MechChem Africa talks to David Claassen, Managing Director of Trafo Power Solutions, about the cost, reliability and safety advantages of using dry-type/cast-resin transformers, particularly in hazardous area applications such as underground mines.
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In principle, the primary function of dry-type power transformers is identical to traditional oil-filled transformers, begins David Claassen of Trafo Power Solutions. “Both are used to step up or to step down an incoming AC voltage to match the voltage and power rating requirements of an application. The primary difference is that oil-filled transformers use oil as an insulation and cooling medium, whereas dry-type transformers use only air,” he says.
Both types have a core, low voltage windings and medium voltage windings, with the turns ratio between these two windings governing the output voltage. But from a construction perspective, the windings are a combination of VPI (Vacuum Pressure Impregnation) and Cast Resin.
“Each individual transformer is purpose designed for optimal cooling around the transformer, either using natural ventilation or by adding forced cooling fans when required," Claassen explains. "In every case, we will calculate the precise cooling requirements of a transformer based on the ambient air in the operating environment so that every transformer we supply will operate safely and reliably.”
And while there is a limitation on the use of dry-type transformers for very large power transformers – very few dry-type manufacturers go above 20 MVA – for all applications around a sweet spot of 10 MVA – which includes most data-centres, shopping centres, municipal distribution networks, motor control centres (MCCs) along with industrial power distribution centres for mines and oil and gas, chemical and petrochemical plants – dry-type/resin-cast transformers are fast becoming the preferred option.
“Globally, our Trafo distribution range of transformers – 66 kV in South Africa and 52 kV in Europe – make up our highest market volumes and it has become a no-brainer to adopt this technology in preference to oil-filled technology,” he notes.
“We at Trafo Power Solutions focus on customising the design of our transformer to best match application and environmental requirements. So we don’t supply standard products that work for everything. There are always differences that need to be taken into account, depending on the load cycle and the ambient conditions, for example. If the transformer is being used in an industrial application, which typically has a lot of variable speed drives that cause high non-linear loading on the transformer, the design needs to reflect that.
“For a PV-plant, on the other hand, which will operate at 100% during the hot and sunny hours of the day but will be under no-load conditions during the cooler nights, different contraction and expansion considerations need to be taken into account. In a nutshell, we can engineer a dry-type transformer for any applications where an oil-filled transformer is used, and these solutions will offer significant advantages over the traditional technology,” Claassen tells MechChem Africa.
Going back to the construction of the dry type transformers, he says that the medium voltage winding is typically sealed in a cast resin block, which gives the transformer a very high withstand capability against humidity and pollutants. During manufacture, the winding is first wrapped with fibreglass meshing. When the resin is poured in, it binds into the meshing, which acts like rebar in reinforced concrete to create a very strong and robust containment structure.
“Under short circuit conditions there is a requirement for the transformer to withstand the shock-loading caused by very high levels of current. On a traditional transformer, short-circuit shock generates movement in the windings that can result in voltage flash. Dry-type transformer construction increases robustness under these conditions and they are able to withstand a high level of short-circuit current,” he explains.
As a result, Trafo Power’s dry-type transformers have class F1 Fire Protection certification, which means that under short-circuit or external fire conditions, the transformer will neither cause nor exacerbate a fire. All the materials and resins used are flame retardant and no harmful gases will be produced.
“Because of poor oil containment on an oil-filled transformer, a relatively small fire in the switchgear can often ignite the transformer oil, which then makes that fire bigger and bigger. Using a dry-type transformer completely overcomes this problem,” he notes.
Describing a recent success where customisation played a critical role, Claassen tells of an installation at an underground goldmine to supply power to a refrigeration plant via MCCs. “We supplied three transformers for the applications, a 250 kVA, a 630 kVA and a larger 3 150 kVA unit. These are all going underground and, due to the dimensional and weight constraints of the cage and the access limitations through the underground decline shaft, our standard 3 150 kV transformer geometry wouldn't fit into the cage from a height or width perspective. So we've custom-designed and manufactured a transformer with a unique geometry and with a purpose-built skid base so we can get it down the mine and slide it along the decline shaft,” Claassen explains.
Compared to an oil-filled design, this was much more easily achieved. More importantly though, an oil-filled transformer would have been a fire hazard and, while they are not yet illegal in our mines, from an insurance perspective an oil-filled transformer can cost more than double to insure. Also, operators need to comply with a whole lot of extra fire suppression, detection and other requirements.
“This is an excellent example of how we prefer to operate, where clients comes to us with their power requirements and voltages, along with their access or site constraints and their specific ambient conditions. We then customise a solution to best suit all of these conflicting needs,” says Claassen.
The cost advantage
In comparing the costs of dry-type versus oil-filled transformers, Claassen says, because they were less common, dry-type transformers used to be much more costly than traditional oil-filled equivalents. This has changed dramatically in recent years and, while the upfront investment in a typical dry-type transformer solution might still be slightly more expensive, this premium is quickly swallowed up by the additional infrastructure costs of isolating an oil-filled transformer in its own building or in a purpose-built bay with a bund wall.
In addition, maintenance requirements and costs for dry-type transformers are significantly lower. “On dry-type transformers there's very little maintenance and service intervals are much longer. The only things that we tend to look at are the terminations and cabling, possibly once every year or so, and to use a leaf blower to clear any build-up of dust that might be affecting the cooling.
Oil-filled transformers require repairs of oil leaks and regular oil sampling to monitor gas and moisture build-up and oil degradation – and this is essential to minimise fire risks. Fire detection and onsite fire suppression equipment is also usually required. So the additional costs of the ‘cheaper’ oil-filled unit are almost always far higher than the slightly increased purchase price of a dry-type transformer.
“In some cases there seems to be a disconnect between keeping the project outlay low and minimising running costs and investment returns,” Claassen argues. “In the mining industry and in data centres, however, where end users are a lot more involved in the project development process, dry-type transformers have proved to be a safer, better and much more cost-effective choice.
“For anyone charged with the responsibility of keeping a facility’s equipment operating safely, reliably and cost effectively for the next 30 years, it makes a lot of sense to get involved in the discussions with the project developers at an early stage so that their future ongoing operations become easier to manage.
“And dry-type cast resin transformers, custom-built to suit specific onsite needs and conditions, make total sense in that regard,” he concludes.
