Industrial Power Transformers -- Special features of transformers for particular purposes [part 8]

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Distribution transformers are normally considered to be those transformers which provide the transformation from 11 kV and lower voltages down to the level of the final distribution network. In the UK this was, until January 1995, 415 V three phase and 240 V phase to neutral. Now it is nominally 400 V three phase and 230 V between phase and neutral. Of course, these are nominal volt ages to be applied at consumers' terminals and there are tolerances to take account of light loading conditions and regulation at times of peak load. Prior to January 1995, most distribution transformers were designed for a secondary open-circuit voltage on principal tapping of 433 V and it remains to be seen whether this situation will change in the long term. At the present time, however, transformer voltage ratios have not changed, although it is possible that some adjustment of transformer off-circuit tappings might have been made at some points of the distribution network. Throughout the following section, therefore, in making reference to distribution transformer LV windings and systems, these will be termed 415 V or 0.415 kV. Except where specifically indicated to the contrary this should be taken as a nominal description of the winding or system voltage class and not necessarily the rated voltage of the winding or system in question.

Distribution transformers are by far the most numerous and varied types of transformers used on the electricity supply network. There are around 500 000 distribution transformers on the UK public electricity supply system operated by the Distribution Network Operators (DNOs) and a similar number installed in industrial installations. They range in size from about 15 kVA 3.3/0.415 kV to 12.5 MVA 11/3.3 kV, although most are less than 2000 kVA, the average rating being around 800 kVA. The vast majority are free breathing filled with oil to BS 148, but they may be hermetically sealed oil filled, dry type, or, occasionally, where there is a potential fire hazard, fire-resistant fluids notably silicone fluid, natural or synthetic ester or high molecular weight hydrocarbons which have a fire point in excess of 300ºC may be specified. Environmentally friendly fluids such as natural esters are also becoming increasingly utilized in the USA in distribution transformers, particularly where there might be a perceived environmental risk in rural areas and by companies who wish to market themselves as 'green.' This section will first discuss oil-filled units in some detail and later high light those aspects which are different for dry-type transformers. Other fluids have been discussed in Section 3. As far as the constructional features of transformers using these are concerned, there are no significant differences com pared with oil-filled units apart from the need to ensure that all the materials used are compatible with the dielectric fluid. Most insulating materials, including kraft paper and pressboard, are satisfactory on this score; if there are problems it is usually with gaskets and other similar synthetic materials.

Design considerations

Distribution transformers are very likely to be made in a different factory from larger transformers. Being smaller and lighter they do not require the same specialized handling and lifting equipment as larger transformers. Impregnation under very high vacuum and vapor-phase drying equipment is not generally required. At the very small end of the range, manufacturing methods are closer to those used in mass production industries. There are many more manufacturers who make small transformers than those at the larger end of the scale. The industry is very competitive, margins are small and turnaround times are rapid.

As a result the main consideration in the design of the active part is to achieve the best use of materials and to minimize costs, and a 1000 or 2000 kVA transformer built in 2006 would, on reasonably close examination, appear quite different from one made as recently as, say, 20 years earlier.


Simplicity of design and construction is the keynote throughout in relation to distribution transformers. Simplification has been brought about in the methods of cutting and building cores, notably by the reduction in the number of individual plates required per lay by the use of single plates for the yokes (notched yokes) rather than the two half-yoke plates as would generally be used for a larger transformer. Nonetheless all joints are still mitered and low-loss high-permeability materials are widely used. Cores are built without the top yoke in place and, when the yoke is fitted, this is done in a single operation rather than by laboriously slotting in individual packets of plates. Core frames have been greatly simplified so that these have become little more than plain mild steel 'U' section channels drilled in the appropriate places, and occasionally some manufacturers may use timber for the core frames. These have the advantage that there are no problems with clearances from leads, for example, to be considered in the design of the unit but they are not so convenient in other respects, for example it is not so easy to make fixings to them for lead supports or to support an off-circuit tapchanger. Timber frames are now generally considered by most manufacturers to be less cost effective than steel channels and are now generally tending to be phased out. It is, of course, hardly necessary to state that distribution transformer cores are invariably of a totally boltless construction. Wound cores, in which the core material is threaded in short lengths through the windings to form a coil (FIG. 24) are common for smaller ratings up to several tens of kVA.

FIG. 24 Wound core for single-phase 11/0.250 kV, 50 kVA, pole mounted transformer. Although not discernible in the photograph, each loop of core steel has an overlapped joint at the upper end. As an indication of its physical size, the core limb is about 10 cm square.

One occasion on which more sophisticated designs are widely used in distribution transformers is in relation to the use of the step-lapped core construction described in Section 4.1. This form of construction is to be regarded as the norm for most distribution transformer cores and was adopted for distribution transformers more rapidly than for larger units. There are a number of reasons for this:

• Joints form a greater proportion of the total iron circuit in the case of a small distribution transformer core compared to that of a large power transformer and so measures to reduce losses at the joints will show a greater benefit.

