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

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 = £

_ _ 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

A

(W) Ao

Ak =650 =6000 B

(W) Bo

Bk =800 =7000

C

(W) Co

Ck =930 =8400

D

(W) Do

Dk =1150 =10500

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

1.052 0.92 = 1.1025 = 0.81 0.252 = 0.0625

Not applicable

www.cibsejournal.com

June 2011

May 2011 CIBSE Journal

49

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