Distribution transformers are vital in our daily lives because they perform the final voltage transformation action within the electric power distribution system. This transformation allows the power to be delivered to customers at a lower voltage level fit for residential, commercial, and industrial consumption. One of the most important parameters of these transformers is their efficiency, which must be standardized to efficiently distribute low-voltage power at any required point of the distribution system. Transformers operate continuously, consuming energy until maintenance is needed. This remains true regardless of loading conditions, e.g., No load, lightly or fully loaded, because of internal and external system losses and energy consumption. Therefore, even small improvements in transformer efficiency can result in substantial energy savings. Since 2007, the U.S. Department of Energy (DOE) has played the role of regulating transformer efficiency and has updated its standards periodically to keep in line with the changing requirements of modern power distribution systems. On January 1, 2016, the U.S. Department of Energy (DOE) introduced its latest efficiency standards for distribution transformers to raise the overall electrical efficiency of critical power distribution equipment. These new standards primarily aimed to benefit consumers by improving the design requirements for transformer devices and reducing the costs of data centers and other commercial applications. Fulfilling these efficiency requirements will also help us reduce energy wastage and the associated greenhouse gases.
Transformer Design Variations
Transformer manufacturers are updating the designs of their transformer devices to comply with the DOE 2016 requirements. These changes may result in an increase in the transformer weight, size, and cost. Additionally, important electrical characteristics such as inrush current, impedance, and available short-circuit current will also change for low-voltage dry-type transformers. These changes will depend on the updated transformer design and the differences between current and updated design parameters. Manufacturers have taken a leading role in implementing the new standards and have started working with customers to plan for the potential impact of this transition to the new standards.
Scope of the New Standards
The implementation of the new DOE standards on January 1, 2016, brought about a complete shift in the manufacturing, selling, and importing dynamics of the distribution transformers market in the U.S. These standards have impacted liquid-immersed medium-voltage distribution transformers, low-voltage dry-type distribution transformers, and medium-voltage dry-type distribution transformers. Under these new standards, the required increases in transformer efficiency vary by kilovolt amperes (kVA) size, voltage rating, and transformer type.
DOE 2016 Standard Scope *
The DOE 2016 rulings apply to the following transformers:
- Transformers made or imported in the U.S. after January 1, 2016
- Low-voltage and Medium-voltage dry-type Transformers
- Liquid-filled Distribution Transformers
- Single-phase: 10 to 833 kVA
- Three-phase: 15 to 2500 kVA
- Primary voltage of 34.5 kV or less
- Secondary voltage of 600 V or less
The DOE 2016 rulings do not apply to the following transformers:
- Grounding Transformers
- Autotransformers
- Isolation Transformers
- Control Transformers
- non-ventilated Transformers
- Rectifier Transformers
- Special Impedance Transformers
- Sealed Transformers
- Testing Transformers
- Uninterruptible Power Supply Transformers
- Regulating Transformers
- Welding Transformers
- Underground mining (medium voltage dry type only) Transformers
- Transformers with tap ranges of at least 20 percent
- Transformers that are used, refurbished, or rebuilt.
* While this list is not exhaustive, it does cover several common exceptions. The new requirements of DOE-2016 standards update the 2007 and 2010 DOE guidelines. The 2007 and 2010 DOE rulings applied to low-voltage dry-type and medium-voltage distribution transformers, respectively, whereas the DOE-2016 rulings apply to both. The primary distinction between the earlier standards and the DOE-2016 regulations is the elevated efficiency requirement for transformers. Nevertheless, apart from that, the DOE-2016 rules share similarities with the previous ones regarding scope, methodology, compliance, and exceptions. The DOE-2016 regulations outline the different efficiency requirements that vary according to transformer type (liquid-type or dry-type), ampere rating, and phases (single-phase or three-phase). Also, efficiency requirements for medium-voltage dry-type transformers vary based on their basic impulse level (BIL). To achieve energy and cooling savings in medium and large data center applications, it is recommended to use high-performance low voltage transformers that meet or exceed the latest DOE efficiency requirements. By doing so, energy losses and heat output can be significantly reduced. The DOE-2007 requirements only applied to low-voltage dry-type distribution transformers. These guidelines required 97.7 to 98.9 percent of single-phase transformer efficiencies and 97.0 to 98.9 percent of three-phase transformer efficiencies. The single-phase transformer efficiency requirements have remained the same in the DOE-2016 rulings for low-voltage dry-type distribution transformers. The three-phase transformer efficiency requirements have increased to between 97.89 and 99.28 percent. A general comparison of the two standards is made in Table 2.
