Since oil does not come in contact with external air, there is hardly any deterioration of oil over a period of time and hence such transformers are virtually maintenance free... - Mukesh D Mulchandani

While purchasing equipment Total Ownership Cost (TOC) plays a very important role. TOC includes not only capital and operating cost but also indirect cost towards maintenance & upkeep of equipment throughout its operating life for trouble free performance. Maintenance of equipment, upkeep, its servicing, procuring and maintaining spares and related overheads increases TOC. Hence while deciding purchase of equipment this cost needs to be evaluated and factored in to arrive at optimum TOC. In any running plant this is a big challenge. Equipment and gadgets which require no maintenance or minimum maintenance and where TOC is lower are always preferred.

Hence the concept of zero maintenance or minimum maintenance is fast catching up. This trend is growing and is quite evident as we already have products such as batteries which are practically maintenance free; LED lights due to its long operational life are virtually maintenance free. The success of maintenance free product is because of the concept of “FIT IT & FORGET IT”. Customers are willing to pay premium if there are no headaches and also operating cost towards maintenance of assets is lower. Technological advancement had made this possible for transformer.


The question which naturally arises is what maintenance free transformer is & what is the need for it? Conventional vacuum impregnated or cast resin dry type transformer is also maintenance free! But these transformers have limitations that these are not intended for installation in outdoor atmosphere which are directly exposed to heat, sun, rain, humidity, dust, corrosive and harsh environment. At the most such dry type transformers can be installed in covered location either in basement of building, inside substation or under a covered roof.

Similarly conventional oil filled transformer though can be installed in outdoor atmosphere directly exposed to sky, but suffer from the limitation that these are not maintenance free. Oil leakage from joints, valves, radiators, conservator etc. is quite common. Besides this since these transformers are equipped with breather they inhale and absorb certain amount of moisture. Oil comes into direct contact with the air as a result, oil needs regular filtration to check & ensure that its BDV & dielectric strength is maintained. Is it possible to get best of both worlds? i.e., maintenance free and at the same time it can be installed outdoor (open to sky)!. Yes, hermetically sealed maintenance free transformer is the solution to this. The next question that arises is: what hermetically sealed means and in what way it is different from conventional oil filled transformer?

Transformer installed outdoor…

Hermetically sealed transformer in appearance wise does not differ much from conventional one. However its design and construction is different.


The construction of tank for such transformer is extremely simple. The bottom, corrugated fins side wall panels and plain wall panels are fabricated separately and mounted on especially designed welding tables and welded together. This forms the tank structure. The skill and workmanship of highly experienced and qualified welders and leak tests during production ensure that the finished tank is leak free. The reliability of tank design is proven by stringent fatigue and over pressure tests. These tanks are also subjected to expansion and contraction cycle which simulate the mechanical load on a tank over its service life. The tank design and corrugated fins takes care of expansion and compression cycle due to variation in temperature of oil. Hence, such transformers are not equipped with conservator and breather. Corrugated fins increase the surface area and take care of dissipation of heat. Corrugated fins once welded become integral part of tank structure.

Once the active part (Core and winding assembly) is dried for a specific time in an oven to remove moisture from the insulating material, it is placed inside the tank. Oil is separately pretreated in a vacuum chamber which involves filtration, drying and degassing to remove moisture and impurities. The transformer is placed in a vacuum chamber and filled with pre-treated oil under full vacuum condition.

This ensures complete impregnation of the insulating material by oil, for maximum dielectric strength. The core is completely immersed in oil and the space between top of oil and tank cover is without any gas cushion, thus preventing built-up of pressure and reducing chances of explosion. The top cover is bolted and assembly is completely hermetically sealed. After a specific stabilization period the transformer is subjected to various tests as per codes and standards.
Since oil does not come in contact with external air, there is hardly any deterioration of oil over a period of time and hence such transformers are virtually maintenance free.

Oil filling in totally controlled atmosphere in a vacuum condition…


The table on the previous page analyses and evaluates conventional oil filled transformer vis a vis hermetically sealed oil filled transformer.

Delivery period

Delivery period depends upon the rating and can vary from typically 6 to 12 weeks, which is much lesser than the conventional oil filled transformers.


Such transformers can find application in industry and installation which are exposed to dust, rain, humidity, moisture & harsh in nature. Typical example can be Utility & distribution companies where transformers are installed outside, by the road side in cities, pole mounted transformer in case of rural electricity distribution, distribution transformer of higher ratings in case of process, chemical, cement & steel plants.


In our country where failures of distribution transformer are quite alarming and where skilled workforce is required to maintain assets the Total Ownership Cost (TOC) is very high. It is prudent that we take into account the technological advancement made globally and introduce, adapt & promote new technology. With the advantages of hermetically sealed maintenance free transformer mentioned above, I am sure industry will wake up to the ground reality and encourage installation of hermetically sealed transformer which are not only energy efficient and have low losses but also makes optimum use of material.

As we are embarking upon our mission of creating SMART CITIES to address the serious issue of urbanization & infrastructure, these transformers are best suited for such applications.

If you want to share thoughts or feedback on this article, please leave a comment below.


  1. Hi
    would you please quote for 500 piece of oil immersed hemetically sealed out door transformer as in attached file

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