Industrial Power Transformers -- Transformer enquiries and tenders [part 1]

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No work of this magnitude would be complete without providing guidance in the procurement of transformers and emphasizing to the reader and potential purchasers the need to ensure that there is a complete understanding between all parties from the outset, of the technical, commercial and legal requirements constituting any contract which may be established between them. Earlier editions included a section covering enquiries and tenders near to the beginning, following the sections on fundamental principles. Whilst there is some value in 'defining the problem' at the outset, moving the topic to near the end means that this section can serve as a resume of the work as a whole with the advantage that the reader should have a clearer understanding of what is involved.

In describing the process of issuing an enquiry it is assumed that the reader has a knowledge of what has gone before. Where important or more complex issues are involved, there is a cross reference to the point in the text where the subject is discussed in detail. Elsewhere, the point of reference should be evident without the need for it to be specifically identified. For all of the technical issues raised the reader will be able to find a fuller explanation by referring to what has gone before.


In the initial stage of an enquiry for a transformer there is nothing so important as a full and explicit statement of the total requirements that, from the user's point of view, have to be met and, from the manufacturers standpoint, have to be considered. This statement generally constitutes the technical specification, guarantees and schedules, which, together with the commercial and contractual conditions will form the basis of a contract between the user and the supplier.

Frequently, enquiries are issued giving insufficient information concerning the relevant details, with the result that errors are made and delays are incurred in projects which could have been avoided if adequate consideration had been given to identifying the user's exact requirements at the outset.

There is a regrettable tendency at the present time, in the interests of obtaining the most economical designs and of permitting open competition, of issuing enquiries for 'transformers to EN 60076' with the intention of allowing manufacturers to follow their 'standard' design practices and these enquiries include little more by way of technical requirements. For a good many years both the British and International Standards have themselves endeavored to emphasize the inadequacy of this approach by providing appendices of information required with an enquiry or order. Whilst following the guidance pro vided by these appendices should ensure that no vital information is omitted or no irretrievable disasters are likely to be uncovered when a new transformer arrives on site, it should be remembered that these documents only identify the most basic technical requirements. There is nothing set down in EN 60076, which covers the detail, such as how the transformer should be painted for example - other than the basic requirement that it must be fit for purpose. But, then, how long does the paint finish have to last for it to be fit for purpose and how severe an environment does it have to survive? This is the type of information, not listed in the appendices to EN 60076, which the purchaser must provide if he wishes to ensure that he will obtain the transformer which most effectively, and in the long term most economically, meets all his needs.

Hence, it should be clear that if it is required that a manufacturer submit his most competitive tender, in response to any transformer enquiry, a fairly detailed specification defining minimum standards must be issued with that enquiry, otherwise a manufacturer may be justified in making his own assumptions with regard to minimum requirements in the interests of competitiveness.

Manufacturers do not have standard designs, except, perhaps, for the smallest distribution transformers. To attempt to do so would require that an enormous number of combinations of ratings, voltage ratios, impedances, connections, terminal arrangements, etc. would have to be covered, to say nothing of the variations of no-load losses and load losses required to cover the varying economic circumstances applying to different customer's duties and applications.

Any new transformer contract thus will generally involve the generation of a new design and it is therefore just as easy to make this design fit the user's exact requirements as it is to try to second-guess these by providing a 'standard' arrangement.

Minimum standards can and should, therefore, be identified by the purchaser without restricting a manufacturer's scope for using his expertise to provide reliable and competitive designs. It must be possible, however, to establish compliance with these minimum standards by means of simple checks or tests and not by attempting to dictate to a manufacturer how a transformer should be designed. In the case of the above example relating to paint finish, it is not restrictive of competition to specify the type of paint to be used, the minimum number of coats and the final paint thickness, but it could be restrictive to specify the precise process by which the paint is to be applied. There is no reason why the prospective user should not also specify that the paint finish should last for, say, 8 years without attention to repainting, since this gives an indication of the quality required, although it is not practicable to enforce such a requirement contractually because manufacturers contractual commitments, in the form of warranties, do not normally last for longer than the first year or so of service.

