NL8900954A - DRY TRANSFORMER WITH COATED WINDINGS, SIMILAR ELECTRICAL CONDUCTORS OR INSTALLATIONS AND METHOD FOR PREPARING THE COATING RESIN. - Google Patents

Description

»·

V

Dry transformer with sheathed windings, comparable electrical conductors or installations and work to prepare the amhiillino resin.

The invention relates to improving the burning behavior of sheathed electrical conductors used in the heat.

In particular, it relates to dry power or distribution transformers, the operating temperatures of which are normally 100 ° C higher than the ambient temperature (operating temperature generally on the order of 140-150 * 0).

Also, and for convenience only, reference is now made to the aforementioned transformers.

In this type of transformers, normally intended for a voltage range of 3 to 36 kV, the winding is soaked in an insulating synthetic resin with dielectric properties and with a thickness of a few millimeters (for example 2 to 5 mm). It is noted that in addition to the electrically insulating function, the resin serves to protect against moisture and dust, which could decrease the breakdown voltage. It also protects the electrical windings from an environment contaminated by aggressive chemical agents and plays an important role in mechanical behavior by ensuring the bond strength in the winding among themselves.

Nevertheless, it is known that, under accidental extreme operating conditions, or due to an anomaly, the transformers can burn. The study of their behavior in the event of fire has made it possible to take into account that, when the casing resin burns, it often continues to burn, even when the ignition, which is the cause thereof, has stopped because, starting from a critical temperature, which is inherent in it, makes the resin 8300354 *% -2 - flammable in air.

At this stage, some comments regarding the construction of these resins may prove useful for the following.

The actual casings of dry transformers are thermosetting resins obtained by heating a starting mixture consisting of a resin (generally an epoxide) and a curing agent such as the anhydride. A classic example is provided by the digly-dicylether derivatives of bisphenol A, recently called DGEBA, and obtained by the reaction of the bisphenol A and the epichlorohydrin (see, for example, U.S. Patent 3,202,947). These resins are usually cross-linked and rendered incombustible and insoluble by the addition of low molecular weight amines and polysulfides (for example, the Araldite resin). Other examples are found under the thermosetting resins. The weight amounts are normally 50% for the resin and 50% for the curing agent.

In general, filled resins are used: the starting liquid liquid receives, before adding the curing agent, a charge, often a mineral charge, for example quartz flour (silica) or glass wool, in weight quantities of the order of 3 for the filling and 1 for the resin. The starting mixture then contains, for example, 20% resin, 60% silica and 20% hardener.

The purpose of this filling is to improve the thermomechanical behavior and to absorb already part of the polymerization heat of the resin, which prevents the formation of crevices. It also aims at mechanical reinforcement, because the unfilled resin can remain soft at the operating temperature of the transformers.

On the other hand, it is preferred to use elastomers, such as silicone resins or polyester resins, which are thermoplastic, especially in the case of sheathing cables or wires, in order to give them a certain flexibility. Examples are given, for example, by E.P.R (ethylene-propylene rubber, E.V.A. (ethylene-vinyl acetate) or branched polyethylene.

The invention does not differ according to the type 8900954? * - 3 - Resin (thermal hard hair, thermoplastic or elastomer), to the extent that this resin is an insulating material, intended to be used "hot" and therefore has a good thermomechanical behavior at the above high working temperatures . Also, for convenience, the constituent material of the final insulating envelope formed from the mixture is referred to interchangeably as "envelope resin" or "fill resin": actual resin, reinforcing fill (if the case arises), and optional curing agent (including conventional additives) such as accelerator, flexibilizer, and so on).

The object of the invention is to improve the burning behavior of encapsulated electrical conductors, in particular the windings of dry transformers, by increasing, at the operating temperature, the thermal stability of the incombustible encapsulating resin used.

To this end, the invention relates to a conductor or electric winding encased in an insulating filled resin, which contains an incombustible substance 20 by formation of water as the temperature rises, such as hydrated alumina, which is characterized by a fraction of the filling , equal to at least 20% of the total weight of the casing resin, consists of the incombustible material, which has been partially dehydrated in advance in an amount such that it does not provoke the premature formation of water in the resin when the cased conductor operates is at normal temperature conditions.

