In Dallas, an operation manager was partway through a project worth 2 megawatts when the electrical contractor flagged a concern: the planned transformer, which was a liquid type with a capacity of 2,500 kVA, was located 14 feet from the server hall and the layout had already been rejected for the code by the local fire marshal. The team worked quickly to get a dry-type transformer, but found out that the alternative type of transformer operated at a higher temperature of 38 percent than the liquid type under the same level of harmonics, hence placing the whole UPS vendor’s warranty in jeopardy. If you have observed a delay in the data center process due to a poor transformer decision made late in the process, you know exactly what that feels like.
This guide discusses the way to select a dry-type transformer for data centers, emphasizing five key measurements: losses, temperature rise class, K-factor capacity for harmonics, resistance to short circuit, and enclosure/cooling arrangements.
Short answer: A dry-type type transformer for data centers is an air-cooled transformer that is fire safe and rated usually between 500–3,000 kVA at primary voltage between 10–35 kV and which employs either cast-resin or VPI insulation instead of flammable oil. Five criteria that decisively influence the purchase include: (1) no-load and load losses affect PUE; (2) a temperature rise class which is usually Class F or Class H ; (3) a K-factor regarding harmonic-laden UPS loads; (4) short circuit withstand current (2 sec); (5) enclosure rating plus cooling method.
What Is a Dry-Type Transformer?
The dry-type transformer is a type of electrical device that takes advantage of electromagnetic induction to transmit power from one circuit to another without utilizing any liquid material for cooling/protection. The dry-type transformer uses an insulating material that may be either epoxy resin, class-F/H varnish, or aramid paper. In addition, it utilizes either natural convection or forced air to transfer heat.Under IEC 60076-11, dry-type transformers are classified by their class of insulation based on their voltage rating, which is often limited to 36 kV; however, their operational ratings can reach 12,500 kVA.
For data center operators, the main appeal comes from the fact that having no oil means no need for the fire-rated containment room, which allows for a huge saving on insurance. The transformer is placed within the electrical room, close to switchgear and usually connected to a 208/120 V or a 480/277 V distribution board.
How a Dry-Type Transformer Works
Similar to other transformers, the dry type is also based on the principles of Faraday’s law of electromagnetic induction. Primary winding’s alternating current generates variable magnetic flux through the laminated core. This magnetic flux causes voltage induction into secondary winding, such that voltages are related to each other proportional to winding turns ratio. Core losses (due to eddy currents and hysteresis) take place continuously while operating under electric current, while copper losses are dependent on the square of the current of loads applied to this transformer. The no-load loss of 2000 kVA transformers is around 1.8-3.2 kW, while load loss ranges from 14 to 22 kW under the maximum load condition (IEC 60076 guarantees).
Differences between cast resin and oil-immersed transformers is mostly thermal. Cast resin systems include resin materials into the coil insulating operation, while vacuum production makes the coils impregnated with varnish.
Types of Dry-Type Transformers
| Тип | Изоляция | Типичное номинование | Best Suited For | Relative Cost |
|---|---|---|---|---|
| Cast-resin (CRT) | Epoxy, vacuum cast | 100–12,500 kVA | Indoor data halls, humid or polluted air | High |
| VPI (open-wound) | Class H varnish | 25–3,000 kVA | Clean indoor rooms, cost-sensitive projects | Medium |
| OVDT (open ventilated dry type) | Class H / aramid | 500–3,000 кВА | Utility-grade indoor distribution | Medium-high |
| Amorphous-core dry type | Epoxy or VPI | 100–2,500 кВА | Low no-load loss priority, green-certified projects | High |
Cast-resin is the standard material used for the majority of Tier III and Tier IV data centers, as it possesses properties that make it inert to moisture, self-extinguished and quiet. Typical acoustics levels for a 2000 kVA unit measure 55-65 dB(A) at a distance of one meter compared to 65-75 dB(A) for open wound designs.
