Transformer design and cost drivers
Right now the transformer prices are just sky rocketing. So talking about transformer cost drivers may be like bringing wood to the forest. However, we hope that the world turns normal again. Then, in normal days, there are specific design features and accessories or options that impact the price of the transformer. We mostly call them cost drivers. This post introduces several categories of potential cost drivers.
Input voltage
Dry type transformers are generally more cost sensitive to primary voltage level (limited insulation withstand). You can already note a diference between 6.0/6.6 kV and 10/11 kV class.
Liquid immersed transformers can be classified into classes e.g. 24 kV, 36 kV, 52 kV, 72 kV or 123 kV (below 24 kV there is hardly any difference in cost).
High voltage increases the physical dimensions of the transformer and also requires more expensive components (e.g. OIP or RIP bushings).
Cooling type
Transformer with lower power rating often use natural cooling (AN for dry type, ONAN and KNAN for liquid immersed type). Higher power rating leads to a higher power density and more demanding cooling. A forced cooling (e.g. AF, ONAF) may be preferred. Realizing large transformer with natural cooling results in higher cost and larger dimensions and weight.
Dry type transformers with double circuit and a heat exchanger (AFAF and AFWF cooling) lead to a more complex enclosure and higher cost.
Short circuit impedance
An optimum value or range of short circuit impedance shall be used for cost-optimized design. Both extremes (very low and very high short circuit impedance) have a cost impact. Those extremes are:
- “short and fat design”
- “tall and slim design”
See [1] for further details.
Insulating liquid
For liquid immersed transformers the type of insulation liquid may be a cost driver. Besides conventional mineral oil of standard (normal) grade a higher grade of mineral oil (e.g. super grade) may be specified. Such mineral oil features better oxidation stability but of course costs more as well.
“Less flammable” liquids are sometimes used to reduce the fire hazard. Those liquids are more expensive than classical mineral oil. Less flammable liquids are either based on synthetic esters or natural esters (→ biodegradable [2]).
Design temperature rise
Some customers may require reduced temperature rise in order to extend transformer lifetime. For example, dry transformer with insulation class H might be designed for temperature rise of class F or even B. That is of course a cost driver and leads to additional design margins and sort of overdimensioning.
Presence of DC component
Magnitude of DC current component affects the design of the magnetic core [3]. To avoid saturation, the cross section of magnetic core must be increased and for larger DC component an air gap may be required.
Tertiary winding
Need for a tertiary winding (e.g. to connect a filter bank or to supply an auxiliary power) complicates the design and increases the cost. The commercial impact depends on the rating of the winding and overall transformer topology.
Winding material
In low and medium power range both copper and aluminum are extensively used as conductors. Copper based transformers are more expensive. As the power goes further up the cost advantage becomes marginal. High power transformers normally use only copper windings.
Extreme ambient conditions
Very cold or very hot ambient impact the transformer design and associated cost. An aggressive/corrosive environment requires more expensive painting system.
Transformer accessories
Certain options and accessories may also be considered as cost drivers. Examples of those are:
- Tap changer (de-energized and especially on-load tap changer)
- Plug-in connectors (e.g. Pfisterer) instead of conventional bushings
- Main terminal boxes
Monitoring and protection devices
Specific (unusual) devices for monitoring and protection may be significant cost drivers. Examples are:
- online PD monitoring
- differential protection of multi-winding transformer
References
[1] VFD transformers – Impedance requirements, https://mb-drive-services.com/transformer-short-circuit-impedance/
[2] Transformer bio-based insulating liquids, https://mb-drive-services.com/transformer-bio-based-insulating-liquids/
[3] DC component in VFD transformers, https://mb-drive-services.com/dc-component-in-vfd-transformers/
[4] Hitachi Energy (former ABB Power Grids), https://www.hitachienergy.com/ch/de/offering/product-and-system/transformers/power-transformers
[5] Trasfor – Engineered dry type transformers, http://www.trasfor.ch/