• Building a small core is so much easier than it is for a large core, so that the more sophisticated construction does not present such an obstacle in manufacture.

• Distribution transformers tend to operate at poor load factors. Although this means that the magnitude of the load loss is not too important, iron loss is present all the time and it is therefore desirable to minimize its impact.

• The competitive nature of the industry, discussed above, gives an incentive to provide low losses and noise levels, both of which are improved by using the step-lap construction.

Distribution transformer cores also represent the only occasion for which the use of amorphous steel has been seriously considered in the UK (and quite widely adopted in other countries, notably the USA). As indicated in Section 3.2, the dimensions of the material currently available is one factor which prevents its use in larger transformers, but nevertheless some of the reasons discussed above for the adoption of the step-lap form of construction, namely the relative ease of building small cores and the importance of minimizing iron losses as well as the competitive commercial situation also provide strong incentives for innovation in core design amongst distribution transformer manufacturers.


FIG. 25(a) Construction of foil winding: Copper or aluminum foil wound into cylinder; Lead welded or brazed to edge of foil; Insulation wound between foil layers


FIG. 25(b) Foil winding in manufacture. Two widths of foil are being wound in axial dimension with diamond dotted presspaper insulation between layers


Foil windings are frequently used as LV windings. In this form of construction the winding turn, of copper or aluminum foil, occupies the full width of the layer. This is wound around a plain mandrel, with intermediate layers of paper insulation, to form the required total number of turns for the winding. Strips of the conductor material are welded or brazed along the edge of the foil at the start and finish to form the winding leads as shown in Figs 25(a) and 25(b). Any slight bulge that this creates in the section of the winding is of no consequence. This arrangement represents a very cost effective method of manufacturing LV windings and also enables a transformer to be built which has a high degree of electromagnetic balance and hence good mechanical short-circuit strength. Diamond dotted presspaper (see Section 3.4) is frequently used as interlayer insulation for these windings which also gives them added mechanical strength. The diamond dotted pattern enables the dry-out process to be carried out more easily than would be the case if the resin bonding material were applied uniformly to the whole surface of the presspaper sheet. Foil windings are produced in this way for use in oil-filled transformers, however, the same construction using class F materials can be used in air-insulated transformers or as the LV windings of cast resin transformers.

Distribution transformers frequently use other types of winding construction not found in larger transformers in addition to the foil windings described above. Because of the small frame sizes resulting from low kVA ratings, the volts per turn is usually very low so that for an HV winding a considerable number of turns will be required. The current is, however, also low and the turn cross-section, as a result, is small. Winding wires are frequently circular in section and enamel covered. Circular cross-section wire cannot be wound into continuous disc windings so multilayer spiral windings are common. These will normally have one or more wraps of paper between layers to give the winding stability and to provide insulation for the voltage between layers. One problem with this arrangement is that when drying out the winding the only route for removal of moisture is via the winding ends so that the dry-out process must allow sufficient time under temperature and some degree of vacuum to allow the moisture to migrate axially along the length of the layers. Frequently the dry-out time for this type of winding might appear disproportionately long for a small transformer.

FIG. 26 Crossover coil

Another alternative for HV windings is the use of 'crossover' coils. This form of construction is shown in FIG. 26 which shows an individual coil.

Each section of the winding, or coil, is itself a small multilayer spiral winding having a relatively short axial length. A complete HV winding will then be made up of perhaps six or eight coils arranged axially along the length of the winding and connected in series as shown in the photograph of a complete transformer, FIG. 27. Crossover coils are easier to dry out than full length multilayer windings since they have a short axial length and, by subdividing the winding into a number of sections, the volts within each section is only a fraction of the phase volts, thus distributing this evenly along the leg. For this reason this form of construction is likely to be used for the higher-voltage class of HV winding, for example at 22 or 33 kV, where a simple layer construction would not provide the necessary clearance distances.

Continuous disc windings may, of course, used for any HV winding which has a large enough current to justify the use of a rectangular conductor. At 11 kV, this probably means a rating of about 750 kVA, three phase, and above could have a disc wound HV winding. At 3.3 kV disc windings could be used for ratings of 250 kVA, three phase, and above. Because of their intrinsically greater mechanical strength, disc windings would be preferred for any transformer known to have a duty for frequent starting of large motors or other such frequent current surges. Offset against this consideration is the fact that disc windings are more costly to manufacture so the final decision might be to avoid the use of a disc winding on the grounds of economy.

FIG. 27 Three-phase 750 kVA, 11 000/433 V, 50 Hz transformer with a mitered core. HV windings in delta, LV in star. Crossover HV coils; spiral LV coils. HV windings fitted with tappings brought up to an externally operated tap selector (ABB Power T&D Ltd)

Pressure for much of the innovation introduced into distribution transformers has come from the competition within this sector of the industry. Although many of the materials and the practices used have some application or spin-off for larger sized units, others can be used only because they are tolerable when currents are small and short-circuit forces, for example, are modest. One such case is in the use of winding arrangements which are square in planform as shown in FIG. 28. By adopting this arrangement the core limb can have a square cross-section so there is no need to cut a large range of plate widths, and the core with its three-phase set of windings is more compact so a smaller tank can be used. This is only permissible because small units with modest short-circuit forces do not need the high mechanical strength provided by the use of windings which are circular in section.