Technical specification

A technical specification has three objectives:

(1) To provide the tenderer, or manufacturer, with all that technical information necessary to carry out his design and which will vary from unit to unit, for example rating, voltage ratio, type of cooling, etc.

(2) To provide the tenderer, or manufacturer, with an indication of the strategic importance of the transformer and the value to be placed on reliability, maintainability and long service life.

(3) To provide the tenderer, or manufacturer, with information which will ensure that the transformer will satisfactorily interface with its associated plant and equipment and that installation and commissioning will proceed smoothly and without undue delays.

Clearly the first two objectives will have a significant bearing on the cost of the transformer and must be met by the enquiry document to enable the manufacturer to prepare his tender. The third will include many items which will have relatively little bearing on the overall cost and which could possibly be resolved during the engineering of the contract. However, it is good discipline to identify in the technical specification all those aspects which should be known at the time of initially drawing this up, since not only does this minimize the use of engineers time during the contract stage and ensure that there are no unnecessary delays during the contract, but it also avoids the risk that these items might be overlooked during the detail engineering of the contract.

Table 1: Schedule of technical particulars for transformer

Preparation of the technical specification

The first step in the preparation of the technical specification is to draw up a check-list of important technical parameters. This check-list may well take the form of a schedule of technical particulars which can ultimately form part of the enquiry document. If the user is in the habit of buying transformers at fairly frequent intervals the form of this schedule can provide the basis for a standard company document. A typical schedule is shown in Table 1 or, alternatively, the appendix listing information required with an enquiry and order in EN 60076 may be used as a starting point.

For some applications, for example for small distribution transformers for which the main technical parameters are very simply determined, it may be possible to complete the schedule of technical particulars directly and without any preliminary work, however for most transformers, although the schedule identifies those particulars which need to be determined, the act of deciding the values of many of these will require a design study.

The result of the design study should be to produce a Document of Design Intent, or a document of 'needs.' This document will identify the basis on which, for example, the rating, impedance, voltage ratio and tapping range have been decided and will probably include, possibly as appendices, any calculations performed to derive these. It might possibly identify the need for closer tolerances on impedance variation than those set out in EN 60076 and, if this is the case, it will include a justification for this. It will probably examine a number of options for the other main parameters, for example the type of cooling, and give the reasons why a particular option has been adopted. In the case of a large important transformer, for example for a generator transformer, it might identify the need for high reliability and availability, justifying the requirement for extra testing over and above that covered by EN 60076. It might also consider the case for the provision of a spare generator transformer and identify whether one is to be included at the outset.

When all the information for the schedule of technical particulars has been decided this can then be compiled for inclusion in the enquiry document. This schedule will provide a useful summary for tenderers and ultimately for the designer, however this should be regarded as no more than a summary, so that the full set of technical requirements should also be set out in narrative form in order that, where necessary, any additional explanation of the requirements can be included.


The narrative part of the technical specification should commence with a descriptive outline of the overall scope of the works. For example, it might say:

This Specification details the requirements for the supply, delivery to site, off-loading onto prepared foundations, erection, preparation for ser vice, commissioning and maintenance for the maintenance period of an ONAN cooled oil-immersed three-phase double wound generator transformer for the connection of an 11 kV gas-turbine generator to the 33 kV network. The transformer shall be supplied complete with the first filling of oil.

Standards and conditions of service

The narrative should then identify the main International and National Standards which are to be applied and the extent to which these are to apply.

This may be followed by a detailed description of the service conditions, for example:

The transformer shall be suitable for outdoor installation under the normal service ambient conditions set out in EN 60076 except as modified by the requirements with regard to rating set out below.

Special requirements

Before detailing the requirements concerning rating it is appropriate to identify any other special requirements, for example in the case of a generator transformer a high reliability and availability will be desirable. Although, as explained above, the best way of ensuring that this is obtained would be to specify more extensive testing of the type discussed in Section 5.3, it could be that for a fairly small generator transformer, as in this example, the extra cost of this could not be justified. Whether additional testing is included or not however, it is worthwhile identifying the requirement for high reliability and availability and, by way of extra emphasis for this, the tenderer might be asked to identify in his tender those design features which he would incorporate for the purpose of obtaining high reliability. It will also be appropriate to include under this heading any requirement for the transformer to operate in parallel with an existing transformer.


