Any transformer steps voltage down or up, and the S22 is no exception — feed the high-voltage winding and you get a lower voltage on the other side; feed the low-voltage winding and you get a higher one. The ratio is simply the turns ratio: U2 / U1 = N2 / N1. What actually decides whether an S22 is the right unit is everything around that ratio — the voltage class you are landing on, the kVA you need after starting and harmonic currents, the impedance, the vector group, the cooling class, and how the grounding and protection are done where the unit will be installed.

Reference points used throughout: IEC 60076-1/2/7, IEEE C57.12.00, IEEE C57.91 loading guide, and where the unit lands in North America, NFPA 70 (NEC) Article 450. Confirm every limit against your own nameplate and the applicable standard before you write a specification.

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1. Yes — and the arithmetic is just the turns ratio

A transformer works in both directions. "Step-down" and "step-up" describe which winding you energise, not two different machines.

A worked example on a real distribution ratio. For 10 kV to 0.4 kV:

The 50:1 example in most Chinese source material (10 kV in, 200 V out) is arithmetically correct but it is not a distribution ratio — nothing on a 0.4 kV system runs at 200 V. It is worth knowing because it shows the mechanism, not because it describes a product.

Two things a transformer cannot do, and both come up constantly in enquiries:

• It cannot transform DC. A transformer needs a changing flux to induce voltage. Steady DC produces no induced voltage on the secondary and simply drives the winding into saturation. This matters enormously for solar and storage projects — see Section 7.
• It cannot change frequency. A 50 Hz design is not a 60 Hz design. Flux is proportional to volts divided by hertz, so running a 50 Hz core at 60 Hz and the same voltage reduces flux (usually thermally safe, but the guaranteed losses and impedance no longer hold), while running a 60 Hz core at 50 Hz and the same voltage increases flux and can drive the core into saturation. Frequency is fixed at the drawing stage.

Our oil-immersed transformer range covers the ratings and voltage combinations discussed below.

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2. What "S22" actually designates

This is where most export enquiries go wrong, because the model code gets read as a specification.

Element	Meaning	Note
S	Three-phase	Single-phase would be D
Absence of a fluid letter	Mineral oil immersed	Contrast: SCB = cast resin dry-type
22	Performance level code for losses	A series designation, not an efficiency grade
Optional Z	On-load tap changing (OLTC)	SZ11, SZ22 and similar
Optional M	Hermetically sealed, no conservator	Changes the protection scheme — see Section 9
Trailing numbers	Rated kVA / primary voltage class	e.g. 1000/10 = 1000 kVA, 10 kV primary

Three corrections to make before this goes on a page:

S22 is not an amorphous-alloy design. Amorphous metal cores in oil-immersed distribution transformers are the SH15 family (and in dry-type, SCBH15). S22 sits in the silicon-steel ladder alongside S11, S13 and S20 — progressively lower-loss core and winding designs on conventional grain-oriented steel. If you are quoted an "S22 amorphous" unit, ask what the core material actually is.

"22" is not a GB 20052 energy-efficiency grade. GB 20052 is China's minimum energy performance standard for distribution transformers and it defines grades 1, 2 and 3, with grade 3 being the entry threshold. The digits in a manufacturer's model code are a series designation. Map any efficiency claim to the actual no-load and load-loss values in the type-test report, and check those against the applicable grade limits for that kVA rating. [Confirm: GB 20052 edition, grade claimed, and P0/Pk values from the test report]

Natural ester is an option, not the standard fill. The default dielectric fluid for an oil-immersed distribution transformer is mineral insulating oil to IEC 60296. Natural ester (IEC 62770) and synthetic ester (IEC 61099) are specified deliberately — usually for fire risk, biodegradability, or moisture tolerance — and they change the thermal limits, the maintenance test suite and the price. If the requirement is ester-filled, say so in the enquiry; it is not something to assume is included.

