LU87494A1 - DRY TRANSFORMER WITH COATED WINDINGS, CONDUCTORS AND SIMILAR ELECTRICAL INSTALLATIONS, AND PROCESS FOR PREPARING THEIR COATING RESIN - Google Patents

Description

* ____-, BL-4227

1. ~ * 7 7 ffJL GRAND-DUCHE DE LUXEMBOURG

Patent N ° .3 ..... 7 ......... Ώ ...-... Μ. t *.

w Minister of ............................................. .... Φ ... ΐΑΐ-ί989 'of Economy and the Middle Classes

Intellectual Property Service

liter delivered --------- j. (gS) LUXEMBOURG

Patent Application ............................................ .................................................. .................................................. ...........—: ---------------------------: _____ (i) I. Request

The company called FRANCE TRANSFO SA - Roman Way - ^ 2) ... R.Qnt „„ .. de, ... S.em.é.ci.Q, uX-t ......- .. ... E “5J2.1û .._ .. MAXZIERES. = LES-ME.TX„ -..._ Éxa.nce _______ represented by ETFREYLINGER & E.MEYERS, Ing ”cons.en PI -46, rue du Cimetière - L — ï338 Luxembourg _____ (3) denosefnt) ce ...... six ....... avril ..... mil ....... neuf ..... Oant._. .g.ua.tre. =. v.ing.t ._ .. iie.uf ______ (4) at ........ 1.5 ... 00 .. hours, at the Ministry of Economy and Middle Classes, in Luxembourg: 1. this request for obtaining a patent for an invention concerning: "Dry transformer with coated windings, conductors and ^ similar electrical installations, and process for preparing ...... their d1enroBageT® resin .............................................. .................................

2. description in language ................................ £ .r§.AÇ..§i.se ................................ of the invention in triplicate; 3. ................ i ............................... .................. drawing boards, in triplicate; 4. the receipt of the taxes paid to the Registration Office in Luxembourg, 0 6.0 3.19 8 9____: 5. the delegation of power, dated .................. .................................................. .................. the___________________________________________________; 6. the successor document (authorization); declares (s) assuming responsibility for this declaration, that the inventor (s) is (are): (6) „PID.AN.CET ...... Brlg.i..fc.ta .._ r ........ 15, rue des Jardins sous ..... la ..... Fontaine -_____ ............... F-57158. ..... Montigny ..... les ...... Metz - France___________________________________________________________ VÀZQÜËZ ...... Miguel - 1, rue du Pré de la 'Darne äSILLY surlfllED .....- F-5753Ö CourceÎïës Chaussy - France CÜRD'TER ...... Pli'iïippe - 9, au ôranà 'Éoirlër du Vigy .......

...........-France.........---------------------------- ---------------------------------------------------------- ------- claim (s) for the above patent application the priority of one (or more) request (s) from (7) ----------------- ------ filed in (δ) ------------------------------ ie (9) -: -: - ---------------------------------------------------------- ------------------------: ------------------------- --- under N ° (10) __ 8 8 0 5, 6. 41___________________________________________________________________ in the name of (U), ......... grance .. ,, ^ an ^ ____________________________________________________________ elect (elect) domicile for him (she) and, if appointed, for his representative, in Luxembourg_________________________ .4.6 .., rue du Cimetière -. = ...- .. L-133.8 .._ Luxembourg · ------------ ------------------------ (12) requests (s) the grant of a patent for the invention for the subject described and represented in the appendices above, with postponement of this issue to ..... _______ S -____ ___________months. (13)

The proxy deotjsatrty: i ._________ ________________________________________________________________________________________________________________________ (14) \ Π. Deposit Minutes

The above application for a patent for invention has been filed with the Ministry of the Economy and the Middle Classes, Intelleptuëfle Property Service in Luxembourg, dated: 06 04 1989 \ 1ji \ Pr. The Minister of EcodWie and Classes Medium, at hours dd / ψρΜ \ "| / Mfe.d.

j ^ J Chef du SERV ^ Aeha, Intellectual Property, W, J -J A68007__A- / \ l_

* · EXPLANATIONS RELATING TO THE FORMUUU ^ I ^ OTPtÆ //] / B

(1) if applicable "Application for certificate of addition to the main patent, to the main patent application No ....... Arou ... .1 .......” - ( 2) enter the surname, first name, profession, address of the applicant, when the latter is an individual or corporate name, legal form, address of the head office, when the applicant is a legal person - (3) enter the surname, first name, address of the authorized representative, industrial property attorney, with special powers, if / where applicable: / represents by ........... acting as a representative ”- (4) date of deposit in full - (5) title of the invention - (6) enter the names, first names, addresses of the inventors on the indication "(see) separate designation (will follow)", when the withdrawal is made or will be done in a seoaré document. or the indication "do not mention", when the inventor maintains or will have a non-mention document to be attached to a designation, ϊ · * f BL-4227

