Transformer Selection Guide
A practical comparison of the five distribution transformer families we supply — what each one is for, where it wins, where it fails, and the seven parameters that should be fixed before anyone quotes a price.
The type follows the site, not the catalogue
Before discussing price, fix six things: where it sits, how it is cooled, what feeds it, what it feeds, how it is protected, and which standard the utility will accept.
Why this guide exists
Most transformer enquiries start with a kVA number. That is the third question, not the first. A 1,000 kVA unit for an outdoor 33 kV feeder, a hospital basement and a shopping-centre underground network are three completely different products with three different price levels — and specifying the wrong one is expensive to undo after it ships.
This page lays out the trade-offs in plain engineering language. We are a Shanghai-based export trading company, not a manufacturer, so we have no reason to push one technology: we source from a large pool of audited factories and recommend the type that fits your site and tender.
All five families, side by side
Typical ranges and trade-offs. Every project is confirmed against the tender sheet, grid code and site conditions before we quote.
| Oil-immersed | Pad-mounted | Compact substation | Cast-resin dry-type | Amorphous alloy | |
|---|---|---|---|---|---|
| Typical rating | 30–4,000 kVA | 15–5,000 kVA | 30–2,500 kVA | 30–2,500 kVA | 30–2,500 kVA |
| Primary voltage | 6–35 kV | up to 34.5 kV | 11 / 33 kV | 10 / 20 / 35 kV | 6–35 kV |
| Installation | Outdoor plinth / pole | Outdoor concrete pad | Outdoor plinth | Indoor room / enclosure | Outdoor plinth |
| Best for | Utility, rural, industry | Underground residential | Turnkey new sites | Hospitals, metro, data centres | Solar, efficiency tenders |
| Purchase cost | Lowest | Medium–high | High (full package) | High | Medium–high |
| No-load loss | S11 → S20 improving | ANSI efficiency levels | Follows transformer | Standard to low | 60–80% lower |
| Overload capability | Excellent | Moderate | Moderate | Moderate (AF improves) | Good |
| Fire / containment | Bund required | Sealed, dead-front | Bund or dry-type | No oil | Bund required |
| Maintenance | Oil checks | Low | Low–moderate | Lowest | Oil checks |
| Footprint | Medium | Small | Large | Medium | Medium |
| Standards | IEC 60076 | ANSI / IEEE C57 | IEC 62271-202 | IEC 60076-11 | IEC 60076 |
Ratings are typical manufacturing ranges, not guaranteed stock. Higher ratings, 60 Hz designs and special vector groups are available on request.
Uses, advantages and limitations
Oil-immersed distribution
Use: utility feeders, rural electrification, plants, mines. Wins: lowest cost per kVA, excellent overload, field-repairable. Limits: oil containment, fire clearance, not for occupied indoor spaces.
Pad-mounted
Use: underground residential and commercial networks, ANSI markets. Wins: lockable, dead-front, small footprint, no fence. Limits: higher price, less heat dissipation, ANSI-specific configuration.
Prefabricated substation
Use: new developments, utility reinforcement, renewable collector sites. Wins: one factory-tested assembly, fast site work, single accountability. Limits: shipping volume, crane access, early configuration lock-in.
Cast-resin dry-type
Use: hospitals, metro, data centres, commercial towers. Wins: no oil, fire-safe, low maintenance, close to the load. Limits: price, sensitivity to dust and humidity, derating at altitude.
Amorphous-alloy core
Use: solar plants, utility feeders and any tender scoring lifetime losses or CO₂. Wins: 60–80% lower no-load loss, savings 24/7. Limits: higher purchase price, payback depends on utilisation and electricity cost, and at high continuous load a low-loss S20 silicon-steel unit can win instead.
Seven parameters to fix before requesting a price
Site & environment
Indoor or outdoor, ambient temperature, altitude, dust, salt, flood risk and fire classification. This decides dry-type versus oil, and the IP / corrosion class.
Grid code & standard
IEC 60076, ANSI/IEEE C57, 50 or 60 Hz, utility-specific loss limits, and any required UL / CE / KEMA evidence.
Rating & load profile
kVA, peak and average load, hours energised, future growth, harmonic content from inverters or VFDs.
Voltages & vector group
Primary and secondary voltage, Dyn11 / Yyn0, tapping range, neutral earthing arrangement and impedance.
Losses & efficiency
Guaranteed no-load / load loss values, or a total-owning-cost formula. This decides S11 / S13 / S20 / amorphous.
Protection & switchgear
HV fuse or RMU, LV breaker and feeder count, metering, relays, SCADA interface, earthing materials.
Commercial scope
Quantity, destination port, Incoterm, required certificates, inspection agency, packing and delivery window.
Spare parts
Gaskets, fuses, elbow connectors, fans, indicators and a spare nameplate — cheap with the order, expensive later.
A tender sheet or single-line diagram answers most of these in one file. If you only have the kVA and voltages, send those — we will work through the rest with you.
Getting the kVA right
Oversizing wastes capital and increases no-load loss for decades; undersizing shortens insulation life. Both are avoidable.
A working method
Sum the connected load in kW, apply a diversity factor appropriate to the site, then divide by the target power factor to get kVA. Add allowance for motor starting currents, harmonic derating and 15–25% future growth — but do not simply double the number.
For a three-phase system: kVA = (kW × diversity) ÷ PF, and I = kVA × 1000 ÷ (√3 × VLL) for the LV breaker rating.
Utilities typically load distribution transformers to 60–80% of nameplate rating in normal service, with short peaks above that absorbed by the oil or forced-air cooling. Solar collector transformers need a different check: long hours at low load make no-load loss the dominant cost.
| Typical target loading | 60 – 80% of rated kVA in normal service |
|---|---|
| Growth allowance | 15 – 25%, unless a second unit is planned |
| Motor starting | Check voltage dip and short-time overload |
| Harmonic loads | Apply K-factor or derate, especially with inverters |
| Altitude | Above 1,000 m, derate or specify high-altitude design |
| Ambient | Above 40 °C average, confirm cooling class with factory |
| Common range | 100 / 250 / 400 / 630 / 1000 / 1600 / 2500 kVA |
Rules of thumb only. We confirm the final rating against the load schedule and the factory thermal calculation.
Five mistakes we see in tenders
Specifying a type before the site
Choosing dry-type for an outdoor dusty site, or oil-filled for a hospital basement, creates avoidable cost and approval problems. Fix the environment first.
Ignoring loss evaluation
Comparing only purchase price ignores 25 years of no-load losses. Where the tender scores owning cost, S20 or amorphous wins on arithmetic.
Mismatched grid code
11 kV / 50 Hz IEC equipment will not satisfy a 13.8 kV / 60 Hz ANSI specification. State frequency, voltage and vector group explicitly.
Forgetting the surrounding equipment
Transformers arrive without HV cables, LV panels or earthing materials unless ordered. Consolidating them in one shipment avoids a second freight and a stalled site.
Accepting documents after payment
Request sample routine and type test reports for the exact series before contract signature. If the documents are not available then, they usually will not be later.
Frequently asked questions
Which type is cheapest overall?
When is amorphous alloy worth the premium?
Can we mix types in one order?
What documents will we receive?
How fast can you quote?
Send the specification, get a straight recommendation
We will confirm the type, compare the relevant audited factories and return a priced technical offer with sample test reports — even if the answer is a cheaper standard unit than you expected.