Table 2. Efficiency comparison of Low voltage dry-type distribution transformer
Similarly, The DOE-2010 requirements only applied to medium-voltage liquid-immersed distribution transformers. These regulations targeted efficiencies ranging between 98.36 to 99.49 percent. The introduction of DOE-2016 guidelines changed these requirements by increasing the efficiency level from 98.70 to 99.55 percent. A comparison of the efficiency requirements for single-phase and three-phase liquid-immersed transformers under both standards is made in Table 3 and Table 4, respectively.
Table 3. Efficiency Comparison of Single-phase liquid-immersed distribution transformer
Phase quantity
kVA
DOE 2010 efficiency percent
DOE 2016 efficiency percent
1
10
98.62
98.70
1
15
98.76
98.82
1
25
98.91
98.95
1
37.5
99.01
99.05
1
50
99.08
99.11
1
75
99.17
99.19
1
100
99.23
99.25
1
167
99.25
99.33
1
250
99.32
99.39
1
333
99.36
99.43
1
667
99.46
99.52
1
500
99.42
99.49
1
833
99.49
99.55
Table 4. Efficiency Comparison of Three-phase liquid-immersed distribution transformers
Phase quantity
kVA
DOE 2010 efficiency percent
DOE 2016 efficiency percent
3
15
98.36
98.65
3
30
98.62
98.83
3
45
98.76
98.92
3
75
98.91
99.03
3
112.5
99.01
99.11
3
150
99.08
99.16
3
225
99.17
99.23
3
300
99.23
99.27
3
500
99.25
99.35
3
750
99.32
99.4
3
1000
99.36
99.43
3
1500
99.42
99.48
3
2000
99.46
99.51
3
2500
99.49
99.53
Liquid-Filled Transformer Design Benefits
From these tables, we must note that the increase in the efficiency requirements appears to be more drastic for the dry-type transformers than the liquid-filled transformers. This will most likely result in a significant increase in the upfront cost of dry-type transformers compared to liquid-filled alternatives. Also, the actual efficiency requirements for liquid-type transformers remain significantly higher than the updated efficiency requirements for similarly sized dry-type transformers. The higher efficiency of liquid-filled transformers will also result in less energy loss and radiated heat inside buildings. Typically, dry-type transformers have a 150oC temperature rise, whereas liquid-filled transformers have a 65oC temperature rise. Both facts serve to reduce the operating and cooling costs of the transformer. Furthermore, when compared to dry-type transformers, liquid-filled transformers offer improved safety for indoor applications and can help extend equipment life. To improve efficiency at higher distribution voltages in large data centers, it is recommended to use high-efficiency liquid-filled transformers that meet the latest DOE requirements.
