Oil-Immersed Transformer: Specifications, Testing Standards & Factory Guide (2026)

Referring to the technical nameplate of a Chinese factory sourced oil immersed transformer, you will find a dozen parameters – half a dozen of which, get only one wrong and your buyer’s party may end up in years of field corrective maintenance, unplanned transformer overheating or in a local grid authority inspection rejection of the unit on arrival. This guide is written for the engineer or procurement officer responsible for specifying, verifying and faithfully procuring in the first instance.

The insulating oil types differ in their fire safety, its dielectric strength and their cost differences, what the ONAN, ONAF, OFAF and OFWF symbols mean in straightforward terms, seven nameplate parameters that you should verify to any factory quotation received, and the five factory acceptance tests you should insist on prior to any transformer shipment (including, for reference, the actual 30 – 1,600 kVA distribution specification data of DEMIKS so you can verify competing quotations against genuine production figures).

What Is an Oil-Immersed Transformer? Core Design and Working Principle

A transformer – often called an oil filled transformer or an oil type transformer – is an electrical device in which the magnetic core and copper windings are completely submerged in a steel tank filled with insulation oil. The purpose of the oil is twofold – firstly it forms the main dielectric insulator between the high and low voltage windings, and secondly it transfer heat away from the core and windings by convection to the tank walls and e×ternal radiators.

The combination of the two functions is the main reason why the oil filled design remains the most common choice for working voltages above 36 kV and for capacity ratings above a few MVA – scale physics is sound. Note that the word “oil” is somewhat ambiguous – the insulation oil could be mineral based, natural ester based, or synthetic ester based. To clarify, all three main sub-types are discussed below.

The main structural components are as follows:

Operating temperature is defined by IEC 60076-1: standard rated conditions assume a maximum ambient of +40°C, with a 24-hour mean not exceeding +30°C. The oil itself must not exceed 85°C at the top surface during continuous rated operation per IEC 60076-2. Above this temperature, insulation paper aging accelerates and service life shortens measurably.

What is the purpose of oil in a transformer?

Oil filled transformers are a subclass of a broader family of oil -insulating transformers, and performs what could be called three simultaneous functions: it shall be the main electrical insulation medium for the units with all gaps between high-voltage and low-voltage winding being filled with dielectric oil to avoid potential arcing between the two conductors of differing volts; it shall be the conductor for the heat that appears from the heat within the core due to resistive losses throughout, carrying this thermal effect away by convection to cooler radiators; and it shall be the medium that averted the arc spreading in event of a internal discharge by quenching the occurrence before it develops into a serious malfunction.

The false understanding is that the oil in isolator only performs this function. In fact oil in the series is the conductor of heat in as much as it is the insulator of heat (as the solid deriseds between the high-voltage and low voltage winding – kraft paper wrapping, pressboard barriers – in tandem forms the total insulation system). Weaken the oil (by moisture absorption, bycontaminant contamination and by oxidation) and the insulation system will fail, even if the kraft paper maintains its non-condensing properties.

Transformer Insulating Oil Compared: Mineral Oil, Natural Ester, and Synthetic Ester

There are three main types of insulating oil used in the production of oil-immersed transformers. Mineral oils account for the installed base of all oil-filled units globally. The natural ester solutions – of the forefront products is Cargill’s FR3 – are growing rapidly in oil-immersed transformers used in fire sensitive atmospheres. The synthetic ester is another option, which is ideally suited for power appliation with cold climates and/or a focus on high performance applications.

For the end user the key properties are dielectric strength, fire point, pour point, biodegradability and relative cost.

Internal links: Browse our oil dielectric strength test guide and the dielectric dissipation factor of transformer oil pages to see the test procedures used to measure oil condition during operation. The insulation oil dielectric strength tester used to perform these tests is in our test equipment range.

Cooling System Classification: What ONAN, ONAF, OFAF, and OFWF Mean for Buyers

The four-letter cooling code – IEC 60076-2 standard – on the transformer nameplate is coded to reveal all the parameters of possible infrastructure requirements and units capacity, when interpreted correctly.

The four codes most frequently encountered, with their typical capacity capabilities and temperature rise limits:

Dual ratings: Large units often have two specification numbers, e.g. 60 MVA ONAN / 80 MVA ONAF. The transformer provides 60 MVA with no forced cooling and 80 MVA with fan assistance. For transformers in substations which regularly draw on peak loads in the summer period the dual ratings allow the operator to activate forced cooling and ideally upgrade to a larger unit when placed. The transformer is purchased once but capacity is switched between operational modes as required.

What is ONAN cooling in a transformer?

