Transformer efficiency: Minimizing transformer losses

Matching a transformer to its anticipated load is a critical aspect of reducing energy consumption.

06/12/2013


Figure 1: How engineers approach electrical designs can significantly affect transformer losses. Courtesy: CFE MediaIn 2002, NEMA issued a Standard TP-1 in support of the U.S. Dept. of Energy’s guidelines for more energy efficient electrical devices. This standard was based on a previous U.S. Environmental Protection Agency study showing that the typical dry type transformer under normal operating conditions was loaded to approximately 35% of its nameplate rating. Therefore, TP-1 established a table of minimum efficiencies for various sized transformers when operating linear loads (see Table 1). These efficiencies are really quite incredible as they range from 97% to 98.8%. What TP-1 does not tell you is that it is very unlikely that you will ever see such efficiencies in actual installations. In addition, TP-1 does not tell you that using these very efficient transformers will impact your electrical designs significantly. 

Because of the differences among the efficiencies shown in TP-1 and what really happens with real transformers in real applications, the approach you take in your electrical design could be significantly different when attempting to design an electrical system with minimized losses. This article offers suggestions regarding how you approach your electrical designs to maintain minimum losses in the system transformers (see Figure 1). It will also show areas in which you will have greater losses than those shown in TP-1—no matter which design direction you might choose.

Linearity

TP-1 was developed using linear loads. However, in today’s business environment, most of the loads are nonlinear (rich in harmonic content). Computers, fluorescent light fixtures, printers, elevators, or variable frequency drives for motors generate harmonics. Applying harmonically rich loads to transformers can double or triple their total losses. For example, a 75 kVA transformer that would normally have 2% losses at 35% loading would actually have 4% to 6% losses. Therefore, the 26 kVA load (35% of the 75 kVA) would have losses totaling more than 1.5 kW.

Table 1: Dry-type, low-voltage transformer efficiency chart. Courtesy: Lovorn Engineering Assocs.Core and coil losses

Transformer losses are a combination of core losses and coil losses. The core losses consist of those generated by energizing the laminated steel core. These losses are virtually constant from no-load to full-load, and for the typical 150 C rise transformer are about 0.5% of the transformer’s full-load rating. The coil losses are also called load losses because they are proportional to the load on the transformer. These coil losses make up the difference between the 0.5% losses for the core and range from 1.5% to 2% of the total load.

Typically, the total losses for a 75 kVA transformer are about 1,000 W at 35% loading or 1.3%. The actual losses when the transformer is fully loaded can be more than 3,000 W for linear loads and 7,000 W for nonlinear loads. This amounts to 4% and 9.3% respectively—considerably more than the NEMA TP-1 table for minimum efficiencies for a 75 kVA transformer. While the overall concept for requiring more energy-efficient transformers is quite good, engineers may want to be very careful about transformer selection when the anticipated operating conditions do not match the base criteria that were used in developing the TP-1 table. 

By selecting transformers with lower temperature ratings, that is, 115 and 80 C rise instead of the standard 150 C rise transformers, the core and load losses will change. To reduce the temperature rise, the core is increased in size. This increases the core losses but reduces the load losses, so, according to the anticipated operating point, the total losses may be higher or lower than the standard transformer. Due to the smaller core losses, the total losses for the 150 C transformer are less than the total losses of the 80 C transformer up to about 60% loading. With transformer loading above 60%, the total losses are less than those of a 150 C transformer of the same size (see Figure 2). 

A good compromise between core and load losses is the 115 C rise transformer. While the core losses are somewhat higher than those in the 150 C transformer, they are less than the 80 C transformer core losses. Correspondingly, the load losses are less than the 150 C transformer, allowing the total losses to be less than those of the 150 C transformer under normal operating conditions (see “Know the loss data, loading when specifying transformers”). 


<< First < Previous Page 1 Page 2 Next > Last >>

Consulting-Specifying Engineer's Product of the Year (POY) contest is the premier award for new products in the HVAC, fire, electrical, and...
Consulting-Specifying Engineer magazine is dedicated to encouraging and recognizing the most talented young individuals...
The MEP Giants program lists the top mechanical, electrical, plumbing, and fire protection engineering firms in the United States.
How to use IPD; 2017 Commissioning Giants; CFDs and harmonic mitigation; Eight steps to determine plumbing system requirements
2017 MEP Giants; Mergers and acquisitions report; ASHRAE 62.1; LEED v4 updates and tips; Understanding overcurrent protection
Integrating electrical and HVAC for energy efficiency; Mixed-use buildings; ASHRAE 90.4; Wireless fire alarms assessment and challenges
Power system design for high-performance buildings; mitigating arc flash hazards
Transformers; Electrical system design; Selecting and sizing transformers; Grounded and ungrounded system design, Paralleling generator systems
Commissioning electrical systems; Designing emergency and standby generator systems; VFDs in high-performance buildings
As brand protection manager for Eaton’s Electrical Sector, Tom Grace oversees counterfeit awareness...
Amara Rozgus is chief editor and content manager of Consulting-Specifier Engineer magazine.
IEEE power industry experts bring their combined experience in the electrical power industry...
Michael Heinsdorf, P.E., LEED AP, CDT is an Engineering Specification Writer at ARCOM MasterSpec.
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
Consulting-Specifying Engineer's Product of the Year (POY) contest is the premier award for new products in the HVAC, fire, electrical, and...
Consulting-Specifying Engineer magazine is dedicated to encouraging and recognizing the most talented young individuals...
The MEP Giants program lists the top mechanical, electrical, plumbing, and fire protection engineering firms in the United States.
How to use IPD; 2017 Commissioning Giants; CFDs and harmonic mitigation; Eight steps to determine plumbing system requirements
2017 MEP Giants; Mergers and acquisitions report; ASHRAE 62.1; LEED v4 updates and tips; Understanding overcurrent protection
Integrating electrical and HVAC for energy efficiency; Mixed-use buildings; ASHRAE 90.4; Wireless fire alarms assessment and challenges
Power system design for high-performance buildings; mitigating arc flash hazards
Transformers; Electrical system design; Selecting and sizing transformers; Grounded and ungrounded system design, Paralleling generator systems
Commissioning electrical systems; Designing emergency and standby generator systems; VFDs in high-performance buildings
As brand protection manager for Eaton’s Electrical Sector, Tom Grace oversees counterfeit awareness...
Amara Rozgus is chief editor and content manager of Consulting-Specifier Engineer magazine.
IEEE power industry experts bring their combined experience in the electrical power industry...
Michael Heinsdorf, P.E., LEED AP, CDT is an Engineering Specification Writer at ARCOM MasterSpec.
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me