The rating of the transformer required will be determined by the magnitude and the nature of the load. Since all except the smallest transformers are designed specifically for a particular contract, there is no reason why the rating specified should not be exactly that required, after making due allowance for any future load growth, where appropriate. There is no need to limit the specified rating to the preferred range of sizes, i.e. 1, 1.6, 2.5, 3.125, 5, 6.25, etc. An important point to be remembered, however, is that the EN 60076 rating is a purely notional quantity and is defined as the product of open circuit voltage times full-load current. This will be greater than the total MVA or kVA consumed by the load, which will be the product of open bus bar voltage after allowing for regulation within the transformer times full-load current.

Gas turbines usually provide their highest output at lower ambients than those of EN 60076, so, for a gas-turbine generator transformer, it will be more important to ensure that these can be obtained than to identify an appropriate EN 60076 rating. Hence the rating requirement might typically be set out in the following manner:

The continuous rated power of the transformer shall be matched to the out put of the associated gas turbine generator, which is as follows:

(a) 29 MVA at an LV terminal voltage of 11 kV and an ambient temperature of 0ºC.

(b) 26.2 MVA at an LV terminal voltage of 11 kV and an ambient temperature of 10ºC.

Under the above operating conditions the winding hot spot temperature shall not exceed the value appropriate to continuous operation under the normal operating conditions and temperature rises of EN 60076. The Tenderer shall state in his tender the winding hot spot temperature for operation at each of the above conditions. The Tenderer shall also state in his tender the equivalent EN 60076 rating of the transformer offered when operating under the normal EN 60076 ambient conditions. The above ratings shall be maintained on all tap positions.

The transformer is not required to have any specific overload capability other than that implied by virtue of its compliance with EN 60076.

The transformer will not be subjected to any unbalanced loading.

The reason for asking that the tenderer should also state a true EN 60076 rating is that this will assist in comparison of tenders, but more will be said later about the tender assessment process.

Rated voltage ratio

The voltage ratio to be specified is that applying on open circuit, so that in the case of a step-down transformer, the secondary voltage specified must make due allowance for regulation, for example in the case of a transformer required to supply an 11 kV system, it is likely that the LV open circuit voltage will need to be 11.5 kV. Ideally the Document of Design Intent should include a calculation of the open circuit voltage required to ensure that the minimum voltage necessary at the terminals of the load can be obtained with minimum supply voltage at the HV winding terminals with the transformer fully loaded.

The calculation should then be repeated for the condition with maximum sup ply voltage applied to the HV terminals and the transformer lightly loaded to ensure that an excessively high voltage does not appear at the load terminals. Ensuring that this does not occur might require the use of an on-load tapchanger on the transformer and these calculations will enable the required extent of the tapping range to be established.

Although in the example given above, the generator transformer has been described as having LV and HV voltages of 11 and 33 kV respectively, these are nominal values. In the case of the LV winding the actual, or rated, voltage may well be the same as the nominal voltage, but, in the case of the HV, the rated voltage will need to be higher than 33 kV because the generator, via its step-up transformer, will be required to export MW and MVArs to a system which is normally at around 33 kV. The Document of Design Intent will similarly be required to include a calculation of the precise voltage required (for an explanation of this see Section 7.1). The tenderer must therefore be given the rated voltages for each winding. In this example the HV rated voltage will be taken to be 34.6 kV.

Flux density

It is also very important that the tenderer is given sufficient information to determine the nominal flux density for his design. Alternatively, it is often simpler to specify the maximum permissible nominal flux density to be used. This latter alternative might be considered by some as no longer an acceptable practice since it is tantamount to telling the tenderer how design the transformer.

As indicated in Sections 7.1-7.3 and elsewhere, flux density is determined by the combination of applied voltage, tap position and frequency. The difficulty of ensuring that the designer is made aware of the most adverse condition which can occur in operation can be appreciated by considering the following typical clause which it would be necessary to include in a specification for the generator transformer used in the example.

