The commissioning report of a 40 MVA device that took the time of 14 months to come in has just been acquired by the substation engineer. The device is supposed to cater to a region that has 60,000 consumers. 28,000 liters of oil are present inside the tank along with 20 tons of core-and-winding assembly and the unit’s cooling system which is responsible for regulating heat with a budget of 65K temperature rise. The engineer realizes the fact that the simplicity of the steel tank has potential complications in the form of dissolved gas chemistry, hot-spot management, moisture equilibrium between paper and liquid and a tap changer that is responsible for switching under load.
The purpose of this article is to provide information about power transformers immersed in liquid, how these devices act such power voltage transformers, the way that the cooling system functions, the difference between mineral oil and ester fluids, the complexity of failure patterns that are linked with the control of the transformer operation and prices associated with machines of various capacities.
In short, a liquid-filled power transformer is a type of transformer which is cooled and insulated using a liquid, in most cases mineral oil which is employed in about 75–85% of units, while when there is a risk of fire or hazardous effects to the environment oils like natural and synthetic esters are used. The liquid cooling method provides greater cooling capability than cooling by means of air,the temperature tolerance is (65K) equal to (65K) and above 50–60K for dry transformers.
What Is a Liquid-Immersed Power Transformer?
An immersed liquid power transformer is the one wherein the active part of the transformer including the winding and core is housed in a steel tank filled with an insulating fluid. The role of the insulating liquid is twofold. First of all, it acts as a dielectric that insulated conductors from the ground and provides for some parts of the insulation together with paper and pressboard. Secondly, it acts as a coolant and carries the heat away from the winding and core to the tank and radiators subsequently cooled down by air.
This combination of two functions is a reason of the high efficiency of liquid immersion. Air is a poor dielectric and also a poor heat conductor making it possible to dissipate heat from the dry type winding with much greater efforts. To add, the thermal conductivity of liquid is about thirty-six times higher than that of air and it takes much more heat per liter of both air. Thus, since several hundred of kVA, practically every transformer is considered to be a liquid type immersed transformer (which is often referred to as oil-immersed transformer).
Why Liquid Immersion Wins at Scale
The reasons why liquid immersion is better when it comes to transformer sizes can simply be stated:
- Thermal headroom: According to IEC 60076-2, the average temperature rise of the windings used in oil-immersed transformers is equal to or below 65 degrees Celsius, which results in higher continuous ratings due to better heat transfer that can be achieved.
- Dielectric strength: Due to oil having a dielectric strength of 40-70 kV/2.5 mm, it can be used for reducing clearance and volume due to fact that it is much higher than that of its air counterpart.
- Overload capability: Liquid transformers are built to withstand overloads for brief periods of time – for instance, as IEC 60076-7 prescribes, overloads in the amount of 130 % can continue for 2 hours.
- Self-healing dielectric: Minor discharges in oil create gas, which dissolves, and the oil returns back to its normal operational state unlike an air gap, which cannot heal itself.
- Diagnosability: The liquid expresses a chemical fingerprint of every process occurring inside it, which makes dissolved gas analysis the most effective early-warning system in electrical engineering.
These advantages are counterbalanced by the known disadvantages of liquid systems, namely the danger of fire and leaks, weight, as well as the complexity of maintenance activities inherent in using machinery that contains liquid. That is why the industry relies on engineering solutions, standards, and strict adherence to the rules to minimize these risks.
How Liquid Cooling and Insulation Work
In liquid-filled transformers, cooling is accomplished with natural or forced ventilation by air. In the basic mode ONAN (oil natural, air natural):
- Heat produced by the core and the windings heats the oil surrounding the transformer which makes it less dense;
- Heated oil is moved through ducts inside the windings to the upper part of the tank;
- Heated oil is released to the conservator and then through radiators, where it is cooled down by air;
- Cooled oil returns back to the transformer’s lower part and flows back into the windings to repeat the cycle.
