Dry-type and oil-cooled units: Choosing Smart city transformers

An industry analysis on how to choose between dry-type and oil-cooled transformers and which one would be widely used in smart cities and its standards defined for the safety and reliability.

Transformers are common and useful devices which take high voltage electricity directly from a power station and convert it to a lower voltage. This allows the energy to be safely and efficiently used by machinery and appliances that can only handle low voltage in spaces such as offices, transportation hubs, schools and factories. Through this process, transformers generate a lot of heat that must be dissipated to keep them running safely. There are two types of transformers being used in the industry currently: dry-type transformers and oil-cooled transformers depending on its heat dissipation method.

Dry-type and oil-cooled transformers
Hazardous risks to humans and the environment exist during handling, maintenance and failure of oil-filled transformers aided by oil fires and spills due to its low flash point of 170° Celsius. Tarun Garg, Head of Engineering and Digitalisation, Transformers, ABB India Limited explains, “Use of dry-type transformers addresses this risk due to safe failure modes associated with flame-resistant solid insulation employed in dry-type transformers.”

On the other hand, mineral oil has favourable cooling properties required for transformers.Dry-type transformers rely mostly on ambient air for cooling. Reduced cooling capacity of dry-type transformers also limits the economically achievable power and voltage ratings for a specified application. Though the dielectric capabilities of solid insulation are comparable to that of oil, the later offers much better performance for cooling as compared to ambient air. This is also the reason that for the same rating of transformer, the size and foot print of oil-filled transformer are smaller than corresponding dry-type unit.

Garg assures, “For smart city applications with requirements of smaller ratings (< 3 MVA), dry-type units are recommended due to hazardous aspects of mineral oil; however as power demand goes up, liquid-filled units offer compactness and better noise performance than dry-type transformers and therefore, for such cases, better alternative could be ester oil filled transformers, which addresses most fire safety concerns.”

Dry-type transformers are safe in terms of oil spillage during any unlikely situation where hot oil may come out of transformer tank in a forceful manner and damage things in the immediate vicinity. Generally shopping mall, commercial establishments, schools, hospitals, public office buildings and parking space are examples of areas where if a transformer is to be located, it is better to select dry-type designs. Smart cities of course need such discipline to establish fire safe installations. However, the size remains restricted up to 1600 kVa.

Safer product is compact sub-station (CSS) which not only encloses the transformer in a smart enclosure but takes care of the complete terminations which otherwise remains exposed and can lead to accidents. Sanjib Mitra, Country Head Sales & Marketing, Electrotherm India Limited, suggests CSS also takes care of the HT and LT switchgear and bring them under same roof in a smart metallic enclosure having metallic partition wall separating each compartment and providing smart entry door. It also includes proper system earthing and fire safety devices inside.

Safety and reliability
IS 11171 and IEC 60076 have nicely defined the design and manufacturing standards of dry-type transformers. From construction point of view, the basic difference remains that it does not have dielectric media like transformer oil in which the active part (core coil assembly) is immerged within. Mitra explains, “The cooling in a dry-type transformer is completely dependent on air flow. It has been observed that if installed in a suffocating atmosphere, dry-type transformers temperature goes up rapidly which sometime leads to coil burning and arcing. So enough natural air flow is required for such stations.”
Forced air flow is advisable to make it further safe. The most important parameter in dry-type transformer design remains the selection of paper insulation which is used for conductor covering. Dry casting under vacuum is also critical safety factor because any remaining pore may establish line up phenomenon with time and thus may lead to failure of coil and fire.
According to Garg, “Apart from electromechanical aspects, surrounding protective devices such as fast-acting fire suppression systems and oil containment structures help reduce fire hazard potential in oil-filled transformers to some extent.” The Indian Electricity Rules 1956 amended Nov 2000 mandated use of dry-type transformers in all indoor applications. However, the rule only states ‘dry-type’ and does not define ‘how fire safe’.

Insulating fluid with cellulose insulation has been the popular insulation system for transformers, successfully used for more than a century
Tarun Garg, Head of Engineering and Digitalisation, Transformers, ABB India Limited

The most important parameter in dry-type transformer design remains the selection of paper insulation which is used for conductor covering
Sanjib Mitra, Country Head Sales & Marketing, Electrotherm India Limited