Loss claims of "15–20% lower no-load loss than S13" and "8–10% lower load loss" are plausible marketing figures and must be verified: [Confirm: no-load and load-loss values for this specific rating from the type-test report, at a stated reference temperature]

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3. Voltage classes: what changes when the unit ships to North America

The ratio is the same; the numbers on the nameplate are not.

Chinese class	North American equivalent	Note
110 kV	115 kV	ANSI class; BIL and clearances differ
35 kV	34.5 kV	IEEE C57.12.00 BIL typically 200 kV for 34.5 kV class [Confirm]
10 kV	12.47 kV or 13.8 kV	13.8 kV is the common industrial primary
6 kV	4.16 kV or 6.9 kV
0.4 kV	480Y/277 V	Canada: 600Y/347 V
0.69 kV	600 V
380 V	480 V	380 V is not a North American utilisation voltage
220 V	240 V / 120 V split	Single-phase residential

Two knock-on effects worth budgeting for:

BIL and clearances are set by the class, not by the ratio. A 34.5 kV class unit built to IEEE practice typically has a higher BIL than an IEC 36 kV unit, which changes bushing creepage, clearances and the physical size of the terminal compartment. Do not assume a dimensional substitution.

Vector group notation differs. IEC Dyn11 is written Dyn1 on ANSI drawings — the same connection, a different clock convention. Yyn0 is written the same way in both. Dyn11 is the usual choice for step-down distribution because the delta primary traps triplen harmonics and gives a path for zero-sequence current, which helps with unbalanced loads. Yyn0 is rarely used in North America and, where it is used, the neutral current is limited — typically to about 25% of rated for larger units [Confirm: per applicable standard and manufacturer]. For export work, our comparison of IEC 60076 and ANSI/IEEE practice covers where the two systems diverge beyond notation.

Tap changing. Off-circuit (de-energised) tap switches commonly give ±2 × 2.5%, sometimes ±5%, and require the unit to be isolated before you move them. On-load tap changers give a wider range under load, commonly ±10% to ±15% in steps, and suit feeders with large or fast voltage swings — long rural lines, weak grids, renewable interconnections. They add cost, a maintenance interval based on operations and time, and an additional failure mode. Where voltage fluctuation is the driver, see the SZ11 on-load voltage regulating transformer.

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4. Sizing: the formula, and where it quietly under-sizes

The basic conversion is right:

That gives the minimum. Four things routinely push the real requirement above it:

Factor	Why the simple formula misses it	What to do
Motor starting	Inrush is 5–7× full-load current; the voltage dip depends on transformer impedance and the source stiffness	Check the starting voltage dip, not just the running kVA
Harmonic-rich load	Drives, rectifiers, EV chargers and electrolysers add eddy losses that do not show on the kW meter	Give the harmonic spectrum; size to IEEE C57.110 or specify a K-rated design
Future load	Most plants grow	Target 70–80% steady loading at today's load, not 95%
Ambient and altitude	Above 1,000 m and above 40 °C the thermal margin shrinks	Specify ambient and altitude in writing; apply IEC 60076-2 correction or buy an altitude-rated design

A sizing rule that has saved more projects than any other: size for the load profile, not the peak. A transformer that sits at 40% load most of the year and hits 100% for two hours is a different machine from one that runs at 85% continuously, even though both might size to the same kVA.

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5. Impedance, vector group and grounding — the decisions that outlive the ratio

Impedance

Impedance voltage (Uk%) sets both the voltage regulation and the fault current. Typical values: 4–6% for distribution step-down, higher for larger units or where fault current must be limited.

