NO165420B - PROCEDURE FOR REPLACING PCB-CONTAINING ASCELLARES IN ELECTRICAL INduction APPLIANCES WITH PCB-FREE DIELECTRIC REFRIGERANTS. - Google Patents

Description

The present invention relates to electrical induction apparatus, for example electrical energy transformers, especially the electric liquid coolants contained in these and especially the coolants which consist of or contain as a component polychlorinated biphenyl, PCB. More particularly, the invention relates to a method for converting PCB-containing electrical induction equipment, for example transformers, into essentially PCB-free transformers in order to make these into so-called "non-PCB transformations" in accordance with US regulations.

Due to the good flame resistance, chemical and thermal stability as well as the good dielectric properties, PCBs have been found to be excellent transformer coolants. US-PS 2,582,200 describes the use of PCBs alone or in admixture with compatible viscosity modifiers, such as trichlorobenzene, and such trichlorobenzene-PCB mixtures are generally called "ascarels". These ascarels may also contain smaller amounts of additives such as ethyl silicate, epoxy compounds and/or substances that are used as scavengers for halogen decomposition products that may arise from potential electric arcing. ASTM. D-2283-75 describes several types of askarels and indicates their physical and chemical specifications.

However, PCBs are listed in the U.S. Toxic Substances Control Act of 1976 as environmental and physiological hazards and, due to their high chemical stability, are not biodegradable. Therefore, they will remain in the environment and even provide biological amplification (accumulation in higher order life through the food chain). According to this, transformers in the USA are no longer manufactured with PCB or askarel fluids. While older units containing PCBs can still be used under certain circumstances, it is necessary to provide special precautions such as pollution dikes and furthermore everything must be inspected regularly. Transformers containing PCBs also have a disadvantage because maintenance that requires emptying the core is impossible, and the transformer owner remains responsible for all environmental pollution including cleaning and clean-up costs due to leakage, tank rupture or other spillage of PCBs, or due to toxic by-product emissions due to fire. To replace a PCB-containing transformer it is necessary to 1) remove the transformer from active service, 2) empty the PCB contents and flush the unit in the prescribed manner, 3) remove the unit and replace it with a. new transformer and 4) transport the old transformer' to an approved place for burial (or another approved device). Only then does the ownership of the transformer remain and the owner is still responsible for future pollution problems it will cause. Liquid waste that forms part of the compensation must be taken care of in specially approved places. Thus, replacing a PCB transformer can be expensive but, more importantly, because most pure PCB or ascaretl transformers are located indoors, in building basements or particularly confined spaces: with limited'access, it may not be physically possible to remove or install a transformer and that it would also not be .desirable for other reasons.

A desired approach to the problem would be to replace the PCB oil with a harmless, compatible fluid. A number of fluid types have been used in new transformers as indicated by Robert A. Westin in "Assessment of ■ the Use of Selected Replacement Fluids for PCB's-in Electrical Equipment", EPA, NTIS, PB-296377, March 1, 1979 ; J. Reason and W. Bloomquist, "PCB Replacements: Where the Transformer Industry Stands Now", "Power<*>", October 1979?, pp. 64-65, Harry R. Sheppard, "PCB Replacement in Transformers", " Proe. of the Am.; Power Conf., 1977, pp. 1062-68; "CBem. Week", 130, 3, 24 (1/20/82); A. Kaufman, . "Chem. Week", 130, 9, 5 (3/3/82); "CMR Chem. Bus.", 20 Oct. 1980, p. 26;: "Chem. Eng." July 18. 1977, p. 57, BE-PS 893,389; "Europv Plastic News", June 1978, p. 56. Among these are silicone oils, for example polydimethylsiloxane oils, modified hydrocarbons (for high flash points, for example RTEmp), synthetic hydrocarbons (poly-a.-olefins), high viscosity esters (for for example dioctyl phthalate and "PA0-13-C", and phosphate esters. A number of halogenated alkyl and aryl compounds have been used. Among these are liquid trichloro and tetrachlorobenzenes and toluenes and mixtures thereof (for example liquid mixtures, of tetrachlorodiarylmethane with trichlorotoluene isomers ). Liquid mixtures of trichloro and tetrachlorobenzene isomers are particularly suitable because of the low flammability (eg high flash point) and similar physical and chemical properties to the ascarols being removed. Other suggested fluids are tetrachlorethylene, eg "Perclene TG", and polyols and other esters.

Of all the non-PCB fluids, silicone oils have been the most generally accepted. Their chemical, physical and electrical properties are excellent. They have a flash point above 300°C and no known toxic or environmental problems. These oils are trimethylsilyl end-blocked poly(dimethylsiloxane):

where n has a value sufficient to give the desired viscosity, for example a viscosity at 25°C of approx. 50 cSt. Commercial silicone oils suitable for use are available, eg "L-305", and others. In addition, US-PS 4 146 491, GB-PS 1 540 138 and 1 589 433 describe mixtures of silicone oils with a number of additives to improve the electrical performance in capacitors, transformers and similar electrical equipment and describe polysiloxanes with alkyl and aryl groups different from methyl.

