Selecting energy-efficient transformers

Engineers should know the design concepts for selecting and sizing transformers to help achieve energy efficiency.

By James Ferris, PE, and Aaron Johnson, TLC Engineering for Architecture, Orland March 12, 2014

Learning objectives

  • Understand the different types and uses of transformers.
  • Know how to select and size transformers.
  • Understand the concepts of transformer protection.

Transformers are perhaps among the most overlooked components within an electrical distribution system. However, they play a key role in our everyday lives. Every house is fed by a single transformer. In our offices, a single transformer may serve the computers for the entire floor. In hospitals, the operating rooms and intensive care units are typically fed from two separate transformers.

If a transformer fails or is improperly sized, catastrophic outages, which are not quick and easy to fix, could occur. Also, transformers don’t shut off; they continue to use current and generate heat 24 hr a day, seven days a week.

Transformer selection

Transformers are available in many different flavors. High- and medium-voltage transformers (primary voltage greater than 600 V) are available as dry type units, but are more commonly liquid filled or liquid immersed. That liquid is commonly petroleum-based oil, but many companies are starting to offer similar products based on biodegradable seed oil. Commercial buildings operate on low-voltage (primary voltage less than 600 V) predominately, and typically use dry type transformers to step down the voltage from 480 V to 208 Y/120 V or 240/120 V. This article focuses primarily on dry type transformers. However, many of the concepts presented apply to higher voltage transformers as well.

To select a dry type transformer, you need to answer three simple questions:

  1. What is the purpose of the transformer?
  2. How do I want the transformer to perform?
  3. What options should I select?

Generally, there are three purposes of a transformer: Change the voltage, isolate power systems, and harmonic accommodation. Voltage can be decreased or increased. These transformers are available in either delta- or wye-connected primary or secondary, depending on the distribution voltage and system requirements. They may be single- or 3-phase, and are available in a variety of sizes (see Figure 1). Isolation transformers can be used in health care facilities to minimize the risk of stray currents in the electrical system, or even on a single load that has very sensitive electrical requirements.

How the transformer performs boils down to temperature and efficiency. Transformers are listed with a rate of temperature rise, typically 80 C, 115 C, or
150 C. This temperature rating is the rise above ambient (see Figure 2). The surface temperature of a transformer with an 80 C rise is significantly less than
one at a 150 C rise.

How engineers approach energy efficiency is evolving. Every other year, the U.S. Dept. of Energy (DOE) revises many of the energy standards that regulate our industry. Transformers manufactured today, and since 2007, are required to meet the criteria defined under NEMA TP-1 2002. In April 2013, a new rule was adopted that implements new transformer standards, effective Jan. 2016. NEMA TP-1 2002 introduced two significant changes to energy efficiency considerations: Minimum efficiency was defined for each transformer size, and the point where that efficiency was measured changed from full load to 35% of transformer capacity. As a part of the origin of NEMA TP-1, research was performed to determine that most low-voltage distribution transformers are, on average, only 35% loaded. Many manufacturers currently offer a NEMA premium efficiency transformer, which was created prior to the implementation of the April 2013 final rule (see Table 1). Please refer to the NEMA class I efficiency chart for further information on the past, present, and future ratings of transformers.

The decision of which transformer to provide becomes a decision of which options to specify. Questions that engineers should ask include:

  • Are the transformer windings aluminum or copper?
  • What kind of enclosure do you need for your application?
  • Is it outdoors, or is a NEMA 1 enclosure acceptable?
  • What are the specific requirements of the manufacturer in terms of required space around the transformer?

Some manufacturers require a 3-in. clearance around transformers, while some require a 6-in. clearance. Unfortunately, sometimes 3 in. can make a big difference in designing electrical rooms.

Transformer efficiency

Consider a facility that has an older 225 kVA transformer that was installed around 1973. It has a 480 V primary and a 208/120 V secondary, and it meets the required clearances. When the issue of replacing the transformer arises, considerations include:

  • The unit is 40 years old, but for this example, assume it has been fully checked for hot spots and is still in good operating condition.
  • The unit is specified at 150-C rise, which means at full load, the surface temperature of the transformer can get quite hot.
  • The unit is more than 112.5 kVA, which per National Electrical Code (NEC), Article 450, requires the room to have a minimum fire rating of 1 hr.
  • The unit is less efficient than the currently available transformer models, but the question is: Does the energy savings alone justify the replacement of this transformer?

