Pablo Castano-Mariscal challenges some widely held misconceptions about dry-type transformers – in particular that they are only for applications at up to medium-voltage where inherent safety is important
Dry-type transformers use solid dielectric materials to insulate the windings and natural air for cooling. This compares with conventional transformers, which are filled with mineral oil to insulate and cool the windings.
Compared with oil, the solid insulation is non-flammable, non-combustible and has no chance of a leakage. This gives dry-type units the significant advantage of having an inherently low risk of safety and environmental impact.
However, the materials and manufacturing process is more costly for dry-type transformers, leading to a higher purchase price than their oil-filled cousins.
As a result, they have always been reserved for installations where fire and environmental protection are top priorities. This typically includes indoor substations or outdoors near sensitive sites. They are also regularly installed as a retrofit solution to comply with the latest building and environmental codes.
At the same time, high safety and low maintenance requirements can offer savings in civil engineering, fire safety systems and maintenance that offset the higher purchase price. These savings can mean that dry-type competes with oil-filled technology on the basis of total cost of ownership (TCO).
Installation savings can be found with dry-type units as there is no need for primary and secondary oil containment, fire suppression systems and perimeter clearance or fire-rated walls. In addition, transportation and installation are also more straightforward, with no oil handling as dry-type units are shipped fully assembled.
Once in operation, maintenance costs are lower as there is no need for oil monitoring or checks on tanks and radiators.
In addition, because they have no bushings, there is no risk of bushing failure, a common failure mode for oil-filled transformers. Lastly, although catastrophic failures or spills are very rare, their cost is built into insurance.
With the potential for reducing budgets in several areas, it is worth considering dry-type transformers on a case-by-case basis to see whether there is a TCO advantage. Taking account of all the lifetime costs, dry-type transformers can be the most economical choice as well as the safest, with potential savings in the region of 11 to 18%.
SAFETY IN “WORLD’S LARGEST” APPLICATIONS
Dry-type transformers are traditionally used most when safety for people, property and the environment are the top priority.
For example, the world’s tallest building, Dubai’s Burj Khalifa, relies on a distribution substation on the 155th floor that was equipped by ABB. The substation provides power for everything from elevators to lighting and HVAC (heating, ventilation and air conditioning) systems. The substation technology includes 72 dry-type transformers that were specially designed for the tower with a flexible enclosure to fit in a compact space.
The world’s largest iron ore mine, Vale’s Carajás mine in Brazil, also relies on a fleet of ABB’s dry-type transformers. It is equipped with 135 ABB cast coil transformers to step power down to supply power for equipment such as excavators, stackers, reclaimers and conveyors.
ABB delivered the transformers as part of a contract for electrical and automation systems to enable a major expansion of the facility.
RATED UP TO 72KV FOR SUB-TRANSMISSION
There has been steady development in dry-type transformer technology in recent years. This has opened up potential for new applications.
Dry-type transformers are now available at higher voltages than ever before – units are now available with ratings of up to 72kV. This makes dry-type transformers suitable for sub-transmission applications for utilities, heavy industry, offshore oil and gas facilities, and managing power at infrastructure such as airports and stadiums.
One utility operator in north America has replaced an oil-filled 27MVA transformer unit with a 30MVA 69kV HiDry transformer. By switching to dry-type unit, the operator eliminated the need for a fire suppression system and removed risk of oil leakage into a nearby waterway.
These are completely free from insulating oil, including the on-load tap changer (OLTC), which is used to vary the voltage output.
TVRT FOR SWITCHING RESISTANCE
In addition, ABB has introduced a new range of dry-type transformers that are designed specifically for applications that experience frequent switching (once per month or more often).
The Transient Voltage Resistant Transformer (TVRT) has potential applications in heavy industry, data centres, traction infrastructure or to supply power for multi-pulse rectifiers. It also has potential to connect solar photovoltaic installations, which can experience sudden drops and peaks in output as clouds pass.
SEALED FOR SUBMERSIBLE APPLICATIONS
Another potential application for dry-type transformers is that they can be supplied in hermetically sealed tanks for harsh environments, for example urban networks that may be vulnerable to flooding, offshore platforms, mines and chemical processing plants that might have a corrosive environment.
One such transformer beneath the streets of Manhattan in New York has survived hurricanes Sandy and Irina and continued to function in spite of being submerged under 20ft of water. The tank is over-pressured with nitrogen and equipped with sensors to detect any loss of pressure that might indicate tank corrosion.
Pablo Castano-Mariscal is with ABB