Claim to the priority of the patent application filed in France on April 22, 1988 under the number 88 05641 Descriptive memorandum filed in support of a patent application for "DRY TRANSFORMER WITH COATED COILS, CONDUCTORS AND SIMILAR ELECTRICAL INSTALLATIONS , AND PROCESS FOR THE PREPARATION OF THEIR COATING RESIN "

FRANCE TRANSFO Roman Way Pont de Semécourt F-57210 MAIZIERES-LES-METZ

»DRY TRANSFORMER WITH COATED WINDINGS. CONDUCTORS OR SIMILAR ELECTRICAL INSTALLATIONS AND METHOD FOR freparation.de LEUR.RESIN DECOBAGE.

5 The invention relates to improving the fire resistance of coated electrical conductors placed in hot service.

It applies in particular to dry power or distribution transformers, whose working temperatures conventionally exceed by more than a hundred degrees 10 centigrade the ambient temperature (working temperature of the order of 140-150 ° C generally ). Also, and for convenience only, reference will be made for the rest of the description to the example of the aforementioned transformers.

In this type of transformer, usually reserved for a voltage range from 3 to 36 kV, the winding is embedded in an insulating synthetic resin with dielectric properties and having several millimeters in thickness (from 2 to 5 mm, for example). It should be remembered that, in addition to its function of electrical insulation, the resin has a protective role against humidity and dust which could lower the breakdown voltage. It also protects the electrical windings against an ambient environment polluted by aggressive chemical agents and plays an important role of mechanical resistance by ensuring the fixity of the turns between them in the winding 25 It is nevertheless known that, under conditions of use "accidental extremes, or following an anomaly, Trans trainers can burn. Study of their fire behavior has shown that when the coating resin burns, it often continues to burn, even if the fire causing it has stopped, because , from a critical temperature of its own, the resin becomes flammable in the air At this stage, a few reminders concerning the constitution of these resins may prove useful for the following.

Current coatings for dry transformers are thermosetting resins, obtained by heating an initial mixture composed of a resin proper (generally an epoxide) and a hardener, such as anhydride. A classic example is provided by the diglycidic derivatives of 2-phenol A, more commonly called DGEBA, and formed by reaction between bis-phenol A and hepichlorohydrin (see USP 3,202,947 for example) . These resins are most often crosslinked and made infusible and insoluble by the addition of amines and polysulphides of low molecular weight (ARALDITE resin (R> for example). Other examples are found among thermosetting polyesters. are typically 50% for the resin and 50% for the hardener.

Generally, filled resins are used: "resin". starting liquid receives, before the addition of the hardener, a filler, often mineral, for example quartz flour (silica), or glass wool, in weight proportions which are of the order of 3 filler for 1 of resin. There will then be 20% of resin, 60% of silica and 20% of hardener in the initial mixture.

This filler has the role of improving the thermomechanical behavior and of absorbing part already of the heat of polymerization of the resin, which makes it possible to avoid the formation of cracks. It also has a role of mechanical reinforcement, because the uncharged resin can remain of soft consistency at the operating temperature of the transformers.

On the other hand, elastomers, such as silicone resins, or polyester resins, which are thermoplastics, are used instead, in the case in particular of coating of cables or wires, in order to allow them a certain flexibility. Examples are given by E.P.R (Ethylene-propy-lene-rubber, ΙΈ.V.A. (Ethylene vinyl acetate), or cross-linked Polyethylene).

The invention does not distinguish according to the type of resin (thermosetting, thermoplastic or elastomer), insofar as this resin is an insulating material intended to be used "hot" and therefore therefore exhibits good thermo-mechanical behavior at the high working temperatures indicated above. Also, for convenience of language, we will indifferently call "coating resin" or "charged resin" the constituent material of the final insulating coating formed by the mixture: resin proper, reinforcing filler (if applicable), and optional hardener (usual 40 additives included, such as accelerator, flexibilizer, etc.).

3 f

The object of the invention is to improve the fire behavior of coated electrical conductors, in particular the windings of dry transformers, by increasing, at their operating temperature, the thermal stability of the flame retardant coating resin used.