In the UK the likely variation of 33 kV system voltage and frequency is given in the Distribution Code. At voltages of 132 kV and above the relevant document is the Grid Code. The following typical clause has been based on an interpretation of the Distribution Code current at the time of writing:

The HV nominal system voltage is 33 kV. It will normally be maintained within +/-6 percent of this value but may occasionally and for short periods reach a level of plus 10 percent above nominal.

The nominal LV terminal voltage is 11 kV. This will be maintained by the action of the generator automatic voltage regulator within a band of +/-5 percent of the nominal value.

The nominal system frequency is 50 Hz.

The transformers shall be capable of exporting full generator output to the 33 kV system and of operation without damage at the loadings indicated above, over the range of power factors from 0.85 lag to 0.95 lead and frequency 47 to 51 Hz under the following conditions:

(i) Frequency range 49.5-51 Hz:

at rated MVA and with rated applied voltages, continuously.

(ii) Frequency range 47-49.5 Hz:

the decrease in transformer throughput MVA shall not be more than pro-rata with the change of frequency.

Operation below 47 Hz down to 40 Hz during extreme emergency system conditions will be for periods not longer than 15 minutes at or about no load with the voltage adjusted pro-rata with frequency. [The requirement for operation below 47 Hz would normally only apply to a generator connected to the main transmission network and would probably not be a requirement for an embedded generator such as this unless it was required to have the capability for operation in an islanded mode. ]

The simpler alternative to the above would be to specify that the nominal flux density should not exceed 1.7 Tesla at rated voltage and frequency, but it must, of course, be recognized that this is transferring to the user the responsibility for ensuring that at this nominal flux density there is no risk of saturation under any operating condition.

Insulation levels

The above detail concerning system voltage would also generally enable the tenderer to decide on the insulation levels required for the HV and LV windings, except that because both the HV and LV system voltages are less than 52 kV, Table 2 of EN 60076, Part 3, allows two alternative impulse withstand voltages for each. Clause 7.1 of that document states that choice between the alternative levels depends on the severity of overvoltage conditions to be expected on the system and on the importance of the particular installation. For a generator transformer this would normally be taken as having a high importance so that the higher impulse levels of 75 kV for the LV and 170 kV for the HV would be appropriate. The narrative part of the technical specification should make this clear.

It is usual to quote insulation levels in terms of power frequency and impulse withstand tests so the wording of the appropriate clause would typically be:

The winding insulation levels shall be:

LV windings - power frequency 28 kV, lightning impulse 75 kV peak

HV windings - power frequency 70 kV, lightning impulse 170 kV peak.

This clause should also indicate whether it is required to make measurements of partial discharge during the induced overvoltage test and whether or not the lightning impulse withstand test is to include chopped waves. For a generator transformer, even one operating at the relatively modest voltages of 11/33 kV, specifying that each of these tests should be carried out would be a way of ensuring high reliability without incurring too much additional cost.


The decision concerning the extent of tapping range required will depend on the likely variation in the supply voltage and the acceptable limits on output voltage taking into account regulation within the transformer over the load range from light load to full load. The influence of these factors on the extent of the tapping range has been mentioned above in relation to voltage ratio. The tappings will be provided on the HV winding unless there is a very good reason for doing otherwise, and they will normally be full-power tappings, that is the product of rated tapping voltage and rated tapping current will remain constant and equal to the rating of the transformer. Thus, for tappings having a lower rated tapping voltage than the rated voltage on principle tap, the rated tapping current will be greater than the rated current on principle tap, and for tappings having a higher rated tapping voltage, the rated tapping current will be less than the rated current on principle tap. EN 60076-1, Clause 5.3, suggests that some economy can be obtained by applying a cut-off to the tapping current at some tapping having a higher rated tapping voltage than the minimum rated tapping voltage. Any saving will, however, be minimal and it is rare for such arrangements to be employed.

