Cast Resin and Dry Type Transformers

In the past, cast resin and dry type transformers have been widely used in a range of applications, but there are some aspects where MIDEL ester fluids can deliver improvements in performance.

Fire Resistance

Transformer fire protection is a primary concern when installing transformers, as they can often be located in high risk areas such as inside buildings, subterranean installations, trains or wind turbines. MIDEL 7131 offers a 100% fire safety record that makes it ideal for use in high risk environments such as underground power transformers.

Environmental Impact

MIDEL 7131 is also notable as an environmentally friendly transformer fluid. It is readily biodegradable and non-toxic, giving it an advantage over alternative dielectric oils.


Potential faults can be detected earlier in MIDEL filled transformers than in cast resin with Dissolved Gas Analysis (DGA) testing, allowing you to fix any damage before the problem becomes costly and time consuming. If a fault is detected, coils can be repaired with relative ease in liquid-filled transformers, whereas with cast resin they will often need to be replaced if a fault develops. MIDEL ester has actually developed a reputation as the ‘fill and forget’ fluid, due to its robust nature, leading to very low maintenance requirements. In particular, cleaning is a much easier task; cast resin transformers must be tended to regularly especially in dusty environments.

Dielectric Oils

Mineral oils have been used for many years to help transformers perform at their maximum level, but studies have since revealed them to be flawed. Ester-based dielectric oils have been shown to be a high performance alternative to mineral oil for use in transformers. Mineral oils have been shown to be highly flammable, they contain corrosive sulphur, have poor moisture tolerance and can damage the environment, all of which makes MIDEL ester based transformer oils a much better alternative. MIDEL’s natural and synthetic esters perform the same task as mineral oil in a safer, more environmentally friendly fashion.

Biodegradable Synthetic Transformer Fluid

Biodegradable synthetic ester MIDEL 7131 performs well in many applications, including high risk locations such as underground or offshore. It can be used in both sealed and breathing transformers, and has been proven up to 433kV. Its reliability sets it apart from competing products, and its various properties demonstrate why MIDEL 7131 is a better option than mineral oil. Firstly, it has a high fire point of >300°C and a 100% fire safety record, ensuring you are protected from the extensive damage a transformer fire can cause. It is also environmentally friendly, non-corrosive and has low maintenance requirements.

Natural Dielectric Oil

Manufactured using renewable materials, MIDEL eN 1204 and MIDEL eN 1215 are ideal products for those seeking environmentally friendly transformer oils. Their excellent biodegradability characteristics reduce damage in the event of a spillage, and laboratory tests have shown them to be non-corrosive. Furthermore, like MIDEL 7131 they have a high fire point. MIDEL natural ester fluids perform best in sealed transformers, and as such are well suited to tasks involving distribution transformers located indoors or in busy areas.

Fluid Filled Transformers and Insulating Oils

Transformers using MIDEL ester-based oils have been shown to perform significantly better than those filled with standard mineral oil, especially in demanding applications such as rolling stock. While mineral oils were for many years the product of choice in transformers, in recent years studies have revealed how ester-based fluids offer a far superior alternative. Whereas mineral oil is highly flammable, MIDEL 7131 and MIDEL natural ester fluids provide a 100% fire safety rating. Mineral oils also contain corrosive sulphur and are potentially damaging to the environment. MIDEL esters can also increase the life expectancy of fluid filled transformers with their greater moisture tolerance.

MIDEL 7131

Our synthetic ester is the ideal choice for a broad range of applications, and works just as well in sealed or breathing transformers. High risk environments such as offshore or underground transformers require a safe, environmentally friendly fluid, and MIDEL 7131 can provide this. It boasts a high fire point of >300°C, while its 100% fire safety record provides the necessary protection for areas where an outbreak of fire could have disastrous consequences. MIDEL 7131 is non-corrosive and offers a low maintenance transformer fluid solution, with its increased reliability and safety setting it apart from mineral oil alternatives.