Replacing PCB-containing ascarels in older transformers with silicone oils or one of the other replacement fluids may look like a simple matter, but it is not. A typical transformer contains a large amount of cellulose insulating material to prevent electrical windings and so on from making unwanted contact and to prevent electrical bridging. This material is naturally moistened with askarel and can contain from 3 to 1256 of the total fluid volume in the transformer. This: absorbed askarel will not be discharged nor can it be flushed out by any known means no matter how effective the means. When the original mass of askarel is replaced with new non-PCB fluid, the slow diffusion process allows old absorbed askarel to gradually leach out and the PCB content in it. new fluid will rise. Therefore, the new coolant is also contaminated.

For the purpose of classifying transformers, US government regulations have designated fluids: with more than 500 ppm PCB as "PCB transformers", in those with 50-500 ppm PCB as "PCB contaminated transformers" and those with less than 50 ppm PCB' as "non-PCB' transformers" . 'While the main expenses follow the first two classifications in case of gambling or the necessity of disposal, the last category goes free of the regulations. To achieve the latter classification, the PCB concentration must remain below 50 ppm for at least .90 days with the transformer operating and applying sufficient energy to realize temperatures of 50<*>C or above. This requires a 90-day average for elution of approx. 0.56 ppm per day. It is believed that most, if not all, states in the United States will adopt regulations that may be the same as, or more stringent than, the federal regulations. Milder rules may exist elsewhere.

There are a number of commercial* backfill procedures on the market including those described in "The RetroSil PCB Removal System", the promotional literature of Dow Corning Corp., 10-205-82 (1982) and the trade literature of Positive Technologies, Inc. in connection with the "Zero/PC/Forty" process. These use initial cleaning procedures with as high efficiency as possible during which electrical equipment is in operation. Most include/a series of flushes with fluids such as fuel oil, ethylene glycol or any number of chlorinated aliphatic or aromatic compounds. Trichlorethylene is a preferred flushing fluid. Some processes such as the Positive Technologies, Inc. "Zero/PC/Forty" process use a fluorocarbon vapor wash alternating with a liquid rinse. When the first cleaning procedure is finished, the transformer is filled with silicone fluid. However effective these elaborate flushing processes may have been expected to be, they cannot remove PCBs absorbed in the interstices of the cellulosic material. As a consequence, the PCB content in the silcone coolant gradually rises as residual PCB is leached out while the transformer is in use. Therefore, if one wishes to reach a PCB-free state ("non-PCB" as defined in US federal regulations), it is necessary to either periodically replace or continuously clean the silicone fluid until a leaching rate of less than 50 ppm for 90 days is reached.

Periodic extraction is very expensive and because both silicone and PCB are essentially non-volatile, distillation cannot be used to separate them in practice and other methods of separation are expensive or ineffective. In the "RetroSil" process, a continuous carbon filtration is used to clean the fluid ("The RetroSil PCB Removal System", Promotional literature of Dow Corning Corp., 10-205-82 (1982); Jaqueline Cox, "Silicone Transformer Fluid from Dow Reduces PCB Levels to EPA Standards", "Paper Trade Journal", September 30, 1982; T. 0'Neil and J.J. Kelly, "Silicone Retrofill of Askarel Transformers", "Proe. Elec./Electron. Insul. Conf.", 13 , 167-170 (1977); W.C. Page and T. Michaud, "Development of Methods to Retrofill Transformers with Silicone Transformer Liquid".. proe. Elec./Electron. Insul. Conf.", 13, 159-166

(1977)). Westinghouse has in US-PS 4 124 834 patented a transformer with a filtration process to remove PCBs from the refrigerant while RTE in EP-PS 0 023 111 has described the use of chlorinated polymers as adsorption media. However, the filters used in these processes are very expensive and the removal of PCBs is very inefficient, which is due to both the lack of selectivity and the very low concentration of PCBs being filtered.

Instead of filtration, processes involving decantation have been proposed, US-PS 4 299 70«4 but these are impractical due to solubility limitations, and are only good at high concentrations; extraction with polyglycols (F.J. Iaconianni , A.J.. Saggiomo and SSWY Osborn, "PCB Removal from Transformer Oil"<1>,, EPRI PCB Seminar, Dallas, Texas, December 3, 1981) or- with supercritical CO2 (Richard P. deFilippi, "CO2 as a Solvent: Application to Fats, Oils and Other Materials", "Chemi.. and Ind.",,19<f.> June 1982, pp. 390-94, as well as chemical de st rue r i rog of PCB with ;:na,tr ium, GB-PS 2 063 908. Neither of these schemes has been foundl to be economically or commercially practical for askarel transformers. However, the filtering scheme could be' a reasonably efficient and more expensive procedure were it not for the fact that the rate of leaching is so slow that it would take many years to reduce the residual PCB content to a point where the final leaching is reduced to an acceptable value (Gilbert Addis and Bentsu Ro, "Equilibrium Study of PCB/s Between Transformer Oil and Transformer Sortid Materials", EPRI PCB Seminar, 3.

December 1981.)

