Dry type transformer technologies
Transformers can be classified based on their insulation system in two major groups: liquid filled and dry type. Liquid filled (immersed) use either mineral oil or some alternative type of insulation liquid (silicon oil or esters). Dry type technology also has several sub-categories. The terms might differ based on manufacturer and the commercial name of their product. Nevertheless, there are three basic dry transformer technologies. Let’s look at them in this article.
There are sometimes heavy discussions about what technology is more suitable for particular application. We try to avoid giving too much personal opinion (just a little different perspective) and instead make a brief description of each technology including some pros and cons. We would like to stress again that the quality and reliability of the transformer does not depend on the insulation technology only. In fact, there are many factors. Having a failed transformer with specific technology does not disqualify that technology.
Vacuum pressure impregnated (VPI) and vacuum pressure encapsulated (VPE)
VPI transformers feature a single coat of polyester varnish. VPE transformers use a multiple coating of silicon resin. Power range of these transformers goes up to 20 – 25 MVA. Insulation class is H (180°C) or C (220°C). Primary voltage reaches up to 36 kV. This technology usually uses UL listed (UL recognized) materials with 220°C class NOMEX.
Vacuum cast coil (VCC)
Vacuum cast coil, also called vacuum cast resin or just cast resin have high voltage windings cast in epoxy and cured (dried) in vacuum. Casting can principally be done horizontally or vertically. Maximum power reaches 30 – 40 MVA. Primary voltage ranges up to 52 kV. Insulation class is F (155°C) or H (180°C).
Resin encapsulated
This insulation applied for high voltage windings combines epoxy resin and fibre glass. The cross wound glass fibre insulation is soaked in epoxy and cured in open atmosphere. Maximum power goes up to roughly 40 MVA. Primary voltage goes up to 52 kV. Some manufacturers offer this type of insulation for up to 72.5 kV class.
All above technologies are available as IEC or ANSI/IEEE transformer design. Their commonalities are environmental friendliness and high level of safety (particularly fire hazard).
Remark on power and voltage rating
Note that above power and voltage ratings are typical seen on the market. The values are subject to change due to continuous product development. It is technologically possible to design and build a dry type transformer with over 100 kV primary voltage. However, unless strictly requested, liquid filled transformer would normally be preferred. Also note that maximum power rating of dry distribution transformer does not mean that one can build multi-winding VFD transformer for the same power.
What dry transformer technology is most suitable for VFD application?
The answer is simple: All mentioned dry transformer technologies are suitable for VFD duty transformers. Depending on installation, power and voltage rating, environment and other factors one technology fits a bit better than the other one.
For example, for integrated transformers VPI technology is practically exclusively used. The integrated transformers normally go up to 11 kV primary input voltage (some products support 13.8 kV input as well). There VPI is a great choice. Moreover, VPI transformer in given voltage and power class features smallest volume, resp. footprint which is of course very important for the integration into the VFD.
For external transformers all three technologies are suitable. In case of higher primary voltage the VCC starts having advantages over VPI.
Resin encapsulation claims to be robust in environment with higher vibrations (either during transport or in operation – e.g. rolling stock) and ambient with extremely low temperatures. VPI on the other hand is excellent for applications with cyclic loading and fluctuating temperature rise (convenient expansion coefficients of material).
References
[1] VFD transformers – Introduction, https://mb-drive-services.com/vfd-transformers-introduction/
[2] Integrated versus external transformer, https://mb-drive-services.com/integrated-versus-external-transformer/
[3] Trasfor – customized dry transformers and reactors, http://trasfor.com/
[4] ABB dry type transformers, https://new.abb.com/products/transformers/dry-type