MIDEL eN natural ester fluids

Our natural ester fluids provide another high performance alternative for fluid filled transformers, and these environmentally friendly transformer oil are made using renewable materials. MIDEL eN 1215 (soybean) and MIDEL eN 1204 (rapeseed/canola) are biodegradable and non-corrosive, so will reduce the environmental impact should any spillage occur. They are also 100% fire safe, as is MIDEL 7131.

Insulating oil for transformers

MIDEL provides insulating oils for transformers that provide a safer, environmentally friendly, high performance alternative to mineral oil. While these mineral oils have previously been widely used, MIDEL esters address many of their key flaws to create a product that is now trusted and highly regarded within the industry. Choose from three MIDEL products, our synthetic transformer oil MIDEL 7131 or our natural ester fluids MIDEL eN 1204 (rapeseed/canola) and MIDEL eN 1215 (soya), all of which can ably perform in a broad range of applications.

Properties of MIDEL 7131

The most important features of our synthetic oil are its fire safety and excellent environmental protection. It is also non-corrosive and has become known as the ‘fill and forget’ fluid due to reports that it requires very low levels of maintenance once MIDEL 7131 has been added. All of these qualities make it a more appealing option than mineral oil, particularly in high risk areas. For example, fire safety is of paramount importance in transformers located indoors, while for offshore projects environmental protection becomes vital in the event of a spillage or leak. MIDEL 7131 has the added advantage of being suitable for both sealed and breathing transformers, while it can also be used in mobile transformers.

Properties of MIDEL eN

Our natural transformer fluid is made using renewable materials, and so is ideal for companies wishing to closely monitor their carbon footprint. It is biodegradable, which again helps to reduce its impact on the environment.

Mobile Transformers

There are many situations in which mobile transformers may be required, often to ensure energy supply is maintained when a main transformer is taken out of commission.

Each component of these mobile transformers is constantly reviewed to make sure they can carry out their job effectively, and as with many applications MIDEL dielectric fluids can help ensure they can continue to operate effectively.

Mobile Transformer Applications

A mobile transformer will sometimes be required at short notice to replace a power supply lost due to a fault with the main sources. This can be caused by an unforeseen equipment failure or extreme weather event. They can sometimes be brought in temporarily to provide back up while the main units are shut down for scheduled maintenance. If there is a delay in building work on a new project, bringing in a mobile transformer is often a cheaper option than constructing a secondary transformer away from the main site. Another circumstance where a mobile transformer may be deployed is if power is required in an isolated area. In these cases, engineers can operate the power remotely.

Using MIDEL Transformer Oil

With mobile transformers often used as a back up to a failing main unit, it is essential that they can continue to work effectively. MIDEL transformer oils offer many benefits that will help to ensure this is the case. Our high performance natural and synthetic esters offer 100% fire safety, full environmental protection and moisture tolerance. They are also very low maintenance, and using MIDEL may mean that your mobile transformer can be designed so it is smaller and has a reduced weight.

Silicone Oil Transformer

MIDEL esters can be shown to provide many advantages over silicone oil transformers, from enhanced performance to easier maintenance. Our products have become the preferred transformer fluid for a wide range of applications, particularly in high risk areas such as underground and offshore.

MIDEL Improved Performance

MIDEL 7131’s superior moisture tolerance gives it the edge over silicone oil and other alternatives such as cast resin transformers. Our dielectric ester-based fluids ensure greater stability over a sustained period, maintaining a high performance level throughout their lifespan. Additionally, silicone oil has also been shown to have a negative effect on the environment. MIDEL has been thoroughly tested and shown to be non-toxic, making it an environmentally friendly choice.

MIDEL Lower Maintenance

Silicone fluid has a tendency to seep through gaskets and seals, and can also form gels under partial discharge conditions. In contrast, MIDEL has a proven track record as a low maintenance transformer oil, gaining a reputation as the ‘fill and forget’ fluid. This infrequent necessity for maintenance work significantly reduces transformer down time.