Table 5: Efficiency comparison of medium-voltage (MV) transformers
Three-phase kVA
Liquid-filled transformer efficiency at 50% load
Dry type transformer efficiency at 50% load
Liquid-filled transformer efficiency advantage
500
99.35%
98.99%
35.64%
1000
99.43%
99.20%
28.75%
1500
99.48%
99.30%
25.71%
2000
99.51%
99.36%
23.44%
2500
99.53%
99.41%
20.34%
Between 1929 and 1977, polychlorinated biphenyl (PCB) insulation was commonly used for indoor liquid-filled transformers. However, in the mid-1970s, PCBs were effectively banned from such applications by the Environmental Protection Agency (EPA), which regulated this technology’s use, storage, and disposal. This increase in the regulation of PCBs made dry-type transformer technology a more compelling alternative to liquid-type transformers for indoor applications. Over the years, substantial research in dielectric fluid and other insulation technology has made liquid-filled transformers prevalent in today’s market. Several environmentally friendly insulation technologies are now available for liquid-filled transformers that meet the requirements of DOE-2016 standards. For example, Envirotemp FR3, a non-toxic high-fire-point dielectric fluid, is increasingly found in more liquid-filled transformer applications instead of mineral oil, whose presence in liquid-type transformers is constantly decreasing. This non-toxic fluid is made from renewable, biodegradable, and soybean-based materials and helps reduce greenhouse gas emissions. The National Electrical Code (NEC) 450.23 provides guidelines for installing transformers inside or near buildings, and indoor Liquid-filled transformers must be installed by following these guidelines. Transformer designs should also comply with F.M. Approval and its Approval Standard 3990 to improve safety from fire hazards. In recent years, several high-profile data centers have had fire incidents due to the use of dry-type transformers. Conversely, liquid-filled transformers using modern insulation technology, like Envirotemp FR3 fluid, have enjoyed an impeccable fire safety record. This is evidenced by the fact that even though more than 500,000 FR3-filled transformers were installed in the last 20 years, no fire incident has ever been reported from these sites.
Additional benefits of liquid-filled transformers include:
- They have a much lower operating temperature than vacuum pressure-impregnated (VPI) and cast resin insulation technologies, which helps keep cooling costs relatively low.
- They are more energy-efficient than dry-type transformers and yield lower losses and operating costs.
- They have a better overload capacity than dry-type transformers because they operate in lower temperature ranges.
- The reduced footprint of the transformer enables smaller electrical rooms.
- The reduced sound level of the transformer results in fewer sound mitigation requirements.
Core Materials Optimization
The introduction of the DOE-2016 standards has allowed the transformer manufacturing industry to reassess, reevaluate and redesign their transformers with different types of core material. Amorphous metals, conventional core steel (CCS), or other materials may be used based on application requirements and economic concerns. Evidence suggests that most manufacturers will use amorphous metals (AM) for three-phase medium-voltage transformer designs to meet the DOE-2016 requirements because of higher secondary currents and voltages. Whereas conventional core steel (CCS) materials will be used to meet the lower requirements of a single-phase transformer system to reduce the cost. For transformers with equivalent DOE efficiencies, the amorphous design may have higher efficiency at low loading levels, while the conventional steel design may be more efficient at higher loading levels. Medium-voltage distribution transformers (single-phase and three-phase) are engineered based on the client’s application requirements. Criteria such as impedance, temperature rise requirements, loading factors, space constraints, overloading, fluid requirements, and more must also be considered when designing transformers to meet the DOE 2016 efficiency level requirements.
Conclusion
The DOE-2016 efficiency standards have changed the U.S. distribution transformer market forever, and the market must now ensure that they comply with them when they manufacture, sell, or import distribution transformers in the U.S. DOE-2016 guidelines are the latest of the efforts made by the U.S. Department of Energy to regulate and improve the U.S. distribution system, and they are likely to further increase the energy-efficiency requirements at some point in the future. Therefore, customers must work with manufacturers that will be able to not only meet the new DOE-2016 requirements but will also be able to accommodate evolving DOE regulations in the future to ensure that the new efficiency standards are met and that the various project, application, and equipment objectives are cost-effectively addressed. Meta Power Solutions is a long-time leader in distribution transformers and takes pride in its ability to provide its customers with innovative and high-efficiency technologies. MPS can optimize transformer designs according to the new DOE requirements while maintaining design attributes specified by the customer. For more information about MPS’s transformer technology offerings, please visit https://metapowersolutions.com/.