Modern designs enable controlling the cooling process with oil flow systems in larger transformers involving pumps to force the oil through the winding ducts (OF modes) and fans in front of radiators (AF modes). The cooling modes in question are reflected in the nameplate designation like ONAN, ONAF, OFAF, etc.
| Cooling code | Significado | Typical capacity uplift | Common use |
|---|---|---|---|
| ONAN | Oil natural, air natural | Base rating | Most distribution and medium power |
| ONAF | Oil natural, air forced (fans) | +20–30% while fans run | Cyclic loads, overload headroom |
| OFAF | Oil forced, air forced | +30–45% | Medium-to-large power transformers |
| ODAF | Directed oil flow, air forced | Highest per-unit rating | Large power transformers >60 MVA |
While a unit with a rating of 40 MVA ONAN may be rated 50 MVA ONAF when using fans, since the fans approximately double the heat dissipated by the air side, insulation works as a composite means. Paper and pressboard make up the insulation barrier, while the liquid occupies all existing free space. It is also important to note that moisture equilibrium in paper and the oil must be taken into account. Paper absorbs water easily and oil is able to dissolve water. In ester-filled transformer application, the oil allows for high moisture absorption by paper which slows down the aging process, while in standard mineral insulating oil units, the water penetrates the paper faster which accelerates the aging of the paper.
Mineral Oil vs. Ester Fluids
Selecting the liquid type is one of the most important specifications for a liquid-immersed transformer. The main options are:
| Fluid | Fire point | Biodegradability | Moisture tolerance | Relative fluid cost | Best fit |
|---|---|---|---|---|---|
| Mineral oil | ~160°C | Slow | Low | Baseline | Standard substations |
| High-temp mineral | ~300°C | Slow | Low | +20–40% | Indoor, higher safety margin |
| Natural ester | >300°C | >90% in 28 days | High | +30–60% | Environmental and fire risk areas |
| Synthetic ester | >300°C | Moderate | High | +60–100% | Indoor, high-voltage, critical assets |
Mineral oil is still considered as the most common option because of being cheap and very broadly used due to its sufficient characteristics when it is effectively installed with proper fire separation and oil containment. Esters are used in cases when it is required to follow local laws or regulations regarding usage of mineral oil- indoors, around water bodies, in compact urban vaults, and actively at the sites using renewable resources. It should be mentioned that esters are quite expensive in comparison to mineral oil- $3000 – $8000 which totally depends on the capacity of a transformer – and such an investment will be paid back in the forms of approval all the permits, insurance conditions and reduced aging of paper.
The Complexity: Failure Modes and Diagnostics
Liquid-filled transformers break down in well-defined ways, so to know how to operate the asset, one has to grasp:
- Insulation aging: The leading cause of failure, due to temperature and moisture influence. Each six degrees (Celsius) of thermal excess halved the lifetime of paper insulation.
- Moisture ingress: Traumas suffered by internal insulation through its breathers, leaks, and aging gaskets. More moisture reduces dielectric strength.
- Partial discharges and arcing: Maintenance of the atmosphere where the gases produced are identical to those that DGA detects.
- Thermal failures: Increase in the temperature of internal oil due to poor contacts or blocked ducts cause gas developments as well.
- Tap changer malfunction: Details inside an oil-filled body that wear down with operation.
Each cause of malfunction leaves a “gas trail” practice: now the liquid can be called a “living diagnostic sample” at a price of $200–600 visit per year. With multi-gas online monitoring at a price of $8000–40000, it has been possible to monitor the status of oil-filled transformers somehow.
Typical Specifications by Rating
| Classificação | Classe de tensão | Impedância | Resfriamento | Typical loss (no-load / load) |
|---|---|---|---|---|
| 500 kVA | 10–35 kV | 4–4.5% | ONAN | ~1.0 / ~5.2 kW |
| 1,000 kVA | 10–35 kV | 4.5–5% | ONAN | ~1.7 / ~10.5 kW |
| 10 MVA | 35–110 kV | 7.5–8% | ONAN/ONAF | ~10 / ~50 kW |
| 40 MVA | 110–220 kV | 10–12% | ONAN/ONAF/OFAF | ~30 / ~170 kW |
| 100 MVA | 110–220 kV | 12–14% | OFAF/ODAF | ~70 / ~380 kW |
The numbers refer to the IEC 60076 specification and change depending on the loss category, material types, and producer. However, the trend is always there: the impedance increases with the power rating in order to keep the currents during a fault under control, the cooling becomes more intensive during operation, and the losses increase at a slower rate than power thus making the efficiency increase with the size: a 100 MVA machine can achieve efficiency exceeding 99.7%.