FIG. 28 400 kVA transformer with square-section core and coils.

This transformer has a core of amorphous steel (see also Fig. 3.8) but the technique can be used to simplify core construction and improve space-factor for any type of transformer where the unit is small enough to limit the short-circuit forces to a modest enough level

FIG. 29 Three-phase 11-33 kV HV off-circuit tap selector (ABB Power T&D)

Leads and tappings

Most distribution transformers will be provided with off-circuit tappings, generally at _2.5 and _5 percent on the HV winding, selectable by means of a pad lockable switch on the outside of the tank operable only when the transformer is isolated. A typical off-circuit tapping selector switch is shown in FIG. 29.

This enables the user, probably no more than once or twice at the time the transformer is commissioned, to select very conveniently the most appropriate LV voltage for its location on the system. At these low current ratings off-circuit switches are not subject to any of the problems of pyrolytic carbon deposits, described in Section 4.6, which beset the high current applications and which, as a result, lead to a preference for tapchanging by the use of off-circuit links on those very much larger units.

Multilayer windings and crossover coils are not as convenient as disc windings as regards the ability to make tapping connections from the outside face of the coil. It is common practice to make a tapping connection within a layer so that the lead is brought out along the surface of the layer and with possibly an additional layer of presspaper insulation above and below it to provide insulation as it crosses the adjacent turns within the layer. The tapping connections can be seen emerging from the ends of the central crossover coils in FIG. 27.

Simplification of the arrangement and method of forming leads internal to the tank has been made possible by the use of round wire rather than flat copper bar for these wherever possible. Round wire or bar, being stiffer, usually requires fewer supporting cleats and since it can be bent with equal ease in all planes it can usually be taken from point to point in a single formed length, whereas flat bar might require several specially formed bends and joints in order to follow a complex route. Joints external to windings are generally formed by crimping and are nowadays rarely brazed. Crimping has the advantage that it avoids the need to bring a blowtorch into the close proximity of windings with its associated risk of fire or, at the very least, overheating of insulation. Crimped joints are also made very much more quickly than brazed or sweated joints, leading to cost savings.

Widespread use is also made of pre-formed insulation sections, for example flexible créped paper tubes threaded onto leads to provide external insulation for these, and corrugated pressboard to form interwinding ducts.


Because of the relatively large numbers made, some flow-line production can be introduced into tank manufacture for the smaller units, notably the 3.3/0.415 kV pole-mounted types. This requires that tanks should be standardized, which means that the fittings provided and the location of these must also be standardized. Internal surfaces, as well as the steel core frames, are usually left unpainted. Although this goes against the principle of preventing oil coming into contact with the catalytic action of the steel, manufacturers claim that with modern oils, for the conditions of operation encountered in sealed distribution transformers (see below) this does not lead to unacceptable levels of oxidation.

A form of construction leading to what are generally known as corrugated tanks is frequently used for distribution transformers. With this type of construction the tank sides are formed from thin sheet steel, generally between 1.5 and 2 mm thickness, folded to form vertical cooling fins. The fins project between 100 and 150 mm and are spaced about 25 mm apart. After the folding operation the top and bottom edges of the fins are seam welded in order to make these oil tight. One or more sides of the tank may be corrugated in this way, and the advantage of the arrangement is that it combines the provision of a cooling surface with a means of providing the tank with an expansion capability to enable it to be hermetically sealed rather than free breathing.

Hermetic sealing of tanks is described below.

Often the provision of a dehydrating breather for small distribution transformers would result in an unacceptably high maintenance liability. These transformers are therefore frequently sealed, with a small cushion of dry air above the oil to allow for expansion and contraction. This limited amount of air in contact with the oil is then considered to present only a modest tendency towards oxidation. However the primary means of providing for expansion and contraction of the oil is the 'concertina' type movement of the corrugations described above. The transformers are filled to a predetermined optimized pres sure related to the filling temperature so that, assisted by the compliance pro vided by the corrugated sides, the range of internal pressure variation over the full range of operating temperatures is kept within acceptable limits. Sealing of the transformers prevents the moisture arising from insulation degradation from escaping, but again, this amounts to far less of a threat to insulation quality than would be the case if the transformers were left to breathe freely without a breather or if a breather, having been provided, was not maintained in a dry condition. In addition, sampling of the oil from hermetically sealed transformers is not really practicable, but then most operators of small distribution transformers would not consider sampling to be economically justified anyway.

Larger distribution transformers, say those of one or two MVA and greater, would probably benefit from having dehydrating breathers fitted provided that these were well maintained, in which case tank internals should be painted to prevent contact between the oil and mild steel components. As the units become larger, the use of a conservator tank to reduce the surface area of con tact between oil and air, and the fitting of a Buchholz relay must be considered, although the precise rating at which these measures become economically worthwhile is a decision for the user.

