In contrast to distribution transformers that are utilized for commercial projects, the data center transformers have a special role: they need to be placed close to the load they support, operate continuously with high efficiency, and tackle loads with a form different than sine wave loads. Hence it is impossible to use oil transformers due to the risk of ignition and three attributes of the transformers: harmonic performance, thermal margin, and partial discharge.
In a nutshell: For power connecting data centers, it is suitable to select a cast-resin dry-type transformer with Class H (180 °C) thermal insulation, K-factor rating of no less than K-13 (usually K-20), 6% impedance, a Dyn11 vector group for UPS input, less than or equal to 10 pC partial discharge test value at 1.1× rated voltage, and according to DOE 2016 or EU Ecodesign Tier 2 in terms of performance. The typical power ratings of data center transformers are between 500 kVA and 3,750 kVA at 10 kV and 35 kV, respectively.
Why Data Centres Use Dry Type in the First Place
The primary factor is the fire safety of the system. Cast-resin windings can self-extinguish and don’t contain any combustible liquids, so the transformer can be positioned in the building close to the load to be served. Thus, there is no need for an oil pit, a safety wall, or a special compartment with good fireproof properties. The transformer is placed in the vicinity of IT equipment in co-location or enterprise data centers where every square meter of white space is valuable; therefore, the distance of low voltage wire run is lowered.
The next driving factor is the maintenance requirements. There is no need in monitoring the oil level, gas relay, or drying the breather. Instead of them, only partial discharge and temperature must be monitored, which is simple as compared to the fluid chemistry process.
On the other hand, dry type transformers do not tolerate heat well. Oil moves and helps to remove heat away from windings, while resin does not do that. Thus, insulation class, temperature rise, and harmonics become much more important in the case of dry type transformers as the distance between normal operation and insulation damage is significantly smaller.
The Three Specifications That Decide Everything
Insulation class and temperature rise
According to ANSI/IEEE C57.12.01, dry type transformers are normally rated at either 80, 115 or 150 K temperature rise at an ambient of 40 Celsius for the 220 Celsius insulation system. Thus, a cooler temperature rise on the same equipment ensures a greater margin to thermal restrictiveness, increasing insulation life and creating capacity for overload.
From the standpoint of the IEC, the insulation classes correspond to Class F (155 Celsius) and Class H (180 Celsius) along with a Class B temperature rise (80 Celsius) observed during the rated load. For data center usage, it is standard practice to require a Class H insulation with Class B of Class F temperature rise, which means insulation characterized by the temperature significantly higher than the one observed during the operation of the transformer, leaving a place to dissipate harmonic losses generated by the equipment in operation.
This is the specification most often modified for the bid to bring down its value and the product that fails eight years later.
K-factor
The K-factor rating determines the level of loss that can be dissipated by a transformer without violating its thermal limitations. It is calculated in accordance with IEEE C57.110 publication. UL 1561 recognizes the K-factor heating ratings, K-1, K-4, K-9, K-13, K-20, K-30, K-40, and K-50 ratings.
The selection depends on the actual load characteristic:
| Lastprofil | Recommended K-factor | Typical harmonic content |
|---|---|---|
| Legacy servers with 6-pulse UPS rectifier | K-13 | 30–40% THD |
| Modern servers with 12-pulse or IGBT rectifier | K-7 | 15–25% THD |
| Fully optimised with active harmonic filters | K-4 | Below 10% THD |
| Mixed loads, high density, unknown future growth | K-20 | Maximum margin |
In the case of a new installation that has not undergone a harmonic analysis, K-13 is considered the cautious standard, while K-20 is the obligatory figure where the load mix remains unknown. The K-20 is also the most commonly specified value for rectifier loads only.
A K-rated transformer comprises more than just a heavier core. It utilizes lower flux density for harmonic flux; it has a larger neutral. A K-13 transformer has a neutral bus rated for 200% of the phase rating as triplen harmonics are summed in the neutral rather than cancelling each other out.
The results in the absence of a K-rated transformer are apparent. Harmonic currents lead to increased eddy losses and, consequently, elevated temperatures and accelerated insulation degradation. The harmonic flux also leads to higher peak voltages, so when all harmonic values add up, the peak value increasing above 1.25-1.5 of the rated crest leads to a very high probability of the occurrence of partial discharges.
