IE891229L - Dry transformer with coated windings, similar electrical¹conductors or installations, and process for the¹preparations of their coating resin - Google Patents

Description

£4367 - ia - The invention relates to the improvement in the fire behaviour of coated electrical conductors which operate hot.

It applies in particular to dry power or distribution transformers, whose working temperatures habitually exceed the ambient temperature by more than a hundred degrees centigrade (working temperature generally of the order of 140-150°C). Accordingly, and solely for reasons of convenience, reference will be made to the example of the abovementioned transformers in the remainder of the description.

In transformers of this type, which are usually selected for a voltage range extending from 3 to 36 kV, the coil winding is embedded in an insulating synthetic resin with dielectric properties and with a thickness of several millimetres (e.g. from 2 to 5 mm) . It is recalled that, in addition to its electrical insulating role, the resin plays a part in protection against moisture and dust, which could lower the breakdown voltage. It also protects the electrical coil windings against an environment contaminated by corrosive chemical agents and plays a major part in mechanical behaviour by ensuring the immobilization of the turns relative to one another in the coil winding.

Nevertheless, it is known that in accidental extreme conditions of utilization, or following an anomaly, transformers can burn. A study of their fire behaviour has made it possible to realize that, when the coating resin burns, it often continues to burn, even if the fire which caused it has ceased, because the resin becomes flammable in air above a critical temperature which is specific to it.

At this stage, a few reminders concerning the constitution of these resins may be found useful in what follows.

Present coatings for dry transformers axe heat-curable resins obtained by heating an initial mixture composed of a liquid resin (generally an epoxide) and a hardener such as a - anhydride. A classical example is provided by bisphenol A diglycidyl ether derivatives, more commonly called BADGE, and formed by reaction between bisphenol A and epichlorohydrin (see, e.g. DSP 3,202,947). These resins are in most cases crosslinked and made infusible and insoluble by addition of amines and of polysulohides of low molecular mass (for example Araldite (Registered Trade Mark). Other examples are to be found among heat-curable polyesters. The proportions by weight are usually 50 % in the case of the resin and 50 % in the case of the hardener.

Filled resins are generally employed: before the hardener is added, the initial liquid resin receives a filler, often inorganic, for example quartz (silica) flour, or glass- wool, in proportions by weight which are of the order of 3 of filler per 1 of resin. In the initial mixture there will then be, for example, 20 % of resin, 60 % of silica and 20 % of hardener.

The function of this filler is to improve the thermomechanical behaviour and to absorb already some of the heat of polymerization of the resin, which maJces it possible to avoid the formation of cracks. Another of its functions is mechanical reinforcement, because unfilled resin remains soft in consistency at the transformers' operating temperature.

From another aspect, elastomers, such as silicone resins or polyester resins which are thermoplastics, are more likely to be employed particularly in the case o£ coating of cables or of wires, so as to permit a degree of flexibility therein. An example is provided by polyesters such as the polyterephthalate of ethylene glycol or of butylene glycol.

The invention does not discriminate according to the resin type (heat-curable, thermoplastic or elasto-meric), insofar as this resin is an insulating material intended to be employed "hot" and therefore consequently has good thermomechanical behaviour at the elevated working temperatures indicated earlier. Accordingly, for convenience of language, the names "coating resin" or "filled resin" will 5 be applied indiscriminately to the material constituting the final insulating coating formed by the mixture of : polymerizable resin proper, reinforcing filler (where applicable), and possible hardener (including usual adjuvants such as accelerator, flexibility promoters, and the like).

The purpose of the invention is to improve the fire behaviour of coated electrical conductors, especially the windings of dry transformers, by increasing the heat stability of the flame-retardant coating resin employed, at their service temperature.

According to a first aspect of the invention there is provided a dry transformer, or similar electrical installation, in which at least one winding is coated with a filled insulating resin containing a substance which is flame-retardant due to water formation when the temperature rises, characterised in 20 that a proportion of the filler equal to at least 20% of the total weight of the coating resin consists of the said flame-retardant substance which has been previously partially dehydrated to such an extent that it does not cause an untimely formation of water in the coating resin capable of 25 being detrimental to the quality of the latter, while the transformer is in service in its normal temperature conditions.