The problem and its cause are discussed by L.A. Morgan and R.C. Ostoff, . "Problems Associated with-. the Retrofilling of Askarel Transformers", IEEE Power Erig. Sbc, V/inter Meeting, N.Y., N.Y., Jan. 30-4. Feb. 1977, pap. A77, pp. 120-9. The solubility of a typical silicone oil in PCB is practically 0, thus less than 0.5$, temperatures up to and above 100°C, while the solubility of PCB in silicone ranges from only. 10$ at 25°C to 1256 at 100i° C ., While this limited solubility does not limit the bulk silicone dissolving t.iilgj engel ig free PCB, it does limit the ability of PCB<1> to diffuse from the pores or spaces in the cellulose materials.

In any given pore filled with PCB, diffusion of PCB out must be accompanied by diffusion of silicone in. At a certain point in the pore there must be an interface between PCB and silicone, across which neither material can diffuse. Due to the fact that PCB is more soluble in silicone than vice versa, PCB will slowly diffuse into the silicone while the interface slowly advances into the pore. The limited solubility limits the rate of diffusion and while this mechanism eventually cleans the pores, for PCBs it is orders of magnitude slower than if the two fluids had been miscible. The high viscosity of silicone (and many other coolants) is also an inhibiting factor. The result is a long leaching period of possibly several years, during which the silicone must be continuously filtered or periodically replaced to remove PCBs from it. Thus, the slow leaching of PCBs out of the solid insulation by means of silicone is worse than no leaching at all because the danger of spillage of PCB-containing material exists over a period of several years. Experimental studies by Morgan and Osthoff have shown, for example, that effective PCB diffusivities into a typical silicone oil were only one-tenth of those into a 10 cSt hydrocarbon oil. Even if one might then prefer to refill with such a hydrocarbon oil, if it were not for the flame risk of hydrocarbons, there is still a problem in separating PCBs from contaminated hydrocarbon oil, which is high-boiling in the same way as PCBs and like the kerosene oil. .

The present invention is based on the fact that there are suitable cooling fluids which are more suitable than silicone oil for use over a limited period of time while the leaching is carried out. They are reasonably volatile for distillation from PCB, easily miscible with PCB and with relatively low viscosity for rapid diffusion into the pores of the insulation. The other constituents of askarel, namely trichlorobenzene and tetrachlorobenzene, have been found to be ideal fluids for this purpose. They can be used as temporary or interim leaching cooling fluids where fire can be a potential hazard, while light hydrocarbons can be used if there is no fire hazard.

Of particular known technique in this area, reference should be made to: US-PS 4,296,436 which describes the removal of residual hydraulic fluid with a special flushing fluid;

US-PS 4,357,115 which describes cleaning the interior of containers with a special solvent;

US-PS 4,299,704 which provides for the removal of PCBs from a transformer by heating those PCBs to promote their dissolution in silicon flux; and

US-PS 4,124,834 which uses silicone fluid and an adsorbent to remove PCBs from. a transformer as the silicone fluid with dissolved PCBs passes through the adsorptive agent.

However, no prior art has been found which describes the concept of achieving a substantially PCB-free transformer by removing, flushing out and elution of ash from transformers containing these by means of an interim dielectric liquid or the step of filling the transformer tank with an interim dielectric coolant" . which is miscible with the PCB contained in the transformer tank, Capable of penetrating the electrical insulation and capable of being separated from the PCB, or the stage of electrical - to operate the transformer while eluting the PCB with an interim dielectric liquid and; continue the electrical operation for a period of time sufficient to elute the PCB impregnated in the solid insulation into the interim coolant, draining the PCB-filled interim coolant, complete this cycle with filling with fresh interim coolant, electrically. operation and draining a sufficient number of times until the elution degree> of PCBs drops below 50 ppm, calculated on the weight of permanent coolant to be used, after 90 days of electrical operation, after which the coolant is then drained from the transformer and then separated from contained PCBs by thus allowing filling of the tank with a PCB-free permanent dielectric jacket fluid that remains substantially PCB-free during subsequent electrical operation.

According to this, the present invention relates to a method for replacing a coolant, containing PCB, in an electric induction apparatus with a tank containing the coolant, an electrical winding and porous solid cellulose-containing electrical insulations 1 the PCB-containing coolant, with an essentially PCB-free high-boiling dielectric permanent coolant to convert the electrical equipment into one, where the elution degree of PCB to the coolant is below the maximum allowable elution degree of the coolant for an electrical equipment judged as non-PCB, the solid porous electrical insulation being impregnated with it PCB-containing refrigerant, and this method is characterized in that it comprises: a) draining the PCB-containing refrigerant from the tank to remove a major proportion of the PCB-containing refrigerant contained therein; b) filling the tank with a temporary dielectric coolant liquid such as halogenated aromatic and aliphatic hydrocarbons or mixtures thereof which are miscible with PCBs, and have sufficiently low viscosity to circulate in the tank and penetrate into spaces in the porous solid electrical insulation, and is capable of being easily separated from the PCB; c) electrically operating said electrical induction apparatus and continuing the electrical operation for a period of time sufficient to elute PCB contained in the PCB-containing coolant impregnated in the porous solid insulation to said temporary dielectric coolant; d) then draining said temporary dielectric coolant liquid containing eluted PCB from the tank; e) repeating the cycles of steps (b), (c) and (d) when the elution rate of PCB to the temporary dielectric coolant exceeds 0.55 ppm PCBB per day, calculated on the weight of said permanent dielekitfcic refrigerant; and f) filling the tank with a substantially PCB-free permanent coolant selected from high-boiling high-viscosity silicone oils, synthetic ester fluids;, poly-cx-olefin oils and hydrocarbon oils to thereby qualify said electrical equipment to non-PCB status;.