Teilentladung
Partial discharge defines to differentiate between a high standard cast-resin transformer and an economically manufactured one. According to IEC 60076-11, a PD rate should be a maximum of 10 pC with an operating voltage of 1.1 times rated voltage for Environmental Category E2 that applies to humid or outdoor transformers. Any factory report that shows a PD reading greater than 10 pC at a test voltage of 1.1×Um shall not be accepted.
When it comes to operation, thresholds change. If the PD value exceeds around 100 pC at operating voltage, the insulation is already damaged enough to raise the question of a replacement. Because progressiveness of PD and the fact that the issue is not visible from the outside imply that factory test should be requested at the moment of purchase and further monitored.
Specification language for a power supply unit at a data Centre should be as follows:
- Insulation, class H (180 °C), temperature rise, F class (100 K) or lower.
- K-factor, at least K-13, or K-20 in case of rectifier or unknown loading, confirmed by factory acceptance at harmonic loading.
- Vector group Dyn11 as UPS input standard.
- Impedance should be 6% for strengthening power supply or 8% if the aim is to reduce short circuit current.
- Electrostatic shield shall be placed between HV and LV coils to avoid zero-sequence harmonics circulation.
- PD shall be factory tested according to IEC 60076-11 standard, with confirmation.
- Winding material is copper, core material is silicon steel, temperature measuring device can be RTD or PT100 with cooling system.
That set of requirements produces a unit that is not the cheapest bid and is the one that will still be in service when the facility is due for its next refresh. The reasoning a buyer needs to work through before writing the spec is set out in this guide to the key metrics and pitfalls when choosing a dry-type transformer for data centres, and it is worth reading before the tender documents go out rather than after the bids come back.
Types of Dry-Type Unit That Fit Data Centre Duty
There are three types of construction which are relevant commercially.
- Cast resin (epoxy) dry type. Windings which are enclosed in cast epoxy resin. It is the best choice for the data centers since it possesses the largest wealth of ability against moisture and contamination, ability of self-extinguishing and ability to withstand the thermal cycling of 2427 duty cycle. Moreover, it is most simple to specify with the use of a high K-factor.
- Vacuum pressure impregnated (VPI). Windings which are impregnated in varnish under vacuum. This is less expensive than the cast resin and mechanically resistant but also more vulnerable to the penetration of moisture in the conditions of humidity or proximity to the sea.
- Ventilated open wound. The least expensive. Rarely this decision is good enough for indoors of a data center.
It comes down usually to the choice of cast resin and the difference in quality of the resin, the winding design and the testing process.
The Unit Substation Arrangement
Data centres are increasingly opting to purchase the complete power chain as one integrated, engineered product instead of acquiring the system as a series of components, such as the medium-voltage incoming section, cast resin transformer and low-voltage distribution system.
The pros of this approach include simplification of the interface and saving space. The cons are the issue of price comparison on single-source products and the fact that specifications of all parts involved become more important when welded into one component.
| Parameter | Typical data centre range |
|---|---|
| Nennleistung | 500–3,750 kVA, larger for hyperscale |
| Primärspannung | Up to 35 kV class — commonly 10, 11, 13.8, 24 or 33 kV |
| Sekundärspannung | 400, 415, 480 or 600 V |
| Phases and frequency | Three-phase, 50 or 60 Hz |
| Insulation / temperature class | Class F (155 °C) or Class H (180 °C) |
| Kühlung | AN natural air, or AF forced air adding roughly 33–50% capacity |
| Harmonic rating | K-13 to K-20, K-30 on request |
| Impedanz | 6% typical, configurable |
| Effizienz | DOE 2016 / EU Ecodesign / NRCan minimums |
| Normen | IEC 60076-11, IEEE C57.12.01, UL 1561, CSA as applicable |
Before using forced-air cooling in design, it is essential to get a proper understanding of how this process works. Forced-air cooling stage helps increase kVA output, but it does this by drawing heat away from the device and not by enhancing the insulation. In a setup with N+1 cooling configuration, this is acceptable. However, in case of using a single fan setup, it is important to understand that an AF value will not be important as soon as one of the devices fails.
This means that the list with kVA output of 500, 630, 800, 1000, 1250, 1600, 2000, and 2500 should not be treated as just a menu that one can simply pick the required value from. The selection follows the load calculation, and a cast-resin platform covering the 6–10 kV distribution ratings — the SCB-series dry-type distribution transformer range is a typical example — will usually have the right frame available in the ladder, which is a consideration when the alternative is a custom build and a longer lead time.