To this end, the subject of the invention is an electrical conductor or winding coated in a charged insulating resin containing a flame retardant substance by the formation of water when the temperature rises, such as hydrated alumina, characterized in that that a fraction of the filler, equal to at least 20% of the total weight of the coating resin, consists of said flame-retardant substance, which has been partially dehydrated beforehand in an amount such that it does not cause untimely formation of water in the resin 15 as long as the coated conductor is in service under its normal temperature conditions.

Another subject of the invention is a process for preparing a charged insulating resin of this type for coating the windings of dry transformers, according to which a charge is added to the initial liquid resin, a fraction of which is at least equal to 20% of the total weight of the coating resin, is constituted by a substance having flame-retardant properties by formation of water when the temperature rises, such as hydrated alumina, but this, after having previously dehydrated partially the flame retardant substance, preferably by heating, so that, when the transformer is used at its nominal operating temperature, there is no untimely formation of water within the resin which can adversely affect its quality.

The invention also relates to a coated dry transformer, at least one electrical winding of which is coated in a mass of flame-retardant insulating resin conforming to that obtained by the process specified above.

The flame-retardant role of alumina trihydrate, as an adjuvant to coating resins for electric cables or for transformers, is already known, for example from the document USP 3,202,947 already mentioned (and the teaching of which is incorporated herein by reference). But, never until now, to the knowledge of the inventors, it has been possible to highlight the advantage that 4 a can paradoxically expect better thermal stability of the resin by dehydrating such an adjuvant partially beforehand, then that it acts as a flame retardant precisely by forming water.

5 The invention will be well understood, and other aspects and advantages will appear more clearly, in the light of the detailed description which follows, given with reference: - Tables I, II and III presented in the body of text 10 and expressing the good fire resistance of a coating resin according to the invention; - in Table IV, also presented in the body of the description, and giving the main characteristics of the profile of the water departure with the rise in temperature 15 for a certain number of flame retardants; - Figures 1 and 2 showing the evolution over time of the weight loss of different flame retardants usable, by formation and elimination of water, resulting from their instability "hot" when heated to constant temperature.

The two essential characteristics of the invention set out above are repeated successively: 1) ADDITION IN SUFFICIENT QUANTITY OF A FLAME RETARDANT SUBSTANCE BY FORMING WATER DUE TO HOT INSTABILITY.

The flame retardant added may be hydrated alumina AlzOs, nHzO with n = 1, 2, or 3 of bihydrated magnesia, zinc borate, or any other material known for its self-extinguishing properties by elimination of water and preferably also capable, like silica, of reinforcing the resin. We can give preference to trihy-draté alumina, which has proven to be the most effective flame retardant, and which in addition, does not emit, or little smoke.

35 The water formation reaction can be written:

2 Al (OH) * ----> AlzOs + 3 HzO

The speed of this reaction increases in the direction of the arrow with temperature, and its endothermicity delays, or even prevents, the reaching of the ignition threshold of the resin.

r 5

It will be seen later that this speed does not change linearly with temperature, but that an intense departure peak of water, characteristic of the substance, forms and of which a decisive advantage is that it appears at temperatures 5 located precisely in the overheating zone at risk, therefore below the critical flammability temperature of the coating resin.

AI (OH)? can be easily mixed with the starting mineral filler, since they are both in the form of powdered solid matter or fine grains.

Instead of 60% by weight of S1O2 in the final resin, it is replaced, for the most part, with the flame retardant substance. This produces, for example, a mixture representing 50% of Al (0H) 3 and only 10% of Si0 =. However, experience has shown that the amount of Al (OH) 3 can be reduced to 25% (therefore 35% of SiO 2), without significantly harming the qualities of fire resistance of coating, due to the presence of the flame retardant substance.

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

Tables I, II and III below provide indications and quantified results of laboratory tests carried out on a dry transformer, showing the effect of flame retardants (here A1 (0H) 3 and Zinc Borate) on the values of parameters recognized as representative of the fire resistance of the materials and in particular of the coating resins of the windings of the dry transformers, namely the Oxygen Index, the Combustion Speed and the Higher Calorific Power.

5 TABLE I: Oxygen Indices fl.O.)

i Measurement temperature at 20 ° C! 80th! 150 ° Ci 200 ° CJ

I_______________________I______I______I______I __I

I (I t I I

5: Qz index (1.0.) I 30! 27! 23.5! 21 | {minimum imposed! ! ! ! !

I ------------------------------------------------- --I

I -------- I

! TEI-HYDRATED ALUMINA |

l_______________________________ I

I I

10! ™ u * oE | 707o de Si0z i "30!" 28! "24!" 19! r I CHARGE I____ _ __ I ____I ____I______l______i if i II t (j-ro-r ^ <20% of Si0z and j ~ sg j - 37; "32! ~ 31!! / ο" i50% of Al (0H ) 3 !!!!!!