Transformer Cooling

It is important that the transformer fluid you use has adequate cooling properties in order to protect your transformer from the damage that can be caused by overheating. MIDEL 7131 has been shown to be a very effective coolant, performing particularly well across a broad range of temperatures that transformers regularly have to operate under. It is important for transformer oils to protect against overheating, as the harm this can cause will dramatically decrease the lifespan of your transformer. Therefore, choosing a fluid such as MIDEL 7131 is likely to save you money in the long term, as it is less likely that you will need to replace your transformer or shut it down temporarily for maintenance.

Transformer Insulation

MIDEL’s natural and synthetic esters are both much more suited to the task of transformer insulation than previously widely used mineral oils. Both offer a number of key features now recognised as vital for carrying out this task safely and in an environmentally friendly way.

Transformer insulating oil from MIDEL

The main advantage of using MIDEL transformer oils over mineral oil is their far greater fire safety. Mineral oils have long since been recognised as a fire hazard, but MIDEL’s natural ester fluids and synthetic ester MIDEL 7131 all possess a 100% fire safety rating.

MIDEL esters are also fully biodegradable, whereas mineral oils are not, so using our transformer insulation fluid is much better for the environment. MIDEL eN fluids and MIDEL 7131 are also suitable in certain specific situations, such as those involving power and distribution transformers, and work better in high temperatures when compared with mineral oils.

Transformer Oils

Transformer oils are ideal for minimising electrical clearances and augmenting cooling performance in a variety of applications. Previously, mineral oils were thought to be the best way of achieving optimum transformer performance, but research has since revealed these to be highly flammable and bad for the environment, while they also contain corrosive sulphur and have a poor moisture tolerance. The MIDEL range uses both natural and synthetic esters as a mineral oil alternative for transformers, achieving the same goal safely in an environmentally friendly way.

Natural Ester Fluids

MIDEL eN fluids are natural transformer ester oils, making them a particularly good choice for those seeking an environmentally sound solution to their dielectric fluid requirements. They are made using renewable materials, and in the event of a leak or spillage their biodegradability greatly reduces the potential damage caused to the surrounding area. MIDEL eN fluids have also been found to be non-corrosive by independent laboratories, while they have a very high fire point and are fully RoHS compliant. These esters are suitable for sealed transformers, and works best in temperate climates, making them a particularly good option for distribution transformers in built-up areas and indoors.

Synthetic Transformer Ester Oils

Synthetic transformer ester fluid MIDEL 7131 is a reliable, flexible product with a strong track record of performing well in a wide range of applications, particularly in challenging situations such as offshore or underground. Like MIDEL eN fluids, MIDEL 7131 has been found to be non-corrosive and environmentally friendly. It also has a high fire point of over 300°C and is very low maintenance, earning a reputation as the ‘fill and forget’ fluid. It works well in both sealed and breathing transformers, and can be used successfully in power transformers, rectifier transformers, pole-type transformers and traction transformers, making MIDEL 7131 the most versatile dielectric fluid on the market.

Transformer Oil Testing

There are many benefits to using MIDEL fluids, such as 100% fire safety, environmental friendliness and low maintenance. All our transformer oils are tested thoroughly to ensure they are capable of meeting these demands.

Testing MIDEL 7131 and MIDEL eN

MIDEL 7131 and MIDEL eN fluids have attained many industry standards to help cement our reputation as a market leader. For example, our fluids have been tested in independent laboratories for corrosive sulphur to ASTM D1275 B and IEC 62535 requirements, and were found to be non-corrosive. Our ester-based oils also conform to the Restriction of Hazardous Substances Directive, with neither containing the flame retardants PBB or PBDE, nor the metals cadmium, lead, mercury and hexavalent chromium.

Dissolved Gas Analysis

Another advantage of using MIDEL transformer oils is that they can be tested during operation in order to spot any possible faults before they become serious. One of the ways this is done is through Dissolved Gas Analysis (DGA), where the identification of key gases and rate of gas production can be used to determine the nature and severity of any problems with your transformer. Using this form of transformer oil testing allows plenty of time to correct the problem and make any necessary repairs, potentially saving you a lot of time and money. This gives MIDEL 7131 and MIDEL eN fluids a clear advantage over alternatives such as dry-type transformers.