The invention is based on the use of suitable temporary or interim coolants as a substitute; for PCB-containing coolants in electrical induction equipment, for example transformers, with a container, for example a tank, containing the coolant and an electric; winding and porous solid cellulosic electrical insulation immersed in and impregnated with PCB during electrical operation of the transformer for a sufficient period of time to elute PCB from the solid electrical insulation contained in the transformer. During operation, the lnterim fluid is exchanged for • to; speed up the elution process, the preferred goal is to elute so much leachable PCB that the transformer can be operated for 90 days and not exceed 50 ppm PCB in the permanent coolant intended for the transformer. After the amount of leachable PCB in the transformer has been reduced to the desired level, the irrlnterim liquid was removed from the tank and then filled. with a PCB-free permanent dielectric coolant compatible with the transformer. A procedure is described below: according to the invention, where a PCB-containing fluid in a transformer is replaced with a permanent PCB-free liquid coolant: 1) The transformer is stopped (de-energized) and the PCB-containing fluid is drained. and. disposed of in accordance with environmentally acceptable procedures .'. The transformer can be flushed with a flushing fluid, such as trichlorobenzene or trichloroethylene, liquid or vapor], to remove "free" PCB fluid. 2) The transformer is filled with a temporary or interim coolant such as trichlorobenzene, TCB, or a mixture thereof with tetrachlorobenzene, which is miscible with or dissolves PCBs and which is capable of penetrating the pores of the electrical insulation and which is also in capable of being easily separated from the PCB, and the electrical operation is resumed. 3) The fluid temperature is monitored and if the electrical load on the transformer does not produce a sufficiently high fluid temperature to provide the desired PCB elution rate, thermal insulation or even external heating may be necessary. Circulation of the fluid through an outer loop and a pump to heat it, or to increase the internal circulation, may also be desirable. 4) The rate of PCB elution into the interim cooling fluid can be determined by periodic sampling and analysis. The accumulated PCB is periodically removed by removing the interim coolant containing the PCB and then distilling the interim coolant, for example trichlorobenzene (TCB) from the PCB. This can happen by shutting down, de-energizing the transformer, draining old fluid for distillation and replacing this with new interim coolant, for example TCB. Alternatively, the transformer can continue in operation while new interim coolant, TCB, is added and old is removed via a slip stream or a circulation loop. 5) PCB-contaminated TCB fluid is distilled to obtain an essentially PCB-free TCB distillate and a PCB bottom product that is contaminated with TCB. PCBs can be disposed of according to approved US procedures, for example by incineration. 6) When the PCB elution rate reaches the desired level, preferably less than 50 ppm PCB calculated on the weight of intended permanent refrigerant for a period of 90 days (for example, an elution rate of 5/9 ppm per day), the permanent refill can be carried out . The transformer is closed, degassed, drained and filled with silicone oil or another permanent coolant compatible with the transformer. It is then brought back into operation. 7) To meet the requirements for "non-PCB" transformers, analyzes must show a PCB content of less than 50 ppm PCB, (calculated on the weight of later permanent coolant) after a period of 9.0* days, after which the transformer is reclassified to PCB-free, that means "non-PCB".

With regard to the flushing step, this, although efficient draining and flushing techniques should be used, does not in itself constitute the invention, but is part of it, in previously known refilling procedure. They are a precursor to the most efficient embodiment of the present invention, but their value has hitherto been overestimated as it is the slow leaching rate and not the flushing efficiency that has been found to limit the degree of PCB removal. a wide range of solvents can be used in the flushing step including hydrocarbons such as petrol, kerosene, mineral oil or the like, toluene, turpentine or xylene, e.-t widely chlorinated aliphatic or aromatic hydrocarbons, alcohols, esters, ketones and so on. From a material handling standpoint and PCB separation considerations, it is practical to avoid using any more chemical types than necessary so that the use of the intended temporary leaching fluid, such as TCB or mixtures thereof/with tetrachlorobenzene, as the first flush is most practical.