The ‘Oil Natural, Air Natural’ rating (hot and cold) is the simplest form of cooling available for oil immersed transformers. Fans and pumps are not used, so auxiliary power requirements are avoided, and if installed correctly the oil transfers heat from the core and coil assembly within the transformer tank by natural convection, through the external radiators by radiation and conduction, and loses heat to the surrounding environment, in a continuous cycle. This form of cooling is specified in roughly equal measures of the largest distribution transformers to around 10 MVA and can power reliably without auxiliary equipment.

Reading the Nameplate: 7 IEC 60076 Parameters Every Buyer Must Verify

When requesting a quotation or datasheet from a Chinese transformer factory it is the specification table, if available, that is critical for understanding the suitability of their product. Seven parameters are used to judge whether the unit will work to meet your application needs:

1. Rated Capacity (kVA or MVA): The rated power level developed for a short time at a specified frequency and voltage, without any damage or exceeding the allowable temperature rises. IEC 60076-1, section 4 gives the rated, guaranteed value at some specified reference conditions.
2. Voltage Ratio (HV/LV in kV): the upper and lower rated tap rated primary and secondary voltage (additional specifies may be given for au/vtuse so you must specify if you have an AC or a through test). Make sure this value exactly matches your grid voltage. An 11 kV, 12 pulse, system cannot work with a 10.5 kV transformer even with the tap down.
3. Short-Circuit Impedance (Zk%): the voltage drop expressed as a percentage at rated load level (IEC 60076-1 cl. 5.4). The impedance value affects fault current level seen at the secondary and the transformer voltage regulation. It is critical to specify the value that is best suited to your transfer-to-coordination scheme
4. No-Load Loss (P, Watts): the core, hysteresis or iron loss. This watts value is exported continually whenever the transformer is energized, regardless of load. It is the primary component of the electrical power supplied to the transformer for its entire 25-40 year anticipated lifetime.
5. Load Loss (Pk, Watts): the full load (rated current) winding copper loss. This watts value scales as the square of the fractional load: at approximately 80 % load, the load loss is approximately 80 % squared = 64 % of nameplate Pk.
6. No-Load Current (I%): the amount of excess excitation inrush current required to energize the transformer compared to perfect (reference, no phasing, infinite conductor) transformer. A higher-than-specified value I% indicates core material quality issues or ISAS mismatch.
7. Transformer Vector Group (Junction Group Label): indicates appropriate phase connection type, phase rotation sense, and phase displacement. Without matching vector groups you will get parasitic circulating currents in the paralleling transformers, causing overheat and state-of-charge problems in the IDC.

Note the following standard distribution transformer specification directly from the DEMIKS factory production data:

Full range: 30–1,600 kVA. Available HV combinations: 6, 6.3, 6.6, 10.5, or 11 kV. See DEMIKS oil-immersed transformer specifications for the complete 17-row table, and our IEC 60076 transformer standards guide for further technical background.

Oil-Immersed vs. Dry-Type: A Quick Buyer’s Reference (+ Application Decision Matrix)

For most buyers, the oil-vs-dry decision comes down to three factors: installation environment, voltage class, and fire safety requirements. This section gives you a quick reference; for the full technical and cost comparison, see our dry-type vs. oil-immersed transformer complete guide.

Application decision matrix – here for just two minutes:

What is the difference between oil immersed and dry transformers?

Oil-immersed transformers submerge the core and windings in insulating oil for combined insulation and cooling. Dry-type transformers use air and resin insulation — no liquid. The practical difference: oil transformers handle any voltage class and cost less per MVA for large ratings; dry-type is preferred indoors wherever building fire codes prohibit oil. See our complete oil vs. dry-type comparison → for full cost-of-ownership analysis. Also see the DEMIKS dry-type transformer range if your application requires indoor installation.

Factory Acceptance Tests: 5 Mandatory Checks Before Your Oil-Immersed Transformer Ships

Every transformer delivered to you from a factory should be supplied pre-checked with a completed factory acceptance test (FAT) sheet before the movement is approved. IEC 60076-1, section 10 lists all the routine tests that every transformer must pass – regardless of manufacturer, size, or price. These tests are neither optional nor recommended. Any factory that cannot supply documented routine test sheets for each item has not yet assembled the transformer.

The application of this five checklist – the DEMIKS Factory Acceptance Checklist – is ordered in the common sequence of IEC-listed routine testing of distribution transformers:

1. 1. No-Load Loss Measurement (IEC 60076-1 cl. 10.4)

   What it tests: core iron quality and magnetic efficiency. This is performing the transformer at rated voltage while the secondary is open-circuited.

   Pass criterion: Measured P shall not be greater than the nameplate value by more than +15% if that allowance is used as defined in IEC 60076-1.

2. 2. Loading Loss and Short-circuit Impedance (IEC 60076-1, 10.3)

   What it tests: Insulation on the winding conductor and accuracy of the impedance. The secondary is shorted and a lower voltage is applied to give rated current. Both copper loss (Pk) and impedance voltage (Zk%) are measured simultaneously in a single test.