Redundancy and Where the Transformers Sit
Selecting the data centre transformer is closely linked to the fault-tolerant topology that it is designed to support. The rating of the transformer is determined by the topology of the setup that is either Tier III or Tier IV based on N+1 or 2N arrangement of transformers.
- N+1- A spare transformer or an ability to lose 1 transformer from a group without tripping the load. This is the usual arrangement in Tier III facilities.
- 2N- Two fully rated but independent paths of supply. This means there can be 2 transformers feeding the same group of servers, which is common in Tier IV setups.
- Block redundancy/distributed redundancy- the power supply from transformers is shared by blocks, which leads to less kVA being installed.
When calculating the transformer size, it must consider the actual IT load instead of connected load. Remember that the physically selected size of the transformer affects the PUE, because even if the transformer is working below its capacity, it still results in significant losses that the facility will have to pay for. The possibility of using a number of small transformers instead of one large unit should be carefully evaluated. This is a design decision, not merely specification.
Where a facility is configured around a defined critical infrastructure topology, the arrangement of incoming supply, transformer and distribution is the part that decides whether a single failure is survivable, and the configuration options are set out in this look at power distribution solutions for data centre and critical infrastructure.
Efficiency Regulation Is Not Optional
The efficiency requirements are obligatory in major markets and forces equipment to be different in size as well.
- USA: Development of the 2016 Department of Energy (DOE) efficiency regulation, based on NEMA TP-1, refers to the distribution transformers. In this case, more core materials and conductors will be needed to manufacture equipment that satisfies current regulations.
- European Union: Regulation EC 548/2014 defines the minimum efficiency requirements for transformers with capacity higher than 1 kVA for 50 Hz circuits. Tier 1 requirements entered into force on July 1, 2015, while Tier 2 ones came into force on July 1, 2021. Tier 2 defines maximum no-load and load losses in watts for medium-sized dry transformers and establishes minimum efficiency for the rated capacity above 3,150 kVA.
- Canada: It sets the necessary minimum efficiency requirements for similar classes of transformers.
There can be two consequences of this requirement. Firstly, the compliance transformer should be larger in size and weight compared to the non-compliance transformer of the same rating. As a result, the construction space may be exposed to some changes. Secondly, efficiency requirement and K-factor pressure are directed in the same way in relation to manufacturing, which means that the compliance transformer is heavier in terms of conductive materials.
Installation Notes Specific to Data Centre Work
- Ventilation and clearances – be sure to check with the manufacturer for minimum clearances in all directions and above as well as against NFPA 70 installation requirements. Because a cast resin transformer dissipates the losses in the room, the air handling system must be sized to handle the losses.
- Floor loading – 2000 kVA of a cast resin transformer and enclosure means heavy equipment. It is important to confirm capacity of installation location, not just that of the entrance.
- Noise rating – quiet version per NEMA ST-20 is important if the transformer is located near occupied space. This is about core clamping and low flux density design.
- Grounding and neutral – A K-rated transformer with 200 % neutral means that the conductor and bus bar need to be sized accordingly for the downstream portion of the installation. This means that choosing the transformer and undersizing the neutral bar is bad practice.
- Monitoring – install RTD or PT100 sensors to all windings and connect them to the building management system or DCIM. This is the cheapest predictive maintenance you can get in dry-type transformers.
- Termination of cables – confirm whether this is cable box, bus duct or open busbar arrangement at the place of connection and check the phase distance at that point instead of a delivery site.
Acceptance Testing: What to Ask For
The factory acceptance test documentation must include at least the following elements: ordinary tests as required by IEC 60076-11/IEEE C57.12.01, turns ratio and polarity information for each tap, winding resistance value, insulation resistance value and polarization index value, values of no load and load losses from guaranteed numbers, results of the applied voltage withstand test and the induced voltage withstand test, result for the partial discharge test with 1.1 times Um indicated in the document as actual number.
If the transformer is K-rated, it is important to indicate some information on harmonics loading as per chosen K-factor value. Moreover, in cast resin transformers that operate in data centers, some checks must be conducted to ensure that there are no voids, cracks, or incomplete resin fill in the winding encapsulation.
At the installation site, before energizing the transformer and starting operation: confirm the vector group, tap position, neutral, and ground connection, insulation resistance as well as reading temperature in all the three phases. In addition, it is recommended to conduct a thermographic test at full load and to repeat it in a month afterward to check the ongoing temperature reading of both phases.