Decision	Lower impedance (4%)	Higher impedance (8%)
Voltage regulation	Better	Worse
Fault current	Higher	Lower
Downstream equipment	Higher SCCR required — costly switchgear	Lower SCCR acceptable
Parallel operation	Must be matched within tolerance	Same
Losses	Slightly lower load loss at the same design	Slightly higher

Worked example, 1,000 kVA at 480 V:

That 4 kA difference is a switchgear specification, a NEC 110.10 interrupting-rating check, and often a five-figure cost difference on the downstream gear. Choose the impedance with the switchgear in mind, not in isolation. [Confirm: calculated fault level against the installed switchgear SCCR]

Grounding

The 4 Ω / 1 Ω targets commonly quoted from Chinese practice do not transfer directly. In North America:

• All grounding electrodes at a building or structure must be bonded together — NEC 250.50. Separate, independent grounding electrodes for power and for lightning protection are not the North American model.
• A single made electrode is acceptable at 25 Ω or less where it cannot achieve the usual 25 Ω threshold with one rod, a second electrode is added — NEC 250.53 and 250.56. [Confirm: AHJ]
• Substation and large-facility ground grids are designed to IEEE 80 against step and touch potential limits, not to a single resistance number. A grid can measure 0.5 Ω and still be unsafe if the surface layer and conductor layout are wrong.

Write the grounding requirement as "designed to IEEE 80 for step and touch potential, with all electrodes bonded per NEC 250", rather than as an ohm value, and have it reviewed by the authority having jurisdiction. [Confirm: AHJ and site soil resistivity]

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6. Temperature limits: the numbers sources usually mix up

The commonly quoted "top-oil ≤95 °C, never above 105 °C, A-class winding rise ≤65 K, F-class ≤100 K" mixes three different things.

Quantity	Typical limit	Source
Top-oil temperature rise	55 K	IEEE C57.12.00 (self-cooled)
Top-oil temperature rise	60 K	IEC 60076-2
Average winding temperature rise	65 K by resistance	IEC 60076-2 / IEEE C57.12.00
Winding hottest-spot for normal insulation life	about 110 °C	IEEE C57.91

Consequences:

• 95 °C and 100 °C are top-oil temperatures at rated load in a 40 °C ambient, arrived at from the rise limits above. 105 °C top-oil is not a standard limit. [Confirm: design basis for your unit]
• "A-class ≤65 K, F-class ≤100 K" is conflating oil-immersed practice with dry-type practice. The 65 K average winding rise applies to liquid-immersed units regardless of the thermal classification discussion. A 100 K rise figure belongs to cast-resin dry-type transformers under IEC 60076-11, a different standard with a different insulation system. Do not put F-class 100 K on a liquid-filled specification.
• What ages the insulation is the winding hot-spot, not the bulk oil. Under IEEE C57.91 the normal-life reference is a hottest-spot of about 110 °C, which corresponds to roughly 180,000 hours, or about 20 years. The hot-spot sits well above the top-oil temperature, so a legal oil reading does not guarantee a comfortable winding.

If the unit is specified with ester fluid, the permissible operating temperatures and the loading guidance differ from mineral oil — IEEE C57.155 for natural ester and IEC 61099 / IEC 61203 for synthetic ester. [Confirm: fluid type and the applicable loading guide]

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7. Where step-down duties get specified wrong: solar, storage and motors

Three applications in the usual S22 application list need correcting, and all three come up in real enquiries.

Solar PV — the transformer does not see DC, and it usually steps up

The description "inverter outputs 800 V DC, transformer steps it down to 380 V AC" is wrong twice over.

• No transformer anywhere in that chain handles DC. The DC-to-AC conversion happens in the inverter. Asking a transformer to "step down DC" is a category error, and it is the single most common misunderstanding in renewable enquiries.
• The grid-tie transformer normally steps up, not down — from the inverter's low AC voltage to the medium-voltage collection or interconnection voltage. A step-down transformer appears in a PV plant only on the auxiliary/station service side, feeding pumps, lighting and controls from the MV bus.
• 380 V is not a North American utilisation voltage. Interconnection at low voltage in North America means 480Y/277 V, or 600Y/347 V in Canada.

Our guide to step-up transformers in renewable energy covers the selection logic, and this 50 MW solar PV step-up transformer project shows what the MV side of that chain looks like in practice.

Battery storage — same structure

Battery rack DC goes to the power conversion system, which is a bidirectional inverter. The transformer sits on the AC side of the PCS and normally steps up to the interconnection voltage. Specifying the transformer starts with the PCS output voltage, its harmonic performance, and whether the interconnection is MV or LV.