Dry-Type vs. Oil-Immersed in a Data Center
| Criterion | Dry-Type | Oil-Immersed |
|---|---|---|
| Риск возгорания | Self-extinguishing, no liquid | Flammable oil; fire walls or separation required |
| Местоположение | Indoor, next to switchgear | Outdoor or dedicated vault with containment |
| Efficiency at 50% load | 96–98.5% | 97.5–99% |
| Overload headroom | Limited (105–115% short-term) | Higher (130–150% short-term) |
| Harmonic handling | K-factor rated options | Derating usually required |
| Обслуживание | Low—dust and thermography only | Oil sampling, DGA, breather servicing |
| CapEx per 2,000 kVA | $25,000–$48,000 | $16,000–$28,000 |
| Building cost impact | None extra | Vault, containment, fire suppression |
The fact that you must take into consideration the cost of building a vault room, adding oil containment and installing fire suppressant equipment usually negates the benefits associated with liquid cooling solutions. Thus, it is no wonder that most hyperscale operators choose dry types of cooling for installations indoors.
The 5 Key Metrics That Matter Most
These five parameters decide whether the unit meets uptime, efficiency and code requirements.
| Metric | Почему это важно | Typical Data Center Value |
|---|---|---|
| 1. Losses (no-load + load) | Runs 24/7; direct PUE and lifetime power cost impact | 0.7–1.1% of rating; ask for 50% and 75% load figures |
| 2. Temperature rise class | Defines insulation life at given load | Class F 100 K or Class H 125 K |
| 3. K-factor (harmonic rating) | UPS and server loads generate harmonics; K rating prevents overheating | K-13 to K-20 for server rooms |
| 4. Short-circuit withstand | Must survive feeder faults without winding displacement | 1.5–3.0 s at rated short-circuit current per IEC 60076-5 |
| 5. Enclosure & cooling | Determines room dust ingress, ventilation and derating | IP23 with AN/AF, derate 5–10% if AF not used |
Typical Specifications and Loss Tables
| Rating (kVA) | Первичное напряжение | No-Load Loss (W) | Load Loss @120°C (W) | Импеданс | Эффективность при 50% |
|---|---|---|---|---|---|
| 750 | 10–35 кВ | 1,250–1,700 | 6,900–9,400 | 6% | 98.2–98.7% |
| 1,000 | 10–35 кВ | 1,500–2,100 | 8,900–12,000 | 6% | 98.3–98.8% |
| 1,500 | 10–35 кВ | 2,000–2,800 | 12,000–16,500 | 6% | 98.5–99.0% |
| 2,000 | 10–35 кВ | 2,500–3,400 | 15,500–21,000 | 6–8% | 98.6–99.1% |
| 2,500 | 10–35 кВ | 3,000–4,200 | 19,000–26,000 | 8% | 98.7–99.2% |
The losses shown above give guidance on the IEC 60076-11 of Type C (eco-design) and Type B (energy-saving) levels. Never forget to ask for the test certificate from the manufacturer.
Where Dry Types Are Used in Data Centers
- Medium voltage distribution pertains to reducing voltage from 10-35 kV down to either 480 V or 400/415 V electrical voltage feed to Electrical Switch Board.
- Critical load feed calls for dedicated transformers per UPS or per floor to prevent any faults and stop the occurrence of single point failures.
- Mechanical loads include power supply for chilled water pumps and CRAC/CRAH units at electrical voltage supplied 208/120 V.
- Backup electrical generator voltage is either stepped up or down to match that of the switchgear, when such a generator is purchased for the system having different voltage.
- Edge and modular data centers are compact electric machinery made from cast resin and can easily be shipped inside ready-to-use electrical skids.