Fluids different: from ordinary volatile trichlorobenzene, TCB or a mixture thereof with tetrahydrobenzene, can be used. The preferred interim fluid has the following characteristics: a) it is compatible with PCB, that is to say preferably dissolving at least 50% of its weight of PCB, preferably at least 90% of its weight of PCB and completely miscible in all proportions with PCB;

b) it has a low enough molecular weight to have good molecular mobility to be able to penetrate into

the pores or spaces in the solid insulating material and it promotes rapid mutual diffusion, preferably with a viscosity at 25°C of 10 cSt or

less and preferably 3 cSt or less;

c) it can be easily separated, for example distilled, thus preferably has a boiling point of 275°C or less and

preferably 260°C or less, from PCB;

d) it is currently considered environmentally harmless; and

e) it is compatible with typical internal elements of transformers. While TCB or mixtures with tetrachlorobenzene

is preferred, a number of alternatives as mentioned above can be used. These include modified and synthetic hydrocarbons and a number of halogenated aromatic and aliphatic compounds. There are also a number of liquid trichlorobenzene isomeric mixtures. The preferred TCB fluid will be a mixture of these isomers with or without tetrachlorobenzene isomers. The advantage lies in the fact that such a mixture has a lower freezing point than the individual isomers, which reduces the chance of solidification in the transformer in very cold places. Furthermore, the mixtures are often the usual result of the preparation and can thus cost less than separated and purified individual isomers.

Because the preferred object here is to leach out the PCB in the fastest practical way, the preferred embodiment includes running the transformer to achieve the fastest possible diffusion rates as indicated in step 3 above. Used at full load, a transformer should automatically provide enough heat for this purpose. However, because many transformers are operated below the rated load and below the specified safe temperature, it is not certain that a sufficiently high temperature, for example at least 50°C, is reached without thermal insulation or, external heating. While this thermal control represents a preferred embodiment of the invention is it possibly also not an essential requirement, the many transformers for which such insulation and heating may be impractical, Leaching at a lower temperature, even ambient temperature, can be carried out but will take longer.

Fluid circulation as indicated in step 3.-Sees possibly but is an advantageous embodiment, as such circulation will prevent the build-up of concentration gradients which can cause a retarded diffusion. Because elution is a slow process, the circulation speed does not need to be high. Violent circulation must of course be avoided to avoid damage to the internal construction of the transformer. It is recognized that many transformers, due to their construction or their location, cannot be easily modified to use a circulation loop and such circulation is not considered a necessary aspect but only as::, an embodiment of the invention to increase elution rates . In most transformers, thermal gradients alone will induce sufficient circulation, especially in cases where a mobile low-viscosity coolant such as TCB is used.

As the PCB content in the TCB or the other interim coolant in the transformer rises, it may possibly reach a point where diffusion no longer serves to leach out the TCB from the cellulose pores or spaces IJ the insulation in the transformer tank. A reduction in the elution rate, determined by sample analysis, is a hint that this may occur. If it is determined that this is happening, it may be necessary, as indicated in step 4, to replace the PCB-filled interim dielectric cooling fluid with new, PCB-free fluid. This is most easily done by stopping the transformer, draining: contaminated leaching fluid (interim dielectric coolant) and replacing this with fresh fluid. As a practical precaution, instead of monitoring the elution rate to determine when diffusion no longer serves to effectively leach PCBs from the pores or spaces of the electrical insulation, it is more practical to set up a schedule for the transformer to regularly replace coolant. If a non-PCT transformer is desired, coolant changes occur after selected periods of electrical operation until the coolant can no longer elute 50 ppm PCB per 90 days of operation. Periods of electrical operation between refrigerant changes can be chosen between 20 days and 1 year (or more if the transformer owner must prevent transformer shutdowns except at rare special times, such as special holiday periods, so that more than 1 year can elapse between shutdowns, and possibly closure can only take place every other year.), preferably 30 to 120 days and most preferably 45 to 90 days.

The contaminated leaching fluid can then be distilled off and condensed for new use, leaving PCB bottom product which is incinerated or otherwise disposed of in accordance with the regulations. While a complete replacement of the interim coolant is preferred, it is possible that the inconvenience of additional shutdowns dictates a different procedure, for example, simultaneously introducing new fresh fluid while removing the old contaminated one while the transformer remains in operation. It is less efficient because new fluid mixes with old in the transformer and fluid with reduced PCB concentration is actually removed. In order to thus remove all PCBs, more leaching fluid will be required than for the preferred procedure. This can be reduced if care is taken to avoid excessive mixing. For example, freshly cooled TCB or other interim dielectric cooling fluid can be introduced into the bottom of the transformer while old, hot, PCB-filled interim dielectric cooling fluid is removed from the top. The density difference will delay the mixing. Regardless of the method used, the process will require repetition until the desired PCB level, namely less than 50 ppm can be maintained for at least 90 days in silicone oil coolant.

While distillation is the preferred method for separating TCB or other refrigerants and PCBs, other methods are conceivable, especially if a fluid different from TCB is chosen as the temporary fluid.

There is some concern that TCB itself, or other chlorinated interim dielectric coolants such as TTCB and the halogenated solvents, may eventually come under suspicion as a health risk, and that the transformer, even if it is free of PCBs, will be contaminated with TCB or other potentially questionable interim flows. The further advantage of the procedure according to the invention is that such contamination can be easily rectified if necessary, because interlms-TCB or the other fluid, is more volatile than the silicone or heavier hydrocarbon fluids, or other relatively high-viscosity permanent coolants used in the transformer and can be distilled from; this. According to this, the chlorinated part of the refrigerant can be replaced and the old batch sent to distillation-^ for easy purification. Two or three such changes over; - a period of several months will give an essentially halogen-free system if this is desired.