A further object of the invention is to provide a process for preparing a filled insulating resin of this type for enveloping windings of dry transformers, for which a filling is added to the liquid starting resin, a fraction of which is equal to at least 20% of the total weight of the coating resin, consists of a substance with incombustibility properties due to the formation of water when the temperature rises, such as hydrated alumina, which is partially dehydrated beforehand, preferably by heating, so that 8900954 · - 4 - during using the transformer at the rated operating temperature, there is no inconvenient formation of water in the resin, which could deteriorate its quality.

The invention also aims to provide an encapsulated dry transformer, at least one electrical winding of which is encased in an insulating, incombustible resin mass corresponding to that obtained by the above-described method.

The incombustible role of aluminum oxide trihydrate, as an additive to resins for sheathing electric cables or transformers, is already known, for example from U.S. Pat. No. 3,202,947, which has already been mentioned above (and the teaching of which is to be incorporated herein) considered). But until now, as far as the applicant knows, the advantage has never been put forward that one can paradoxically obtain better thermal stability of the resin by pre-partial dehydration of such an additive when operating for incombustion by forming water.

The invention will be better understood, and other aspects and advantages will become more apparent, from the following detailed description, with reference to: Tables I, II and III, which follow below and show the good burning behavior of an enveloping resin according to the Invention; Table 4, which provides the main characteristics of the water loss profile as the temperature rises for a given number of incombustibles; Figures 1 and 2, showing the development over time of the weight loss of the various useful incombustibles, by forming and eliminating water, which is due to the instability "in the heat" when placed on a constant temperature heated. The two essential features of the invention are discussed successively.

8900954? ft - 5 - 1) Addition of a sufficient amount of a substance, which makes it incombustible by water formation due to heat instability. 5 The added incombustible substance may be hydrated alumina, Al 2 O 3, nH 2 O with n = 1, 2 or 3, magnesium oxide dihydrate, zinc borate and any other material known for its self-extinguishing properties by removing water, and preferably, in addition, is capable of strengthening the resin, like silicon oxide. Alumina trihydrate is preferred, which has proven to be the most efficient incombustant and, in addition, produces little or no smoke.

The formation reaction can be written as follows: 15 2 Al (OH) 3 ----- ·> A1203 + 3 H20

The rate of this reaction increases with temperature in the direction of the arrow and its endotherraity slows down, and even prevents reaching the ignition threshold of the resin.

It will be apparent from the following that this velocity does not proceed linearly with temperature, but that it forms a strong initial peak of water, which is characteristic of the substance and of which it is decisive that it occurs at temperatures in the superheat zone with ri-25 sico, so below the critical ignition temperature of the casing resin.

The Al (OH) 3 can easily be mixed with the starting mineral fill, since they both exist in the form of solid powder or fine grains.

Instead of 60 parts by weight of SiO 2 in the final resin, the major part can be replaced by incombustible material. Thus, for example, a mixture with 50% Al (OH) 3 and only 10% SiO 2 is obtained. However, experience has shown that the amount of Al (OH) 3 can be reduced to 35% (i.e. 35% SiO 2), without the properties of good fire behavior of the casing, due to the presence of the incombustible substance, to decrease.

These values, determined for an initial fill of 8900954 * - 6 - 60% by weight, can be modified when this amount varies,

Tables I, II and III provide numerical indications and results of 5 laboratory tests on a dry transformer showing the effect of incombustibles (here Al (OH) 3 and zinc borate) on the parameter values recognized as they are representative of the burning behavior of the materials and in particular of resins for encapsulating the windings of dry transformers, ie the oxygen index, the burning rate and the upper calorific power (pouvoir calorifique superior).