Motor starting — a different device

"Providing reduced-voltage starting for large motors" is not the distribution transformer's job. Reduced-voltage starting uses an autotransformer starter, a reactor starter, a soft starter or a variable-frequency drive. The distribution transformer does have to survive the event: starting inrush causes a voltage dip proportional to transformer impedance, and repeated heavy starts are a thermal duty. Size and select the impedance accordingly, but do not list motor starting as a transformer function.

Furnaces and welders — special-purpose, not catalogue

Very low secondary voltages at very high current are furnace-transformer territory: high impedance to stabilise the arc, heavy bracing for repeated short circuits under IEC 60076-5, and often an integrated reactor or on-load tap changer. They are engineered per installation, not picked from a distribution catalogue.

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8. Oil-immersed versus dry-type for the same step-down duty

Attribute	S22 oil-immersed	SCB18 cast resin dry-type
Cooling	ONAN (oil natural, air natural), ONAF with fans [Confirm: available stages]	AN, with AF fan stage
Typical location	Outdoor or in a ventilated substation	Indoor, dry and non-corrosive
Short-time overload	[Confirm: per IEEE C57.91 / IEC 60076-7 loading guide — depends on pre-load, ambient and duration, not a fixed 1.5× for 2 h]	[Confirm: per the dry-type loading guidance for the insulation system]
Maintenance	Periodic oil sampling and DGA; cooling and sealing checks	No oil, but cleaning of insulation surfaces is the highest-value task
Noise, 1,000 kVA	[Confirm: declared as sound power level LWA to IEC 60076-10]	[Confirm: LWA to IEC 60076-10 — typically a few dB above oil-immersed, not below]
Capital cost, same rating	Lower	Higher [Confirm: percentage differential for the target market]
Fire and environment	Mineral oil combustible; ester fluids or a bunded, drained installation where fire risk drives the design	No combustible liquid; fire performance rated F1 under IEC 60076-11

Notes on the claims in that table:

• Overload figures of "1.5× for 2 hours" are not a specification. Permissible short-time emergency loading under IEEE C57.91 or IEC 60076-7 depends on the pre-existing load, the ambient temperature, the duration and how much insulation life you are willing to spend. Ask for the loading guide calculation with the actual duty cycle; do not accept a single multiplier.
• Noise must be declared as sound power level LWA (IEC 60076-10), not as a dB reading at one metre. In a free field, sound pressure at 1 m is roughly LWA − 11 dB, and hard reflective surfaces in a substation push it up. A quoted "60 dB" without saying which quantity it is cannot be compared with anything.
• "Maintenance-free" is marketing. Oil-immersed units need periodic oil testing and sealing checks. Dry-type units need their insulation surfaces cleaned — conductive dust plus humidity is the most common early failure mode. Both need terminal torque checks and insulation measurement.
• IP ratings do not describe a transformer. An outdoor oil-immersed unit is weatherproof by construction; IP and NEMA ratings apply to enclosures, terminal chambers and marshalling boxes. IP23 means protected against spraying water up to 60° from vertical — it is not a general outdoor rating. Outdoor accessory enclosures in North America are typically specified NEMA 3R.

For the dry-type side of the comparison, see the SCB18 cast resin dry-type transformer; for what each cooling designation actually guarantees, see the cooling classes guide.

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9. Oil, sealing and what maintenance actually involves

Fluid	Standard	Why it is chosen	What changes
Mineral oil	IEC 60296	Default; lowest cost; longest service history	Combustible; conventional test suite
Natural ester	IEC 62770	High fire point, readily biodegradable, tolerates more moisture	Different limits and test suite (IEEE C57.155); material compatibility checks
Synthetic ester	IEC 61099	High fire point, very stable	Different maintenance guidance (IEC 61203); higher cost

Sealing philosophy changes the protection scheme. A hermetically sealed unit has no conservator and therefore no gas collection path, which means a Buchholz gas-actuated relay cannot be fitted. Protection then relies on a pressure-relief device, a sudden-pressure relay, and the upstream electrical protection. That is a protection decision to make with the protection engineer, not a maintenance convenience.