Other preferred permanent refrigerants that can be used in place of the last fill of silicone oil include dioctyl phthalate, modified hydrocarbon oils, such as "RTEmp", poly-a-olefins, such as "PA0-13-C", synthetic ester fluids and any other compatible permanent fluids. It is also preferred that the permanent dielectric fluid is characterized by a high boiling point compared to the temporary dielectrics; solvents so that this can be separated from the permanent fluid if the need arises, and also to avoid releasing permanent fluid due to volatilization in the event that the transformer tank breaks.

While the following is proposed, and in some cases used, as permanent dielectric-; fluid, they are less preferred than the relatively high-viscosity high-boiling permanent dielectric fluids: tetrachlorodiarylmethane with or without trichlorotoluene isomers, freon, halogenated hydrocarbons, tetrachlorethylene, trichlorobenzene isomers, and tetrachlorobenzene isomers. The trichlorobenzene isomers, tetrachlorobenzene isomers and mixtures thereof have high flammability ratings and other physical properties similar to askarel and are therefore preferred among the less preferred permanent fluids. The invention shall be illustrated by the following examples. In the examples, the following abbreviations are used:

TCB = trichlorobenzene

TTCB = tetrachlorobenzene

TCB = mixture of 30-35 wt-* tetrachlorobenzene, TTCB, in trichlorobenzene, TCB (containing an effective amount of a chlorine scavenger 1 form of an epoxy-based

inhibitor)

PCB = polychlorinated biphenyls

ppm = parts PCB or TCB mixture per million parts

refrigerant, on a weight basis

Askarel = is askarel type A with 60 weight-* "Aroclor 1260" and

40 weight-* TCB

"Aroclor 1260" = polychlorinated biphenyl containing 60 wt-*

chlorine

"L-306" = a silicone oil within the scope of formula A above

with a viscosity of 50 cSt at 25°C.

A "cycle" is the period of time between replacing the refrigerant. A "part" of a cycle is a part of a cycle where the degree of leaching into the refrigerant is markedly different from the degree in earlier or later parts of the cycle.

The examples 1. 2. A. B and C.

Each of the four transformers identified in Table I was drained, flushed, and filled with refrigerant intended for cycle 1 in each case in Table I.

In each case in example A and; 1, the coolant in the transformer before draining and flushing* was in mineral oil (Exxon "Univolt" inhibited oil j;e, transformer quality) containing the initial PCB concentration shown in Table I. Transformers #459 and #461 in examples A and 1 were once ascarel-filled transformers which were previously connected to mineral oil and contained 9,150 and 7,800 ppm PCB respectively. Transformer 45991: example A was drained, spray rinsed with mineral oil and then filled with fresh mineral oil. Transformer 461 in Example 11 was drained, flushed with TCB mixture and then filled with fresh TCB mixture. The transformers were run at approx. 80°C in the deifcrt time space indicated in Table I. In Example 1, the transformer was drained of TCB mixture and refilled with TCB mixture twice more to give two complete cycles and one ongoing cycle.

In Examples 2, B and C, each respective transformer was part of a bank of three identical Westinghouse transformers rated at 333 KVA each. Each of these transformers contained approximately 860 L of Type A askarel which was a 60:40 weight ratio refrigerant "Aroclor 1260" (PCB) and trichlorobenzene. These three transformers were all tied to the same three-phase bench and were all operated at the same capacity and subject to all the same conditions except as noted below. The normal operating time for the transformers in examples 2, B and C was far below the nominal and the annual mean operating temperature was approx. 40°C except as otherwise stated. Transformer 669 Example 2 and 667 Example C were drained of askarel, spray rinsed twice with TCB mixture and then refilled with TCB mixture. Transformer 668 Example B was drained, spray rinsed twice with silicone oil "L-305". and finally filled again with "L-305". All transformers were re-energized and fluid samples were taken periodically to examine the PCB content.

All five transformers were run and the PCB concentrations determined at the end of the various daily intervals indicated in Table I and the daily ppm rate of increase for PCBs in the refrigerant was calculated and converted to a silicone oil refrigerant basis (if not already on such a basis) as indicated in table I.

In Example 2, transformer 669 was run for 96 days, then drained, flushed with TCB mixture and refilled with TCB mixture and operation was continued for cycle 2 and then the same sequence was repeated for cycles 3 and 4 for the transformer. In Example 2, at the end of cycle 4, transformer 669 was drained and flushed with TCB mixture and then filled with TCB mixture and continued under electrical operation. The daily intervals, the PCB concentrations at the end of these intervals and the total elution rate and converted elution rate expressed in ppm per day is given in Table I for each cycle.

In example B, transformer 668 was initially drained of askarel, flushed twice with "L-305" and filled with fresh "L-305". At the end of the 390th day, the transformer was flushed again, spray-rinsed with "L-305" and refilled with fresh "L-305" and continued to operate in cycle 2. The day intervals, the PCB concentrations at the end of the day intervals, and the total degrees of elution in ppm per day is listed accordingly in table I.