FIG. 30(a) Packaged substation having 11 kV switchgear consisting of two ring-main units and circuit breaker feeding transformer on right of photograph and 415 V feeder pillar for outgoing circuits on left, all mounted directly onto tank of 11/0.415 kV skid-mounted transformer. Incoming 415 V disconnector is on right-hand side of feeder pillar (Schneider Electric)


When used for 415 V local distribution purposes, ground-mounted distribution transformers with ratings from about 315 kVA are frequently supplied as part of a complete packaged substation unit. That is, 11 kV ring-main units, transformer isolating switch or circuit breaker and protection equipment, transformer and 415 V distribution panel are all included in a single package usually mounted on a skid base and ready to be placed on a prepared foundation.

This has the advantage that the connections from the HV switchgear to the transformer and from transformer to 415 V distribution panel are all internal and factory made. FIG. 30(a) shows a typical arrangement of packaged substation and the electrical connections of this are shown in FIG. 30(b). Eleven kilovolt cables are terminated to each side of the ring-main unit and the 11 kV tee-off connections from this are taken directly through internal 11 kV bushings into the transformer tank. On the 415 V side busbars emerge for direct connection onto the outgoing fuseways.

Nowadays the switchgear for packaged substations almost invariably consists of SF6-insulated sealed-for-life maintenance free units, with protection for the transformer and LV busbar zone provided by fuses for transformer ratings up to about 1.5 MVA, and above this by circuit breakers. At least one UK manufacturer has produced a protection device which uses the action of a fusible element to trigger an SF6 rotating-arc interrupter to give one-shot discriminating protection for the transformer in the event of a fault either internally or on the 415 V busbars. The fusible element provides the basis for the time graded discrimination and the whole unit is enclosed in a sealed-for-life SF6 module. It is a fairly simple step to progress from this arrangement to one in which the module is housed within the transformer tank to produce a 'self protecting transformer.' Such an arrangement is shown in FIG. 31.

FIG. 30(b) Electrical arrangement of packaged 11 kV/415 V substation shown in FIG. 30(a)

Dry-type and cast resin transformers

Dry-type transformers, particularly those using cast resin insulation, are now widely used in locations where the fire risk associated with the use of mineral oil is considered to be unacceptable, for example in offices, shopping complexes, apartment buildings, hospitals and the like. The background to this development and the factors requiring to be considered in installing cast resin transformers within buildings have been discussed at some length in Section 6.1. This section describes the special features of cast resin transformers them selves. Although there are other types of dry insulation systems available, nowadays for most purposes dry type means cast resin, so the following paragraphs are written primarily as referring to cast resin.

FIG. 31 800kVA unit substation transformer with integral HV protection (Schneider Electric)

In 2004 IEC issued a standard for dry-type transformers as Part 11 of the IEC 60076 series covering power transformers. The following discussion of dry-type and cast resin transformers relates to transformers complying with IEC 60076-11, except where indicated otherwise.

Complete encapsulation of the windings of a power transformer in cast resin is an illogical step to take, because, as explained on a number of occasions elsewhere in these pages, one of the main requirements in designing transformer windings is to provide a means of dissipating the heat generated by the flow of load current. Air is a very much poorer cooling medium than mineral oil anyway, without the additional thermal barrier created by the resin. All air cooled transformers are therefore less efficient thermally than their oil-filled counterparts and cast resin are poorer than most. Hence they will be physically larger and more costly even without the added costs of the resin encapsulation process. In addition, the absence of a large volume of oil with its high thermal inertia means that cast resin-insulated transformers have shorter thermal time constants which limit their overload carrying capability. More will be said about overload capability below.

The incentive to develop an economic design of cast resin transformer was provided by the outlawing of polychlorinated biphenyls (PCBs) in the late 1970s on the grounds of their unacceptable environmental impact. Alternative non-flammable liquid dielectrics have all tended to have had some disadvantages, with the result that users have come to recognize the merits of eliminating the liquid dielectric entirely. (A possible exception is natural ester fluid (see above and Section 3.5) which is gaining fairly wide acceptance in the USA.) Nevertheless, cast resin does not represent an automatic choice of insulation system for a power transformer. Cast resin transformers are expensive in terms of first cost. They are less energy efficient than their liquid-filled equivalents.

In their early days there were suggestions that their reliability was poor and even that their fire resistance left something to be desired. In recent times, however, their qualities of ruggedness, reliability and excellent dielectric strength have come to be recognized as outweighing their disadvantages and their use has become widespread in situations where these properties are most strongly valued.

Resin encapsulated windings

Cores and frames of cast resin transformers are very similar, if a little larger, than those of oil-filled distribution transformers. It is in the design of the windings that cast resin transformers are unique. Four hundred and fifteen volt LV windings are usually foil wound, as described above for oil-filled transformers, and are non-encapsulated, although they are frequently given a coating of the same resin material as that used for the HV winding in order to provide them with an equivalent level of protection from the environment.