Test intervals. Dissolved gas analysis is typically annual to every three years depending on criticality and class — a three-year interval is the long end, not the default for a critical unit. [Confirm: interval against the owner's maintenance policy] Oil quality testing (acidity, dissipation factor, breakdown voltage, water content) follows IEC 60422 for mineral oil and IEEE C57.106 for processing guidance. Ester-filled units use a different suite — do not apply mineral-oil thresholds to an ester sample.

More background is in the resource library.

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10. Standards and compliance map

Topic	IEC	North America
Ratings, temperature rise	IEC 60076-1, IEC 60076-2	IEEE C57.12.00
Loading and thermal ageing	IEC 60076-7	IEEE C57.91
Short-circuit withstand	IEC 60076-5	IEEE C57.12.00 / C57.12.90
Tap changers	IEC 60214-1	IEEE C57.131
Insulating oil — new / in service	IEC 60296 / IEC 60422	ASTM D3487 / IEEE C57.106
Ester fluids	IEC 62770 (natural), IEC 61099 (synthetic)	IEEE C57.155
Energy efficiency	GB 20052 (China MEPS, grades 1–3)	DOE 10 CFR Part 431 Subpart K (§431.197 liquid-immersed distribution)
EU market	—	EU Ecodesign Regulation 2019/1783 (PEI)
Installation	—	NFPA 70 (NEC) Article 450; vaults in Part III where indoors [Confirm: AHJ]
Grounding	—	NEC Article 250; IEEE 80 for grid design

Two cautions for export projects:

• GB 20052 is a Chinese standard and overseas buyers do not recognise it. Quote it alongside IEC loss values and, where the destination requires it, DOE or EU Ecodesign parameters. Always map a claimed grade to the actual measured no-load and load losses.
• Efficiency regulation coverage is narrower than people assume. DOE liquid-immersed distribution transformer requirements and EU Ecodesign tiers apply to defined kVA and voltage bands; large power transformers and many industrial units fall outside them. Where a unit is outside coverage, efficiency has to be enforced contractually — stated P₀ and Pₖ limits at a stated reference temperature, measurement tolerance, plus a loss-capitalisation evaluation so that a genuinely lower-loss bid wins on evaluated price rather than on purchase price. [Confirm: coverage for the target rating, voltage and market]

UL listing is a separate question from IEC compliance. A unit can be fully compliant with IEC 60076 and still not be UL listed, and listing is a specific, named scope. If a North American installation requires listing, ask for it explicitly and get the file number. [Confirm: listing scope and file number, or state available on request]

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11. What to send with the enquiry

Give these ten items and you will get a firm, comparable quotation instead of a budget estimate:

1. Rated kVA and the load profile, not just the peak
2. Primary voltage and secondary voltage, with system nominal and maximum
3. Frequency — 50 or 60 Hz, stated once and never changed
4. Vector group — Dyn11 (ANSI: Dyn1) unless there is a reason otherwise
5. Impedance voltage, chosen with the downstream switchgear SCCR in mind
6. Tap changer — off-circuit or on-load, and the range and step size
7. Cooling class, ambient temperature and site altitude
8. Dielectric fluid — mineral oil, or ester where fire or environment drives it
9. Enclosure and terminal arrangement, plus any NEMA or IP requirement for the chambers
10. Standards, efficiency requirement, and any listing or type-approval needed

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12. FAQ

Can an oil-immersed transformer be used to step voltage down? Yes. Any transformer is bidirectional: energise the high-voltage winding to step down, or the low-voltage winding to step up. The ratio equals the turns ratio, U2 / U1 = N2 / N1, and the current moves inversely. The rating, the insulation level and the thermal design are what make a given unit suitable for a given duty, not the direction of power flow.

