Transformers and Substations Handbook 2014

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

1

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 ($) Comparison: Additional cost compared with standard unit ($) Energy cost/ year ($) Annual energy cost saving compared with standard unit ($) Payback period (years)

1366 (R13463)

1979 (R19505) 643 (R6337)

2064 (R20343) 728 (R7175)

3214 (R31678) 1878 (R18510)

1336 (R13463)

1979 (R19505) 643 (R6337)

2064 (R20343) 728 (R7175)

3214 (R31678) 1878 (R18510)

1336 (R13463)

1979 (R19505) 643 (R6337)

2064 (R20343) 728 (R7175)

3214 (R31678) 1878 (R18510)

1336 (R13463)

1979 (R19505) 643 (R6337)

2064 (R20343) 728 (R7175)

3214 (R31678) 1878 (R18510)

1964 (R19364)

1363 (R13441) 600 (R5923)

1220 (R12030) 744 (R7337)

725 (R7149) 1239 (R12214)

1205 (R11882)

8 60 (R848561)

772 (R7615) 432 (R4267)

459 (R4526) 746 (R7355)

663 (R6539)

502 (R4949) 161 (R1589)

452 (R4460) 210 (R2079)

268 (R2647) 394 (R3448)

1205 (R11882)

322 (R3179) 69 (R688)

292 (R2882) 99 (R985)

185 (R1831)

344 (R3396)

206 (2036)

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.

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.

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Transformers + Substations Handbook: 2014