Transformer 667, Example C, was initially drained of askarel, flushed twice with TCB mixture and filled with TCB mixture. At the end of the 96th day, it was drained, rinsed and filled with fresh TCB mixture. Subsequent cycles are as indicated in Table I. For example A, transformer 459, the leaching rate was reduced to an average PCB leaching rate of 0.36 ppm per day in the second part of the first cycle. This was below the 0.55 ppm/day or 50 ppm for 90 days required for reclassification as non-PCB. Accordingly, on day 220 the mineral oil was drained and replaced with permanent "L-305" silicone coolant. As can be seen from Table I, the total amount of PCB removed during cycle 1 was equivalent to 475 ppm in the volume of refrigerant contained in the transformer. This is less than the 1220 ppm removed during the same interval with the PCB mixture in 461, a transformer of similar size and type, and suggests that the mineral oil was not as effective as the TCB mixture as a leachant.

In contrast, transformer 461 was treated with TCB mixture. On day 68, the TCB solvent was drained off and reintroduced into the same transformer. On day 165, the TCB mixture was drained off and replaced with fresh TCB mixture. While the total degree for cycle 3 was 1.64 ppm per day the degree decreased during this cycle and when it came to day 245 the PCB elution degree was reduced to 0.05 ppm per day, which had gone below the target of 0.55 ppm PCB per day. Accordingly, the fluid can at this point be replaced with a permanent silicone coolant.

Transformer 669, example 2, was initially filled with askarel. Therefore, this had much more PCB in the insulation than 461, example 1. Therefore, many more cycles and a significantly longer period of time were needed to leach out to an acceptable level. When the leaching rate falls below the target of 50 ppm 1 90 days, the TCB mixture can be replaced by silicone. The conversion figures in the last column of table I show a continuous reduction with time and the target should be reached approx. day 600. It should be pointed out that transformers 667, 668 and 669 were expected to be among the most difficult to treat.

They are spiral-wound transformers in which the paper insulation and therefore the diffusion path length can be tens of centimeters. In contrast, many transformers are of "pan-cake" construction where the path lengths are less than 2% cm.

Examples B and C, transformers 668 and 667, are comparable examples because in example B the original coolant was silicone oil and in example C a change to silicone oil occurred before the elutions reached 0.55 ppm PCB per day.

Transformers 667 and 669 were initially filled with askarel replaced with TCB compound while transformer 668 was initially filled with askarel replaced with "L-305" silicone oil. Figure 1 compares the PCB analyzes for the first cycle of these three transformers. This data is converted into the total number of real grams of PCB removed. While approx. 60,000 to 70,000 grams of PCB was rapidly removed within the first 28 days, the subsequent removal was much slower and the degree is indicated by the straight line drawn through the points. It is believed that the bulk of PCBs retained in the loose insulation is easily extracted regardless of solvent, but that it is the PCBs retained in the more tightly wound paper and cardboard insulation that is limiting for the process, in which case the effectiveness of the eluents varies. Figure 1 shows this difference. While the data points are somewhat scattered due to the difficulty of accurate PCB analysis, it appears that silicone needs 400 days to remove the same amount that the PCB mixture can remove in 60 days. A comparison of the slope of the lines shows that the TCB mixture is approx. 8.5 to 9.0 times as effective as "L-305". The key point of the invention is that the efficiency ratio is so high. Thus, a process that can take 5 to 10 years with silicone alone can be completed much shorter with a temporary solvent such as a TCB mixture.

Figure 2 shows the effect of the transformer temperature on the degree of PCB elution. Both 667 and 669 were at approx. 400° C during the first cycle. They were replaced with fresh TCB mixtures after 96 days. Due to the approach of winter and the transformers not being used to the extent necessary to keep the temperature high, it was predicted that low temperature could affect the elution. According to this, the 667 was heated extra by conducting heat to the cooling fins. Transformer 669 was left unheated for the sake of comparison. The temperature at 667 was on average approx. 55° C, while in 669 it had a mean of approx. 23° C. Again, the data points are rather scattered, but it is clear that the warmer transformer eluted PCB faster than the cold one by a factor of 1.6. However, this factor need not be linear and as a result the gain need not be as dramatic for higher temperatures. Further elution was carried out at 85°C if possible and thus most of what is listed in Table I was carried out at this higher temperature.

As a further example, the following examples 3 to 5 shall be presented. While they do not represent results from real transformers, they are based on the performance to be expected from the process according to the invention under the conditions indicated below for each example applied to transformers from which it is relatively easy to elute PCBs by the method of the invention compared to the transformers used in examples 1, 2, A, B and C.

In each of examples 3 to 5, a transformer was used with a volume capacity of 910 1 fluid, where the interior of the transformer held approx. 27 1 in the cellulosic material, i.e. the paper as insulated in the windings and containing 910 1 more or less, of an askarel of 50* PCB (500,000 ppm), except for example 4 where the transformer contained 910 1 more or less , mineral oil Containing 10,000 ppm PCB.

