250 kVA 11 kV Oil-Immersed Step-Up Transformer for Solar PV & Wind Power Projects – Grid-Tied / Off-Grid, IEC 60076

Model:S22-M-250/11 (S20-M-250/11 and S13-M-250/11 optional; amorphous SBH25-M-250/11 on request)

Rated Capacity:250 kVA

Phase:Three-phase

Frequency:50 Hz / 60 Hz

Rated High Voltage (HV):11 kV (10, 12, 13.8, 20, 22, 22.9, 24, 33, 35 kV optional)

HV Tapping Range:±2×2.5% off-circuit (±5% optional, OLTC on request)

Rated Low Voltage (LV):0.4 kV (0.415, 0.48, 0.69, 0.8 kV optional)

Direction of Use:Step-up (0.4 → 11 kV) and step-down (11 → 0.4 kV) in the same unit

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Quick Answer

A 250 kVA 11 kV oil-immersed transformer for a renewable project is a hermetically sealed three-phase unit that steps inverter or turbine output up from 0.4 kV — 0.69 kV on wind — to 11 kV for the grid connection, and back down to 400/230 V when the site runs in island mode. It suits a 200–250 kWp PV array, a single medium wind turbine or a hybrid mini-grid. Because it stays energised 8,760 hours a year while the plant produces at a 12–20% capacity factor, no-load loss is about 65% of the annual energy lost. Specify GB 20052-2024 Class 1 (S22-M-250/11, 220 W no-load), Dyn11 and a sealed corrugated tank.

Product Overview

This is a three-phase, 50/60 Hz ONAN oil-immersed transformer rated 250 kVA with an 11 kV and a 0.4 kV winding — 0.69 kV for wind turbines and 0.8 kV for a 1500 V PV system — used in either direction: stepping inverter or turbine output up to 11 kV for export, or 11 kV down to 400/230 V when the site runs as an island. Both windings are copper, sealed in a corrugated-fin tank with no conservator, so the oil never contacts air and needs no sampling or filtration over a 25-year design life. Oil temperature monitoring with alarm and trip contacts plus RS485/Modbus RTU are standard, with optional 4G or LoRa DTU. Tested to IEC 60076-1/-2/-3/-5/-10, wind duty to IEC 60076-16, energy performance to GB 20052-2024.

What GB 20052-2024 Changed for Renewable-Duty Transformers

China’s GB 20052-2024 took effect on 1 February 2025 and, for the first time, brings photovoltaic, wind and storage transformers from 6 kV to 35 kV into the mandatory efficiency scheme, expanding the tables from 28 to 42 categories. Grades are NX1 (best), NX2 and NX3 — S22, S20 and S13 on silicon steel; SBH25, SBH21 and SBH15 on amorphous cores. For an overseas buyer the value is that the grade is verifiable against a type test report, and that Class 1 here comfortably meets EU Ecodesign Tier 2.

Why Solar and Wind Sites Choose Sealed Oil-Immersed Over Dry Type

Our recommendation: for an outdoor PV array, wind cluster or hybrid mini-grid, specify a hermetically sealed oil-immersed transformer; keep cast resin dry type for indoor rooms, plateau sites above roughly 3,000 m, and where the fire or environmental code rules out mineral oil.

Why oil wins here: 220 W no-load loss against higher dry-type figures, no night-time condensation in a sealed tank, oil mass absorbing cloud transients and gust peaks, and IP54 cable boxes that stand in the open without a room.Where oil loses: near forest or a water catchment, use dry type or natural ester; above roughly 3,000 m, use dry type.

Silicon Steel or Amorphous Core at 250 kVA

250 kVA is where amorphous starts to pay, and renewable duty favours it: the core cuts no-load loss from 220 W to ≈110 W, while load loss rises slightly to ≈2,400 W and first cost typically runs 15–30% higher. Decision rule, not a payback claim: on PV duty it saves about 18,600 kWh over 20 years against S22 — roughly USD 2,240 at USD 0.12/kWh. Take it if the quoted premium is below that figure; otherwise stay with S22. Loss and running-cost figures sit beside the silicon-steel grades in the table below.

Energy Efficiency Grades and the 20-Year Running Cost

On a renewable site the transformer is energised far longer than the plant produces — a PV array runs at a 12–20% capacity factor while the transformer sits on 11 kV for all 8,760 hours of the year — so the no-load figure, not the purchase price, decides the 20-year cost.