Table I: Oxygen indices

Measuring temperatures 20 ° C 80 ° C 150 ° C 200 ° C

15 Required minimum oxygen index (1.0.) 30 27 23.5 21

Aluminum oxytrihydrate

Content of 70% SiO 2 -30 vv28 ^ 24 -19 the total 20% SiO 2 and -39 i-37 ^ 32 -31 20 filling 50% Al (OH) 3 70% 60% SiO 2 -26 / / / 50% SiO 2 and 10% Al (OH) 2 ^ 25 / / / 40% SiO2 and —27 / / / 25 20% Al (OH) 3

Content of 35% SiO 2 and —30 v — 28 - ^ - 25 —20 the total 25% Al (OH) 3 filling 25% SiO 2 and 60% 35% Al (OH) 3 28-31 28-31 24-27 19 -22 30 20% SiO2 and 40% Al (OH) 3 30-33 29-33 25-28 23-24 10% SiO2 and 32-35 30-32 27-29 24-26 50% Al (OH) 3 35 8900854. «- 7-Zinc borate

Content of 10% SiO2 and v-32.5 no values the total 55% Borate filling 10% SiO2 and ^ 3 0.3 no values 5 65% 50% Borate 60% Borate w33.6 no values

The coating material was prepared in the following manner: the mineral fill (here the silicon oxide), after being mixed with the incombustible substance in an adequate amount, is half-axaxed with the liquid resin (epoxide resin, marketed by the Swiss company Ciba-Geigy under the name "Araldite CY 225") and the other half with the curing agent, also in the liquid state (anhydride marketed by the aforementioned company under the name "Durcisseur HY 227").

The two mixtures are then combined and the whole is maxaxed, then placed in an oven (80 to 150 ° C) for curing.

For these tests, the measurement methods were according to the standards UTE NF T51-071 at 20 ° C and EDF HN20 M40 at 80, 150 and 200eC.

Note: The ranges of values indicate that the measured I.O. in this case depends on the origin of the commercially available aluminum oxide used.

The cases indicated by a "/" indicate the measurements that have been judged useless. The corresponding values, taking into account those obtained at 30-20 ° C, are not acceptable, because they are too far below the established minimum I.O.

It is immediately seen that for a total charge of 60%, the minimum level of Al (OH) 3 to be observed is 20-25%. Below that, the determined minimum values of the oxygen index are no longer sufficiently ensured. Complementary measurements, not mentioned here, show that with a global fill of 70% by weight (first part of the table), the threshold of Al (OH) 3 drops to 20%. It is also noted that as far as the zinc borate is concerned, the threshold minimum is much higher: 50% by weight minimum, which, as is known, signifies a much greater efficiency of the aluminum trihydrate.

The burning rates given in Table II are measured in the device used to determine the oxygen indices on the test specimens in the form of elongated plates, with a length of 100 mm, a width of 6.5 mm and a thickness of 4 mm. The test samples have two 10 longitudinal marks one after the other, with the first located at 10 mm from one end and the second at 60 mm. At ambient temperature, the combustion time of the sample between the marks is noted and the average combustion rate (in mm / s) is derived from this as a function of the oxygen content in a combustion mixture of oxygen and nitrogen.

Table II: Burning rate 02 content 35% 40% 45% 50% 60%

Maximum permitted combustion rate (in mm / s) 0.15 0.30 0.45 0.6 0.9

Aluminum oxide trihydrate

Content of the 70% SiO2 0.28 0.37 0.47 0.58 1.05 total filling 20% SiO2 + 0.06 0.10 0.13 0.19 0.25 25 70% Al (OH ) 3 60% SiO2 v ~ 0.35 ^ 0.50 M), 60 ^ 0.75 ^ 1.0 50% SiO2 + ^ 0.35 \ -0.45 i-0.60 - ^ 0.75- ^ 1.0 10% Al (OH) 3 40% SiO 2 + ^ -0.20 ^ 0.35 ^ 0.45 - ~ 0.55 o% 1.030 20% Al (OH) 3