Efficiency and Lifetime Economics
The cost of liquid-stuffed transformers relies on losses, and these losses are significant. For example, if we were to take a 40 MVA electrical equipment with 30 kW no-load and 170 kW load loss at 60% utilization:
- The no-load loss would be:30 kW held for 8,760 hours: 30 × 8,760 = 263 MWh/year.
- The load losses would be:170 kW × 0.36 × 8,760 hours = 536 MWh/year.
- The resulting number is:The sum of these is about 800 MWh/year valued at $56,000-$80,000 based on $0.07-0.10/kWh.
In 25 years, the total losses would amount to $1.4-$2.0 million – this is several times higher than the purchase value of the equipment. Thus, this is the main reason why companies take losses into account when estimating energy expenses and prefer to acquire units with higher prices but lower losses. There is a rule of thumb, for every difference of 10 kW of losses, the costs need to be evaluated at $7,000 – $8,700 per year of continuous no-load operation or adjusted according to the utilization ratio for load losses.
| 40 MVA example | Loss (kW) | Annual energy (MWh) | 25-year cost @ $0.08/kWh |
|---|---|---|---|
| No-load loss (P0) | 30 | 263 | $525,000 |
| Load loss (Pk) at 60% load factor | 170 | 536 | $1,072,000 |
| Total | — | 799 | $1,597,000 |
| Reference purchase price | — | — | $220,000-$400,000 |
Brand Landscape and Prices
The liquid-immersed transformer market involves almost all transformer manufacturers without exceptions. The typical FOB price details:
| Marca | Origem | 1 MVA | 10 MVA | 40 MVA |
|---|---|---|---|---|
| Hitachi Energy | Japão/Global | $22,000–$34,000 | $55,000–$82,000 | $250,000–$400,000 |
| Siemens Energy | Alemanha/Global | $21,000–$32,000 | $50.000–$75.000 | $240,000–$380,000 |
| ABB | Suíça/Global | $20,000–$31,000 | $52.000–$78.000 | $240,000–$390,000 |
| Schneider Electric | França/Global | $19,000–$29,000 | $48.000–$72.000 | — (distribution focus) |
| Hyundai / Toshiba | Korea/Japan | $18,000–$28,000 | $46.000–$68.000 | $220,000–$350,000 |
| Jiangsu Subian Electric Power | China | $14,000–$22,000 | $28.000–$48.000 | Up to 110kV class on request |
The price differentials across product ranges are observed mostly in expensive products where engineering complexity, test history and global service come into play. On the other hand, at low-end products price differentials have almost collapsed due to standardization of the technology and increase in certification availability.
Jiangsu Subian Electric Power is a Chinese company engaged in the production of distribution transformers of a liquid-immersed type with the capacity ratings for 35-110 kV, IEC 60076 compliant. Subian’s product line includes the transformers made using mineral oils as well as esters and having different on-load voltage regulator configurations and efficiency ratings. The products are successfully used by utilities and EPC contractors all over Asia and Africa.
Operating a Liquid-Immersed Unit Well
The efficient operation of an oil-filled transformer should be considered a routine task, not a project:
- Conduct oil sampling using IEC 60422 yearly. DGA testing should be completed, and the oil should be tested for dielectric strength, water content, acidity, and tan delta. In case any ratio changes, keep the regular sampling.
- Quarterly inspection should be conducted for the following: silica gel in the breather, oil level, leaks, radiator cleanliness, and surrounding vegetation.