It is the HV winding which is truly resin encapsulated. Apart from the problem of heat dissipation, the other problem arising from resin encapsulation is the creation of internal voids or minute surface cracking of the resin. Voids can arise due to less than perfect encapsulation or they can be created due to differential thermal expansion between winding conductors and the resin, which may also lead to surface cracking. Surprisingly, the resin has a greater coefficient of expansion than the conductors. The coefficient of expansion of aluminum is a closer match to that of resin than is copper, and aluminum is therefore the preferred winding material. This may be either wire or foil.

If wire is used this will normally be round in section, with a thin covering of insulation. This will probably be randomly wound to the required build-up in diameter, either over a plain mandrel or one which is notched at intervals so that the turns progress from one end of the winding to the other to provide an approximately linear voltage distribution along the axial length of the winding.

If the winding is wound from foil, then a number of narrow foil-wound sections will be connected in series in a similar manner to the method of connecting crossover coils described earlier. Each foil-wound section will be machine wound with two layers of melamine film between foils to provide the interturn insulation; two layers being used to avoid the possibility of any minute punctures in the film coinciding and creating turn-to-turn faults. The melamine film is exceedingly flimsy and the foil must be free from any edge defects which could cut through the film, and a very high level of cleanliness is necessary during the foil winding process to ensure that no particles are trapped between foil and film which could also lead to breakdown by puncturing the film. The winding process usually takes place within an enclosed winding machine which is pressurized with air to above the pressure of the winding room and dry filtered air is blown across the surfaces of the foil and films at the point within the machine where these are brought together. After winding, the foil sections must be kept in a carefully controlled ambient temperature to ensure that the winding tension remains within close limits so as to ensure that there is no relative axial movement of foils and film.

The encapsulation process involves placing the wire or foil-wound sections within steel molds into which the resin may be admitted under high vacuum. Resin, hardener and fillers, that is the material which gives the resin its bulk, are mixed immediately prior to being admitted to the mold. To ensure that the filler material is fully mixed, part quantity can be fully mixed with the resin and the remainder fully mixed with the hardener before the two are then mixed together. The windings are located within the molds by means of axial strips of resin material of the same quality as that used for encapsulation.

These are placed between the winding and the outer mold so that the resin covering the inner surface of the winding, which will be subjected to the HV to LV test voltage, will be totally seamless. It is important that the resin should penetrate fully the interstices between the conductors if the winding is of the wire-wound type. In some processes this is assisted by initially admitting low viscosity resin into the mold. This is then followed by the encapsulation resin which displaces it except in any difficult to penetrate places, which, of course, was the purpose of the low viscosity resin.

The resin hardening process is endothermic, that is it generates heat. In order to ensure freedom from stress within the cured resin to minimize the likelihood of resin cracking, it is necessary to carefully control the tempera ture of the curing process by cooling as and when required. Achieving the precise temperature/time relationship is critical to the integrity of the encapsulated winding so that this process is usually done under microprocessor control.

It is usual to provide cast resin transformers with off-circuit tappings on the HV winding at _2.5 and _5 percent of open-circuit voltage. These are selected by means of bolted links on the face of the HV winding. The windings are mounted concentrically over the core limb with shaped resilient end blocks, usually of silicone rubber, providing axial location and radial spacing.

The HV delta connection is made by means of copper bars taking the most direct route between winding terminals.


The complete unit is normally mounted on rollers so that it can be easily moved around for installation and is fitted within a sheet-steel ventilated enclosure. Since cast resin transformers are frequently associated with a 415 V distribution switchboard, the enclosure can be made an integral part of this, with the transformer LV busbars connected directly to the switchboard incoming circuit breaker or switch fuse. The HV supply cable may be glanded and terminated within the enclosure with cable tails taken directly to the winding terminals. If the cable comes from below, so that the cable tails pass in front of the face of the HV coil, the transformer manufacturer will specify a minimum clearance between these and the coil face. Some users may prefer to keep the cable termination external to the transformer enclosure and mount a cable box on the outside with through bushings taking the connections into the enclosure so that these may be linked across to the winding terminals. FIG. 32 shows a cast resin transformer core and windings and this figure shows how such a transformer can be incorporated in a 415 V switchboard.

Problems with cast resin transformers

Despite the problems associated with cast resin which have been briefly mentioned already, the history of cast resin transformers has been remarkably successful and catastrophic failures have been few.

