Power Distribution and Management

Thinking Beyond First Costs When Specifying Cast Resin Transformers

When it comes to specifying cast resin transformers the capital expenditure, or CAPEX, is often the primary – or only – consideration. However, for owners, the operational expenditures (OPEX) over time can have a more significant financial impact. In fact, long-term OPEX impacts related to equipment losses, insulation levels and other factors can dwarf the initial investment over a transformer’s lifespan.

The recent white paper, “How to Specify Cast Resin Transformers for Commercial and Industrial Buildings,” goes into detail on the full range of factors engineers should consider when designing and specifying such equipment. This post touches on the key parameters affecting a transformer’s cost.

Specify Transformer White PaperRated power
Transformers can normally operate for about 20 years under a continuous load corresponding to rated power, and at an average daily ambient temperature of 20˚C. If the chosen rated power is too low (high nominal transformer load), safety problems can arise, and if rated power is too high (low nominal transformer load), both CAPEX and OPEX will be suboptimal. All known current and future parameters regarding the connected load need to be considered when determining rated-power requirements.

Level of Losses
A transformer’s total cost of ownership (TCO) is dependent on its no-load losses and load losses, and on the load factor. These parameters are especially significant where electricity costs are high – Figure 1 illustrates how high European prices can be, as an example. As a result, designers and specifiers should explore the following parameters for every project to better understand how their selection could affect their client’s TCO:

  • Electricity rates
  • Interest rate on any borrowed funds
  • Transformer average load factor (considering that load losses vary with the square of this value)
  • Expected transformer lifespan
Price of Electricity

Figure 1-Cost per kWh in EU countries. Source: eurostat

Insulation level
Service continuity and safety for transformers certainly depend on specifying an appropriate level of insulation – dielectric failures for overvoltage, material failure or an unanticipated event can bring service to an abrupt halt. To maintain the highest level of service, specifiers should request a series of evaluations, including induced tests at 2,5Un, lighting impulse testing on an insulation sample, and special partial discharge testing at the nominal temperature rise.

Degree of Protection
This factor relates to the design of a transformer’s enclosure and is a critical consideration for the safety of facility personnel – even in resin-based designs, it is deadly to touch a dry-type transformer. Designers can choose from a range of standard degrees of International Protection (IP). IP31 is often a good compromise rating, as it provides a good level of protection while still allowing adequate air circulation around the core and transformer winding.

Environmental conditions, fire behavior and seismic withstand
A range of testing should be performed to ensure a transformer will perform well, given specific conditions presented by its installed location. These could include exposure to high humidity and temperatures, as well as the potential for seismic activity. Additionally, because cast resin transformers are dedicated for use in buildings where fire risk is unacceptable, specifiers should require F1 transformers with adequate certification.

Conformity to standards
Standards help ensure both safety and service life, so specifiers need to be familiar with the applicable tests used to determine standards compliance. Additionally, specifiers should request all related documentation to prove compliance has been met.

Maintenance and reliability
Cast resin transformers are low-maintenance equipment, requiring only regular cleaning if they are exposed to dust. They can be equipped with thermal sensors, however, which can keep operators aware of coil temperatures through local or remote monitoring/alarming. A new concept under development also allows measurement at several other points to help detect aging contacts, enabling predictive and proactive maintenance to further improve service continuity and optimize OPEX.

This is certainly not an exhaustive list of parameters specifiers should consider in their selection of cast resin transformers. The paper covers a much broader range of issues and implications in the white paper. For more information, you can download the white paper here. And you can also visit our Consulting Engineer Portal for more resources and to connect with our engineering specialists.


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