- The servicing of tap changer should take place based on its use. Tap changers of oil-filled types usually serve between 50,000 to 100,000 switches.
- Temperature readings need to be taken: top oil temperature and winding temperature in compliance with IEC 60076-7 loading curves. Investigate the cause of an upward draft if one occurs.
- Testing of the protective devices needs to occur. Testing needs to include checking Buchholz relay gas and surge trips, the pressure relief valve, and both temperature alarms.
Perguntas Frequentes
Why are most large transformers liquid-immersed rather than dry-type?
Liquid cooling and insulation have proved far superior, since oil has a thermal conductivity approximately 30 times that of air and offers much greater dielectric strength. From a certain power level, liquid immersion offers better performance, reducing installation footprint and reducing cost. Dry transformer type is usually used in places where fire concern and containment preclude the use of oil — for less than about 5 MVA.
What is the difference between oil-immersed and liquid-immersed?
There is none. Oil-immersed is the old term for oil transformer technology; liquid-immersed is widely used so as to cover oil, natural ester fluids, synthetic ester fluids, and silicone fluids as well. Therefore, in this case, the recommended term is liquid-immersed. However, once the fluid is chosen, it is advisable to mention it and put it into the specification.
How much does a liquid-immersed transformer cost?
Prices vary considerably depending on power rating, manufacturer, number and types of extra features; however, as a rough estimate the following costs can be indicated: for 500 kVA models they cost anywhere from 6000 to 16,000, while 1000 kVA units cost around 14,000 to 34,000 and 10 MVA cost anywhere between 28000 to 82000. One must also take into consideration the fact that ester fluids are more expensive than mineral oil, and the price difference can reach from 30 to 100 percent.
Are ester-filled transformers worth the premium?
The milestone of the use of esters is that in cases where ordinary mineral oil will be difficult to use due to fire regulations or environmental standards, ester fluids will be the best alternative. The reason is that they have several advantages including fire point higher than 300°C, biodegradability and retardation of the paper aging process. Although fluids are more expensive by 30 to 100%, their use can be justified due to possible permits from relevant authorities and due to insurance reasons that may finally prove the worth of the higher price.
How often should transformer oil be tested?
For transformers in standard operating mode the testing frequency should be annual as prescribed by the IEC 60422 standards. In other cases, if any major changes occur above the baseline level, then monthly testing should be done to identify changes and relevant tendencies.
Referências
- IEC 60076-1: Power transformers – General — Ratings, temperature rise, and testing requirements for liquid-immersed power transformers.
- IEC 60076-3: Insulation levels and dielectric tests — Impulse and dielectric test requirements by voltage class.
- IEC 60296: Mineral insulating oils specification — Quality requirements for new transformer oil.
- IEC 60422: Supervision and maintenance guidance for mineral insulating oils — In-service oil testing limits and schedules.
- IEEE C57.12.00: Liquid-Immersed Distribution and Power Transformers — North American general requirements for liquid-immersed units.
- IEEE C57.104: Interpretation of Gases Generated in Oil-Immersed Transformers — DGA interpretation and action levels.
- Jiangsu Subian Electric Power official site — Liquid-immersed transformer products, fluid options, and test documentation.
Conclusão
Liquid-filled transformers have a firm grip over the power grid as they maintain the best efficiency possible for cooling and insulating substantial electrical machines. While some aspects — chemistry of fluids, management of hot spots, regulators, monitoring — can be complicated, the rules and procedures make it easy enough to handle them, providing an efficiency advantage that dry type systems cannot achieve.
Main points to remember:
- Liquid cooling makes it possible to achieve >99% efficiency in medium and large transformers while not providing any advantage for dry-type machinery at this power rating.
- Mineral oil is the traditional liquid; ester fluids justify their $30-$100 percent-based price practically everywhere in case of fire and ecological issues.
- Annual DGA test can cost $200-$600 but it brings the highest profit in the electrical engineering.
- Get ready to pay between $6,000-$400,000 neglecting losses while taking into account the performance of a transformer.