ELECTRICAL SERVICES WHOLE-LIFE COSTS
The supply schematic for this building
is shown in Figure 1. The client’s brief is to provide maximum up-time catering for the following failure scenarios: a. One HV feeder failed – the other HV feeder to step in via the changeover switch, 100% supply restored via the healthy standby transformer, the system can be classed as having a permanent and healthy supply;
b. One transformer failed – LV supply to affected area via the healthy standby transformer, 100% supply restored, the system can be classed as having a permanent and healthy supply;
c. One HV feeder/One Transformer/ Changeover switch failed – one standby generator set steps in, 100% supply restored with 50% of the supply being classed as a temporary supply; and,
d. Two HV feeders/three transformers failed – two standby generators step in, 100% supply restored and the system is classed as having two separate temporary supplies To cater for the above scenarios, the required rating of equipment would be:
Tx1 = Tx2 = Tx3 = 800kVA (each has a prospective fault MVA = 13.4)
G1 = G2 = 800kVA (each has a prospective fault MVA = 5.4)
The following will examine the costs
and CO2 emissions associated with the no-load and on-load losses of the proposed arrangements. Table A shows the typical
losses for different energy efficiency class transformers. For the purposes of this article, it is
assumed that the average daily load profile of the building is operating at a power factor of 0.95 lagging, at 105% load for 1.5 hours, 90% load for 14.5 hours and 25% load for the remaining eight hours. Since the on-load losses are related to the loads that are switched on, it will either increase or decrease in proportion to the square of the load current when the transformer is supplying loads at above or below the rated value of the transformer. Table B shows the on-load losses adjustment factors for 105%, 90% and 25% load factors. A flat rate electricity tariff of £0.25p per
kWh is used as the average price over a total building life period of 25 years. The price is averaged to include the relevant maximum demand and standing charges, and climate change levy. Since the standby transformer is
permanently energised to be ready for stepping in at any time, its standby energy cost is
Cost yrrun = kWh× (hours No Load losses ) ×365 ×
_ £ off load _ _ 1000
the annual running cost for the two duty transformers can be estimated by the expression*:
Cost yr kWh× _ TCO PP A rated no load losses B rated on load losses where TCO=total life-time cost, PP=purchased price, = + × _ _ + × _ _ _ run = £
(hoursOn load × Load lossesFL ) + (hours No Load losses) ×365
_ _ 1000
*
www.leonardo-energy.org uses a different expression to arrive at a more accurate total life-time cost of the transformers: _
A=tariff rate for no-load operation, B=tariff rate for on-load operation. Readers who are interested in the method should refer to the report given in the Leonardo-energy website.
off load _ × _ _ _
Electrical engineers are well placed to offer a best-value, whole-life approach
Table A: No Load and On Load losses for 800kVA transformers to EN50464-12 Transformer
Energy Class losses
800kVA
No Load On Load (100%)
A
(W) Ao
Ak =650 =6000 B
(W) Bo
Bk =800 =7000
C
(W) Co
Ck =930 =8400
D
(W) Do
Dk =1150 =10500
Table B: On-load losses adjustment factors
Demand in kW 800kVA
No of operating hours
0.95 x 1.05 x 800 = 798 1.5 0.95 x 0.90 x 800 = 684 14.5 0.95 x 0.25 x 800 = 190 8
kWh/day
1197 9918 1520
On-load losses Adjustment factor
1.052 0.92 = 1.1025 = 0.81 0.252 = 0.0625
No-load losses adjustment
Not applicable
www.cibsejournal.com
June 2011
May 2011 CIBSE Journal
49
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