Content of the 35% SiO2 + ^ 0.18 ^ -0.28 ^ 0.38 ^ 0.49 W), 95 total filling 25% Al (OH) 3 60% 25% SiO2 + 0.14-0.20 - 0.27- 0.34- 0.65- 35% Al (OH) 3 0.17 0.23 0.30 0.37 0.59 35 20% SiO2 + 0.13- 0.19-0, 24 - 0.30 - 0.48 - 40% Al (OH) 3 0.16 0.20 0.27 0.35 0.56 10% SiO 2 + 0.07 - 0.14 - 0.20 - 0, 26- 0.37- 50% Al (OH) 3 0.10 0.16 0.23 0.29 0.40 8900954? * - 9 -

Zinc borate

Content of the 10% SiO2 + ^ 0.16 ^ 0.21 ^ 0.30 \ ~ 0.8 total filling 55% borate 65% 5 Content of the 10% SiO2 + ^ -0.25 ^ 0.43 v> 0.65 W), 98 total filling 50% borate 60% 60% borate v ~ 0.25 \ ~ 0.31 c ~ 0.52 w 0.96

Note: The pairs of values have the same meaning as in the previous table.

As can easily be seen, the values shown in Table II support the values of Table I, in that, with regard to the criterion "combustion rate", they also show that the limit value to be respected for the amount of A1 (0H) 3 in the starting mixture 25 wt% (a little less than 50% for the borate).

These conclusions remain fully valid in view of Table III below, which lists the results of the series of tests conducted on the third parameter, upper calorific power (P.C.S.). As can be seen, the maximum permitted value of 11 MJ per kg of substance is never exceeded.

Table Ills Upper calorific value fP.C.S.1 (Test method: adiabatic calorimetry 25 according to UTE NP M 03-005.)

Content of the Content of the total filling: 60% total filling 70%

Aluminum oxide trihydrate 30 70% Si02 20% Si02 60% Si02 40% Si02 35% Si02 25% Si02 10% SiO2 + + + + + + 50% Al 10% Al 25% Al 35% Al 50% Al * ~ 8 w7 i ~ ll want will want wn 35 Zinc borate 10% SiO2 + 55% Bor. 10% si02 + 50% Borate 60% Borate w 10 wants 8900 954 - 10 -

These tables clearly show the influence on the burning behavior of the addition to the starting resin, in adequate amounts, of a non-combustible substance by the formation and elimination of water, according to the invention.

These results show a strong release of water from the casing resin side when it reaches abnormally high temperatures. Such a phenomenon gives a self-controlling absorption of heat, which retards and inhibits combustion of the filled resin and therefore gives the intended self-extinguishing character.

It is noted that an abundant formation of water molecules in the mass itself is not without consequences for the quality of the encapsulating resin. It degrades during such a process and can generally no longer be reused for the sequel. The transformer must then be replaced or reconditioned.

The weight loss of the incombustible substance as the temperature rises is a good indication of its water-forming ability. It is noted that, as can be seen from the specifications of the suppliers, in the case of Al (OH) 3 the weight loss at 300eC is already close to 30%. Incidentally, at this temperature it is practically only produced in the form of a narrow peak and a large amplitude, which shows the liveliness and intensity of the phenomenon, when this temperature level, which is characteristic of the incombusting agent used, is reached.

This is correct, whatever the variety of the commercial aluminum trihydrate used, as shown in Table IV below.

This table of values is given for information only, from data from suppliers of the Al (OH) 3, which is sold under the name "Alcoa" in varieties, the commercial references of which identify them in the columns of the table are listed.

89 00 954 - IX -

Table IV; Analysis of the endothermic peaks of water formation (the temperature increases are from 25 to 600 ° C with a constant increase of 10oC / min.) 5 M15 S65 / 40 C31 M6 M15S S65 / 150 Medium soda

Start temp. of the peak (eC) 196 205 216 180 195 188 197 10 Final temp. from peak (eC) 372 385 353 382 355 370 353 MAx.temp. from the peak ("O 316 312 314 314 311 309 312 ΔΕ from the peak 15 (in kJ / g) 1.03 1.01 1.07 1.06 1.03 1.0 1.02

In order for the expected effects for the type of incombustible substance of the invention to be completely satisfactory, it is necessary that not only the strong initial peak of water occurs at adequate temperatures, that is, between the normal point of functioning in the heat and that where the casing can ignite, but also that water formation does indeed only occur in the case of abnormal overheating. In other words, one should avoid any risk of premature degradation of the envelope resin, which could arise from an untimely formation of water from the ambient temperature to the normal operating temperature in the heat of the transformer.