I_______l „„ _____________ ί ______ I______i______I______I

there i i i i i 15! ! 60% SiOz! "26! /! / \ / \ I I _______________ I ______ i __.____ I______l______l

<1 I I I I I

! ! 50% SiOz and! "25! /! /! /!!! 10% AMOH) *!!!!!!! I_______________l______I______I_____I______l

fl I I I i I

20! ! 40% of SiOz and! "27! /! /! /!!! 20% of Al (OH> 3!!!!! I I _______________ I ______ I ______ I ______ I _____ | ttliiii jt" ux o = j35% of SiOz and! "30!" 28! "25!" 20! | 25% d / Al (OH) 3!!!!! Op; * total i_______________i______i______i______i_____i 6 v | ii | ili! MX! 25% SiOz and! 28-31! 28-31 [24-27! 19-22!!! 35% dJAl (OH> 3!!!!! I I________________I______I______J _____ I _ __ t II ((lit!! 20¾ of SiOz and! 30-33! 29-33! 25 -28! 23-24! 30!! 40% of AKOH) ^!!!!!

I I _________________I______I ______1______I______I

II I I I I I

! ! 10% of SiOz and! 32-35! 30-32! 27-29! 24-26! ! ', 50% Al (0H> 3 11!!! I ---------------------------------- -------------- l

i ------------—--—--—------------------------------ --I

35! ZINC BORATE!

I _____________________________________, _________ „_________ I

i 1 1 GH RATE! 10% SiOz and! "32.5!! Ίτστ.όβχ! 55% Borate!! No values!

I___________________________ ________________________I

(i 40! rate of 10% of SiOz and! "30. 3!! [charoe [50% of Borate!! no values!

I I I

| TOTAL 1 ------------------------------------------- 1! Μ "î60% of Borate!" 33.6! No values! «7

The coating material was prepared in the following manner: the mineral filler (here silica), after having been mixed with the flame retardant in an adequate amount, was kneaded, for half, with the liquid resin (epoxy resin). 5 dique marketed by the Swiss firm CIBA-GEIGY under the name "ARALDITE CY 225") and, for the other half, with the hardener, also in the liquid state (anhydride marketed by the aforementioned company under the name "DURCISSEUR HY 227 "). The two mixtures were then combined and the whole was kneaded, then placed in the oven (80 to 150 ° C.) for solidification.

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

Rq.- The ranges of values reflect the fact that 15 the I.O. measured in this case depended on the origin of the commercial alumina that was used.

- The boxes marked with a "/" translate measures considered useless. The corresponding values, taking into account that obtained at 20eC, are immediately non-ac-20 countable, because too below the minimum I.O. imposed -

We can immediately see that for a total load of 60% the minimum content of Al (0H) 3 to be respected is 20-25%. Below the minimum values' imposed on the Qz Index are no longer effectively guaranteed. Additional measurements, not taken into account here, have made it possible to show that, with an overall load of 70% by weight (1st part of the table), the threshold value of Al (0H) 3 drops to 20%. We also observe that, as regards Zinc Borate, the minimum threshold is much higher: 50% by weight at least, which translates, as we knew, a greater efficiency of Alumina trihydrate.

The Combustion Speeds recorded in Table II below were measured in the apparatus used for the determination of Oxygen indices on elongated platelet test tubes, 100 mm long, 6.5 mm wide and 4 mm thick. The test pieces have two marks one after the other depending on the length, the first being located 10mm from one end, the second 60mm.

We note, at room temperature, the combustion time of the test piece between the two marks and we deduce the 40 Average Combustion Speed (in mm / s) depending on the rate 8 TABLE II: Combustion Speeds.

! O2 rate 1 35% 40%! 45%! 50%! 60%! I _____________________ I _______ I ______ I _______ {______ I__ i

I I I I I I ~ I

J Combustion Speed! J! ! ! ! 5! maximum allowed! 0.15 | 0.30! 0.45! 0.6! 0.9! ! (in mm / s)! ! ! ! ! \! TEI-HYDRATED ALUMINA!