Defining the tapping range requirements also provides the balance of the information required by the tenderer to enable him to establish a value for flux density. Typically, again considering the generator transformer of the above example, the specification might say:

Full-power tappings shall be provided on the 34.6 kV winding for a variation of the no-load voltage over the range +4.44% to -15.54% in 18 steps of 1.11%.

This wording provides sufficient information, taken in conjunction with that relating to power factor, applied voltage and frequency given above. It should be noted that the above arrangement of tappings does not provide round percentages at either end of the tapping range but the overall range is approximately 20 percent and the extreme taps are a whole number of steps removed from the principle tapping. An alternative way of specifying tappings, used in EN 60076 and much of Europe, is to quote the number of tappings in each direction and the size of step. The above arrangement would thus be described as 36.4 kV + 4 x 1.11%, -14 x 1.11%.

System grounding and short-circuit levels

Even when a transformer has uniformly insulated windings throughout, as will be the case for the 11/33 kV generator transformer of the example, it is customary to provide details of the system grounding. This enables the tenderer to be assured that there will be no condition arising in service which might stress the transformer to a higher level than that for which it has been designed. This is relevant since, if the tenderer is ultimately given a contract to supply the transformer he will be required to provide a warranty of at least 12 months, possibly longer.

Similarly it is customary to provide information concerning system fault levels which will enable the tenderer to calculate the currents which the transformer will be required to withstand in the event of a short circuit on either set of winding terminals with system volts applied to the other winding. EN 60076, Part 5, allows the impedance of the supply to be taken into account in calculating short-circuit current and gives values for supply impedance which may be assumed in the absence of any information provided by the user. The supply impedance is usually small compared to that of the transformer how ever, and margins of safety are generous, so that the manufacturer is, in most cases, effectively ignoring the impedance of the supply.

If the transformer is required to withstand a short-circuit on its secondary terminals for longer than the 2 seconds implied by compliance with EN 60076, then this should be specified. Any other special requirements with regard to fault withstand capability should also be stated, for example if the transformer were required to provide power for frequent direct-on-line starting of large motors. It might be that the tenderer might be asked to supply evidence, either from test reports or calculations, of the transformers ability to withstand short circuit or frequent imposition of motor starting loads.

Load rejection

As explained in Section 7.1, generator transformers can be subjected to sudden load rejection leading to a short-term increase in the voltage applied to the LV terminals. Clause 8.3 of EN 60076, Part 1:1997, identifies this requirement for generator transformers and states that these shall be able to withstand 1.4 times rated volts for 5 seconds at the terminals to which the generator is connected.

It is helpful to remind tenderers of this, in the case of an enquiry for generator transformers, by identifying that they may be subjected to sudden load rejection and to include any details of the resultant overvoltage requirement particularly if a more specific figure than that quoted in EN 60076 is available.

Figure 1 - Typical impedance envelope as used to specify acceptable impedance variation with tap position.


As explained in Section 6.6 which deals with system faults and their effects on transformers, system designers are constantly striving to achieve the best com promise between the lowest level of impedance which will nevertheless limit fault currents to an acceptable magnitude and the highest level which can be tolerated without resulting in excessive system regulation. As a result they are invariably aiming to restrict manufacturers to the tightest possible tolerance limits on impedance values. Impedance tolerances can frequently be restricted to narrower limits than those set out in EN 60076, but there is usually a price to be paid. It is desirable therefore that close tolerances on impedance are only specified if there is a very good reason for doing so. A significant proportion of the variation in impedance is due to change of tap position. Although designing for a closer overall tolerance might be difficult or involve additional cost, it is sometimes possible to arrange that the impedance characteristic is of a shape which causes the least problems for the system designer. For an explanation of this the reader is referred to the discussion concerning variation of impedance with tap position contained in Section 4.6. If impedance and the variation thereof is so important that normal EN 60076 tolerances are not acceptable, then the best way of specifying this is by means of an envelope of acceptable values as shown in Fig. 1.