   Zk% should be within 10% of nameplate and Pk should be within + 15% of nameplate.

3. 3. Voltage Ratio and Phase Displacement Check (IEC 60076-1 cl. 10.2)

   What it tests: Turns ratio accuracy and vector group correctness (Dyn11, Yyn0, etc.). Performed at each tap position. Ratio error must be within ±0.5%.

   Pass criterion: Ratio error <0.5% at all taps; installed vector group to be confirmed as per nameplate.

4. 4. Induced Overvoltage Withstand Test – AC Hipot (IEC 60076-1 cl.10.5)

   Purpose of the test:.50. test one turn-to-turn and one layer-to-layer insulation integrity. Applying a voltage twice rated voltage across the secondary winding for 60 sec. This exerts a higher than normal stress on the internal insulation.

   Pass criteria: No flashover, no partial discharge at any time, no longterm operation of the protection relay during the 60 sec application time.

5. 5. Separate-Source Voltage Withstand Test (IEC 60076-1 cl. 10.6)

   What it tests: Earth insulation between each winding and tank.The insulation between each terminal bundle of the windings and the tank must be insulated and maintained at earth potential. A known AC voltage is applied for 60 seconds from each terminal bundle, individually, to the tank earth via an external source. For a primary 11 kV winding, the test voltage is 28 kV per IEC 60076-3.

   Pass criterion: no flashover or sustained arc through the whole time of application.

How to Evaluate a Chinese Transformer Factory’s Test Capability

Ask any potential suppliers for evidence of three of the technical capabilities before placing an order for:

• In-house high-voltage test laboratory (rated at your unit voltage class). A transformer with 35 kV class insulation requires a test lab capable of applying 70–100 kV. Request the list of equipment with calibration dates.DEMIKS has high-voltage test equipment with a test voltage range suitable for distribution and power transformer class.
• Oil analysis ability – the in-house DGA equipment, or documented association with a IEC accredited oil testing laboratory. See our transformer test equipment series for details on the instruments in manufacturing testing.
• Calibrated measuring equipment – Wattmeters, voltage transformers, and current transformers (used in FAT) must be calibrated to a national standard. Request calibration certificate dates.

For testing methodology details, our guides cover how to test a transformer, types of transformer testing, the tangent delta test, and the temperature rise test procedure in full technical detail.

2025–2026 Industry Outlook: Renewable Energy Is Driving Oil-Immersed Transformer Demand

Based on market analysis by Fortune Business Insights the total global oil filled transformer market in 2025, was valued at an estimated $47.61 billion, with forecasts to be $51.50 billion in 2026- a CAGR of about 5.8% according to IntelMarketResearch. Note these are Tier 3 market research numbers and should be viewed as directional rather than exact.

What is the important number to procurement planning is not the headline number. The main demand driver 2025-2026 is for grid integration in relation to renewable energy sources- not the conventional grid expansion & upgrading works as in earlier times. Every utility scale solar parks and onshore wind farms need power step-up transformers to connect their output with grid transmission. Grid scale battery storage plants need dedicated sub-station transformers rated at 5-100 MVA. As more wind & solar comes into operation in Southeast Asia, Middle East, Europe & North America, that is the main piece of enabling equipment to grow.

Other secondary demand stems from the rise of EV charging infrastructure (grid-scale charging depots need 500 kVA-2 MVA distribution transformers), data center power density requirements, and grid modernization – replacing transformer stocks bought in the 1980s before they reach the inevitable end of their cycle.

From the Chinese oil-immersed transformer manufacturer perspective: lead times for distribution transformers extending from the usual 4-6 weeks to 8-12 weeks in 2024-2025. This was caused by the combined effect of Chinese domestic grid investment and high margins from export demand in Southeast Asia putting pressure on capacities at mid-tier factories. This condition has not fully unwound.

Frequently Asked Questions About Oil-Immersed Transformers

Related Technical Guides

• The Ultimate Guide to Transformer Test Equipment
• Partial Discharge Testing: Procedures and Pass Criteria
• Temperature Rise Test Procedure for Power Transformers
• Transformer DC Resistance Test: Method and Interpretation

References

1. IEC 60076-1:2011 – Power transformers Part 1: General. International Electrotechnical Commission. iec.ch
2. IEC 60296:2020—Fluids for electrotechnical applications: unused mineral insulating oils. International electrotechnical commission. iec.ch
3. IEEE C57.12.00-2021—General requirements for liquid-immersed distribution, power, and regulating transformers. IEEE. ieee.org
4. Fortune Business Insights—Oil filled transformer market size, share & growth analysis, 2025. (Tier 3 market research; cited with hedging)
5. IntelMarketResearch—Oil immersed transformer market, CAGR forecast, 2025. (Tier 3 market research; cited with hedging)