According to a second aspect of the invention there is provided a process for the preparation of a filled insulating 30 resin for coating electrical windings of dry transformers, or of other similar electrical installations, according to which there is added to the initial liquid resin, in addition to the filler, a substance which is flame-retardant due to water formation when the temperature rises, which process is characterised in that the said substance is added in a proportion of at least 20% of the total weight of the coating resin and in that the said substance has been previously partially dehydrated to such an extent that there is no untimely formation of water capable of being detrimental to the behaviour of the coating resin, while the electrical installation remains in service at its nominal operating temperature.

A further subject of the invention is a coated dry transformer in which at least one electrical winding is coated with a bulk of f lame-retardant insulating resin in accordance with that obtained by the process specified above.

The flame-retarding role of alumina trihydrate, as an adjuvant in coating resins for electrical cables or for transformers, is already known, for example from document USP 3,202,947, referred to already.

However, so far as the inventors are aware, it has never hitherto been possible to bring to light the advantage which could be paradoxically expected of a better heat stability of the resin by partially dehydrating an adjuvant of this kind beforehand, whereas it acts as a flame-retardant precisely by forming water.

The invention will be properly understood and other aspects and advantages will appear more clearly in the light of the detailed description which follows, given with reference: - to Tables I, II and III, presented in the body of the text and expressing the good fire behaviour of a coating resin according to the invention; - to table IV, also presented in the body of the description, and giving the main characteristics of the profile of the loss of water with the rise in temperature for a number of flame-retardant substances; - to Figures 1 and 2, showing the change with time of the loss in weight of various flame-retardant substances which can be employed, due to the formation and removal of water resulting from their "hot" instability when they are heated at a constant temperature.

The two essential characteristics of the invention, stated earlier, are considered in successions 1) ADDITION OF A SUFFICIENT QUANTITY OF A SUBSTANCE WHICH IS FLAME - RETARD ANT DUE TO WATER FORMATION DDE TO INSTABILITY WHEN HOT.

The £ lame-retardant substance which is added may 5 be alumina hydrate Al^Oj.nHjO with n ■ 1, 2 or 3, magnesia dihydrate, zinc borate, or any other substance known for its self-extinguishing capacity properties due to elimination of water and additionally preferably capable of reinforcing the resin, just like silica. Preference may 10 be given to alumina trihydrate, which has been found to be the most effective flame-retardant and which, in addition, gives off little or no smoke.

The water formation reaction can be written: 2 Al (OH) 3 » A120j + H20 The rate of this reaction increases in the direction of the arrow with temperature, and its endo-thermicity delays, or even prevents, the ignition point of the resin from being reached.

It will be seen later that this rate does not 20 change linearly with temperature, but that an intense water loss peak is formed, which is characteristic of the substance and whose decisive advantage is that it appears at temperatures which lie precisely in the overheating range at risk, and therefore below the critical ignition 25 temperature of the coating resin.

The AJL(OH)3 can be easily mixed with the starting filler, which is preferably inorganic, since both are in the form of powdered or finely granular solid matter.

Instead of 60 % by weight of Si02 in the final 30 resin, a major part thereof is substituted by the flame-retardant substance. For example, a mixture representing 50 % of. Al(OH)} and only 10 % of Si02 is thus produced. However, experience has shown that the quantity of Al(OH)3 could be lowered as far as 25 % (and thus 35 % of 35 Si02) without significant detriment to the good fire behaviour qualities of the coating, which are due to the presence of the flame-retardant substance.

These values, established for an initial loading of 60 % by weight can, of course, be modified if this proportion varies.