If the transformer has any multiple rating arrangement, as, for example, is provided by ONAN/ODAF type cooling, or should the transformer have any special emergency rating in addition to its normal EN 60076 value, then it must be made clear to the tenderer under which condition the losses are to be guaranteed. Referring, once more to the generator transformer of the example, losses are most important at the gas-turbine generator full-load condition. The tenderer should therefore be instructed that the losses are to be guaranteed at this loading. This is an appropriate place also to indicate to the tenderer the value to be placed on losses and whether these will be taken into account for tender assessment purposes. A suitable clause to be included in the specification for the generator transformer of the example would be:

The guaranteed losses of each transformer on principal tapping and at a winding temperature of 75°C shall be stated by the Tenderer. Load losses shall be guaranteed at the maximum rated power of 29 MVA. These guaranteed losses will be used as the basis for evaluation of tenders, for acceptance or rejection of the completed transformer or for variation of the Contract Price in accordance with the capitalized cost of losses specified in the Enquiry.

Sound power level and vibration

The tenderer should be advised of any noise constraints existing at the pro posed site and whether any form of sound attenuating enclose is required for the transformer either initially or in the future. The tenderer should be asked to give guarantees of maximum sound power level when measured in accordance with EN 60076-5.

Transformer construction

The technical specification may also identify any constructional requirements which are considered important. This will include whether radiators may be tank mounted or whether a separate free-standing cooler bank is required. The manufacturer will probably ensure that there is an adequate number of removable covers for access to such items as winding temperature indicator current transformer connections and core and core-frame ground connections as he is likely to need access to these at the stage of installing the transformer in its tank. It is nevertheless worthwhile reiterating this requirement as well as specifying that these should be provided with lifting handles and be light enough in weight to be easily removed by one person. This usually means no heavier than 25 kg.

Consideration should be given as to whether it is required that the core and core-frame ground should be brought outside the tank to enable the insulation resistances to be periodically checked without lowering the oil. The cost of pro viding this feature is not large in relation to the overall cost of the transformer and the advantages of bringing out these connections on any transformer rated more than a few MVA is becoming increasingly recognized.

The section of the specification relating to construction should also identify whether a welded flange for the tank cover is acceptable. Such an arrangement has the benefit that the possibility of oil leaks is eliminated, but it has the disadvantage that, should it be necessary to gain full access to the tank for any reason, (as distinct from the limited access which can be gained by removing an inspection cover) this will involve cutting of the weld and re-welding when any work is completed. Where such an arrangement is adopted, it is usual to specify that the design of the welded joint shall be suitable for opening, by grinding or otherwise, and subsequent re-welding on at least three occasions.

Cleaning and painting

Included under the heading of construction are the requirements concerning cleaning and painting. If the transformer is to be installed at a particularly hostile site, for example close to the sea, then this should be stated. Internal and external surfaces of all equipment other than that having machined mating surfaces should be shot blasted or cleaned by other similar process and the first protective paint coat applied on the same day without any outdoor exposure.

External surfaces should then be suitably primed followed by two further coats of weather and oil-resisting paint. These should be of contrasting colors so that full coverage can be easily established and specifying a minimum total thickness of 0.13-0.15 mm will ensure a good durable protection.

The internal surfaces of tanks, core frames and any vessels or chambers which are to contain oil should be sealed by means of a single coat of oil-resistant paint or varnish, the main objective of this being to prevent the catalytic action of the steel on the oil. As indicated in the section dealing with distribution transformers, this paint treatment is often omitted in the case of distribution transformers without, it is claimed by manufacturers, any deleterious effect on the long term quality of the oil.

The advantage to be gained by galvanizing any thin sheet steel components, particularly panel type radiators, prior to painting is becoming increasingly recognized. These should be hot-dip galvanized to the appropriate ISO Standard prior to the application of a paint finish in a similar manner to that described above for the other external surfaces. Without galvanizing, any loss of paint which might occur after leaving the factory quickly results in rusting. This usually occurs in the crevices between the panels so that adequate preparation for repainting is virtually impossible with the result that, even if an attempt is made to repaint these, it is likely to be unsuccessful and the radiator life becomes seriously restricted. The only disadvantage of galvanizing is that it is more difficult to achieve a good bond for the subsequent paint coating.