I___________________________________________________________I

10! TrtlJX ~ =! 70% of SiOz 1 0.28! 0.37! 0.47! 0.58! 1.05! I CHARGE I_____________I ______I _ I_____ | _ _ | ____ |

II I I I I I I

! TOTfiL i 20% SiOz + ‘0.06! 0.10! 0.13! 0.19! 0.25! ! 70¾ 150% d-’ÄKOH) ^! ! ! ! ! ! I _______ I _____________ l _______ I ______ I _______ I ______ I _______ (

II I I I I I I

15! 160% SiCh! ~ 0.35! "0.50!" 0.60! ~ 0.75! ~ 1.0! I I_____________I________ I______I________I__ _ I ___ |

I I I II I “* I I

! 150% SiOz + ', "0.35 Γ0.45' Γ0.60!" 0.75! "1.0!!! 10% Al (0H) 3 |!!!!! I I _____________, __ I _______I_______I_______I______I_______t

II I I I I I I

20! | 40% of SiOz +! "0.20 ί" 0.35 Γ0.45! "0.55!" 1.0! ! | 20% Al (0H) 3 | ! ! ! !

1 I I I I I I I

! ™ ux! 35% SiOz +! "0.18 Γ0.28 | ~ 0.38! ~ 0.49!" 0.95! ! CHftTOB! 25% d5Al (OH) ®! ! ! ! ! !

O C I TOTAL I _ ______ I _______ I ______ I _______ I ______ î _______ I

6v | i ““ * | | | j I i! 60-,! 25% of SiOz +! 0.14-! 0.20-! 0.27-! 0.34-! 0.56-! ! i35% Al (OH) 3! 0.17! 0.23! 0.30! 0.37! 0.59!

I I____ I_______I______I_______I______I_______ I

there 1 1 1 1 1 1! ! 20% SiOz + | 0.13-! 0.19-î 0.24-! 0.30-i 0.48-! 30 ! 140% AKOH) *! 0.16 to 0.20! 0.27! 0.35 to 0.56!

I I___ I_______I______I_______I______I I

I I “I II II I

! 110% SiOz + 1 0.07- J 0.14- ', 0.20-! 0.26- | 0.37-! ! 50% Al (0H) 3 d 0.10! 0.16 to 0.23! 0.29! 0.40! 35! ZINC BOKATE!

I___________________________________________________________I

i i

| T "ux GM.10% of SiOz +! ! "0.161" 0.21 1 "0.301 ~ 0.8 1! TOT-6S5 !! 553â from Borate !!!!!!

t___________________________________________________________I

1 1 40! TAux »e! 10% SiOz +! ! "0.25!" 0.43! "0.65!" 0.98! ! charge [50% Borate! ! ! ! ! !

I I. I

l TOTAL (------------------------------- 1! 160% Borate!! "0.25!" 0.31! "0.52 ! "0.96! 9"

Rq. the indications of pairs of values have the same meaning as in the previous table.

As can easily be seen, the values given in Table II corroborate those in the table by showing 5 that, with regard to the criterion "combustion rate" also, the "floor" value to be observed for the quantity of Al ( OH) 3 in the starting mixture is substantially 25% by weight (slightly less than 50% for Borate).

These conclusions remain entirely valid in view of Table 10 below, recording the results of the series of tests carried out on the 3rd parameter retained, the Higher Calorific Power (P.C.S.). As we can see, the maximum allowable value of 11 MJ per kg of material is never exceeded.

15 TABLE III: Higher Calorific Power (P.C.S.) (Test method: adiabatic calorimetry according to standard UTE NF M 03-005.)

Charging ratej Total charging rate: 60% 20 total: 70% d

ALUMINA TRI-HYDRATE

_ n- f Γ I 1 70% Si02 1 20% SiQ * || 60% SiQ *. 40% Si0z | 35% SiOa | 25% SiQ2 | 10% Si02: 25 | + If. + ί + + | + ------------- 4 ------- 1 ------- D -------- 1 ------ d --- ------ ~ 8 ~ 7 II ~ 11, ~ 11 I ~ 11 I - 11 I - 11

1 II I I I I

30 ------- L ------ Ji -------- i ------- It ------- i ------- 1- --------

BORATE DI ZINC

il ~ i 10% Si0z + 55% Bor. | j 10% SiQ * + 50% Borate | 60% Borate --------------- j, ------------------------- | --- -------------- ______________1_________________________i_________________

It is therefore clear to see clearly on these tables the influence on the fire resistance of the addition to the starting resin, in adequate proportions, of a flame-retardant material by formation and elimination of water, in accordance with the invention .

10 '

These results reflect a strong release of water from the coating resin when it reaches abnormally high temperatures. Such a phenomenon acts as a self-controlled heat absorber, which delays and slows the combustion of the charged resin, and gives it the desired self-extinguishing character.

Of course, an abundant formation of water molecules within the mass itself is not without consequences for the quality of the coating resin. This degrades during such a process and, generally, cannot be reused for the rest. The transformer must then be replaced or reconditioned.