Tables I, II and III below provide indications f and nutaerical results of tests carried out in the labora tory on a dry transformer, showing the effect of flame-5 retardant substances (in this case A1(0H)3 and zinc borate) on the values of parameters which are recognized as representing the fire behaviour of materials, and in particular of coating resins for dry transformer windings, namely the Oxygen Index, the Rate of Combustion and TABLE 1: Oxvqen index fO I.) 40 measurement temperature minimal Oxygen Index *C 30 80*C 27 150*C 23.5 I 21 200 #C ALOMINA TRI-HYDRATE LOAD • ro * 70% of SiOa - 30 " 28 ~ 24 " 19 % of SiOa + 50% of Al(OH)3 " 39 - 37 ~ 32 - 31 60% of SiOa ~ 26 / / / 50% of SiOa + 10% of A1(0H>3 - 25 / / / 40% of SiOa + 20% of A1(0H>3 ~ 27 / / / % of SiO* + 25% of A1(0H)3 " 30 - 28 ~ 25 ~ 20 % of SiOa + 35% of A1(0H)3 28-31 28-31 24-27 19-22 % of SiOa + 40% of A1(0H)3 -33 29-33 -28 23-24 % of SiOa ♦ 50% of A1(0H)3 32-35 -32 27-29 24-26 ZINC BORATE !10% de SiOx et 32.5! |55X de Borate i ! pas de valeurs F XI LOAOl % de SiOa et !" 30.3! 50% de Borate ! ! pas de valeurs 60% de Borate !~ 33.6! pas de valeurs The coating material was prepared in the following manneri after having been mixed with a suitable quantity of the flame-retardant substance, the inorganic filler (here, silica) was blended, one half with the liquid resin (epoxide resin marketed by the Swiss firm Ciba-Geigy under the name "Araidite cy 225" ("Araidite" is a Registered Trade Mark), and the other half vich the hardeiier, also in the liquid state (an anhydride marketed by the above-aentioned firm under the name "Haxdener HY 227"). The two mixtures were then combined and the whole was blended and then placed in an oven (80 to I50°C) for setting.

The methods of measurement in these tests were in accordance with UTB Standard NF TS1-071 at 20*C and EOF Standard HN20 M40 at 80, ISO and 200*C.

Note - The ranges of values reflect the fact that the measured 0.1. depended in this case on the source of the commercial alumina employed.

- The boxes marked with a */* reflect measurements deemed to be useless. The corresponding values, bearing in mind that obtained at 20*C, are wholly unacceptable, bein? too far below the stipulated minimum O.I.

It is immediately apparent that in the case of a total loading of 60 % the minimum content of Al(OH), to be conformed to is 20-25 %. Below this, the stipulated minimum values of the 0, index are no longer ensured effectively. Additional measurements, not reported here, have made it possible to demonstrate that with an overall loading of 70 % by weight (1st part of the table), the threshold value of Al(OH), drops to 20 %. It is also seen that la the case of sine borate the minimum threshold is much highers at least SO % by weight, which reflects a greater effectiveness of alumina trihydrate, as was known.

The Rates of Combustion reported in Table ZX below were measured In the apparatus employed for the determination of the Oxygen Indices on test specimens in the shape of small elongate plaques 100 mm in length, 6.5 mm in width and 4 mm in thickness. The test specimens incorporate two marks following each other along the length, the first being situated at 10 mm from one end, 5 the second at 60 mm. The combustion time of the test specimen between the two marks is noted at ambient temperature and the mean Rate of Combustion (in zom/s) is deduced from this as a function of the proportion of oxygen in a combustive mixture of oxygen and nitrogen.— TABLE II: Rate of Combustion Oxygen per cent % 40% 45% . 50% 60% max. Combustion rate autorised (i~mm/s) 0. 15 0. 30 0. 45 0.6 0. 9 ALUMINA TRI-HYDRATE r i ui.cn 70% of SiOa 0. 28 0. 37 0. 47 0. 58 1. 05 LOAD • TO X % of SiOa + 50% ofAl(0H)s 0. 06 0. 10 0. 13 0. 19 0. 25 60% of SiO* '0. 35 "0. 50 "0. 60 "0.75 1"1. 0 50% of SiO* + 10%of A1(0H>3 "0. 35 "0. 45 "0. 60 "0.75 "1.0 40% of SiOa * 20%of A1(0H)3 ~0. 20 ~0. 35 "0. 45 "0. 55 "1.0 r X UL.CR UOAO * % of SiOa -25%of Al(0H)s "0. 18 ~0. 28 "0. 38 "0. 49 "0. 95 60 X % de SiOa + 35% d'Al(0H)3 0. 14 0. 17 0. 20 0. 23 0. 27 0.30 0.34 0.37 0. 56 0. 59 % de SiOa + 40% d'Al(0H)s 0. 13 0. 16 0. 19 0. 20 0.24 0. 27 0.30 0.35 0. 48 0. 56 % de SiOa + 50% d'Al(0H)a 0. 07 0. 10 0. 14 0. 16 0.20 0.23 0.26 0.29 0. 37 0. 40 ZINC BORATE p* x ui.cn L0A040X 110% de SiOa ;55% de Borate! "0.16! "0.21 1 1 ! "0.30! "0.8 1 I i 1 40 rtuueiv UOAO • % de SiOa + ! 50% de Borate! : "0. 251 "0.43 1 t !"0.65 1 1 !" 0.98 1 1 4»0 X 60% de Borate! -0.25! "0.31 J "0.52! "0.96 Note: The meaning of pairs of values, where shown, is the same as in the preceding Table.

