Solar Farm Step-Up Transformer: 2026 Buyer's Guide

============================================================

Solar Farm Step-Up Transformer: 2026 Buyer's Guide

Solar photovoltaic (PV) projects rarely connect to the grid at the inverter's native AC voltage. A ground-mounted 50 MW solar farm in Chile or a 5 MW rooftop array in Vietnam both require a dedicated step-up transformer to raise the inverter's 0.4 / 0.69 / 1.0 kV output to a usable medium-voltage (MV) or high-voltage (HV) level for grid interconnection. Choosing that transformer correctly is the difference between a 25-year asset and an early replacement.

XIAOPAI has been manufacturing and integrating oil-immersed power transformers up to 230 kV / 200 MVA for export markets since 2003, and we deliver dedicated step-up units for PV plants in Spain, Chile, Vietnam, South Africa and the Middle East. This guide is the same checklist our engineering team uses when reviewing an EPC's substation RFQ.

--------

1. WHY A SOLAR FARM NEEDS A STEP-UP TRANSFORMER

--------

A solar inverter outputs low-voltage AC (typically 0.4 kV, 0.69 kV or 1.0 kV) at a power factor near unity. The grid-side Medium Voltage (MV) collection network in most countries is 10 kV, 20 kV, 33 kV or 35 kV; for plants over 50 MW it steps up again to 110 kV or 230 kV for HV transmission. Without a transformer, the inverter's low-voltage AC cannot travel far - voltage drop on long MV feeders would exceed 5 % before reaching the Point of Interconnection (POI).

The step-up transformer also isolates the inverter DC side from the grid, blocks DC injection that would saturate upstream transformers, and provides a zero-sequence path for ground-fault detection.

--------

2. TYPICAL VOLTAGE LADDER AND CONFIGURATION

--------

For a 1-5 MW ground-mount PV plant:

• Inverter output: 0.4 kV / 0.69 kV AC

• Step-up LV/MV transformer: 0.4 / 0.69 kV -> 10 kV or 20 kV

• MV collection ring main unit feeds the plant substation

• Substation transformer steps up again if exporting to 33 / 35 kV

For a 50-200 MW utility-scale PV plant:

• Inverter output: 0.69 kV

• Pad-mounted step-up transformer per inverter block: 0.69 kV -> 33 / 35 kV

• Main step-up transformer at POI: 33 / 35 kV -> 110 kV or 230 kV

XIAOPAI delivers both ends - the pad-mounted LV/MV step-up (usually oil-immersed sealed type, 1000-2500 kVA) and the main substation step-up (35 kV -> 110 kV or 230 kV, up to 200 MVA). See our 230 kV 200 MVA unit shipped to Spain for a reference design.

--------

3. OIL-IMMERSED VS DRY-TYPE VS PREFABRICATED SUBSTATION

--------

For PV plants, the three common packaging options are:

• Oil-immersed sealed transformer (S-M series): lowest cost per kVA, longest thermal life, but contains mineral oil and needs fire-protection clearances

• Dry-type transformer (SC-B series with epoxy resin or SCBH15 amorphous): no oil, indoor installation possible, slightly higher cost, 25 % lower no-load loss with amorphous core

• Prefabricated substation (YB series): integrates MV switchgear + transformer + LV panel in one IP54 enclosure, fastest on-site installation, ideal for desert or coastal sites

For outdoor PV plants the oil-immersed sealed type remains the most common choice on a $/kVA basis. For indoor inverter stations or sites with strict fire codes, dry-type amorphous is increasingly specified. Prefabricated substations win when the EPC needs to compress civil works and minimise on-site labour.

A more detailed comparison of capacity limits, fire clearances and lifetime cost is in our dedicated guide: Dry-Type vs Oil-Immersed Transformer: True Cost & Capacity Limits (2026).