15 The loss in weight of the flame retardant during the rise in temperature is a good indicator of its capacity to form water. It should be noted, as the suppliers' specifications show, that in the case of Al (OH) s, the weight loss is already close to 30% at 300eC. Moreover, it takes place practically only at this temperature in the form of a narrow peak of large amplitude, which testifies to the liveliness and intensity of the phenomenon, when this temperature level, characteristic of the flame retardant used, is reached.

. 25

This is true regardless of the variety of commercially available tri-hydrated alumina used, as shown more precisely in Table IV below.

This table of values is given for information only, based on the indications of the suppliers of Al (0H) 3 sold under the name "ALCOA" according to varieties, the commercial references of which are given in the columns of the table to identify them.

11 TABLEAP IV: Analysis of the endothermic peaks of water formation (the temperature increases were carried out from 25 to 600 ° C under a constant progression of 10 ° C / min.)

5 M15 S65 / 40 C31 M6 M15S S65 / 150 MEDIUM

SODA

____ __________________________________

Temp. start 196 205 216 180 195 188 197 peak (° C) 10 Temp. end 372 385 353 382 355 370 353 peak (° C)

Temp. max. 316 312 314 314 '311 309 312 peak (° C) 15 ----------------------------------- ------------------------. Peak HA 1.03 1.01 1.07 1.06 1.03 1.0 1.02 (in kJ / g)

For the effects expected by the type of flame retardant recommended by the invention to be fully satisfactory, it is not only necessary that the intense starting water peak develops at suitable temperatures, that is to say between the point normal hot operation and that where the coating can ignite, but still that a formation of water 25 is really realized only in case of abnormal overheating. In other words, any risk of premature degradation of the coating resin which could be caused by untimely formation of water from ambient temperature to that of normal hot operation of the transformer should be avoided.

It is this difficulty which is resolved by the second main characteristic of the invention, namely, moderate pre-aging of the resin, as will be seen in more detail below: 35 2) PRIOR PARTIAL DEHYDRATION OF THE QUANTITY OF SUBSTANCE. ADDED FLAME RETARDANT.

This is to cause dehydration beforehand to prevent it from occurring later in the transial, since it is also sought at high temperature, when the transformer heats up abnormally and there is a risk of ignition. to fear.

This dehydration can advantageously take place by prior heating of Al (OH) *. The goal is not to completely eliminate water that can form in a range of temperatures from ambient to; the operating temperature "hot" of the transformer (and which we will call in a pictorial way "volatile water" to mark the fact that it must start at low temperature), but a sufficient elimination so that the water "volatile "Residual is present in too small an amount to lead to degradation of the resin. It has been observed, in fact, that, when the alumina trihydrate had not been preheated before being added to the mineral filler, Fragments of the resin coating the electrical windings of test transformers intervened at operating temperature, which forced them to be scrapped.

A convenient way to achieve partial dehydration of the flame retardant by preheating is to consider its weight loss curve over time. For a substance used for the first time, it is advantageously possible to proceed in two stages: a first, on an analysis sample, intended to determine the quantity of water which is eliminated during an extended stay at constant temperature, which is that (or close to that) of normal hot operation of the transformer; - A second step, this time processing all of the material, consisting of heating it to relatively high temperature in order to quickly reach, therefore under industrial conditions, a weight loss corresponding to the elimination value of water determined in the previous phase.

Of course, this second step will be repeated with each resin preparation, while the first is only really necessary once to characterize a type of flame retardant that has never been used before.

The curves of Figures 1 and 2 illustrate this first phase of study on samples by showing the pace, 13 at certain temperature values, of the evolution of weight loss as a function of time for certain temperature values. The curves are configured on some of the varieties of alumina trihydrate "ALCOA" in Table IV identified 5 by their commercial references at the right end of each curve. Three temperature values were considered: 140, 160 and 180eC in order to properly cover the conditions; usual "hot" operation of transformers. To avoid unnecessary overloads, the three corresponding families of curves have been separated in the two figures: FIG. 1 groups the families at 140 ° C. (broken lines) and 160 ° C. (solid line); the family at 180 ° C. appears alone in solid lines in FIG. 2.

As can be seen, all these curves are advantageously of general logarithmic appearance with very rapid growth at the start, followed by a plateau slightly inclined on the horizontal, this plateau being all the sooner reached as the temperature work is high. Thus at 180 ° C (fig. 2), most of the "volatile" water (approximately 80%) is already formed after only 140 hours, out of the more than 700 hours of total duration of the tests. Weight loss ranges from around 5.3% to 2.5%, depending on the type of alumina.

