Selecting and sizing transformers to achieve energy efficiency

Electrical engineers must know when and where to specify dry-type transformers. Energy efficiency guides play a vital role in this process.

By Matt Zega, PE, RTM Engineering Consultants, Chicago June 19, 2017

Learning objectives

  • Learn to select and specify dry-type power transformers.
  • Apply knowledge of cost and payback of transformers to their selection and specification.
  • Outline all codes, standards, and regulations for power transformers.

Transformers are integral components of any power distribution system. As a consulting engineer, selecting and specifying the proper transformer for the operation is an essential part of electrical system design. The focus of this article is on dry-type power distribution transformers and how to select the right transformer for the job.

There are many things to consider when specifying a dry-type power distribution transformer (see Figure 1). Weighing each of the options to select the right transformer for the job can be a balancing act. The efficiency of a transformer is a critical design component, and understanding the physics surrounding efficiency is important. Key factors to consider that are related to efficiency include transformer loading, the presence of system harmonics, K-factor, and temperature rise. Another major movement in the world of transformer efficiency centers on the U.S. Department of Energy (DOE) federal efficiency regulations. As regulations become stricter, and as manufacturers adopt and adapt to these new regulations, engineers should explore how transformers are evolving to keep up with these standards. Lastly, the payback and upfront cost of transformers are aspects a consulting engineer should review when selecting a more energy-efficient transformer.

Transformer build and efficiency

When considering a transformer selection, it’s important to understand exactly how transformers work and what contributes to their efficiency. This is the first component of a quality product specification. When you understand the physics behind how a transformer operates and what makes it efficient, this gives you the ability to constructively review the product specification and compare it against others. In this day and age, manufacturers are already up against rigid federal guidelines in terms of producing efficient transformers. Understanding the basic principles behind transformer build and operation is a fundamental building block for making the correct transformer selection.

A sign of inefficiency when it comes to transformer operation is the presence of excess heat. Any heat generated by a transformer is a direct result of inefficiency and losses within the transformer. Some of the key contributing factors to transformer inefficiency are conductor and core losses.

Conductor losses are attributed to lack of winding efficiency. The windings are typically made up of either copper or aluminum. The construction material is certainly a consideration, as it relates to conductor losses. While copper conductors have better current-carrying properties, aluminum conductors can closely match the current-carrying properties of copper when they are sized properly. The weight of copper as compared with aluminum can be more than three times greater, but the weight of the copper is offset by its current-carrying abilities. When comparing the two, strictly on a weight basis, aluminum has better current-carrying capacity than copper. This is an important consideration when it comes to transformer selection. Most manufacturers offer transformers with copper or aluminum windings, providing the same efficiency ratings, while the copper-wound transformers can be more expensive given the higher comparative cost of copper.

The losses attributed to winding resistance also are critical components to the efficiency of a transformer. While no conductor is ever 100% efficient, properly sized winding conductors play a huge role in transformer efficiency. Manufacturers can perform testing as well as properly size the conductors based on and around a transformer specification, but it is also the role of the consulting engineer to understand these specifications and how the transformer was constructed and designed to perform under certain conditions.

When it comes to core losses, the main factors are flux leakage, eddy currents, and excitation. Flux leakage is the magnetic flux created by the primary transformer winding that does not pass over to the secondary winding. Transformer manufacturers work to limit the amount of flux leakage in their transformer designs. The winding and core manufacturing processes can play a role in limiting flux leakage.

Eddy currents are generated from changing magnetic fields. These circulating currents are wasted energy and generate heat at the transformer core. Core losses also can be generated through hysteresis. When the transformer core undergoes the changing electrical waveform, losses are experienced due to the change in electrical polarization (see Figure 2).

Excitation current is the amount of current required to excite the transformer and create current flow from the primary winding to the secondary winding. This can be referred to as no-load losses, as this is the amount of current it takes to magnetize and energize the core creating the flow of current. The excitation current experienced at no load also is the same amount of excitation current experienced at full load.

As a final thought, in summarizing the physics behind transformer efficiency, it should be noted that the optimal performance and efficiency of a transformer occurs when the conductor and core losses are equal.

Transformer K-factor

Another factor to consider in relation to transformer efficiency is the level of potential harmonics on the electrical system. Harmonics that are generated by nonlinear loads can contribute to efficiency losses in transformers. Harmonics in an electrical system also can diminish the standard life expectancy of a transformer. Implementing a K-rated transformer can help improve the efficiency of a system and also prolong the useful life of the transformer. Knowing and understanding the K-factor rating is important in selecting the proper transformer.

What precipitates the need to select a transformer with K-factor is the presence of harmonics in the electrical system. The use of solid-state electronics is a major cause of harmonics. It’s important to note that while intermittent motor loads also can introduce harmonics into an electrical system, it is solid-state switching electronics and variable-speed drives that introduce the most prevalent and damaging harmonics. Knowing what causes harmonics can help you to not only properly select a transformer, but also to compare other components of the electrical system, such as variable speed drive selection.

Another important concept to recognize is that a K-factor transformer does not reduce harmonics or filter them out. K-factor transformers are built to withstand the heat buildup that harmonics can create, ultimately making the voltage-transformation operation more efficient.

A K-factor of one (K-1) provides no harmonic mitigation and is designed for relatively linear loading. As the K-factor of a transformer increases, the better the transformer is built to withstand nonlinear loading and related harmonics in the electrical system. K-factor transformers come in various ratings, typically from K-1 all the way up to K-50. Typical manufactured K-factor ratings are K-1, K-4, K-9, K-13, K-20, K-30, K-40, and K-50 (see Table 1). Although not explored in detail within this article, upsizing of the neutral conductor should be considered as nonlinear loading increases.