As can be easily concluded, the values given in Table II corroborate those of Table I in showing that, in respect of the "Rate of Combustion" criterion as well, the "bottom" value to which the quantity of A1(0H), in the starting mixture should conform is substantially 25 % by weight (a little below 50 % in the case of the borate).

These conclusions remain wholly valid in the light of Table III below, showing the results of the series of tests carried out on the third parameter selected, the Upper Heating Power (U.H.P.). As can be seen, the maximum permitted value of 11 MJ per kg of material is never exceeded.

TABLE III: Upper Heating Power (Test method: adiabatic calorimetry according to UTE Standard NF M 03-005.) Filler Load : !! Filler Load : 60% 70% ALOMINA TRI-HYDRATE 70XS10* 20XSiOs + 50% Al. 60% SiOa 40%SiQ* + 10X Al. 35X SiQs ♦ 25X Al.

%Si0a + 35X Al. 10X Si02 ♦ 5OX Al. ~ 8 ~ 7 " 11 ~ 11 ~ 11 ~ 11 ~ 11 ZINC BORATE %Si0*+55XBor. 10X SiOa + 50X of Borate 60X of Borate - 10 ~ 11 " 11 These tables clearly show, therefore, the influence on the fire behaviour when a material which is flame-retardant due to water formation and elimination is added in suitable proportions to the initial resin.

These results reflect a high release of water by the coating resin when the latter reaches abnormally high temperatures. A phenomenon of this kind acts as a self-controlled heat absorber which retards and has a braking action on the combustion of filled resin and endows the 10 latter with the required characteristic of self-extin-guishability.

Of course, an abundant formation of water molecules actually within the mass is not without consequences for the quality of the coating resin. The latter 15 decomposes in the course of a process of this kind and, in general, can no longer be reemployed subsequently. The transformer must then be replaced or reconditioned.

The loss in weight of the flame-retardant substance with the rise in temperature is a good guide to 20 its water-forming capacity. It will be noted, as shown by the suppliers' specifications, that in the case of A1(0H)3 the loss in weight is already close to 30 % at 300°C. Furthermore, it takes place practically solely at this temperature, in the form of a peak which is narrow 25 and of high amplitude, which bears witness to the energy and the intensity of the phenomenon when this temperature.-level, characteristic of the flame-retardant agent employed, is reached.

This is verified, whatever the variety of commer-30 cial alumina trihydrate employed, as is shown more accurately by Table IV below.

This table of values is given merely by way of . information, on the basis of data from suppliers of Al(OH), sold under the name "Alcoa" (Registered Trade 35 Mark) in varieties whose commercial- references are repeated for identification in the columns of the table.

TABLE IV: Analysis of endothermic water formation peaks (The temperature increases were performed from 25 to 600°C at a constant rate of 10°C/min. ) i 1 l 1 M15 1S65/40; C31 I l > < 1 « 1 M6 : 1 t 1 1 M15S ;S65/150JMEDIUM ; ; SODA Temp, start peak ! rc> : 196 ! 205 ! 216 1 l < < 1 » 1 ! 180 ! i i l l 195 : 188 1 1 ! 197 i i Temp, end peak ! (#C) : 372 ! 385 ! 353 1 l < l * ; 382 ; t * t i 355 : 370 1 1 353 1 1 top Temp, of! peak (*C) ! 316 : 312 ; 314 I l I t ; 314 ! 1 1 1 1 311 309 t 1 ! 312 i l "H of ( in peak kJ/g) : 1.03! 1.01 !1.07 1 < 1 1 ; 1.06! 1 1 1 1 1.03 : 1.0 i i i CM 1 O 1 «-» 1 1 For the effects expected for the type of flame-retardant substance recommended by the invention to be fully satisfactory it is not only necessary that the intense water loss peak should be developed at saitable temperatures, that is to say between the normal hot operating point and that where the coating can ignite, but also that a formation of water should actually take place only in the case of abnormal overheating. In other words, it is appropriate to avoid any risk of premature decomposition of the coating resin, which could be caused by an untimely formation of water between the ambient temperature and that of the normal hot operation of the transformer.