A structured commissioning sequence is worth following rather than improvising, and the routine tests that a dry-type unit should go through on arrival are set out in this seven-step routine test programme for dry-type transformers. Skipping the tests that are awkward to repeat after energisation is how a defective unit reaches full load before anyone knows.
Brands and How the Range Maps
The market for dry-type data center facilities can be divided into two groups: first is the construction of the entire power train and integration of the transformer into the overall design by global electrical companies, and second is the manufacturing of the transformer on specialized manufacturers’ facilities according to their specifications.
In the case of the specialized option, the key aspect would be whether the supplier has its whole production area for this purpose. Suppliers who can do cast resin dry-type projects from 6-10 kV to 35 kV with pre-tested DC solution would be working with the entire project from the same manufacturing premises instead of sourcing from different producers every time. This holds more value with phased projects than with one-time orders since the next batches of transformers produced have to work together. Subian fits this model as it is a Chinese company with dry-type and oil-immersed transformers fulfilling electrical, high-voltage and amorphous alloy transformers for international projects.
Speaking about the competitive analysis, SCB13 and its equivalents in cast resin technology is the platform on which all data center buyers compare the offers of local markets since the parameters used for each type are very similar even though the certification, methods of production may be different. That comparison is worked through in this note on how SCB13 dry-type transformers compare with international cast-resin designs, including where the differences are real and where they are documentation.
Normal price positioning works in a usual way: global brands at the top, specialist producers significantly below and the gap is becoming wider with the increase of kVA rating and the amount of customisation involved. What a buyer should not do is to compare a fully specified K-13 Class H unit with a K-1 Class F catalogue cost and thus surmise that that the cheaper product is indeed a bargain.
FAQ
Why do data centres use dry-type instead of oil-filled transformers?
Fire safety and site. Cast-resin winding does not burn and has no combustible liquids, so this equipment can be installed indoors and near the load without an oil containment facility. Maintenance is easier since there is no oil testing or gas relay or breather. This implies lower temperature tolerance that makes insulation type, temperature rise and K-factor more important for dry-type data centres than for oil-filled distribution devices.
What K-factor do I need for a data centre transformer?
K-13 is the conservative option for a facility with no harmonic study and is appropriate for legacy servers using 6-pulse UPS rectifiers with THD of 30–40%. K-20 is the correct option for loads dominated by rectifiers or loads that may grow in the future. K-7 is appropriate for new installations using 12-pulse or IGBT rectifiers with THD of 15–25%, while K-4s are used only where active harmonic filters have brought down the THD to 10% or below. Always round up to the next standard rating.
How do I size a transformer for a data centre?
Consider the actual load of the IT equipment, as well as that of the UPS design. Apply redundancy through either an N+1 or a 2N system. Air caching solutions are required only for those areas where it is redundant. An excessively loaded device will have high no-load losses, thus smaller transformers with high efficiency in partial load operation can be better than one larger device in terms of capital expenditure and efficiency measures (PUE).
What partial discharge level is acceptable?
According to the stipulations of IEC 60076-11, at Environmental Category E2, the allowed PD level is not above 10 pC at 1.1 of the rated voltage. Any factory certificate indicating a higher value must be invalidated. Under operating conditions, values above approximately 100 pC at the nominal operating voltage means the insulation condition must be considered unsatisfactory and that the insulation will have to undergo further evaluation and likely replacement.
Referenzen
- UL Solutions — UL 1561 for dry-type general purpose and power transformers, including recognised K-factor ratings
- Eaton — dry-type transformer application guidance, K-factor selection and harmonic loss explanation
- Schneider Electric — dry-type transformer specification data, temperature rise classes and efficiency compliance
- US Energy Information Administration — data centre electricity consumption and load growth analysis
- US Department of Energy — transformer efficiency standards and distribution transformer energy conservation rulemaking
- Uptime Institute — data centre tier classification, redundancy topologies and availability research
- ASHRAE — thermal guidelines and technical resources for data processing environments
- International Electrotechnical Commission — IEC 60076-11 for dry-type power transformers and IEC 60076-1 general requirements
Fazit
A dry-type transformer chosen for a data center has three key elements: the insulation category that exceeds significantly the temperature range of the transformer; the K factor that classifies the level of harmonic distortion for the operating load; and partial discharge testing. Properly adjusted insulation with a combination of thermal monitoring, energy efficiency, and extensive factory testing should ensure that the dry transformer will work for twenty years without failures. Any problems that might arise will become apparent only after eight years as a winding fault and by this time all savings provided by the contract will have been spent already.