--------

4. KEY PARAMETERS TO SPECIFY

--------

A PV transformer RFQ should always include:

• Rated power (kVA / MVA) and cooling class (ONAN / ONAF / OFAF)

• Voltage ratio, e.g. 0.69 / 10.5 kV or 33 / 110 kV, with tap-changer range (+/- 2 x 2.5 % typical)

• Short-circuit impedance Uk, usually 6.0-8.0 % for LV/MV units, 10-12 % for HV units

• No-load loss P0 and load loss Pk at 75 deg C (drives LCOE)

• Vector group Dyn11 is the most common for PV step-up (provides a neutral for ground-fault detection)

• Insulation level (BIL / LIWV) for HV side

• Frequency 50 Hz or 60 Hz, ambient temperature, altitude (derating above 1000 m)

• Standards: IEC 60076 series (for export markets), IEEE C57.12.00 (for North America)

--------

5. STANDARDS MAP: IEC 60076 AND IEEE C57

--------

For European, Asian, African and Latin American PV projects, IEC 60076 is the mandatory family. The relevant parts:

• IEC 60076-1: general requirements, rating, tests

• IEC 60076-2: temperature-rise limits (insulation class)

• IEC 60076-11: dry-type transformers specifically

• IEC 60076-16: transformers for wind turbine and photovoltaic applications - this one is mandatory reading for any solar plant engineer

For US/Canadian projects, IEEE C57.12.00 (general) and IEEE C57.12.90 (test code) define the parallel requirement set.

XIAOPAI designs both to IEC and to IEEE/CSA on request - the difference shows up mostly in impedance tolerance, nameplate wording, and routine test scope. A unit that ships to Chile will be dual-labelled IEC/IEEE and tested to both codes.

--------

6. APPLICATION ENGINEERING BY INDUSTRY

--------

Different PV project types stress the transformer differently:

• Desert plants (Middle East, Atacama): 50 deg C ambient, sand ingress, derate by 8-10 %

• Coastal plants (Vietnam, Philippines): salt fog, specify C5-M corrosion protection

• High-altitude (Andes, >3000 m): derate by 1 % per 100 m above 1000 m, specify higher BIL

• Cold-climate (Northern China, Canada): specify low-temperature ester oil or special cold-start kit

We have shipped to all four categories - see Transformer Applications by Industry for a deeper sector breakdown.

--------

7. FAQ

--------

Q1: Can one step-up transformer serve multiple inverters?

A: Yes, this is common in 1-5 MW plants. A single 2.5 MVA LV/MV transformer with Dyn11 vector group can aggregate four 500 kW inverters via a 0.4 kV AC bus. Above 5 MW it becomes economically better to use one transformer per inverter block to limit fault current and outage scope.

Q2: Do I need an amorphous core transformer for PV?

A: If the transformer is energised 24/7 (baseload), amorphous saves 70-80 % no-load loss and pays back the cost premium in 3-5 years. If the plant is on a daily on/off cycle (morning energise, evening shutdown), the standby losses are smaller and standard silicon steel is usually sufficient.

Q3: What fire-protection clearance applies to oil-immersed transformers outdoors?

A: IEC 61936-1 and most national codes require a 3-10 m clearance between the transformer and adjacent structures depending on oil volume. Dry-type or ester-oil transformers can be placed closer to buildings.

Q4: How long is the typical delivery time?

A: Standard 0.69/10 kV or 0.69/35 kV units in 1000-2500 kVA: 30-45 working days from PO. Custom 110 kV or 230 kV main step-up: 90-120 working days. We confirm the schedule before PO and hold to it.

Q5: Does XIAOPAI supply the MV switchgear and inverter skid too?

A: For power transformers and packaged substations, yes - we integrate IEC 62271-100 compliant vacuum circuit breakers and the LV panel as a one-stop delivery. The inverter skid itself is normally sourced from the inverter OEM (Huawei, Sungrow, SMA, etc.) under the EPC's procurement.

RELATED GUIDES

- Transformer Applications by Industry | XIAOPAI

- 230kV 200MVA Transformer Shipped to Spain | XIAOPAI

- Dry-Type vs Oil-Immersed Transformer: True Cost & Capacity Limits (2026)‍

IEC 60076-1:2011 (Power Transformers - General)‍

IEC 60076-16:2011 (Wind/PV Transformers)‍

IEEE C57.12.00-2021 (Liquid-Immersed Transformers)

============================================================