Most manufactures of panel radiators are, however, able to carry out some artificial weathering or similar process which results in a greatly improved bond.


For any transformer manufactured in the UK it is likely that the initial filling of oil will be uninhibited napthenic to BS 148 unless the purchaser has specified otherwise. If the transformer is manufactured outside the UK it will be necessary to specify the type of oil to be supplied. If the transformer is to be installed in the UK then the use of uninhibited napthenic oil is to be preferred since this is least likely to create problems should it ever be necessary to request one of the UK oil suppliers to remove the oil from the transformer and take it for reprocessing.


The technical specification should list the fittings to be supplied with the transformer. These should be selected from the following.


Most transformers used in the UK, with the exception of distribution transformers of around 1.6 MVA or less, are likely to benefit from the fitting of a conservator. Transformer breathing systems are discussed in Section 4.8. If a conservator is specified it should have a capacity of about 7.5 percent of the total cold oil volume within the transformer. A removable end cover should be provided to allow the interior to be cleaned out if necessary and the conservator should be provided with a sump to contain any solid deposits either by extending the feed pipe inside the conservator or by bringing this in through an end wall. The extension or wall entry should be such as to provide a minimum sump depth of 75 mm or one-tenth the diameter of the conservator, whichever is less.

It will be necessary to provide a means for filling the conservator which must be airtight and weatherproof when closed. An oil level gauge will be required. This may be simply a prismatic sight glass or it may be of the type having a dial pointer which is magnetically coupled to an internal float operating within the vessel. If a sight glass is used it is advantageous for this to be angled downwards slightly to aid viewing from the transformer plinth level.

Prismatic gauges are the most foolproof but the magnetically operated type can be arranged to provide remote indication of oil level by means of micro switches and/or transmitters/transducers. Whichever type of gauge is used it must have a mark corresponding to the 15° C oil level and may also be marked with, say, -10°C and +80°C levels.

For most transformers the conservator will also be fitted with a desiccant-filled breather. For transformers of 275 kV and above a refrigeration breather may be fitted as an alternative.

A transformer having an on-load tapchanger will normally have a second small conservator for the diverter-switch oil. This frequently takes the form of a sectioned-off portion of the main conservator at one end of the main conservator.

Cooling equipment

If tank-mounted radiators are permitted these should be detachable to allow replacement or repair in the event of a leak and these will thus require a butterfly type isolating valve at each of the points of connection to the tank.

Where separate free-standing coolers have been specified these will need expansion devices in the inlet and outlet pipes to the transformer.

For a transformer having forced cooling, the extent of standby capacity should also be identified, for example if the transformer is required to deliver full rated output with one oil pump and/or one fan out of action this should be clearly stated.

If the transformer is to be water-cooled, the need for cooler standby capacity is more important since a water cooled transformer has no naturally cooled capability and without standby a loss of cooling represents a loss of transformer. The Document of Design Intent should therefore make an assessment to decide the extent of standby required, usually this means whether this is to be 2 by 100 percent coolers or 3 by 50 percent. Alternatively the tenderer could be asked to provide alternative prices for each option so that this can be decided at the tender assessment stage. It will also be necessary to provide the tenderer with an analysis of the water quality to enable cooler tube and tube plate material to be decided.

Consideration should also be given to the use of double tube, double tube plate, coolers, particularly if the cooling water is at high head or is of a chemically highly aggressive nature.


The number, size and location of all valves required for maintenance and operation of the transformer will need to be identified. These should comply with the appropriate national or international standard. European Standards 1171, 12334, 12288 and 593 are the ones applying in Europe and the UK at the time of writing. It will avoid the risk of operating errors if valves are standardized to all have the same direction for operation. Clockwise to close is the convention usually adopted. Isolating and filter valves should preferably be of the wedge gate variety. Up to 75 mm nominal bore these will be of copper alloy. For some applications, for example for the individual isolation of tank-mounted radiators, should it be necessary to remove these, as mentioned above, butterfly valves may be acceptable. When a radiator has been removed, unless it is replaced immediately the closed butterfly valve will be covered with a blanking plate as additional security. The necessary blanking plates should be included with any maintenance spares supplied with the transformer. If the transformer is to be provided with a separate free-standing cooler bank, consideration should be given as to whether it is required to provide isolating valves to enable this to be mounted at either end of the tank. Any valves which are open to the atmosphere should also be provided with blanking plates.