It is this difficulty, which is solved by the second main feature of the invention, that is, a moderate pre-aging of the resin, as will be shown in detail from the following: 2) Partial pre-dehydrogenates of the additive amount of incombustible dust

This is to establish a pre-dehydration to prevent it from finally occurring in the trans 8900954-12 formator. However, this dehydration should only be partial, since it is pursued at a high temperature, when the transformer heats abnormally and the danger of ignition is feared.

This dehydration can advantageously take place by preheating the A1 (0H) 3. This objective to be achieved is not total removal of water, which can form in a range of temperatures, from ambient temperature to the operating temperature in the heat of the transformer (and which will be figuratively called "volatile water" to explain the fact emphasize that this must escape at low temperature), but sufficient removal so that the "volatile" water residue is present in too small an amount to lead to degradation of the resin. In fact, it has been observed that when the alumina trihydrate is not preheated prior to addition to the mineral fill, cracks of the resin encapsulating the electrical windings of the test transformers occur at the operating temperature, requiring the latter to be put at the waste.

An easy way to achieve partial dehydration of the incombustible material by preheating is by studying the weight loss curve as a function of time. For a substance used for the first 25 times, it is advantageous to proceed in two steps: the first, determining on an analysis sample the amount of water that disappears during a prolonged stay at a constant temperature, which is (or near that) of normal operate in the heat of the transformer; the second stage, this time treating the total of the material, consisting of heating at a relatively high temperature in order to achieve quickly, thus under industrial conditions, a weight loss corresponding to the value of the water removal determined in the previous phase.

It should be noted that this second step must be repeated with each resin preparation, while the first 89 00 95 4-13 is only needed once to characterize a type of incombustible substance that has never been used before.

The curves of Figures 1 and 2 illustrate this first phase of studying the samples by showing the evolution of weight loss as a function of time for certain temperature values at certain temperature values. The curves are those of some of the alumina trihydrate 10 "Alcoa" varieties of Table IV, which are indicated with their commercial references at the right end of each curve. Three temperature values have been considered: 140, 160 and 180eC, to include the usual operating conditions "in the heat" of the transformers. To avoid unnecessary overprints, the three corresponding groups of curves on the two figures are separated: figure 1 contains the group at 140 * C (discontinuous lines) and 160eC (solid line)? that at 180 ° C is only shown in solid lines in figure 2.

It can be seen that all these curves have a general logarithmic course with a very rapid rise at the beginning, followed by a part that slopes slightly with respect to the horizontal, which part is reached earlier, the higher the operating temperature. Also at 180 ° C (Figure 2), the bulk of the "volatile" water (about 80%) is formed after only 140 hours, over the more than 700 hours total duration of the runs. The weight loss is between about 5.3 and 2.5%, depending on the type of aluminum oxide.

The existence and stability of the planar parts are verified by testing aluminum oxides which have been preheated to a much higher temperature. Tests were carried out on two samples of the "M15" variety, one of which was held at 180 ° C for 18 hours and the other at 35 200 eC for 18 hours. The results are shown in Figure 1 in the form of two straight lines (A) and (B) with ordinates at the origin of 1.6 and 4.2% weight loss at 180 and 200 ° C, respectively. They are seen to be substantially horizontal 8900954> 14, which well reflects the insensitivity of the samples to a second heating at a much lower temperature, due to the fact that virtually all "volatile water" at 140 or 160 ° C has been effectively removed during the first 5 heating.

It can also be seen in Figure 2 that after 18 hours at 180 ° C, a sample "M15" is barely halfway up its curve of rapid growth in weight loss by eliminating water.

It will be clear that these curves have a favorable characteristic allure, which makes it very easy to find in advance the partial degree of dehydration to be reached. For example, one can choose as criterion the beginning of the flat part and as the amount of volatile water to be removed take the value given by the ordinate of this start of the flat part.