Transformers have losses, and those loses are given off into the room in the form of heat. If you want to know what the transformer losses are, you can either obtain information from the manufacturer’s cut sheet or perform a series of no-load and loaded tests on the actual transformer. For this example, a data sheet from a transformer from that era was used for comparison. Table 2 compares a standard TP-1 efficient model and a transformer available from that era. Note that transformer losses reviewed between 1973 and 2003 appear to have similar efficiency and loss characteristics. It was not until the introduction of TP-1 that transformers were re-evaluated and efficiencies saw a big adjustment.

For the 225 kVA example, today’s unit is more efficient by 728 W. Keep in mind the different components associated with this energy loss. The energy loss is given off into the room in the form of heat, and that heat is then cooled by the HVAC system, which itself has losses. The owner pays for the energy required from the transformer—plus the losses of the transformer. The owner also pays the utility company for the energy required to drive the fans, chillers, and pumps for the additional capacity. In effect, the owner pays for these losses twice. As part of this exercise, and working in conjunction with our HVAC brethren, we were able to expand this chart to compare how many more cfm are needed to keep the electrical room at 75 F based on 55 F supply air temperature (see Table 3).

The efficiency of replacing a 225 kVA transformer with a new 225 kVA unit can result in a reduction of approximately 113 cfm in air conditioning. This isn’t a big difference, won’t have a huge impact on the utility bills, and won’t pay for the new transformer in savings. So let’s add a new wrinkle to the case study. When the U.S. Environmental Protection Agency did the TP-1 study, it found that most transformers are only 35% loaded, and in most cases this is true. If we put a 30-day load reading on this transformer, we can determine the existing load per the NEC, and have the flexibility of sizing the transformer at existing load +25%. A 225 kVA transformer at 35% + 25% per NEC would result in a 98 kVA required transformer size. The next size transformer available is 112.5 kVA. Changing our comparison from a 225 kVA existing to a 112.5 kVA new transformer means that we can reduce the airflow required to cool the room by more than 300 cfm. Losses for the new transformer are 53% of the losses of the existing transformer. This is beginning to look like a viable option for the owner to consider.

Transformer protection

For the remainder of this article, assume we selected a dry type transformer rated 112.5 kVA with a 480-V primary and a 208 Y/120 V secondary. Because the unit is installed indoors, it requires a NEMA 1 enclosure. We’ve selected aluminum windings because space is of no concern, and it’s more cost effective for our size of transformer.

Overcurrent protection shall be selected and sized on the primary and secondary in accordance with the NEC table 450.3(B), which indicates that the secondary must be sized no greater than 125% of the full load of the transformer (312 FLA). We select a 400-A breaker because it is the next available size. The primary can be selected at up to 250% of the transformer rating (135 A), so we can select anything from a 175-A breaker to a 300-A breaker. We typically keep the primary at 125% as well because it helps simplify selective coordination, considering that these breakers can be set to overlap each other. For our example, we will select a 175-A primary breaker and a 300-A secondary breaker. When we define the breaker settings and sizes, we need to review them against two distinct points: the transformer damage curve and transformer inrush.

Transformer damage curve: The transformer damage curve is an ANSI standard curve that all transformers are measured against. It indicates the level of current over time the transformer can withstand and typically is shown as a sloped line on time-current curves. We must ensure that the overcurrent protection will trip before this current is reached.

Transformer inrush: Every transformer has windings, which means it is inductive. Because transformers are inductive, they experience an inrush of current when power is applied. Because a transformer operates on magnetic principles, there can be differences in transformer starting current, depending on the phase angle of the voltage when the transformer is first energized. The starting current can vary from full load current to 20 times the transformer’s full load rating, and can essentially appear as a fault. Manufacturers can provide the maximum inrush current for each of their units, and it should be considered when selecting the size of the primary overcurrent protection. In our coordination study, the breaker will not trip from transformer inrush current (see Figure 3).

Transformer selection, sizing, and protection

Transformers discussed in this article are dry type, but the theory remains the same for larger transformers of different types. However, some of the protection schemes are more advanced and different materials are used in transformer construction. Now that you are armed with additional information, it’s time to go out in the world and check those lonely transformers that are protecting so much.

James Ferris is an electrical project engineer with TLC Engineering for Architecture. He specializes in power distribution for health care facilities. Aaron Johnson is a mechanical project engineer and project manager at TLC Engineering for Architecture, where his focus is on HVAC design.