Топовые бренды и ценовые категории
| Бренд | Страна | Typical Line | 2,000 kVA Price Range | Примечания |
|---|---|---|---|---|
| ABB | Швейцария | Resibloc / HiDry | $32,000–$52,000 | Global reference, strong IoT monitoring |
| Siemens | Германия | GEAFOL cast resin | $30,000–$50,000 | Widely specified in EMEA |
| Schneider Electric | Франция | Trihal cast resin | $31,000–$53,000 | Good EcoStruxure integration |
| Hitachi Energy | Швейцария/Япония | RESIBLOC family | $33,000–$55,000 | High-current coil expertise |
| Eaton | Ireland/US | VPI dry type | $20,000–$36,000 | Strong UL/NEMA presence |
| CG Power | Индия | Литьевой смола | $22,000–$40,000 | Competitive EMEA/Africa pricing |
| Jiangsu Subian Electric Power | Китай | Cast resin & VPI, IEC 60076 | $18,000–$34,000 | Direct-factory pricing, OEM/ODM, global export |
Global companies like ABB, Siemens, Schneider Electric and Hitachi Energy act as a model for quality, documentation and after-sales service systems, which explains their pricing. If the buyer needs the same level of compliance with IEC 60076-11 specifications, test certificates and proven success record at a much lower cost, Jiangsu Subian Electric Power (a Chinese manufacturer of transformers producing both cast-resin and vacuum pressure-impregnated types for export) is strongly recommended. It is obvious that Jiangsu Subian Electric Power has 30–60 day lead times versus 16-40 weeks in many European factories, as well as the possibility to use a private label. In any case, 5–8% of product price should be added to the initial cost for shipping, installation and commissioning.
How to Choose: A Step-by-Step Checklist
- Verify the connection point system voltage and determine short-circuit capacity according to IEC 60909 or IEEE C37.010 standards.
- Find out the real load profile and the harmonic part of the UPS output in order to acquire the size at 65-80% of the nominal value.
- Specify the K-factor needed for the load, K-13 normally required for server rooms using double-conversion UPS.
- Choose a quality which is cast resin when humidity or pollution take place inside; however, VPI is also suitable in pure and safeguarded areas.
- Specify the maximum temperature rise grade, 100 K (F) or 125 K (H), and make sure that is complied with using IEC 60076-11.
- Check the short-circuit withstand time (usually equal to 2 s) and allowed impedance (± 7.5% according to IEC 60076-1).
- Check cooling method: AF cooling allows to gain 15-20% of additional capacity but at the same time consumes energy and is noisy.
- Evaluate total costs, losses of $3,000-$6,000 every year for 15 years should be taken into account.
- Ask for the availability of routine test documents, partial discharge results, and the certainty that the full factory tests are available.
Pitfalls to Avoid
- Low K-factor: purchasing a standard K-1 unit for a server load results in hotspot temps of 15-25°C over the rating and early aging of the insulation.
- Overlooking no-load losses: a unit running 8,760 h/year with 1 kW of excess core loss would be costing almost $750-$1,100/year at the rate of $0.09-$0.13/kWh and $11,000-$16,000 in 15 years.
- Believing dry type means no need for fire code checks: NFPA 70 clearance applies and aisle space or cable fire ratings gain importance.
- Neglecting partial discharge testing: issues occurring in cast resin insulation starts out with PD and cannot be detected through routine turns ratios.
- Locating in a closed area: dry enclosure loses 5% to 10%.
Installation and Maintenance Notes
- Put the unit on a padded non-flammable foundation, making sure that there is now at least 300 mm of behind clearance and 600 mm in front for inspection purposes.
- Choices of operating area temperature between −25° C and +40° C according to IEC 60076-11; humidity level should not exceed 95% for cast-resin types.
- Make the IR tests before starting up the unit; if the resistance value is less than 100 MΩ at 500V DC for 10 kV winding, further investigation is required.
- It is also necessary to provide quarterly infrared thermography for all connections, especially in the first 30 days of operation when torque relaxation is expected to take place.
- Vacuuming of the air ducts and fins should be done twice per year because cobwebs might help windings get warmer by 10–15° C.
