Battery storage and solar plants load a transformer in ways a passive network never does: through inverters, at high load for long uninterrupted stretches, with harmonic content and — for BESS — power flowing in both directions many times a day. A standard distribution transformer will work, but it will run hotter and age faster than its nameplate suggests.
What inverters do to a transformer
Inverters produce current with harmonic content on top of the fundamental 50 Hz. Harmonics raise eddy-current losses in the windings and stray losses in the structural parts, and that heat is not evenly distributed — it concentrates at the winding hot spot. The result is a transformer that reaches its thermal limit below its rated kVA.
The usual answer is a k-factor rating, or an equivalent derating, which specifies extra thermal margin for harmonic heating. Give the expected harmonic spectrum rather than a k-number alone, so the designer can dimension the windings for your actual case.
What is specific to battery storage
A BESS transformer cycles. It charges and discharges, often several times a day, and it can sit at or near full load for hours in either direction. Continuous cycling stresses the insulation and, across a long service life, matters more than the occasional peak.
Inverters can also inject a small DC component. DC bias pushes the core towards saturation, raising magnetising current, noise and losses. Where the inverter specification allows a DC offset, say so at enquiry stage — the core can be dimensioned for it.
Solar plants have their own profile
A PV transformer sees a daily curve rather than a flat load: nothing at night, a broad peak around midday, and often reactive power support outside generation hours. No-load losses run around the clock while the plant only earns during daylight, so a low no-load figure is worth real money over a 25-year plant life.
Site conditions matter as much as electrical duty. Solar farms are outdoors, often remote, sometimes dusty or coastal. Corrosion protection class, ambient temperature range and whether the unit is supplied as a skid, kiosk or containerised package belong in the specification alongside the kVA figure.
Configuration choices worth making early
- Dual- or multi-winding designs let one transformer serve several inverters and keep the low-voltage cabling short.
- Vector groups are chosen so that the phase shift between windings cancels selected harmonic orders across paralleled inverters.
- A delta winding traps triplen harmonics and keeps them out of the grid.
- Impedance is set with the inverter fault-ride-through requirement and the grid operator's short-circuit limits in mind, not only voltage regulation.
The practical trade-off is between a cheap standard unit and one specified for the duty. A transformer bought purely on kVA and price will run, but harmonic heating, cycling and DC bias eat into its life — and on a 25-year asset that is an expensive saving. Share the inverter datasheet, the expected load profile and the site conditions at enquiry stage, and the design can be matched to them. Protrafo supplies BESS and solar transformers as oil-filled or cast-resin units, with live oil- and winding-temperature monitoring available through Protrafo Insights where cycling duty makes trending worthwhile.