All valves should be padlockable in both the open and closed positions so as to avoid the risk of any unauthorized interference.

The following valves should be provided as a minimum.

Isolating valves

(i) On the conservator side of any gas and oil actuated relay,

(ii) A valve for draining the conservator sump -- usually 50 mm will be adequate, (iii) A valve at the lowest point of each main oil pipe -- usually 50 mm will be adequate,

(iv) A valve at the lowest point of any oil-filled chamber -- usually 50 mm,

(v) A valve for draining the main tank. This should be 80 mm as a minimum, larger on large transformers.

As a cost saving measure some users will accept the use of a screw-in plug for draining short pipes and small oil-filled chambers. These have the disadvantage that when loosened there is no control over the escaping oil and they are therefore best avoided.

Filter valves

(vi) A filter valve is required at the top and bottom of the main tank, sensibly diagonally opposite to each other, and in the top and bottom headers of any separate cooler bank. Filter valves will normally be 50 mm and should be fitted with adaptors for flexible hoses and be complete with covers and gaskets.

Gas and oil actuated relays

A gas and oil actuated relay will be required in the oil feed pipe to each conservator, that is, main conservator and any tapchanger diverter-switch conservator where provided. In the case of some small single compartment tapchangers the conservator and gas and oil actuated relay are built into the tapchanger itself.

To ensure correct operation in the event of an oil surge, gas and oil actuated relays should be fitted into a straight run of pipework having a minimum length of about five times the internal diameter of the relay on the tank side of the relay and three times the internal diameter of the pipe on the conservator side of the relay. The pipe should be arranged at a rising angle of between 3º and 7º to the horizontal.

In order to assist with routine testing of the relay as well as the venting of it in the event of gas collection, separate pipes terminated in pet-cocks should be brought down to a suitable height above plinth level. The pet-cocks should be provided with end covers and be lockable in the closed position. The pipe for air injection should be provided with a suitable one-way valve, as close to the relay as possible, to prevent oil seepage down the pipe.

The tenderer should be given details of the requirements regarding type and duty of alarm and trip contacts to be provided on the gas and oil actuated relay.

Pressure relief device

All transformer tanks should be provided with a pressure relief device to reduce the likelihood of tank rupture in the event of a severe internal fault. This must be provided with deflector pipework to ensure that any oil released is safely directed to within 1 m of plinth level.

The tenderer should be given details of the requirements regarding type and duty of alarm and trip contacts, if any, to be provided on the pressure relief device. It is generally considered that alarm contacts only are required, since, although spurious operation of the device is unlikely, the number of trip sources should be restricted to a minimum to avoid unnecessary tripping and any internal event which operates the pressure relief device is almost certain to operate the Buchholz relay also, thus tripping the transformer by that means.

Winding or oil temperature indicators

Requirements for winding temperature indicators should be set out. On less important transformers or those rated below about 5 MVA, some economy can be obtained by the use of oil temperature indication only. Otherwise winding temperature indication should be provided for each winding. It is desirable that winding temperature indicators should have a means of checking the operation and setting of the contacts, usually via a spring loaded knurled knob, external to the indicator case. This should have wire and lead sealing facilities to prevent unauthorized interference. The indicators should be scaled from about 30°C to 150°C and have independently adjustable alarm and trip contacts. The range of adjustment for both sets of contacts should cover from about 80°C to full scale. If possible the enquiry document should advise the initial settings to be provided.

The tenderer should be given details of requirements regarding the type and duty of the alarm and trip contacts to be provided on the winding temperature indicators.

The winding temperature indicators should be provided with isolating and test links so that these may be checked for correct operation whilst the transformer is in service.

The tenderer should also be advised of any requirement for remote indication of winding temperature including details of the type of system to be used and the type of transmitter to be provided.

cont. to part 2 >>

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