It is this difficulty which is resolved by the second main characteristic of the invention, namely a moderate preaging of the flame-recardant, as will be seen in detail below : 2) PARTIAL PRELIMINARY DEHYDRATION OP THE QUANTITY OF THE FLAME-RETARDANT SUBSTANCE WHICH IS ADDED.

This involves causing a preliminary dehydration in order to avoid its being produced subsequently in the transformer. However, this dehydration must be only partial, since it is required otherwise at a high temperature, when the transformer heats abnormally and when there is a danger that ignition may take place.

This dehydration can advantageously be performed by heating the A1(0H), beforehand. The objective to be attained is not a complete elimination of the water capable of being formed in a temperature range from the ambient to the "hot" operating temperature of the transformer (and which will be colourfully referred to as "volatile water" to emphasize that it must be released at low temperature), but a sufficient elimination in order that the residual "volatile" water may be present in too small a quantity to give rise to a decomposition of the resin. It has been possible to observe, in fact, that when the alumina trihydrate had not been preheated before being added to the inorganic filler, bursting of the resin coating the electrical coil windings of test transformers occurred at the operating temperature, which made it necessary for them to be rejected.

A convenient way of successfully producing the partial dehydration of the flame-retardant substance by preheating is to consider its curve of weight loss as a function o£ time. In the case of a substance employed for the first time, this can be advantageously done in two steps: - a first step, on an analytical sample intended for determining the quantity of water which is eliminated over a prolonged period at a constant temperature which is that (or close to that) of the normal hot operation of the transformer; - a second step, this time of treating all the material, consisting in heating it to a relatively high temperature in order to attain quickly - and hence under industrial conditions - a loss in weight corresponding to the water elimination value determined in the preceding stage.

This second step will obviously be repeated at each preparation of resin, whereas the first is actually 5 required only once, to characterize a type of flame- retardant substance which had never been employed previously.

The curves of Figures 1 and 2 clearly illustrate this first stage of investigation of samples, showing, at 10 certain values of temperature, the rate of change in the loss in weight as a function of time for certain values of the temperature. The .curves correspond to some of the varieties Of "Alcoa" (Registered Trade Mark) aLumina tri-hydrate of TabLe IV, indicated by their commercial references at the right- hand end of each curve. Three temperature values have been considered: 140, 160 and 180°C, so as to cover properly the usual "hot" running conditions of the transformers. To avoid unnecessary overloads, the three corresponding groups of curves have been separated in the 20 two figures: Figure 1 shows together the groups at 140°C (broken lines) and 160°C (solid line); Figure 2 shows only the group at 180°C, using a solid line.

As can be seen, all these curves are advantageously generally logarithmic in form, with a very fast 25 increase at the beginning, followed by a plateau which is slightly inclined to the horizontal, this plateau being reached proportionately earlier, the higher the working temperature. Thus, at 180°C (Fig. 2), most of the "volatile" water (approximately 80 %) has already formed 30 after only 140 h, out of more than 700 h of the total test period. The loss in weight then ranges approximately between 5.3 % and 2.5 %, depending on the type of alumina.

The existence and the stability of the plateaux 35 is verified by testing aluminas which have been preheated to a higher temperature. Tests have thus been carried out on two samples of the "MIS' variety, one of which was kept at 180°C and the other at 200°C, for 18 h. The results appear in Fig. 1 in the form of two straight lines (A) and (B) whose original ordinates are 1.6 % and 4.2 % of weight loss in the case of 180 and 200°C respectively. They can be seen to be virtually horizontal, which clearly reflects the insensitivity of the samples to a second heating to a lower temperature, as a result of the fact that virtually all of their "volatile water" at 140° or at 160°C has effectively been eliminated during the first heating.

Furthermore, in Fig. 2 it can be seen that after 18 h at 180°C, an "M15" sample is barely midway along its curve of rapid increase in weight loss due to elimination of water.