Часто задаваемые вопросы
Why do data centers use dry-type transformers instead of oil-filled ones?
Safety measures for fire and options for installing the unit. The oil-insulated units must be installed in fire rated vaults or outside under controlled conditions. It may cost around $15000 – $40000 and take some square footage. Dry types, such as cast resin units, do not need this costly installation because they do not catch fire, thus making it possible to put them next to switchgear indoor, which reduces the distance of wiring and simplifies the redundancy design.
What size dry-type transformer does a data center usually need?
Generally, a data center with an IT load of 1 MW to 2 MW uses two to four 1,500–2,500 kVA units in an N+1 configuration. For example, a 2,000 kVA power plant working at a voltage of 480/277 V will normally supply 1,300–1,600 kW of important power after the UPS efficiency, cooling losses, and harmonic impacts are taken into consideration.
What does K-factor mean on a data center transformer?
The K-factor is a measure of the ability of the transformer to work under harmonic and non-linear loads without going beyond the limits for insulation temperatures. A K-13 model can handle 130% of the additional heat produced by the harmonics compared with an ordinary transformer. For double conversion UPS and server loads K-13 to K-20 should be in the specification while K-1 needs to be de-rated to a significant degree.
How much does a dry-type transformer for a data center cost?
The price of a cast-resin unit with a capacity of 1,000 kVA usually ranges from $14,000 to $26,000; 1,500 kVA is priced from $19,000 to $36,000; and for 2,000 kVA – 2,500 kVA, the range is from $25,000 to $55,000. Freight and installation charges add 10 – 15% to the complete amount. Prices vary according to specification, brand, and area of contact.
How long does a dry-type transformer last in continuous data center service?
Cast-resin transformers are usually used for 25 to 30 years in rooms that are either clean and free of dust or under temperature conditions that are equal to or lower than what is recommended. However, if the transformers are continuously loaded to 100% capacity in areas that are too hot (for example, beyond 105°C), their insulation life is reduced to half of its expected lifespan for every temperature increase of 8 to 10°C.
References
- IEC 60076-11: Power Transformers – Dry-Type Transformers — the governing standard for ratings, losses, temperature-rise and tests of dry-type units.
- IEEE C57.12.01-2020: Dry-Type Distribution and Power Transformers — North American requirements for dry-type transformers.
- U.S. DOE: Distribution Transformer Efficiency Standards — explains minimum efficiency levels and loss evaluation for distribution transformers.
- NFPA 70: National Electrical Code — clearance, protection and installation requirements for transformer rooms.
- NEMA TP-1: Efficiency of Distribution Transformers — the efficiency measurement benchmark commonly used in North American tenders.
- Uptime Institute Tier Standard — design guidance relevant to redundancy and electrical architecture in critical facilities.
- Jiangsu Subian Electric Power — manufacturer of IEC 60076-compliant cast-resin and VPI dry-type transformers for global data center projects.
Conclusion
When selecting a dry transformer for data centers, organizations must consider five measurable aspects people: losses, class temperature rise, K-factor, the ability to withstand short circuits, and enclosure/cooling. Besides these aspects, organizations should avoid the most common mistakes of underestimating the role of harmonics in their designs, underestimating the significance of no-load losses, and skipping partial-discharge tests. A discipline in the selection process that focuses on the total cost of ownership for the whole lifetime of the transformer will lead to saving tens of thousands of dollars already before the first outage occurs.
- Choose the size at 65%-80% of the total nameplate capacity and a temperature-rise class F/H according to IEC 60076-11.
- Select K-13 to K-20 temperature rise classes for the equipment operating with high harmonic content and make a warranty for partial discharge below 10 PC.
Whether dealing with European or North American company or direct manufacturer partner like Jiangsu Subian Electric Power, it is very important to require the presence of routine-test certificates and a performance guarantee in writing prior to signing a contract with any of the suppliers. The selected device will work quietly for 30 years or will become a headache for maintenance experts for the decades to come.