI
Each string inverter (e.g., Sungrow SG250HX, SG350HX; Huawei SUN2000-330KTL) connects to a dedicated pad-mount step-up transformer, or multiple inverters share a transformer in a skid arrangement. For central inverters (above 1 MVA), a single larger transformer (1.6–4.0 MVA) steps up the full inverter output.
Transformer kVA rating must accommodate the inverter AC output with appropriate margin for harmonic loading and DC/AC ratio oversizing:
Standard transformer sizes in the range relevant to solar (per IEC 60076-1): 100, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500 kVA. Always select the next standard size above the calculated minimum — transformers should not be loaded above 100% of rated kVA on a continuous basis.
| Parameter | Dry-Type (Cast Resin, AN/AF) | Oil-Immersed (ONAN/ONAF) |
|---|---|---|
| Available ratings | Up to 10–15 MVA (typically <3.15 MVA for solar) | Any rating; >1 MVA preferred |
| Voltage class | Up to 36 kV (LV side up to 15 kV) | Up to 220 kV and above |
| Fire risk | Self-extinguishing; suitable for indoor, urban | Oil fire risk; requires bunded area |
| Environmental | No oil spill risk; no dielectric fluid disposal | Oil containment required; periodic oil testing |
| Overload capability | Limited (10–20% short-term) | Good (20–40% short-term, ONAN) |
| Losses at 75% load | 0.8–1.5% (no-load + load) | 0.5–0.9% (no-load + load) |
| Installed cost (per kVA) | Higher (+15–30%) | Baseline |
| Maintenance | Very low; no oil checks | Annual oil sampling; Buchholz relay inspection |
| Best application | C&I rooftop, indoor MV rooms, urban | Utility-scale ground-mount, remote sites |
For utility-scale ground-mount solar, oil-immersed ONAN (oil natural/air natural) or ONAF (oil natural/air forced) transformers are the standard choice due to lower losses and lower cost. Dry-type transformers are preferred for C&I rooftop installations, indoor MV rooms, and sites where fire risk is a concern (schools, hospitals, populated areas).
Beyond kVA rating and voltage ratio, specify the following in your transformer procurement document:
Impedance voltage (Uz%): Typically 4–6% for distribution transformers used in solar. Lower Uz% (4%) reduces voltage drop at full load but increases prospective short-circuit current (relevant for protection coordination). Higher Uz% (6%) limits fault current but creates larger voltage variation under load cycling. Confirm with the utility protection engineer before specifying.
No-load losses (NLL) and load losses (LL): Solar plant transformers spend significant time at partial load (nights at zero, mornings/evenings below 30% load) and are at full load only for peak hours. Energy-weighted losses should be minimized. Specify:
Vector group: For solar plants, Dy11 (delta primary, star secondary with neutral) is most common for LV/MV step-up transformers. The delta winding provides a path for triplen harmonic currents (3rd, 9th harmonics common in PWM inverter output) to circulate without appearing on the MV network. Always specify Dy11 or Dyn11 for inverter step-up transformers.
Temperature rise class and ambient rating:
Solar inverters using transformer-less topologies (all modern string inverters including Sungrow SG250HX, Huawei SUN2000 series, SMA Sunny Tripower series) can inject a small DC component into the AC output. IEC 61727 and IEEE 1547 limit DC injection to 0.5% of rated AC current. However, even this small DC component can cause partial saturation in the transformer core, increasing no-load losses and noise.
To mitigate DC injection effects:
When multiple inverters of different power levels connect to a single transformer (common in 1.5–4 MVA blocks with several 250–350 kW string inverters), split-winding or dual-secondary transformers allow independent connections while sharing a single primary. Benefits include:
Specify split-winding rating and short-circuit impedance between secondaries (typically 8–12%) to ensure adequate fault current differentiation for protection coordination.
All step-up transformers for solar plants should comply with:
Factory acceptance tests (FAT) for each transformer should include: ratio test, vector group verification, no-load loss measurement, load loss measurement at 75% and 100%, insulation resistance, dielectric withstand (applied voltage and induced voltage), partial discharge test (oil-filled), and temperature rise test (typically type test, not routine for all units).
Econo Solar procures step-up transformers from leading Chinese manufacturers — including TBEA, China XD Group, and Sieyuan Electric — all IEC 60076 certified and EU Eco-Design Tier 2 compliant, at direct-factory prices. We handle full FAT supervision, third-party inspection, and shipping for solar EPC projects worldwide. Request a transformer procurement quote for your project.
For rooftop commercial solar projects, dry-type (cast resin) transformers are strongly preferred for three reasons: they eliminate oil fire risk in occupied buildings, they require no bunded (containment) area for oil spill, and they need minimal maintenance (no oil sampling). The higher cost of dry-type transformers (+15–30% vs oil) is justified by reduced civil works, easier regulatory approval, and lower insurance premiums in urban environments. For rooftop projects below 1 MVA, 630 kVA dry-type transformers are the standard solution.
Dy11 (delta HV, star LV with neutral, 30° phase shift) or Dyn11 (with accessible neutral on LV star winding) is the standard specification for solar inverter step-up transformers. The delta HV winding provides a circulating path for triplen harmonics (3rd, 9th order) generated by PWM inverter switching, preventing these harmonics from propagating onto the MV grid. The star LV winding with neutral suits the 3-phase 4-wire connection required by most modern string inverters. Confirm with the utility whether earthing on the MV side requires Dyn11 specifically.
DC/AC ratio (or inverter loading ratio) affects the transformer in terms of peak current demand, not the rated sizing basis. The transformer is sized to the inverter's rated AC output power — the maximum AC power the inverter can produce regardless of DC input. Since the inverter clamps its output at rated AC power when DC input exceeds the inverter's AC capacity (common with DC/AC ratios of 1.2–1.4), the transformer never sees more than the inverter's rated AC output. However, at high DC/AC ratios, the inverter operates at rated output for more hours per day (clipping is reduced), so transformer thermal load duration increases — consider requesting a transformer with a higher temperature class (F or H) if DC/AC ratio exceeds 1.3.
Econo Solar supplies IEC 60076-certified step-up transformers, inverters, and complete BOS equipment directly from Chinese manufacturers — with full FAT supervision and global logistics support.
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