It will immediately be understood that these curves have a characteristic favourable form which enables the degree of preliminary partial dehydration which must be achieved to be very easily deduced. Thus, for instance, the beginning of the plateau can be chosen as a criterion and the value given by the ordinate at this beginning of the plateau can be taken as the proportion of "volatile" water to be eliminated.

Thus, the figures show that, in the case of the "S65/40" variety, 2.5 % of weight loss can be taken, for a transformer operating temperature of 140°C, 4 % at 160°C and 5.5 % at 180°C.

Similarly, the "M15" variety will put up with a preliminary weight loss of 2 % for a transformer operation at 160°C and 3.5 % at 180°C.

At 140°C, the curve for this variety is less typical in form. It will be noted, however, that the value of 0.7 % obtained after approximately 500 h will be perfectly suitable.

To make the concept more concrete, it can therefore be said in general that the preliminary weight loss ranges approximately between 0.5Z and % for all the flame-retardant substances selected for implementing the invention.

The subsequent conversion to the industrial stage consists merely in consequently dehydrating the bulk of flame-retardant substance using methods of fast heating in an oven, with monitoring of the loss in weight, for example by gravimetry with the aid of an automatic balance whose pan is placed in the oven enclosure. ' By way of information, in the case of aluminas of 5 the "MIS" type, it has been possible to obtain the loss r in weight of 3.5 %, required for a transformer operation at 180°C, after only 6 hours' heating at 200*C.

This heating operation will, of course, be proportionately faster, the higher the heating tempera-10 ture. However, for obvious reasons of maintaining a high water formation capacity, necessary in the event of abnormal overheating, care will be taken not to greatly exceed, and preferably to remain below, the onset temperature of the peak of the intense water elimination which 15 is characteristic for the flame-re tardant substance employed and some values of which are given in Table IV above.

If the need to operate using gravimetry is undesirable, especially because of excessively large 20 quantities of material which it might be necessary to process, it will be advantageous to proceed via an intermediate step which makes it possible to convert the loss in weight which is aimed at into a heating time.

This can be done with the aid of a second sample for 25 measuring the flame-retardant substance in question, for which the weight proportion of water to be eliminated is known and which is subjected to a fast heating at a specified high temperature. This heating will be carried out with continuous weighing of the sample to make it 30 possible to measure the time needed to arrive at a weight loss corresponding to the known proportion of water to be ♦ eliminated. The measured value defines the duration of the heating operation at a temperature which is strictly identical with that of the above intermediate operation, 35 to which the bulk of the flame-retardant substance to be processed will be subjected.

If need be, the correct performance of the operation may be confirmed by gravimetric determination at a high temperature of, for example, 1200°C, of the residual water content of a sample taken for this purpose from the,bulk of the flame-retardant substance which is processed. By comparing with the total initial water content (usually approximately of the order of 20 to S 35 % by weight) which will have beeh determined previously in a similar manner on a reference sample, it can be deduced therefrom that the quantity of "volatile" water actually eliminated is well in accordance with that aimed at.

It will be noted that the heating time is not completely independent of the particle size of the material. In the course of tests it has been possible to observe that a coarse particle size resulted in a greater loss in weight than a fine particle size, for a given 15 heating time.

It will also be observed that the weight loss values determined above by reading curves for tests carried out on samples do not in any way represent an upper limit not to be exceeded. Only, as these values 20 correspond to the beginning of the plateau, there is in principle no point in prolonging the heating in order to gain a few tenths of a per cent which, in any case, are probably too insignificant to produce a water release capable of damaging the quality of the coating resin. 25 It will certainly have been understood that these curves, which apply to the normal operating temperatures of the transformer when hot, reflect the behaviour of so-called "volatile" water at these temperatures. At higher temperatures the plateaux are situated at higher 30 levels and are reached more quickly, especially at the peak temperature characterizing the f lame-retardant substance employed, which, as we have seen, is situated in the neighbourhood of 300*C for all the A1(0H), varieties investigated.

Pretreated in this manner, the flame-retardant substance becomes ready for use. Zt remains to complete the preparation of the eoatina resin in the following preferred manner: the filler, which is preferably inorganic, after having been intimately mixed with a suitable quantity of partially 40 prehydrated flame-retardant substance, is divided into two equal parts. One is then introduced into the bath of pure resin ^ and the other into the hardener, also in the liquid state. The two mixtures are blended separately to form * 5 two solid-liquid suspensions, and are then combined into a single mixture which is in its turn blended to ensure good homogeneity. The resulting pulp is then poured into a mould in which the electrical coil winding of the transformer which it is intended to coat has been ar-10 ranged beforehand. After pouring, the mould is placed in an oven for the resin to solidify. After removal from the oven and cooling, the block of resin incorporating the coil winding is demoulded and can then be fitted into the transformer intended to receive it.

