Combined transformer
Our recent post [1] walked the reader through the two main VFD transformer options: integrated transformer and external transformer. Today we will introduce a solution that aims to leverage on advantages of both solutions: combined transformer. The name might be a bit misleading. We can also call it semi-integrated transformer if you like.
Before going deeper let’s quickly recap the pros and cons of classic solutions:
Integrated transformer allows quick and easy installation. Customer basically just connects three cables to the input terminals and three cables to the motor. The protection functions such as thermal monitoring or overcurrent protection are typically included. The transformer is of dry type and located in the VFD enclosure. The design is usually standardized. The cooling is air forced resulting in a compact design. Possible drawbacks are increased footprint of the VFD (more space in electric room), additional heat losses (larger air conditioning), limitation of input voltage range (11 or 13.8 kV) and possibilities to customize.
External transformer provides full flexibility. The technology can be either dry type of liquid filled (mineral oil or synthetic liquids). There is virtually no limitation of input voltage or power rating. High level of customization is possible. The location can be indoor or outdoor. The external transformer requires more project engineering to ensure appropriate system integration. There might be larger amount of cables between transformer and VFD depending on the rectifier pulse number / number of transformer windings. The protection shall be reviewed separately.
Click on picture below if you wish to read the full article on integrated versus external transformer.
In the summary of [1] we stated our opinion that VFD with integrated transformer is very elegant in lower power range due to its simplicity, quick installation and minimum increase of VFD dimensions. In higher power range the external transformer appears a better choice since larger integrated transformer produce considerable losses and have more significant impact on dimensions. It is illustrated in figure 3. The VFD on the left side is capable of up to approx. 2 MVA and dimensions are marginally larger than the same VFD without transformer. VFD on the right hand side is rated up to 7 MVA. The transformer is large enough to have its own dedicated cabinet.
Combined transformer
There is a transformer variant combining some benefits of integrated and external transformers. We call it combined transformer or semi-integrated transformer. What is it and what are the main features?
The concept is based on dry type water-cooled transformer. In this way the heat loss is transferred into the cooling water and does not heat up the building. In turn the demand for air conditioning is drastically reduced. The water circuit is common for the VFD and transformer. The combined cooling gave the name to this transformer solution. The location of the transformer is nearby the VFD, but it is not necessarily directly attached. It can be located for instance beneath the VFD (one floor lower). This flexibility is much appreciated in case of space limitation.
Technical data in a nutshell
– Power rating up to 18 MVA (higher on request)
– Input voltage up to 13.8 kV (higher on request)
– Dry type technology with cast resin insulation
– Windings of aluminum or copper
– Water cooling with water to air heat exchanger (AFWF)
– Enclosure with high ingress protection (IP54 or NEMA 3R)
Options:
– Surge arresters
– Transformer protection relay
– Outdoor installation
– Offshore installation
– Special coating
There is still the limitation on the input voltage level, at least when the transformer is located indoors. In case of outdoor installation the level could be increased to 20 kV or even 33 kV. Beyond that level the dry type technology gets very costly and oil filled transformer might be the preferred choice.
Comparison
In the end you are probably most interested what variant of the input transformer is the best. Hopefully we don’t disappoint you but there is no universal answer. It depends on your project specific needs and restrictions. We have stated our general opinion (integrated transformers tendentially preferred for lower power while external transformers more suitable in higher power range). Below table benchmarks the three variants integrated, external and combined (semi-integrated) transformer.
Table 1: Comparison of integrated, semi-integrated (combined) and external transformer
As you can see, the combined transformer solution is a good compromise in many aspects. Of course, not everyone likes compromises. The solution will not be the first choice in every project. However, it is an interesting alternative to the basic two solutions.
References
[1] Integrated versus external transformer, https://mb-drive-services.com/integrated-versus-external-transformer/
[2] Trasfor – Custom built dry transformers, http://www.trasfor.com/
[3] Product portfolio of medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
