Transformers specifically designed for renewable energy plants

An increasing demand for the use of dry-type transformers in renewable energy plants has been noted as a significant trend.

Dry-type distribution transformers are seen as a more suitable alternative to oil transformers, especially in the distribution power range between 50 kVA and 10 MVA.

Trafo Power Solutions attributes its focus on application engineering as having allowed it to supply customised dry-type transformers for a range of sectors, including renewable energy, and emphasises that it is able to supply dry type transformers up to a power rating of 25 MVA.

The company states that it designs fit-for-purpose transformer solutions tailored to the requirements of each project and that renewable energy applications have specific demands, for example a solar power generating plant requires a transformer to be energised from 0% to 100% on a daily basis.

“This leads to wide fluctuations in temperature inside the transformer which, in turn, causes the resin around the windings to expand and contract considerably,” explains Trafo Power Solutions. The windings in dry-type transformers are insulated in a cast resin or epoxy material, rather than oil, and rely on air movement for cooling.

“Such fluctuations could result in the resin cracking, so the design must specify the appropriate class of insulation to cope with these conditions,” explains the company.

Trafo Power Solutions designs windings for either a Class F or a Class H insulation. In addition,  the mixture of the resin is specially formulated to accommodate the duty cycle specific to renewable energy applications.

The standard design, using Class F, can deal with temperatures up to 155 oC.  In some low voltage and high voltage installations the design includes a resin mix to Class H specifications that gives it the capacity to withstand temperatures of up to 180 oC.

Another aspect of renewable energy applications is the non-linear load that inverters add to the mix. The transformers need to be designed with a K-Factor much higher than used for standard distribution type loads as well as the addition of an electrostatic shield between the primary and secondary windings to eliminate potentially damaging leakage currents.  

With an oil-cooled unit, the temperature fluctuation arising from the rapid and regular energising and de-energising cycles causes more gases to be emitted within the transformer tank, which could lead to a number of problems.

The cast resin material used in Trafo’s dry-type transformers meets the specifications of the International Electrotechnical Commission for fire class F1. The units are therefore said to present minimal fire risk, allowing them to be used indoors safely and without environmental protection like bund walls for potential oil spills.