Transformer K-factor and harmonic analysis can be complicated, and it is not the intent of this article to go into detailed calculations. K-factor calculations can be extremely useful in determining the exact rating for transformer selection. In addition to being familiar with K-factor calculations, what can be even more helpful is to measure power distribution systems within existing buildings so that you can obtain an exact sampling of the harmonic content. You can obtain your own testing equipment, such as a handheld oscilloscope/power harmonic analyzer. You will need to work with a certified electrician and follow all OSHA and arc flash regulations. Most electricians will have their own testing equipment, but having one at your disposal that you can share with an electrician, gives you the added benefit of device familiarity and taking the digital content and images back to your office for further analysis.

Transformer federal regulations

The DOE has been leading the charge in increasing the efficiency of transformers, so much so that Energy Star ratings of transformers no longer exist as of 2007, when the DOE first published energy standards that exceeded Energy Star. With the increased regulations and required energy efficiency, manufacturers have been rethinking the design of transformers to meet the ever-demanding regulations on transformer efficiency.

One of the biggest changes that transformer manufacturers have adapted in the past 10 years is the loading design criteria. In the past, manufacturers have designed transformers to operate in the 80% to 100% load range. Following DOE changes, manufacturers are now required to design their transformers at 35% of the nameplate-rated load. This not only has changed how manufacturers approach their transformer construction and design, but it also alters the methodology on how consulting engineers approach the sizing and selection of transformers.

Table 2 shows the DOE federally mandated transformer-efficiency standards. These federally mandated standards are for transformers tested at a 35%-rated load. The table also illustrates that as the transformer grows in size, the efficiency increases.

As consulting engineers, we should consider the maximum connected load as well as the code-allowable demand load and evaluate how close the demand is to the desired 35% target. It is important to note that with these rigid efficiency regulations, options can be limited in terms of selecting more efficient transformers than what federal regulations mandate.

Transformer cost and payback

As a consulting engineer, there is a tremendous benefit to reviewing transformer cost with the owner and builder. With transformer efficiency increasing, there is great opportunity to save on electricity usage through energy-efficient equipment. It’s paramount that the consulting engineer understands the cost implications on the front end and can convey the payback analysis on the back end.

Transformer cost analysis needs to be reviewed on a case-by-case basis. The example discussed in the “Transformer case study” is intended to provide a basic understanding of cost-analysis calculations. These calculations can be helpful, especially when reviewing transformer replacement.

In some cases, the owner may be on the fence about replacing a piece of aging equipment. On one hand, the owner may hope to get another 10 years out of the equipment, knowing that this gamble could lead to downtime in the future if the equipment fails. Often, running a simple cost analysis can make these decisions more straightforward. In this case, replacing a transformer rated at 98.5% efficiency with a new transformer rated at 99.14% efficiency can have a payback of fewer than 10 years.

While every minute detail of cost analysis has not been explored in this example, this high-level calculation is something that an owner with potentially limited electrical knowledge will be able to understand. Given the number of variables, each selection and payback analysis can be very different.

Whether you are considering a new transformer with copper or aluminum windings, given all transformers must meet and comply with the DOE regulations at a minimum, you can be confident your new transformer will be efficient.

Taking a deeper dive into the payback of transformers, it is relatively simple to create a spreadsheet to run these types of analytics on a project-to-project basis. In reviewing payback, Table 3 helps illustrate how to complete a transformer payback analysis.

Start by reviewing the transformer-efficiency differential. In this example, you can use simple multiplication to calculate the transformer losses based on the percent efficiency of the transformer. If you start by multiplying the transformer power rating by the percent loading, you will get the actual power of the transformer. After obtaining this number, multiply the actual power by the percent losses of the transformer to obtain total power losses. Keep in mind that the transformer efficiency given by the manufacturer is based on a certain percent loading, and the efficiency may decrease as you further load the transformer. For this example, we are calculating the percent loading at 80%. It’s important to note that while transformers are designed around 35% loading, it’s often the case that transformers are overloaded or underloaded.

After you multiply the transformer power rating by the percent loading and percent losses, you have the total transformer power losses. With the transformer losses, you can convert the power losses into dollars lost by reviewing the transformer operating time and utility costs in kilowatt hours. Multiply the transformer total power losses by the number of hours a day the transformer will be operational, along with 365 days per year and the utility rate–$0.09/kWh was used for this calculation. Obtaining actual numbers from the local utility provider will allow you to be more precise with this calculation.

In this example, the operating cost alone is at a $1,556 premium with transformer-A installed. If the owner chooses to replace the existing transformer-A with a new, more efficient transformer-B, the total cost of the transformer plus installation labor, in this case, is $12,700. Given the $1,556 premium of transformer-A, the payback period for both equipment and labor is 8.2 years.

As mentioned earlier, while these numbers are not set in stone, estimating the loading and operational times can cause variation in the calculation. This simple calculation should be in every engineer’s toolbox when designing and specifying transformer equipment. If you have a local equipment representative to poll for transformer pricing, that can be a key ingredient for pulling together accurate equipment cost numbers.

Best practices

There are many factors that should be considered when selecting a transformer, namely transformer loading, the presence of system harmonics, K-factor, and temperature rise. Federal efficiency regulations and a cost and payback analysis are also considerations that should be taken into account. Knowing and understanding the building blocks for selection makes you a more valuable engineer to not only your company, but also your clients and owners. Engineering should involve a knowledge-sharing process to help the team make the best possible decisions for their facility and help elevate your project to save the owner money and maintain a safe and efficient electrical system.

Matt Zega is an associate with RTM Engineering Consultants. He specializes in electrical power distribution for commercial, industrial, and health care facilities.