The existence and the stability of the bearings are verified by testing aluminas previously heated to a higher temperature. Tests were thus carried out on two samples of the variety "M15" maintained for 18 h., One at 180 ° C the other at 200 ° C. The results appear in fig.l in the form of two lines (A) and (B) having ordinates to

the origin of 1.6% and 4.2% of weight loss for 180 and 200 ° C

30 respectively. One notes their quasi-horizontality, which reflects well the insensitivity of the samples to a second heating at lower temperature, due to the fact that almost all of their "volatile water" at 140 ° or 160 ° C has actually was eliminated during the first heating.

35 We observe, moreover, in FIG. 2, after 6 p.m. at 180 ° C, a sample "M15" is barely halfway up its rapid growth curve of weight loss by elimination of water 14

It will be immediately understood that these curves have a favorable characteristic appearance which makes it possible to deduce very easily the degree of partial partial dehydration which must be achieved. We can thus, for example, choose 5 as a criterion, the start of the plateau and retain as the proportion d 'volatile' water to be eliminated, the value given by the ordinate of this beginning of the plateau.

Thus, the figures show that, for the “S65 / 40" variety, it is possible to retain 2.5% weight loss, for a transformer operating temperature of 140 ° C, 4% at 160 ° C and 5.5% at 180 ° C.

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

15 At 140 ° C, the curve of this variety has a less typical appearance. Note, however, that the value of 0.7% obtained after 500 h. approximately will be perfectly suitable.

To fix the ideas, it can therefore be said overall that the weight loss prior to targeting ranges between 0.5% 20 and 10% approximately for all the flame retardants selected for the implementation of the invention.

The transition then to the industrial phase consists simply in dehydrating the mass of flame retardant substance accordingly by rapid heating techniques in the oven, with monitoring of the weight loss, for example by gravimetry using an automatic balance, the tray is placed in the oven enclosure.

By way of information, in the case of "M15" type aluminas, the weight loss of 3.5%, sought for operation of the transformer at 180 ° C., could be achieved after 6 heating hours only at 200 ° C.

This heating operation will of course be faster the higher the heating temperature. However, for obvious reasons of preserving a high potential for water formation, necessary in the event of abnormal overheating, care will be taken not to exceed much, and preferably to remain below, the temperature at the start of the peak. intense water removal characteristic of the flame retardant used and some values of which are given in Aaii TXT ai_v> 4 * 15

If one does not wish to have to operate by gravimetry, in particular because of too large quantities of material which could possibly be to be treated, it will be advantageous to proceed by an intermediate step making it possible to translate the targeted weight loss into a duration of This can be done using a second measurement sample of the flame retardant concerned, of which the weight proportion of water to be removed is known and which is subjected to rapid heating to a determined high temperature. This heating 10 will operate with a continuous weighing of the sample in order to be able to measure the time necessary to achieve a weight loss corresponding to the known proportion of water to be eliminated. The value of the measurement defines the duration of the heating operation, at a temperature strictly identical to that of the above intermediate operation, to which the mass of flame retardant substance to be treated will be subjected.

If necessary, it can be ensured that the operation is carried out correctly by determining by gravimetry at high temperature, for example 1200 ° C., the remaining water content of a sample taken for this purpose from the mass of flame retardant. processed. By comparison with the initial total water content (usually of the order of about 20 to 35% by weight) which has previously been determined in an analogous manner on a reference sample, it is deduced therefrom that the quantity of water "volatile" actually eliminated is well in line with that targeted.

It will be noted that the heating time is not entirely independent of the particle size of the material. It has been observed, during the tests, that a coarse particle size distribution results in a greater weight loss than a fine particle size for a given heating time.

It will also be observed that the weight loss values determined above, by reading the test curves made from samples, in no way represent an upper limit not to be exceeded. However, since these values correspond to the start of the plateau, there is in principle no use in continuing the heating for a long time to gain a few lOths of a percent, which, in any case, are probably too insignificant to cause water to flow out. λ y \ ____ i_ -I - J_ · '1 _ J ~ Ί A Λ> 16

It will certainly be understood that these curves, valid at normal operating temperatures of the hot transformer, reflect the behavior of so-called "volatile" water, at these temperatures. At higher temperatures, the stages 5 are at higher levels and are reached more quickly, in particular at the temperature of the peak characteristic of the flame retardant used and which, as we have seen, is in the vicinity of 300 °. C for all the varieties of Al (0H) 3 studied.

