Dry Type Transformer for Solar Plant & BESS Inverter Duty

A dry type transformer for solar plant and battery storage duty is not a standard distribution transformer with a different nameplate. The load it feeds is a power electronic converter, the current waveform is not sinusoidal, the power can flow in either direction, and the loading changes with the weather. Those four facts change the specification, and IEEE C57.159 exists precisely because early PV projects kept getting them wrong.

This guide covers where the transformer sits in a PV or BESS plant, what inverter duty actually demands of the design, what IEEE C57.159 does and does not settle, and how to size a unit for a source that is rarely at full output and never at a constant one.

Why solar and storage need different transformer thinking

Three characteristics separate renewable duty from conventional distribution duty.

• The source is the converter. The transformer’s primary is connected to an inverter or power conversion system, not to a utility bus. Harmonic content, DC offset and switching transients come from the converter, and the transformer is the first passive component that has to absorb them.
• Power flow is bidirectional on storage. A BESS transformer charges and discharges, so the design has to be evaluated in both directions rather than as a one-way step-up.
• The duty is thermally cyclic. A PV transformer rises with the sun and falls away every evening. Daily expansion and contraction, plus repeated hot-spot excursions, is a different ageing mechanism from a steady industrial load.

Where the transformer sits in a PV or BESS plant

In a distributed PV plant, inverter output — typically 400, 480, 600 or 690 V AC — is stepped up to a medium-voltage collector level, commonly 11, 33 or 35 kV, and then aggregated to the point of interconnection. The transformer at each inverter or inverter block is the inverter transformer; the unit at the plant substation is a different machine with a different specification.

Signal the interconnection and protection scope early. On the MV side, the transformer has to be coordinated with the collector switchgear, and our medium and high-voltage power solutions overview describes how transformer, switchgear and protection are normally packaged together for a plant of this type.

Inverter duty: what it actually demands from the design

Harmonic and stray losses

IEEE C57.159 is explicit that harmonics affect a transformer in a specific way. Eddy currents and stray losses are present in every transformer, and the majority are caused by fundamental-frequency current — but those losses increase when significant harmonic content is present. If the inverter feeding the transformer produces more than a standard level of harmonics, stray and eddy losses rise accordingly.

The guide makes a point that is easy to miss: the effect of the increased load losses on efficiency is usually not the concern. The real concern is the rise in winding hot-spot temperature and in hot spots in metallic parts, because those reduce insulation life, and localised extreme overheating can end in catastrophic failure. The recommended mitigation is straightforward but often ignored — increasing the transformer’s power rating reduces the load losses at operating current and can compensate for the higher temperatures.

Two further points from the guide apply directly to specification. In multi-winding configurations, eddy and stray losses need special attention wherever there is an appreciable difference in load between the LV windings — which is exactly the situation in a multi-inverter block with uneven string output. And stray losses in the electrostatic shield can push local temperature above the insulation thermal class limit, so the shield is not a free addition.

How much derating applies

The standard method for quantifying harmonic heating is IEEE C57.110, which defines the harmonic loss factor and the resulting capability limits. Worked examples using a six-pulse rectifier spectrum show how differently the two families behave: dry-type transformers carried roughly 77% of rated current in the same case, while liquid-immersed units carried around 87%. In other words, oil handles harmonic heating better than air, and a dry-type unit needs either more kVA or a harmonic-rated design to do the same job.

The C57.110 method caps the harmonic order considered at the 25th, because skin effect makes eddy-loss estimates increasingly conservative above the 11th to 19th harmonic. The practical consequence for a PV or BESS specification is that the derating calculation should use the actual measured or expected inverter spectrum, not a generic assumption.

Switching transients and insulation

Inverter output includes switching transients that a plain distribution transformer was never specified to see. C57.159 notes that as distributed PV scale has moved from distribution to transmission level, a 34.5 kV secondary winding voltage has become common, and suggests that users may specify test voltages of the next higher voltage class to compensate for possible transient overvoltage, waveform distortion and other power quality effects. The guide also recommends full phase-to-phase insulation, to protect against ground faults.

IEEE C57.159: what it covers

It is a guide, not a code. It does not replace the product standard, and it does not tell you a kVA figure. What it does is name the failure modes that a conventional specification will not catch — which is worth reading before you copy last year’s transformer clause onto a new project.

Sizing a solar or storage transformer

Three adjustments sit on top of the ordinary load calculation.

• Inverter clipping and DC/AC ratio. Plant design deliberately oversizes the DC array relative to inverter AC rating, so the inverter runs at or near its AC limit for the middle of the day. The transformer should be sized on that AC limit plus margin, not on array nameplate.
• Altitude and ambient derating. Many plants sit hotter than a standard design ambient, and mountain sites sit higher. Both reduce the effective capacity of an air-cooled unit.
• Harmonic derating. Apply the C57.110 or C57.159 loss calculation, and if the result leaves the unit above its thermal limit at rated current, increase the rating rather than accepting a shorter life.

