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Transformers + Substations Handbook: 2014
Noteworthy is the fact that the TP-1 (Energy Star) efficiency, cop-
per-wound unit, loaded at 75% of its nameplate capacity (column 7),
saves over US $88 (ZAR 867) a year compared with an aluminium-wound
TP-1 model (column 6), but costs only US $85 (ZAR 837) more initially.
At only 50% loading, the copper TP-1 unit (column 11) saves about US
$50 (ZAR 492) a year compared with the same aluminium unit (column
10). No-load loss is reduced from 350 to 320 watts because the great-
er conductivity of copper windings allows a smaller core to be used,
so energy continues to be saved, even at light loading levels.
For greater savings, the premium efficiency, copper-wound unit
saves over US $401 (ZAR 3 952) a year at 75% loading (column 8),
compared with the aluminium TP-1 model (column 6), and only costs
an additional US $1 235 (ZAR 12 172).
Minimising owning cost
Whenever possible, compare competing transformer models by asking
for the load and no-load losses in watts and look at the total cost of
ownership. Given their life span, buying a unit based only on its initial
cost is uneconomical and foolish.
Transformer life cycle cost takes into account the initial transform-
er cost and also the cost to operate and maintain the transformer over
its life. This requires that the Total Owning Cost (TOC) be calculated
over the life span of the transformer. With this method, it is possible
to calculate the real economic choice between competing models.
A basic version of the TOC formula would be:
TOC = initial cost of
transformer + cost of the no-load losses + cost of the load losses
No-load losses are constant whenever the transformer is energised.
Specifying copper windings can minimise both the load loss and the
no-load loss, by allowing for a smaller core. If the load is known or can
be predicted, choose a transformer that will be loaded to about 75%
of its nameplate rating. Oversizing the unit increases the no-load loss-
es, as well as the purchase price, unnecessarily.
If the actual losses in watts are not available, and you are seeking
the transformer with the lowest losses, choose a transformer with
80°C rise, with M6 steel grade core or better, and copper windings.
Conclusion
Transformers remain a fundamental part of electrical distribution sys-
tems. The correct sizing for the load they are expected to carry and the
material used in their internal windings can dramatically impact their
life time and cost. It is worth reiterating that the recommended loading
for economic sizing of a transformer is typically around 75% of name-
plate and a premium efficiency, copper-wound, unit will result in sig-
nificant savings in the long run.
Reference
[1] NEMA TP 1: 2002. Guide for determining energy efficiency for
distribution transformers.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
StdAl
TP-1
Al
TP-1
Cu
Prem
Cu
Std
Al
TP-1
Al
TP-1
Cu
Prem
Cu
Std
Al
TP-1
Al
TP-1
Cu
Prem
Cu
Std
Al
TP-1
Al
TP-1
Cu
Prem
Cu
% of name
plate load
100
100
100
100
75
75
75
75
50
50
50
50
35
35
35
35
Core
loss (W)
375
350
320
190
375
350
320
190
375
350
320
190
375
350
320
190
Conductor
loss
2829 1874 1670
993
1591
1054
940
559
707
469
418
248
1591
176
157
113
Total
loss (W)
3204 2224 1990 1183 1966
1404
1260
749
1082
819
738
438
1966
526
477
303
Efficiency
loss (%)
95.9 97.12 97.42 98.45 96.62 97.56
98.69 97.19 97.86 98.07 98.84 96.62 98.04 98.04 98.86
Transformer
cost ($)
1366
(R13463)
1979
(R19505)
2064
(R20343)
3214
(R31678)
1336
(R13463)
1979
(R19505)
2064
(R20343)
3214
(R31678)
1336
(R13463)
1979
(R19505)
2064
(R20343)
3214
(R31678)
1336
(R13463)
1979
(R19505)
2064
(R20343)
3214
(R31678)
Comparison:
Additional
cost
compared
with standard
unit ($)
643
(R6337)
728
(R7175)
1878
(R18510)
643
(R6337)
728
(R7175)
1878
(R18510)
643
(R6337)
728
(R7175)
1878
(R18510)
643
(R6337)
728
(R7175)
1878
(R18510)
Energy cost/
year ($)
1964
(R19364)
1363
(R13441)
1220
(R12030)
725
(R7149)
1205
(R11882)
8 60
(R848561)
772
(R7615)
459
(R4526)
663
(R6539)
502
(R4949)
452
(R4460)
268
(R2647)
1205
(R11882)
322
(R3179)
292
(R2882)
185
(R1831)
Annual
energy
cost saving
compared
with standard
unit ($)
600
(R5923)
744
(R7337)
1239
(R12214)
344
(R3396)
432
(R4267)
746
(R7355)
161
(R1589)
210
(R2079)
394
(R3448)
69
(R688)
99
(R985)
206
(2036)
Payback
period (years)
1.07
0.98
1.52
1.87
1.68
2.52
3.99 3.45
4.76
9.20
7.29
9.09
Table 1 - courtesy: Olsun Electrics, Richmond, IL.
• Al (Aluminium)
• Cu (Copper)
• Std (Standard)
• Prem (Premium)
Notes:
• Standard and AluminiumTP-1 units are 150°C rise, CopperTP 1 unit is 115°C rise,
Premium unit is 80°C rise.
• Loss values at 100%, 75% and 50% nameplate load are at reference temperature
• Loss values at 35% nameplate load are at 75°C in accordance withTP-1
• Energy cost assumed to be $0,07/kWh
• Conversions from US$ to ZAR - 18 July 2013
Table 1: Payback time comparison for 75 kVA dry-type transformers.