GB 20052-2024 Grade	Model	No-load loss P0	Load loss Pk (75 °C)	Annual loss – PV duty	Annual loss – wind duty	20-yr cost – PV	20-yr cost – wind
Class 3 (NX3)	S13-M-250/11	290 W	3,200 W	≈ 3,980 kWh	≈ 5,702 kWh	≈ USD 9,553	≈ USD 13,685
Class 2 (NX2)	S20-M-250/11	260 W	2,560 W	≈ 3,430 kWh	≈ 4,807 kWh	≈ USD 8,231	≈ USD 11,537
Class 1 (NX1)	S22-M-250/11	220 W	2,330 W	≈ 2,976 kWh	≈ 4,229 kWh	≈ USD 7,142	≈ USD 10,150
Class 1 amorphous	SBH25-M-250/11	≈ 110 W	≈ 2,400 W	≈ 2,044 kWh	≈ 3,335 kWh	≈ USD 4,905	≈ USD 8,004

Moving from Class 3 to Class 1 saves about 20,100 kWh over 20 years on a PV site and 29,500 kWh on a wind site — roughly USD 2,410 and USD 3,535 at USD 0.12/kWh. On PV duty no-load loss alone is about 65% of the annual total, so improving load loss alone is the wrong lever here.

Grid-Connected vs Off-Grid: What Changes

The transformer itself is bidirectional and needs no change between the two modes. What changes is the earthing, the protection scheme and, on weak feeders, the tapping range.

Item	Grid-connected (grid-tied)	Off-grid / island
Power flow	0.4 kV → 11 kV, export to the utility	Both directions: 0.4 → 11 kV by day, 11 → 0.4 kV for site loads
Neutral and earthing	As the utility requires	LV neutral must be earthed (TN-S or TT) — Dyn11 provides it
Tapping range	±2×2.5% normally sufficient	Wider range or OLTC if island voltage swings
Short-circuit level	Utility contributes fault current	Inverters only — low; Uk% affects protection sensitivity
Night behaviour	Idle, no-load loss only	May be switched off with the island

Sizing: What Array and Inverter Does 250 kVA Match?

Match the transformer to inverter AC output, not array size: kVA ≈ inverter AC kW ÷ 0.9, so 250 kVA covers about 225 kW of inverter capacity — with the usual 1.1–1.2 DC/AC oversizing, a 250 kWp array, typically eight to ten 25–30 kW string inverters on one LV board, a single 225 kW central inverter, or a PV + battery + genset block.

Key Features

Dual-direction by design — 0.4 kV to 11 kV for export and 11 kV to 400/230 V for island loads in one unit; Dyn11 traps triplen harmonics and gives island loads a neutral.
Low no-load loss for renewable duty — 220 W against 290 W for Class 3, about 65% of annual losses on PV duty. GB 20052-2024 Class 1 (NX1) as standard, under the renewable-duty category the 2024 edition introduced for PV, wind and storage transformers from 6 kV to 35 kV; type test report provided. Amorphous SBH25 core optional, cutting no-load loss by roughly half.
Sealed corrugated tank, no conservator — no oil sampling or filtration over the 25-year design life, no moist air drawn in overnight, and enough thermal mass to smooth cloud transients and gust peaks.
Inverter-ready — copper windings both sides for lower loss and better short-circuit withstand (IEC 60076-5), designed for harmonic current per IEEE 519 / IEC 61000-3-12 and IEEE C57.110 and for DC injection; natural ester option, readily biodegradable, fire point above 300 °C.
Harsh sites and remote monitoring — C4 / C5-M coating for coastal and desert duty, high-altitude design above 1,000 m, -25 °C to +40 °C ambient with -40 °C and +50 °C versions; oil and winding temperature, load current and voltage on RS485/Modbus RTU, with optional 4G or LoRa DTU.

Typical Applications

Commercial and industrial rooftop or ground-mount PV arrays of roughly 200–250 kWp feeding an 11 kV feeder
Single medium wind turbines and small wind clusters with 690 V generator output
Hybrid PV + battery + diesel mini-grids for villages, farms, ranches, mines and telecom sites
Off-grid islands, mining camps, irrigation stations, solar-plus-storage microgrids and weak-grid or long-feeder connections

Standards and Certifications

Designed and tested to IEC 60076-1/-2/-3/-5/-10 and IEC 60076-16 (wind), plus GB 1094 and GB/T 6451. Energy performance meets GB 20052-2024 Class 1 (NX1) — Class 2 (NX2) with the S20 design — under the renewable-duty category; inverter harmonic duty per IEEE C57.110, IEEE 519 and IEC 61000-3-12. Manufactured under ISO 9001, ISO 14001 and ISO 45001; CE conformity and third-party type test reports (CNAS-accredited, KEMA or equivalent) available on request.

Q1: What size solar array or inverter does a 250 kVA transformer match?

Match the transformer to the inverter AC output, not to the DC array size. Rule of thumb: transformer kVA ≈ inverter AC kW ÷ 0.9, so 250 kVA covers about 225 kW of inverter capacity, which with the usual 1.1–1.2 DC/AC oversizing means a 250 kWp array. It is also the common rating for eight to ten 25–30 kW string inverters on one LV board, or for a hybrid PV + battery + genset block where the transformer carries site load in island mode.