The possibility of voids and of resin cracking is one problem which has been identified. One measure which can help to resist cracking is the incorporation into the resin of some reinforcement, such as, for example, glass fiber. This may be distributed uniformly within the resin as an additional filler, or it may be included simply at strategic locations where the tendency to crack is considered to be the greatest. In addition small quantities of plasticizers can be added to the resin to give it some resilience. The presence of voids or cracks can be detected by partial discharge testing and the only certain indication of the absence of these is that the winding remains discharge free, that is indistinguishable from background, at twice normal volts. The creation of cracks, or voids, is aggravated by thermal stresses. Stresses induced by differential thermal expansion will be greatest at high temperature, but the resin is more brittle, and therefore more likely to crack, at low temperature. Thermal shock, such as that induced by the sudden application of full load to a cold transformer or mechanical shocks received during shipment in low temperatures may be particularly dam aging in this respect. Special tests in IEC 60076-11 for proving the quality of dry-type transformers include resistance to the effects of thermal shock. These will be discussed below.

FIG. 32 11 kV/433 V cast resin transformer (Schneider Electric)

Another concern attached to cast resin was that although it might not self ignite, if a cast resin transformer was engulfed in a fire, resins were generally considered to be the type of materials which would burn to produce more heat and/or generate copious quantities of toxic fumes. The fire properties of the resin are, however, largely determined by the type of filler used. Fillers are usually mainly silica, but it is possible to add quantities of other materials to the silica which greatly improve the fire performance. This is also a property which can be tested and it is nowadays the usual practice for manufacturers to submit a prototype winding to a fire resistance type test such as that included in the abovementioned standard and further described below.

Cast resin transformers are also considered to have poor overload capability due to their short thermal time constants referred to above and also because of the combined effects of poor cooling of the conductors and the limitations imposed on operating temperature due to the need to limit thermal stresses.

Manufacturers have however improved overload performance over the years by changes in the constitution of the resin. The plasticizers mentioned above assist overload capability by improving the cracking resistance, and the thermal conductivity of the resin can also be influenced by use of a suitable filler.

If it is required to subject a cast resin transformer to cyclic rating which takes it above its nameplate continuous rating the manufacturer should always be consulted. It must not be assumed that standard loading guides are applicable to cast resin transformers. At the time of writing, Spring 2007, IEC have in preparation a loading guide for dry-type transformers. This will be IEC 60076, Part 12.

Forced circulation of the cooling air will, of course, improve the heat transfer from the windings so that it is possible to obtain a dual rated cast resin transformer which achieves its higher forced cooled rating by means of fans, usually mounted off the lower core frames, and directing air over the windings.

Another possible difficulty with cast resin transformers is that, because of the high cost of molds, there is a strong incentive for manufacturers to pro duce a limited range of standard designs, so that if alternative impedances, non-standard ratings or non-standard losses are required, then these cannot be obtained in cast resin. For the vast majority of applications, however, where a standard unit can be used this will not present a problem.

Testing of cast resin transformers

Most of the testing carried out on cast resin transformers is very similar to that which would be carried out on an oil-filled distribution transformer. They can be impulse tested, if required, and if the transformer is to be installed within an enclosure, the impulse test should be carried out with the transformer in its enclosure. Similarly, if a temperature rise test is to be carried out, then this should be done with the transformer installed within the enclosure and, as indicated in Section 5, it is desirable that the temperature rise test on any dry-type transformer should be done using one of the methods involving excitation of the core at normal voltage. Short-circuit testing is also possible, though unusual, but if carried out, this should be in accordance with EN 60076-5 and one of the acceptance criteria is that that partial discharge measurements must be repeated following the short-circuit application, whereupon the same partial discharge acceptance levels apply as those applicable before the short circuit test.

The most important tests on cast resin transformers, however, are those which the manufacturer carries out in order to prove his resin encapsulation system. These are identified as Special tests in IEC 60076-11 and will normally be done when a new system is developed, or changes made to manufacturing procedures, and not on the transformers of a particular contract, however it is important that any potential user of a cast resin transformer should satisfy himself of the relevance of these special tests to prove the resin system being offered. These consist of thermal or climatic proving tests, environmental, and fire resistance tests, and are set out in IEC 60076-11. There are a number of different duties identified under each of the above headings. These are as set out below:

Two climatic classes are defined as follows:

Class C1 The transformer must be suitable for operation in an ambient not below -5ºC, but may be exposed in transport and storage to ambients down to -25ºC.

Class C2 The transformer should be suitable for operation, transport and storage in ambients down to -25ºC.

Three environmental classes are defined:

Class E0: No condensation occurs on the transformer and pollution is negligible. This corresponds to the conditions generally prevailing in a clean, dry indoor installation. No special testing is required to prove suitability for this class.

Class E1: Occasional condensation can occur on the transformer, such as for example when it is de-energized. Limited pollution is possible.

Class E2: Frequent condensation and/or heavy pollution might be expected.

Two fire behavior classes are defined:

Class F0: There is no special fire risk. Except for the characteristics inherent in the transformer no special measures are taken to limit flammability. Nevertheless, the emission of toxic sub stances and opaque smoke, in the event of the unit becoming engulfed in fire, shall be minimized.

Class F1: The transformer will be subject to fire hazard and should have restricted flammability. The emission of toxic sub stances and opaque smoke shall be minimized.

The special tests specified to demonstrate compliance with the above classes are described below.