The figures also show that, for the variety "865/40", one can take 2.5% weight loss at a transformer operating temperature of 140 ° C, 4% at 160eC and 20 5.5% at 180 ° C.

Likewise, the "M15" variety accommodates with a pre-weight loss of 2% when the transformer is operating at 160 ° C and 3.5% at 180 ° C.

At 140eC, the curve of this variety has a less typical allure. It should be noted, however, that the value of 0.7% obtained after 500 hours may be approximately perfectly suitable.

Thus, to record the thoughts, it can be broadly said that the intended weight loss in advance ranges between about 0.5 and 10% for all of the incombustibles used to practice the invention.

Subsequently, the transition to the industrial phase simply consists of dehydrating the mass of the incombustible substance by rapid heating methods in the oven, with supervision of the weight loss, for example by gravimetry, using an automatic balance, the scale of which is in the oven space placed. In the case of aluminum oxides of the "M15" type, the weight loss of 3.5% required for operation of the transformer at 180eC can be achieved by heating for 6 hours only at 200 ° C. * C.

This heating operation will of course be faster the higher the heating temperature. For reasons of retaining a strong potential of water formation, which is necessary in case of abnormal overheating, care must be taken not to start the peak temperature of strong removal of water, which is characteristic of the incombustible substance used exceed too much and preferably remain below it and some of its values are listed in Table IV.

If one does not want to work with gravimetry, in particular due to excessive amounts of dust, which may have to be treated, it has the advantage of working via an intermediate stage, which makes it possible to translate the intended weight loss into a heating time. This can be done with the aid of a second measurement sample of the relevant incombustible substance, of which the weight amount of water to be removed is known and which is subjected to rapid heating at a determined elevated temperature. This heating takes place at continuous weighing of the sample in order to be able to measure the time taken to achieve a weight loss corresponding to the known amount of water to be eliminated. The value of the measurement determines the duration of the heating operation at a temperature identical to that of the intermediate operation below, to which the mass of the incombustible substance is subjected before being treated.

If necessary, one can be sure of the; good execution of the operation, by gravimetry at a high temperature, for example 1200 ° C, determining the residual water content of a sample which has been drawn for this purpose from the treated incombustible substance. By comparing with the total water content at the beginning (usually in the £ 9,00954.

* - 16 - order of about 20 to 35%), which has previously been determined analogously to a reference sample, it can be deduced that the amount of effectively "volatile" water removed corresponds to the target amount.

It has been found that the heating time is not entirely independent of the granulometry of the material. It has been observed during tests that in a given heating time a coarse granulometry causes a greater weight loss than a fine granulometry.

It has also been observed that the previously determined values of the weight loss, by reading the test curves made from samples, do not in any way show an upper limit which cannot be exceeded. However, since these values correspond to the beginning of the flat part, there is in principle no point in prolonging the heating for a long time to recover a few tenths percent, which in any case are too insignificant to cause a loss of water. , which prevents the quality of the casing resin.

It will be understood that these curves, which apply at normal transformer temperatures in the heat, reflect the behavior of the "volatile" water at these temperatures. At higher temperatures, the flat areas are at higher levels and are reached more quickly, in particular at the temperature of the characteristic peak of the incombustible substance used and which, as has been seen, is close to 300 ° C for all studied varieties of Al (OH) 3.

The incombustible substance thus pretreated is ready for use. Completion of the coating resin preparation remains in the usual manner: the mineral filling, after being intimately mixed with an adequate amount of pre-partially dehydrated incombustible substance, is divided into two equal parts. One part is then placed in the pure resin bath and the other part in the curing agent, also in the liquid state. The two mixtures are individuallyaxed to obtain two solid liquid suspensions and then mixed into a single mixture, which is in turn malaxized to ensure good homogeneity. The pulp obtained is then poured into a mold in which the electrical winding of the transformer which is desired to be encapsulated is placed in advance. After casting, the mold is put in the oven to cure the resin. After being taken out of the oven and after cooling, the resin block containing the winding is taken out of the mold and can then be mounted in the appropriate transformer.

