What Is a Distribution Transformer?
Distribution
transformers reduce the voltage of an electric utility power
distribution line (4-35 kilovolts) to lower secondary voltages
(120-480 volts) suitable for customer equipment. Utilities
and commercial and industrial users purchase more than one
million new distribution transformers annually. Most liquid-immersed
transformers are owned by utilities and used in outdoor
applications (e.g., on power distribution lines). Virtually
all dry-type transformers are owned by commercial and
industrial  customers.
While distribution transformers are relatively efficient devices
-- generally delivering considerably more than 90 percent
of their input power -- they are constantly energized, and
therefore constantly experience some energy losses. As a result,
even small changes in efficiencies can add up to large energy
savings. The overall savings achievable from more efficient
transformers are substantial.
Energy-Efficient
Transformers
In 1996,
the National
Electrical Manufacturers Association (NEMA) developed
a voluntary standard, NEMA Standard TP 1-2002, Guide for
Determining Energy Efficiency for Distribution Transformers
(see NEMA Document
Center). This industry standard essentially defines energy-efficient
transformers. NEMA provides minimum-efficiency values
for single- and the three-phase
transformers most commonly used in buildings. These were developed based on
a goal of a three-year simple payback to the end-user. NEMA
recommends that users perform lifecycle costing to select
the most cost-effective transformer (i.e., accounting for
both initial and operating cost).
Transformer Energy Losses
Although transformers are
relatively simple devices, their efficiency is a more
complex product of a variety of design and operating
parameters. Distribution transformers are composed of
two basic parts: a core made of magnetically permeable
material, and a conductor for windings, typically made
of a low resistance material such as aluminum or copper.
Energy losses in distribution
transformers arise from both of these components. Core
or "no load" losses occur continuously as the transformer
stands by ready to serve a demand. Winding losses or
"load losses" result from resistance in the windings
when there is a load on the transformer. In most dry-type
transformers applications, core losses dominate because
average loads are very low.
|
Efforts to Promote
Energy-Efficient Transformers
Since 1996, there have been major efforts to promote TP 1 transformers.
The programs listed below promote transformers that meet the
NEMA Standard TP 1-1996 energy efficiency levels.
- ENERGY
STAR Commercial and Industrial Transformers Program:
This national labeling program
makes it simple for specifiers, contractors, and building
professionals to identify efficient TP1 transformers. When
EPA first launched the ENERGY STAR program, three manufacturers
agreed to produce and market qualifying products. Currently
there are more than 20 manufacturing
partners, including some of the top names in the industry.
- Federal Energy
Management Program's (FEMP's) Buying
Energy-Efficient Products Series: FEMP, which
assists agencies in meeting a federal requirement that they
purchase products in the top 15 to 25 percent of energy
efficiency, recommends that federal facility buyers purchase
TP 1 transformers (both low and medium voltage).
NYSERDA
has published a comprehensive but user-friendly lifecycle
cost calculator that allows comparisons both between models and between transformer
sizes. In addition, ENERGY
STAR and several manufacturers (e.g., Cooper Industries
and Square D) provide lifecycle costing tools.
During
the 2003 legislative session, Congress included a provision
within the Energy Policy Act of 2003 (H.R. 6) that would
legislate minimum energy efficiency standards for low-voltage
dry-type transformers. While the Act did not make it through
both houses of Congress, these energy efficiency provisions
were not controversial and are expected to succeed in next
year's legislative session. Stay
tuned to find out if minimum-efficiency standards for distribution
transformers are enacted in the 2004 session.
Several states (California, Massachusetts,
Minnesota, New York, and Wisconsin) also have either established
TP 1 as their state minimum-efficiency standard, or included
TP 1 as a requirement in their commercial energy code. Additionally, the
American Society for Heating, Refrigeration and Air-conditioning
Engineers (ASHRAE) and the International Energy Codes Council
(IECC) are both considering proposals to adopt the NEMA
standard, which would have a major impact on the many states
and jurisdictions that reference these standards.
Choosing Efficient Transformers
- Always purchase ENERGY STAR-compliant,
TP 1 products for use in buildings. Instruct
your supplier or electrical contractor to install only
low-voltage transformers that are ENERGY
STAR-compliant and medium-voltage transformers that
have been evaluated based on their lifecycle cost. Include
a requirement for ENERGY STAR transformers in your purchasing
specifications so that all future low-voltage transformer
purchases will automatically be ENERGY STAR-compliant,
as well as a requirement that medium-voltage transformers
be evaluated based on their lifecycle costs.
- Check with your local utility
on transformer promotions. Your local utility or state
energy office may offer financial or technical assistance
with the purchase of ENERGY STAR transformers. CEE also
offers periodic updates
on what its members are doing to promote more energy-efficient
transformers.
- In
general, specialty transformers such as "K-rated" and
"low-harmonic" are not needed in office building and similar
applications;
choose Energy Star/TP 1 instead.
Efficiency Levels of Three Phase
Dry-Type Distribution Transformers
| Rated Capacity (kVA) |
Efficiency |
| 15 |
97 |
| 30 |
97.5 |
| 45 |
97.7 |
| 75 |
98 |
| 112.5 |
98.2 |
| 150 |
98.3 |
| 225 |
98.5 |
| 300 |
98.6 |
| 500 |
98.7 |
| 750 |
98.8 |
| 1000 |
98.9 |
| 1500 |
- |
| 2000 |
- |
| 2500 |
- |
Source: NEMA.
2002. Guide to Determining Energy Efficiency for Distribution
Transformers. NEMA Standard TP 1-2002. Arlington,
Va: National Electrical Manufacturers Association.
|