Environmental test

The transformer under test is placed in an environmental chamber which is maintained at a relative humidity above 93 percent. This can be achieved by the use of atomisers or sprays but these must not spray directly onto the transformer nor should there be drips onto the transformer.

The transformer must remain in this chamber, de-energized for a minimum period of 6 hours. Within 5 minutes thereafter the transformer is to be submit ted to an induced voltage test as follows:

• With windings intended to be connected to a system which is solidly grounded or grounded through a low impedance, these should be energized at a voltage of 1.1 times rated voltage for 15 minutes.

• With windings intended to be connected to a system which is isolated from ground or grounded through a considerable impedance, each HV terminal in turn must be connected to ground and the other two terminals raised to a volt age to ground of 1.1 times the rated voltage. This test may be done as a single phase test or three-phase.

The above electrical tests should preferably be done in the environmental chamber. No flashover should occur and a visual inspection should not reveal any serious tracking.

The environmental test is not exclusively aimed at cast resin, of course and, as an indication of the ability of cast resin to withstand very adverse environmental conditions (and the relative ease with which the above IEC 60076 environmental requirements should be withstood) can be gained from the 'damp heat test' which was devised by the CEGB in the late 1970s for single-phase cast resin generator voltage transformers of 33 kV class. In this test the transformer is placed in a climatic chamber which is maintained as near as possible at 100 percent relative humidity for a period of 4 hours. The transformers are physically small enough to have reached temperature equilibrium with the chamber at the end of this period. At the end of the 4 hours the temperature of the chamber is raised by approximately 3ºC whilst maintaining the relative humidity at 100 percent. The transformer is thus cooler than its surroundings and its surface quickly becomes wet with condensation. In this condition a voltage of 1.2 times rated voltage is to be applied to the transformer for 1 hour, followed by a final 5 minutes at 1.9 times rated voltage; no breakdown, surface tracking or external flashover is to occur.

Climatic test

The test differs for the two classes of transformers. For C1 class transformers these are to be placed in a test chamber arranged so that the ambient can be measured at a minimum of three positions 10 cm from the external surface and at half the transformer height. The ambient is taken to be the mean of these readings. The temperature in the chamber is then to be taken down gradually to -25 +- 3ºC over a period of 8 hours and then maintained at this temperature for at least 12 hours until a steady-state condition has been reached. The temperature is then increased gradually up to -5 +- 3ºC in about 4 hours. This temperature is to be maintained for at least 12 hours or until a steady-state condition has been reached. A thermal shock is then applied by circulating a current equal to twice the rated current until the winding under test reaches a mean temperature equal to the rated average winding tempera ture rise plus 40ºC (the maximum ambient temperature in service). The cur rent circulated may be DC or AC and the temperature is to be measured by increase in winding resistance, or by sensors calibrated against winding resistance rise plus 40ºC, before the test.

At least 12 hours after the test the transformer is to be submitted to the dielectric routine tests appropriate to its rated insulation level, but at voltages reduced to 80 percent of the standard values. Transformers having windings contained within solid insulation, that is cast resin type, should also be subjected to partial discharge measurements at 80 percent of the rated induced overvoltage level, for which the measured values should not exceed those pre scribed for the routine tests.

On visual examination the windings should show no abnormality, such as cracks or slits.

For Class C2 transformers the test is similar to the above except that the temperature is not increased after the period at -25ºC and before the application of the thermal shock.

Flammability of cast resin transformers

The metal parts of a cast resin transformer, such as aluminum and steel, account for around 89 percent of its total weight. The insulating materials amount to only about 11 percent. Of this, less than half can be considered flammable because about two-thirds of the resin compound is silicon dioxide filler -- quartz powder -- and much of the insulation material of the LV winding is glass based. Hence not more than 5-6 percent of the total weight of the transformer comprises flammable substances which could supply energy in the event of the transformer being engulfed in a fire. Nevertheless manufacturers are keen to demonstrate the low fire risk associated with their cast resin transformers by testing, and IEC 60076-11 sets out details of a fire behavior test.

Tests for determining flammability are difficult to frame and to interpret because standardized test conditions often bear no relationship to real circum stances. However, two conditions must be met for stable self-sustaining combustion; the temperature of the material must be raised to the fire point (defined in Section 3) -- in the case of cast resins used in transformers this is usually about 450ºC -- and the combustion must produce an adequate supply of heat to sustain itself. The IEC 60076-11 test is derived from the testing procedure established for electric cables, and in addition to seeking to test whether the material will add to the heat of a fire, it also aims to test whether the materials will release excessive smoke as a result of the fire. The test involves heating the transformer windings in a chamber by burning alcohol and by providing additional electric heating. The quantity of alcohol is arranged to burn for about 20 minutes and the electric heating is maintained for a further 20 minutes. During the test the temperature rise in the chamber must not exceed 420 K and this must start to fall 5 minutes after the radiant heating panel has been switched off. The mean optical transmission factor through the smoke in the measuring section of the chamber during the period 20-60 minutes after the beginning of the test must not be less than 20 percent.