It will be understood that the invention is not limited to the following examples, but includes numerous variants and equivalents containing the characteristics recited in the claims.

In particular, the invention is not limited to the actual transformers. By this term used here, it is to be understood in a broader sense the entirety of inductive electrical appliances or devices which can operate at relatively high temperatures of 100 to 200 ° C, as described above, and the electrical windings of which can be incorporated in a block of insulating resin.

Likewise, although the invention was initially conceived for the applications of thermosetting resins (sheathing of inductive windings, especially of transformers), in fact it relates to all dielectric sheathing materials. It is of particular importance in the case of electrical installations with nominal long-term operation at high or moderately elevated temperature. This means that it is particularly advantageous to use them for the coating resins which already have a good thermomechanical behavior.

35

8960954.V

Claims (11)

1. Dry-encapsulated transformer or analog electrical installation, at least one winding of which is enclosed by a filled insulating resin, which contains a substance which makes it incombustible by the formation of water when the temperature is raised, characterized in that a fraction of the filling , at least equal to 20% of the total weight of the casing resin, consists of the incombustible substance, which has been partially dehydrated in advance by an amount such that it does not cause water to prematurely form in the casing resin, thereby reducing its quality, when the transformer is used at its normal temperature conditions. 2. An electrical conductor encased in a filled insulating resin containing a substance which makes it non-flammable by the formation of water as the temperature rises, characterized in that a fraction of the filling, at least equal to 20% of the total weight of the casing resin, consists of the incombustible material, which has been partially dehydrated in advance by an amount such that it does not cause the premature formation of water in the casing resin, which may reduce its quality when the resin is normal temperature conditions are applied. Dry-transformer or conductor, which is encapsulated according to claim 1 or 2, characterized in that the incombustible substance has a weight loss of about 0.5 to 10% compared to the non-hydrated initial state. 89 00 954 »- 19 - Dry transformer or conductor encapsulated according to claim 1 or 2, characterized in that the incombustible substance is alumina trihydrate. 5. A process for preparing a filled insulating resin for the coating of conductors or electrical windings, of dry-coated transformers, or of other analog electrical installations, according to which a substance is added to the starting resin in addition to the filling. incombustible by the formation of water when the temperature rises, the process being characterized in that the substance is added in at least 20% of the total weight of the coating resin and the substance is partially dehydrated in advance in such an amount that no untimely formation of water which is detrimental to the behavior of the casing resin if the electrical installation is left in use at its normal operating temperature. 6. Process according to claim 5, characterized in that the incombustible substance is pre-subjected to a prior dehydration treatment until it shows a weight loss between about 0.5 and 10% of the original weight. 7. Process according to claim 5 or 6, characterized in that aluminum oxide trihydrate is used as the incombustible substance. 8. Process according to claim 5, characterized in that the degree of partial dehydration of the incombustible substance used for the first time is determined by taking a sample and subjecting it to prolonged heating at a constant temperature. which is lower than that of the peak of water removal from the incombustible substance used and the rate of development of the weight loss of the sample determined with time. Process according to claim 5 or 8, characterized in that the partial dehydration of the incombustible material is carried out by subjecting it to rapid heating until the weight loss corresponds to the amount of water to be removed according to a predetermined amount of a sample whose weight loss development has been monitored as a function of time during a prolonged heating at a temperature lower than the temperature of the peak of removal of water from the incombustible agent used. Method according to claim 9, characterized in that the rapid heating takes place for a time predetermined by a heating operation, carried out at an identical temperature and carried out while weighing a sample of the combustible substance used, until this has a weight loss corresponding to the amount of water that it is desired to remove. 11. A method according to claim 10, characterized in that the amount of water to be removed has been predetermined by following the evolution with time of weight loss of a sample, which is subjected to a prolonged heating at a constant temperature, which is lower than the peak temperature of removing water from the incombustible substance used. 25 ƒ IJ -o-o-o-o-o-o-o-o-o- 30 35 89 00 9 54.