Dry-type transformers, for a variety of reasons,
have largely replaced oil-filled units within industrial, commercial
and institutional buildings in many countries. However, unlike
oil-filled units in the utility grid, little attention is paid
to the energy efficiency of dry-type units. Unlike motors, they
have no moving parts to wear out, and so they are expected to
last 20 years or more.
Efficiency is seldom specified when buying dry-type transformers.
Values of 95% or higher are typical, and differences between
high- and low-efficiency units are only 1% to 2%.
While high efficiency transformers are somewhat more expensive
to purchase than low efficiency units, "first cost" is surely
not the last cost of any transformer. Complete life-cycle costs
must be carefully examined along with the economics of high-efficiency
dry-type transformers.
Life-Cycle Costing
When a lower cost, 98% efficient unit and a higher cost unit
at 99% are compared, that means that the losses are actually
cut in half.
Conductor losses, also known as coil losses or load losses,
should not be overlooked - for one reason because such losses
vary by the square of the load. That means a fully loaded transformer
has four times the load losses compared with one running at
50% of its design load. In many applications, transformers are
heavily loaded. But even at lower load factors, if the units
are not efficient (and many are not), losses mount rapidly.
Thus, the extra cost of a high efficiency transformer should
be balanced by the savings in operating the unit - savings which
go on year after year - quickly paying back the extra first
cost.
Other benefits pile up as well. Efficient transformers run cooler,
and thus more reliably, because of decreased stress on insulation
materials. As a result, they will have a higher overload capacity,
an important issue in dry-type transformers. The ultimate result
can be units with a smaller kVA rating actually doing the same
job, with attendant first-cost savings.
Efficiency vs. Temperature
Efficiency and temperature are related but separate issues.
Inefficiency is purely and simply the generation of waste heat-
I2R losses in the transformer windings. To the extent heat is
generated, the unit is inefficient. The temperature rise, on
the other hand, results not only from how much heat is generated,
but also from how much is removed.
An inefficient dry-type transformer can, in fact, run cool by
incorporating into its design larger air passages or fan-cooling.
High-efficiency transformers eliminate the need for these extra
cooling costs.
The Economics of Efficiency
The examples below are of high-efficiency, copper-wound transformers
designed to achieve an 80°C temperature rise and high efficiency.
These are compared to standard, run-of-the-mill units using
aluminum, the low-cost winding material, and designed to a 150°C
rise.
The 1,500 kVA unit represents a typical, large dry-type transformer
used for industrial and some larger commercial applications.
In these applications, duty cycles can be quite high. Both 65%
and 85% are used in the examples. Paybacks in high-electricity-cost
areas can be as fast as one year for the design shown.
For light commercial applications, such as lighting circuits
in office buildings, smaller units, such as the 75 kVA example
shown, are used. Here, duty cycles would typically be lower.
The sample calculations are for 50% and 75% duty cycles. In
these applications, however, the customer is likely to pay a
higher electricity rate than is the case in industrial applications.
But here, again, paybacks can be short - as little as 1.1 years.
Once the unit's first cost premium is paid back, those energy
savings continue to accumulate for the decades the transformer
will be in service.
Energy Efficiency Means Annual Savings, Rapid Payback.
A
simple analysis to help understand the dramatic effect
of
energy efficiency on transformer operating costs and payback. |
|
Manufacturer
A - 1,500 kVA* |
|
Standard
(Aluminum) |
High Efficiency
(Copper) |
Standard
(Aluminum) |
High Efficiency
(Copper) |
| Load Factor** |
65% |
85% |
| Efficiency
| 98.64% |
99.02% |
98.47% |
99.02% |
Temp. Rise
(100% load) |
150° C |
80° C |
150° C |
80° C |
| Core Loss |
4.3 kW |
5.5 kW |
4.3 kW |
5.5 kW |
| Conductor Loss |
9.1 kW |
4.1 kW |
15.5 kW |
7.1 kW |
| Total Loss |
13.4 kW |
9.6 kW |
19.8 kW |
12.6 kW |
| Power Saving |
– |
3.8 kW |
– |
7.2 kW |
| First Cost |
$16,750 |
$22,650 |
$16,750 |
$22,650 |
| Cost Premium |
– |
$5,900 |
– |
$5,900 |
| Benefits
of Using High-Efficiency Copper-Wound Dry-Type Transformers |
Electrical
Energy Cost |
Annual Savings |
Payback Period |
Annual Savings |
Payback Period |
| $0.05/kWh |
$1,660 |
3.5 y |
$3,150 |
1.9 y |
| $0.07/kWh |
$2,330 |
2.5 y |
$4,420 |
1.3 y |
| $0.09/kWh |
$3,000 |
2.0 y |
$5,680 |
1.0 y |
|
Manufacturer
B - 75 kVA* |
|
Standard
(Aluminum) |
High Efficiency
(Copper) |
Standard
(Aluminum) |
High Efficiency
(Copper) |
| Load Factor |
50% |
75% |
| Efficiency
| 97.24% |
98.61% |
96.61% |
98.38% |
Temp. Rise
(100% load) |
150° C |
80° C |
150° C |
80° C |
| Core Loss |
0.34 kW |
0.21 kW |
0.34 kW |
0.21 kW |
| Cond. Loss |
0.73 kW |
0.32 kW |
1.64 kW |
0.72 kW |
| Total Loss |
1.07 kW |
0.53 kW |
1.98 kW |
0.93 kW |
| Power Saving |
– |
0.54 kW |
– |
1.05 kW |
| First Cost |
$890 |
$1,790 |
$890 |
$1,790 |
| Cost Premium |
– |
$900 |
– |
$900 |
| Benefits
of Using High-Efficiency Copper-Wound Dry-Type Transformers |
Electrical
Energy Cost |
Annual Savings |
Payback Period |
Annual Savings |
Payback Period |
| $0.05/kWh |
$240 |
3.8 y |
$460 |
2.0 y |
| $0.07/kWh |
$330 |
2.7 y |
$640 |
1.4 y |
| $0.09/kWh |
$420 |
2.1 y |
$830 |
1.1 y |
* Actual examples
of 1,500 kVA, 15 kV - 277/480 V, and 75 kVA, 480 V - 120/208
V, transformers.
** A combination of duty cycle and percent of full loading.
|
|
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