Thermal cycling and its consequences

A PV transformer goes through a full thermal cycle every day. The hot spot that matters is the one reached at midday in summer, at maximum ambient, with the array at full production — and it is reached thousands of times over the equipment life. This is why a declared temperature rise below the insulation class limit is worth more on a solar plant than on a steady-duty industrial site, and why the thermal class discussion in our guide to transformer insulation class and temperature rise is directly relevant to this choice.

Dry type or liquid-immersed for a PV or BESS plant?

The honest answer is that most pad-mounted inverter transformers are liquid-filled, because harmonics, outdoor environment and cost per kVA all point that way. Dry type wins in a specific set of circumstances.

Where a plant does use liquid-filled units, the selection logic is closer to mainstream distribution practice, and the comparison in our guide to selecting an oil-immersed transformer type covers the voltage class and cooling choices. For MV interconnection at 35 kV, the reference data in our 35 kV oil immersed power distribution transformer specification shows the ratings and construction typically used at that level.

BESS-specific selection points

1. Evaluate both directions. Charge and discharge modes can have different loss and harmonic profiles. Specify on the worse case, not the average.
2. Check the operating range, not the rated point. Storage spends much of its life part-loaded, so part-load efficiency drives the real energy cost.
3. Confirm impedance suits the PCS. Impedance affects voltage regulation during rapid power changes and interacts with converter control behaviour.
4. Decide the thermal class with cycling in mind. A storage duty cycle can be far harder than a PV duty cycle.
5. Agree the auxiliary supply. Protection, monitoring, cooling and fire systems all need a supply that survives a trip.

Specification checklist

Whatever the technology, one specification discipline pays for itself on every renewable project: state the load profile and the harmonic spectrum alongside the kVA. A quotation that only fixes the rating and the ratio leaves the two variables that actually decide the unit’s life entirely to the supplier — a point that shows up in the final number as surely as it does in the loss figures, as our power transformer price guide sets out.

Conclusion

Solar and storage transformers are converter-duty machines. Read IEEE C57.159 before writing the specification, apply the IEEE C57.110 loss method to your actual inverter spectrum rather than assuming it away, remember that dry-type units derate further than liquid-filled ones under the same harmonics, and size on the converter AC limit after derating. Then add the two requirements that PV and BESS duty always brings: full phase-to-phase insulation for transient and ground-fault exposure, and a declared temperature rise with enough margin to survive a full thermal cycle every day for twenty years. For indoor and enclosed auxiliary positions, the cast resin data in our SCB series dry type transformer range is a practical reference point.

FAQ

What is IEEE C57.159?

IEEE C57.159-2016 is the IEEE Guide on Transformers for Application in Distributed Photovoltaic (DPV) Power Generation Systems. It gives recommendations for specifying, designing and applying step-up and step-down liquid-immersed and dry-type transformers in distributed PV plants, focusing on inverter transformers with an LV primary connected to the inverter.

Why does inverter duty matter to a transformer?

Because inverter output is not sinusoidal. Harmonic content increases winding eddy and stray losses, which raises winding hot-spot temperature and hot spots in metallic parts. Efficiency is rarely the problem; reduced insulation life and localised overheating are.

How much does a transformer have to be derated for inverter harmonics?

It depends on the harmonic spectrum. In worked examples using a six-pulse rectifier spectrum, dry-type transformers carried around 77% of rated current and liquid-immersed units around 87%. Calculate against your actual inverter spectrum using the IEEE C57.110 method rather than applying a blanket derating figure.

Should a solar or BESS transformer be dry type or oil-immersed?

Most pad-mounted inverter and storage transformers at plant scale are liquid-immersed, because harmonic capability, outdoor suitability and cost per kVA all favour them. Dry type is the better choice for indoor or enclosed positions, rooftop and space-constrained sites, and locations where a flammable liquid is unacceptable.

What is an energy storage transformer and how is it different?

A BESS transformer couples a power conversion system to the collector network, so it must be designed for power flow in both directions and evaluated across the full operating range, including the part-loaded conditions where storage spends most of its time.

What is an inverter duty transformer?

It is a transformer whose primary is fed directly by an inverter rather than by a utility bus. The specification has to account for the converter’s harmonic spectrum, switching transients and DC offset, and it is the machine that IEEE C57.159 addresses most directly.

Should I specify a higher test voltage for a PV transformer?

Worth considering. C57.159 notes that with 34.5 kV secondaries now common, users may specify test voltages of the next higher voltage class to cover possible transient overvoltage, waveform distortion and other power quality effects. Full phase-to-phase insulation is recommended in any case.