Class 220 dry-type transformers

Class 220 dry-type transformers are those based on glass fiber reinforced boards, aromatic polyamide paper conductor insulation and similar materials capable of operating at temperatures up to around 220ºC. These are described in Section 3. They have now been somewhat eclipsed by cast resin encapsulated types. However, they do have some advantages over cast resin; they are a little more compact and thus lighter, they generally have lower losses and are up to 20 percent cheaper than cast resin, and, most significantly, they have better overload and short-circuit withstand capability. Although they are not capable of withstanding the same extreme environmental conditions as cast resin, present day dry types are greatly superior in this respect to those of the 1960s when they were initially introduced. At that time, the conductor insulation or 'paper' covering was largely asbestos based in order to be able to achieve the required temperature withstand capability. Even when properly impregnated, this material was inclined to absorb moisture, which greatly reduced its insulation properties. It was therefore very important to ensure that transformer windings were properly dried out before energizing, and even whilst in service it was important to ensure that transformers were given a good dry environment. The availability of aromatic polyamide paper from the mid-1970s greatly improved this situation.

The construction of class 220 dry-type transformers is very similar to oil-filled units. They may have conventional helically wound LV windings or these may be foil wound. For all but the lowest ratings the HV winding conductor will be rectangular in section so that HV windings may generally be disc wound. Disc windings are to be preferred to multilayer helical type, since the former arrangement gives a uniform distribution of the phase voltage through out the axial length of the winding thus ensuring that the electrical stresses are minimized. As previously mentioned, air is a poorer cooling medium than oil and in order to ensure adequate cooling air flow through the windings vertical ducts should be a minimum of 10 mm wide and horizontal ducts a minimum of 6 mm. FIG. 33 shows the core and windings of a typical class 220 dry-type transformer.

FIG. 33 Core and windings of a three-phase class 220 dry-type transformer in its enclosure (Brush Transformers Ltd)

The modern aromatic polyamide papers are far less inclined to absorb moisture than the earlier asbestos based materials. They will absorb about 1 percent moisture for each 10 percent relative humidity, so that at 95 percent RH they will contain 8-10 percent water. However, even at this level of moisture con tent their electrical properties will be very little impaired. Table 2 gives the electrical properties of NOMEX [Dupont's registered trademark for its aramid paper.] paper type 410, 0.25 mm thick, at varying levels of relative humidity.

Table 2 Variation of electrical properties of NOMEX® paper type 410, 0.25 mm with variation in relative humidity

Such a good resistance to humidity might create the impression that varnish impregnation of a winding having aromatic polyamide insulation is unimportant.

This is far from the case and it is necessary to properly vacuum impregnate the windings with a silicone varnish if partial discharges, which will ultimately lead to breakdown, are to be avoided. The object of the impregnation should be to ensure that the insulation structure is free from voids, particularly in the areas of high electrical stress, for example at the ends of windings in the close vicinity of conductors, or between the open ends of layers, if a layer-type winding is used. The dielectric constant of the aromatic polyamide is between 1.5 and 3.5, depending on the density of the material. The figure for air is, of course, near to unity. In any composite insulation structure, that is aromatic polyamide with air-filled voids, the electrical stress in each component material will divide in inverse proportion to their dielectric constants, so that the stress in the voids may be between 1.5 and 3.5 times that in the solid insulation and the reason why these can become a source of partial discharge is thus quite clear.

Installation of class 220 dry types

The method of installing class 220 dry-type transformers is very similar to that used for cast resin transformers. The transformer core and windings will normally be mounted on rollers and housed in a sheet-steel ventilated enclosure incorporated into the LV switchboard with its LV busbars connected directly to the switchboard incoming circuit breaker. It is not so convenient to provide molded HV connections directly onto the winding as is the case with cast resin and, in addition, the paper-insulated windings are more easily damaged than those of a cast resin transformer so it is best to avoid carrying out any unnecessary work in the close vicinity. It is desirable, therefore, that the HV supply cable is not terminated internally within the enclosure but connected into an externally mounted cable box. Adequate access to the enclosure should be provided, however, to enable the windings to be cleaned and inspected about once per year. This should preferably be a vacuum cleaning rather than by blowing out dust deposits - a procedure which may embed foreign material in undesirable locations.

Fire resistance

As in the case of a cast resin transformer, a class 220 dry type contains very little insulation material, probably no more than 3-4 percent of its weight, so the quantity of combustion products resulting from being engulfed in a fire will be very small. The manufacturers of NOMEX claim that it does not melt or support combustion. At high temperature, it will degrade and give off gases which are composed of combinations of its constituent oxygen, carbon, hydrogen and nitrogen in concentrations which are dependent on the conditions, such as temperature, availability of oxygen and other materials present. Table 3 gives details of the products of combustion from NOMEX aramid paper at 900-1000ºC for the cases of both excess and insufficient air.

Table 3 Combustion gases from NOMEX® aramid

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