The decision concerning the power transformer choosing will involve selecting the most acceptable combination of commercial, technical and programme aspects.
In making this study the objective must be to obtain answers to technical and commercial questions.
- Are there any statements made in the covering letter, descriptive material or tender schedules which suggest that the equipment supplied will not be in accordance with the specification?
- Are the impedance values given in the tender schedules in accordance with those specified? Are the impedances on extreme tap positions, including any possible tolerances, within acceptable limits? If zero-sequence impedance is important, is the value offered acceptable?
- Has all the specified testing, in particular type testing, been included in the offer?
- Has the waiving of any type testing been claimed? If so, is the supporting evidence included and is it acceptable?
- Has the tenderer taken due account of any special requirements included in the specification, for example special overloading capability?
- Will the transformer fit in the site?
- Has the tenderer included for all the specified fittings, marshalling cubicle, valves, anti-vibration mountings, etc.?
- Does the pattern of terminations offered comply with the specified requirements with regard to, for example, bushing shed profile, palm configuration, type of cable boxes?
- Has the tenderer included all the special descriptive information requested in the enquiry document, for example the measures incorporated to allow for a high level of harmonics in the load, or to cater for frequent severe overloads?
- Does the offer meet any specified noise level requirements, including the effect of a noise attenuation enclosure where appropriate? If a noise enclosure will be required, has it been included in the tender price?
- Has the tenderer included for all the necessary site work, including delivery and site erection?
Occasionally the descriptive material provided by one tenderer can raise questions in relation to the other tenders, for example some of the tenderers might comment that a specified overload duty at 10°C will require an increase in the rating at normal IEC 76 ambient. This then raises the question as to whether a tenderer who has made no comments at all in relation to the specified overload duty has taken this into account in preparing his design. Similarly, it is sometimes the case that setting out the information provided by the tenderers in their completed schedules of technical particulars will highlight an anomaly in some of the data provided by one of the tenderers and raise the question as to whether his offer is in compliance with the specification.
In the example of the embedded generator, the transformer rating was specified at an ambient temperature of 10°C but it was proposed that tenderers should be asked to quote the rating of the transformers offered at normal IEC ambient. It would be expected that the reduction in rating resulting from the increase in ambient temperature from 10°C to an annual average of 20°C and a daily average of 30°C would be quite modest and very nearly the same for all tenders, but the one for which the reduction is least might be taken as an indication that this is the design which is least stressed thermally, and, as indicated elsewhere in this work, lowest thermal stress is likely to lead to longest life. Such considerations would only, of course, be relevant in differentiating between tenders which were very similar in other respects.
When the purchase of a transformer is considered, as with most other items of plant or equipment, there are two aspects to be taken into account:
- The initial capital cost.
- The running cost which in the case of a transformer is the cost of supplying the losses.
In the typical tender assessment exercise discussed in the preceding section, notional values were placed on each kilowatt of the guaranteed losses for the transformers tendered as a means of comparison of the tenders on a common basis. That is, a cost was assigned to the value of one kilowatt of no-load loss and also to the value of a kilowatt of load loss during the operating life of the transformer. In the example no-load loss was costed at a considerably higher value, £3000 per kW, than load loss at £650 per kW, and although there was no mention in the example of the type of transformer being considered, this could be the case for a typical transmission transformer operating in a multiple transformer substation in the part-loaded condition, so that in the event of losing one transformer in the substation, the remainder must be capable of carrying the total substation load without becoming overloaded. In addition, the daily load cycle has a daytime peak and a very much lower value at night.
As a result the transformer spends much of the time at less than half load, and the average load losses are less than one-quarter of their magnitude at nominal rated power, which is the rating for which the load losses are quoted and guaranteed by the manufacturer.
While this illustration explains why the cost of the load losses is so much less than the no-load losses, it does not explain how their actual value is derived. To do this it is necessary to examine the subject a little further and assess the likely cost of a kilowatt of loss over the lifetime of the transformer.
The simplest method would be to calculate the total energy consumed in losses over, say, a 25 or 30 year life and cost this at today’s energy price. This calculation can be worth carrying out if for no other reason than the fact that the answer can be quite surprising.
Generally the accountants’ view prevails so that the cost of making provision for the lifetime cost of losses is expressed in terms of the sum which must be set aside now to pay for these. This can be calculated from the following expression:
C is the cost per £ annual cost of losses
a is the rate of interest payable for loans at the date of purchase (expressed on a per unit basis)
b is the rate of interest obtainable on sinking funds (expressed on a per unit basis)
n estimated lifetime of the transformer in years
Typically, and for the purpose of illustrating this example, ‘a’ might be taken as 9% for a large organization seeking a long-term loan and ‘b’ as 7%. For a value of ‘n’ equal to 25 years ‘C’ is then 0.1058
Hence the capitalized value of no-load loss is 422/0.1058 = £3988/kW
and the capitalized value of load loss is 87.46/0.1058 = £827/kW
These are the values for losses which it would be reasonable for an organization such as the one described to use in its assessment of tenders for an additional transformer.