Q2: Can one transformer be used for both grid-tied and off-grid renewable projects?

Yes — a transformer is bidirectional, and this unit is specified for both directions. In grid-tied mode it steps inverter output up from 0.4 kV to 11 kV for export; in island mode it steps the 11 kV mini-grid down to 400/230 V for site loads. What changes between the two modes is not the transformer but the earthing and protection: the LV neutral must be earthed (TN-S or TT) in island mode — Dyn11 provides it — and anti-islanding protection is required only in grid-tied mode.

Q3: Why does no-load loss matter more than load loss on a solar or wind transformer?

Because the transformer is energised far longer than the plant produces. A PV array runs at a 12–20% capacity factor, yet the transformer sits on 11 kV for all 8,760 hours of the year, including every night. On a 250 kVA unit, no-load loss contributes about 65% of annual losses on PV duty and about 46% on wind duty. That is why the S22 Class 1 design at 220 W matters more here than it would on a conventionally loaded distribution transformer, and why an amorphous core — which cuts no-load loss by roughly half — is worth pricing on low-capacity-factor sites.

Q4: What voltage does a 250 kVA renewable transformer step up to?

On this unit, 0.4 kV to 11 kV as standard. 11 kV is the common distribution voltage across the UK, much of Asia, Africa, the Middle East and Australia; 10 kV is standard in China and 13.8 kV in parts of the Americas. The HV winding is configurable from 10 kV to 24 kV, with 33 kV and 35 kV for longer feeders. Wind configurations typically use 0.69 kV on the LV side; 1500 V PV systems use 0.8 kV.

Q5: Can the LV side be 0.69 kV for wind or 0.8 kV for a 1500 V PV system?

Yes to both. 690 V is the standard generator output of most medium wind turbines, and 800 V is the usual AC output of a 250 kW string inverter on a 1500 V DC system. Voltage ratio, vector group, tapping range and frequency are all configurable at order. Tell us the inverter or turbine model and we will match the LV winding to it.

Q6: Does GB 20052-2024 apply if my project is outside China?

Not as a legal requirement, but it is a useful procurement specification. China’s GB 20052-2024 took effect on 1 February 2025 and was the first edition to bring photovoltaic, wind and storage transformers from 6 kV to 35 kV into the efficiency scheme, with its own limit tables rather than the general distribution tables. The grades are named NX1, NX2 and NX3. For an overseas buyer the value is that the grade is verifiable — you can ask for the type test report — and that a Class 1 unit comfortably meets EU Ecodesign Tier 2 levels at this rating. If your project has to clear a utility interconnection review or an efficiency subsidy application, specifying the grade by name is what gets checked.

Q7: Should I choose an amorphous core instead of silicon steel?

Take the amorphous option when the site idles for most of the year — a PV array with a low capacity factor, or an off-grid mini-grid. An amorphous core cuts no-load loss by roughly half, and on renewable duty the transformer is energised 8,760 hours a year. The decision rule: on PV duty the amorphous design saves about 18,600 kWh over 20 years against S22, roughly USD 2,240 at USD 0.12/kWh. If the quoted premium is below that, it pays back inside the design life. We will run the same calculation with your tariff and capacity factor.

Q8: Do solar inverters damage the transformer through harmonics or DC injection?

Modern inverters stay within IEEE 519 and IEC 61000-3-12 limits, with THDi typically below 3–5%, so harmonic heating is far less severe than on DC fast-charging sites. Two inverter-related effects still need to be addressed at specification: DC current injection, which biases the core and raises magnetising current, audible noise and no-load loss; and the aggregate harmonic current of many inverters on one transformer, which we account for in the winding design per IEEE C57.110. Both are handled in the design rather than by oversizing.

Q9: Does a renewable step-up transformer need an on-load tap changer?

Usually not. Off-circuit taps of ±2×2.5% cover normal grid variation. An OLTC or a wider ±5% range is worth paying for when the plant connects to a weak rural feeder, because exporting power raises the voltage at the point of connection and a long feeder can push it beyond limits at midday. Ask the utility for the fault level and the voltage rise study before deciding.

Shipped oil-filled and factory-sealed on a steel-reinforced wooden pallet, wrapped in moisture-barrier film with desiccant, HV and LV bushings plus cable boxes protected by plywood guards, fitted with lifting lugs and shock/tilt indicators. Suitable for sea, road and rail transport; full export plywood crate available on request. Where destination regulations require, the unit can be shipped drained with oil in sealed drums. Final gross weight, crate dimensions and packing list confirmed before shipment.