Can it step down DC, for example 800 V DC from a solar inverter? No. A transformer requires a changing magnetic flux; steady DC induces nothing on the secondary and drives the core into saturation. In a PV or storage plant the DC-to-AC conversion happens in the inverter or power conversion system, and the transformer sits on the AC side. The grid-tie transformer in a PV plant usually steps up to the medium-voltage interconnection, not down.

Is S22 an amorphous alloy transformer? No. Amorphous metal cores in oil-immersed distribution transformers are the SH15 family; the S22 sits in the silicon-steel ladder with S11, S13 and S20. If a quotation describes an "S22 amorphous" unit, ask for the core material in writing.

Is "22" an energy-efficiency grade? No. It is a manufacturer's performance-level series designation. Energy-efficiency grades come from GB 20052 (grades 1 to 3, with grade 3 the entry threshold) or, for export markets, from DOE 10 CFR 431 Subpart K or EU Ecodesign. Always convert any grade claim into measured no-load and load-loss values for the actual rating.

Does an S22 come filled with natural ester as standard? No. The default fill is mineral insulating oil to IEC 60296. Natural ester (IEC 62770) and synthetic ester (IEC 61099) are specified deliberately, usually for fire performance or environmental reasons, and they change the thermal limits, the maintenance test suite and the price. State the fluid in the enquiry.

How do I size one — is S = P / cosφ enough? It gives the minimum, and it is the right starting point: 800 kW at 0.85 power factor needs about 941 kVA, so you select 1000 kVA. It does not account for motor starting inrush, harmonic-rich loads, future growth, ambient temperature or altitude. Target roughly 70–80% steady loading at today's load and check the starting voltage dip separately.

What impedance should I choose, 4% or 8%? Lower impedance gives better voltage regulation but higher fault current, which pushes the required interrupting rating of the downstream switchgear up. On a 1,000 kVA, 480 V unit, 5% gives roughly 24 kA of prospective fault current against about 20 kA at 6%. Choose it together with the switchgear SCCR, and match impedances if units will run in parallel.

What is the correct temperature limit? For a liquid-immersed unit, the top-oil temperature rise limit is 55 K under IEEE C57.12.00 or 60 K under IEC 60076-2, and the average winding rise is 65 K by resistance. In a 40 °C ambient that puts top-oil at about 95 °C or 100 °C at rated load. "105 °C top-oil" is not a standard limit, and "F-class 100 K" belongs to cast-resin dry-type practice under IEC 60076-11, not to liquid-filled transformers.

Can a sealed transformer have a Buchholz relay? No, not in the usual arrangement. A gas-actuated relay needs a gas path from the tank to a conservator. A hermetically sealed unit has no conservator, so protection relies on a pressure-relief device, a sudden-pressure relay and the upstream electrical protection. Decide the sealing philosophy with the protection engineer.

Why do North American grounding requirements differ from a 4 Ω target? Because the North American model is different in kind, not just in number. NEC 250.50 requires all grounding electrodes at a structure to be bonded together, so independent power and lightning grounds are not used. A single made electrode is acceptable at 25 Ω or less, and large facility and substation grids are designed to IEEE 80 against step and touch potential rather than to a single resistance figure. Write the requirement as an IEEE 80 design with NEC 250 bonding, and confirm with the authority having jurisdiction.

Is a dry-type transformer quieter than an oil-immersed one? Usually not — oil tends to damp the core and provide better acoustic isolation than air, and the difference is typically a few dB in the oil-immersed unit's favour at the same rating. Compare on declared sound power level LWA to IEC 60076-10, not on a quoted dB figure with no stated quantity. And remember that a dry-type unit on its AF fan stage is materially louder than on AN.

What should I send to get a quotation? Rated kVA and load profile, primary and secondary voltages, frequency, vector group, impedance, tap changer type and range, cooling class with ambient and altitude, dielectric fluid, enclosure and terminal requirements, and the standards plus any listing or approval needed. Browse the full product range or recent project work for reference configurations while you put it together.