IS It is obvious that the invention could not be limited to the examples described above, but extends to many alternative forms or equivalents, so long as the characteristics listed in the claims which follow are observed.

In particular, the invention is not limited to transformers as such. This term, as employed here, should be understood, in fact, to mean more broadly all the inductive electrical apparatus or equipment capable of operating at relatively high temperatures, from 100 to 25 200°C as we have seen, and whose electrical windings can be encased in a block of insulating resin.

Similarly, although the invention was originally conceived for applications of heat-curable resins (coating of inductive coil windings, particularly in. trans-30 formers), it relates, in fact, to all insulating coating materials. It offers an especially marked advantage in the case of electrical installations which nominally function for long periods at an elevated or moderately elevated temperature. This means that it will be par-35 ticularly advantageous to employ it for coating resins which are already capable of good thezmomechanical behaviour.

Claims (12)

1. A dry transformer, or similar electrical installation, in which at least one winding is coated with a filled insulating resin containing a substance which is flame-5 retardant due to water formation when the temperature rises, characterised in that a proportion of the filler equal to at least 20% of the total weight of the coating resin consists of the said flame-retardant substance which has been previously partially dehydrated to such an 10 extent that it does not cause an untimely formation of water in the coating resin capable of being detrimental to the quality of the latter, while the transformer is in service in its normal temperature conditions.

2. A dry transformer according to Claim 1, characterised in 15 that the said previously partially dehydrated flame- retardant substance exhibits a loss in weight of from 0.5 to 10% relative to its initial undehydrated state.

3. A dry transformer or conductor coated, according to Claim 1, characterised in that the flame-retardant substance is 20 alumina trihydrate.

4. A process for the preparation of a filled insulating resin for coating electrical windings of dry transformers, or of other similar electrical installations, according to which there is added to the 25 initial liquid resin, in addition to the filler, a substance which is flame-retardant due to water formation when the temperature rises, which process is characterised in that the said substance is added in a proportion of at least 20% of the total weight of the 30 coating resin and in that the said substance has been previously partially dehydrated to such an extent that there is no untimely formation of water capable of being detrimental to the behaviour of the coating resin, while the electrical installation remains in service at its nominal operating temperature.

A process according to Claim 4, characterised in that the flame-retardant substance is subjected to a preliminary partial dehydration treatment until it exhibits a loss in weight of between 0.5 to 10% of its initial weight.

A process according to Claim 4 or Claim 5, characterised in that alumina trihydrate is employed as the flame-retardant substance.

A process according to Claim 4, characterised in that the degree of preliminary partial dehydration of the flame-retardant substance which is employed for the first time is determined by holding a measurement sample at a constant temperature equal to or lower than that of the peak of elimination of water belonging to the flame-retardant substance employed, and by considering the rate of change in the loss in weight of the said sample with time.

A process according to Claim 4 or Claim 7, characterised in that the preliminary partial dehydration of the f lame-retardant substance is performed by subjecting it to a heating until its loss in weight corresponds to the quantity of water to be eliminated according to a proportion determined beforehand on a measurement sample whose change in the loss in weight has been followed as a function of time during holding at a constant temperature equal to or below that of the peak of elimination of water belonging to the flame-retardant substance employed. - 22 -

9. A process according to Claim 8, characterised in that the duration of said heating is determined by a heating operation conducted at an identical temperature and performed with weighing of a sample of the flame-resistant substance employed until this sample exhibits a weight loss corresponding to the quantity of water which it is desired to eliminate.

10. A transformer substantially as hereinbefore described with reference to the accompanying drawings.

11. A conductor substantially as hereinbefore described with reference to the accompanying drawings.

12. A process substantially as hereinbefore described with reference to the accompanying drawings. Dated this 18th day of April 1989 CRUICKSHANK & CO. Agents for the Applicant 1 Holies Street Dublin 2 AB\RETYPE1-95\103I».FEB