Sizing Distribution Transformers for Solar and Storage Projects

Why renewable projects are different

A distribution transformer in a factory or a commercial building runs close to its rating for much of the day. A transformer on a solar farm does not. Output ramps up in the morning, peaks around midday and falls away in the evening, so the unit spends a large share of its operating hours well below rated capacity. Figures typically quoted for PV plants put the annual average loading below half of the nameplate rating.

Battery storage behaves differently again. A storage system charges and discharges repeatedly through the day, so its transformer sees more operating cycles and a higher average loading than a PV unit of the same rating.

Both cases lead to the same conclusion for specification: the unit is energised for far more hours than it is fully loaded. The loss that accrues around the clock is the no-load loss in the core, not the load loss in the windings.

Start from the inverter or the PCS, not the transformer

The low-voltage winding has to match the AC output of the inverter or power conversion system. That is commonly 0.4 kV for string and mid-size inverters, with 0.69 kV and 0.8 kV used by larger central units. The high-voltage winding matches the plant's collector system, which is usually 10 kV, 20 kV or 35 kV depending on plant size and local practice.

Two details are often settled too late. The first is the vector group and neutral arrangement: the transformer's earthing configuration has to agree with the plant's grounding design. The second is what happens when units are operated in parallel. Where several transformers feed one feeder, or several inverters feed one transformer, the impedance and vector group have to match, and that is a specification decision rather than a detail to fix on site.

Which loss figure you are actually paying for

No-load loss is present whenever the transformer is energised. Load loss varies with the square of the loading. On a plant that runs at low average loading, no-load loss can account for the larger share of lifetime energy cost — which is why a lower-loss core design sometimes costs more at purchase and less over the life of the project.

Many tenders handle this through capitalised loss evaluation: a monetary value per watt is applied to each loss figure and added to the purchase price before offers are compared. If your tender does not use capitalised losses, be aware that the lowest purchase price is not automatically the lowest total cost.

Conditions that change the design

Ambient temperature. IEC 60076-1 treats normal service conditions as a maximum ambient of 40°C, with a 24-hour average of 30°C and an annual average of 20°C. Sites that exceed this need the unit derated, or ordered for a higher ambient class.

Altitude. Normal service conditions are stated up to 1,000 m. Above that, air insulation strength decreases and cooling is less effective, so insulation levels and temperature rise both need correction.

Harmonics. Inverter and PCS output carries harmonic content that a conventional distribution transformer is not designed around. Additional harmonic losses raise winding temperature and may require derating. If your inverter datasheet gives a current distortion figure, pass it to the transformer supplier at the enquiry stage.

Site conditions. Desert and high-irradiance sites combine high ambient temperature with solar gain. Containerised or indoor dry-type installations need the airflow path planned before the unit is specified. Coastal projects need the corrosion class for the tank and fittings settled early, because it affects delivery time as well as price.

What to confirm before you order

· Rated capacity and the low-voltage / high-voltage ratio

· Tap range, typically ±2 × 2.5%

· Vector group and neutral arrangement

· Impedance, and whether units will be paralleled

· No-load and load loss figures, and the ambient / altitude correction applied

· For dry-type: cooling class (AN or AF) and rated output under forced cooling

· Scope of tests, and whether third-party inspection is required

· Energy efficiency level required in the destination market, where one is regulated

On that last point, if the project is in the European Union, compliance with IEC 60076 alone is not sufficient. Commission Regulation (EU) 2019/1783 sets maximum loss levels for small, medium and large power transformers placed on the EU market, with the EN 60076 series as the harmonised standards. The required loss level should be stated in the specification from the outset, because it changes the core design rather than the finish.