'10 The flame retardant thus pretreated becomes ready for use.It remains to complete the preparation of the coating resin in the usual way: the mineral filler, after having been intimately mixed with an adequate amount of flame retardant substance partially pre dehydrated, is divided into two equal parts. One is then introduced into the pure resin bath, and the other into the hardener, also in the liquid state. The two mixtures are kneaded separately to form two solid-liquid suspensions, then combined into a single mixture as l 'is kneaded in turn to ensure good homogeneity. The resulting pulp is then poured into a mold, in which the electrical winding of the transformer that is to be coated has been placed beforehand. After pouring, the mold is placed in the oven for solidification of the resin. After discharging and cooling, the resin block 25 incorporating the winding is removed from the mold and can then be mounted in the transformer intended to receive it.

It goes without saying that the invention cannot be limited to the examples described above, but extends to numerous * 30 variants or equivalents, in so far as the characteristics set out in the claims which follow are respected.

In particular, the invention is not limited to transformers proper; it should be understood, by this designation used here, more broadly all of the inductive electrical apparatus or equipment able to work at relatively high temperatures, from 100 at 200eC as we have seen, and whose electrical windings can be embedded in a block of insulating resin.

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Similarly, although the invention was initially conceived for the applications of thermosetting resins (coating of inductive coils, in transformers in particular), it in fact relates to all dielec-5 tric coating materials. It is of particular interest in the case of electrical installations having nominal long-term operation at high or medium temperature. This means that it will be particularly advantageous to use it for coating resins which already have good thermomechanical properties.

Claims (11)

1. Dry coated transformer, or similar electrical installation, at least one winding of which is coated in a charged insulating resin containing a flame retardant substance by formation of water when the temperature rises, characterized in that a fraction of the charge , at least equal to 20% of the total weight of the coating resin, is constituted by said flame retardant substance, which has been partially dehydrated beforehand in an amount such that it does not cause untimely formation of water in the coating resin which may degrade the quality thereof, as long as the transformer is used in its normal temperature conditions. 2. Electrical conductor coated in a charged insulating resin containing a flame retardant substance by formation of water when the temperature rises, characterized in that a fraction of the charge, at least equal to 20% of the total weight of the coating resin, is constituted by said flame-retardant substance, which has been partially dehydrated beforehand in an amount such that it does not cause untimely formation of water in the coating resin which can degrade the quality thereof , as long as the conductor is used under normal temperature conditions. 3. Dry transformer, or conductor, coated according to claim 1, or according to claim 2, characterized in that said flame-retardant substance has, compared to its * non-dehydrated initial state, a loss in weight of 0.5 to 10% approximately. . 4. Dry transformer, or conductor, coated according to claim 1, or according to claim 2, characterized in that the flame retardant is alumina trihydrate. 19 r * 5. Process for the preparation of a charged insulating resin for coating electrical co-inductors or windings, coated dry transformers, or other similar electrical installations, according to which the initial resin is added, in addition to the charge, a flame retardant substance by formation of water when the temperature rises, process characterized in that said substance is added in an amount of at least 20% of the total weight of the coating resin, and in that previously partially dehydrated said subs-tance in an amount such that there is no untimely formation of water which could be detrimental to the strength of the coating resin, as long as the electrical installation remains in service at its nominal operating temperature . 15 6) Method according to claim 5, characterized in that the flame retardant is subjected to a partial partial dehydration treatment until it has a weight loss of between 0.5 to 10% of its initial weight. 20 7} Process according to claim 5 and 6, characterized in that alumina trihydrate is used as the flame retardant. 25 8) Method according to claim 5, characterized in that one determines the degree of prior partial dehydration of the flame retardant which is used for the first 5 times, by taking a measurement sample which is subjected to a prolonged heating at constant temperature, below 30 that of the clean water removal peak of the flame retardant used, and taking into account the shape of the change in weight loss of said sample over time. 9. Method according to claim 5 or 8, characterized in that one carries out the partial partial dehydration of the flame retardant substance by subjecting it to rapid heating until its loss in weight corresponds to the amount of water. to be eliminated according to a proportion previously determined on a measurement sample, the evolution of the weight loss as a function of which has been followed over time, during prolonged heating at a temperature below that of the peak of elimination of clean water at the flame retardant used. 20 V ”- * 10. The method of claim 9, characterized in that said rapid heating is carried out for a period which has been predetermined by a heating operation carried out at an identical temperature and carried out with weighing of a sample of the flame retardant used until the latter has a weight loss corresponding to the amount of water which it is desired to eliminate. 11. The method as claimed in claim 10, characterized in that said quantity of water to be eliminated has been determined beforehand by monitoring the development over time of the weight loss of a measurement sample that the it was subjected to prolonged heating at constant temperature, lower than that of the peak of elimination of clean water to the flame retardant substance used.