Figures 3 to 5 are concentration charts for PCB in ppm in the temporary dielectric fluid, TCB, in the transformer listed on a vertical logarithmic scale against elapsed days or soaking time and graphically show the expected results sought to be achieved according to the invention.

Example 3

In example 3, the transformer is first de-energized. It is then drained for askarel, the latter of which is finally disposed of in an approved manner. The transformers are spat out with a small amount (for example 113 1) of trichlorobenzene to reduce the residual content of askarel in the free fluid system to 0.5* of the original value. The system is then logically inspected for leaking bushings or other physical problems that may require repair at this time.

The transformer is then filled with 910 1 trichlorobenzene, TCB, (or alternatively a trichlorobenzene-tetrachlorobenzene mixture), sealed, and, after careful testing, energized. Because the flushing is not completely thorough, the original PCB level in the new fluid in the transformer is estimated to be 2500 ppm, that is 0.5* of the original PCB level. It is assumed that PCB retained in cellulose material is leached to a degree that varies from 0.001 to 0.01* per day. While these values may seem arbitrary, they are likely to be achievable in easily leachable transformers and higher and lower grades will only affect the time required to achieve total leaching and not the principle procedure. The top curve indicated in the diagram in Figure 3 shows the concentration, on a logarithmic scale, of PCBs that can be expected to be found in the transformer fluid as a function of time. In current commercial application of the process, it would not be necessary to determine all these concentrations. However, one would like to take samples of the old fluid being replaced and determine its PCB concentration. This is shown above by the open circles in Figure 3. While the exact length of the leaching periods is arbitrary, experience with a given type of transformer will suggest the most practical time period expressed in total process time and total number of fluid replacements. In this case a 60 day elution period was used. At the end of 60 days, the transformer was de-energized once more, the fluid drained and a sample taken for analysis. The system can be flushed again with approx. 113 1 The TCB and the flushing fluid together with the fluid mass are brought to a place where the TCB can be recovered by distillation and the residual PCB is disposed of using approved methods.

The transformer was filled again with TCB and this time the expected starting PCB concentration due to residual former fluid is approx. 83 ppm. Again, the expected PCB concentration follows the second highest curve in Figure 3 for the next 60 days (to 120 days), after which the TCB in the transformer is replaced as before with one exception. Because the drained TCB fluid has a concentration of PCBs that is less than the starting value for the first filling, the drained liquid does not need to be sent to a facility for separation, but can instead be used as the first filling for a second PCB transformer to be converted to non-PCB -states. This saves valuable distillation time and energy as well as transport and processing costs.

The refilling process is repeated one more time. Table II provides a list of expected analytical results which are represented by the circles in the diagram in Figure 3. It is clear from the data in Table II and Figure 3 that the fourth refill does not rise above 500 ppm PCB content, the prescribed value for the category of non-PCB transformers. After 180 days, the transformer was therefore filled again with its permanent fluid, a silicone oil, for example "L-305". The PCB value expected to be reached after a further 60 days (240 days) is only 16 ppm and after the prescribed 90 day period, 270 days, it is still expected to be only 18 ppm. Thus, the transformer can be reclassified as a non-PCB transformer.

Example 4

In example 4, 60-day periods were used, but flushing of the transformers is eliminated. It is assumed that 98* of the fluid can be properly drained off, leaving 2* to remain in the transformer. In this case, the initial concentration will be 2* of the previously drained fluid instead of the 0.5* in example 3. The procedure in example 3 is repeated in this example.

The results that would be expected for Example 4 are given in Table 3 and are shown in the diagram in Figure 4. Note that the target is still achieved and the system can be refilled with silicone or another permanent oil after 180 days. The lack of highly effective flushing is expected to lead to a somewhat higher PCB content in the final fluid, but this does not significantly change the achievement of a non-PCB transformer.

Example 5

The shape of the concentration curves in figures 3 and 4 may lead one to believe that the fluid replacements should be carried out more often, for example every 30 days instead of every 60 days. Example 5 is identical to Example 4 except that 30 day periods were used. The expected analytical results are given in table 4 and the points are shown in figure 5. The trend is obvious from the diagram in figure 5. The first refill shows a reduction almost as good as that in example 4 but after that the curve started to fall off . The sixth refill can be carried out with the permanenté fluid and some time is saved, approx. 30 days, at the cost of two additional TCB refills. This example shows the availability of time against the number of fillings and the choice depends on what you attach the most importance to in the particular case.

The present invention is not limited to use in transformers but can be used in the case of any electrical induction apparatus that uses a dielectric coolant including electromagnets, liquid-cell electric motors and capacitors, for example ballasts used in fluorescent lights.

Claims (12)

1. Method of replacing a PCB-containing coolant in an electrical induction apparatus with a tank containing the coolant, an electrical winding, and porous solid cellulosic electrical insulations in the PCB-containing coolant with a substantially PCB-free high-boiling dielectric permanent coolant for to transform the electrical apparatus into one, where the degree of elution of PCB to the coolant is below the maximum permissible degree of elution of the coolant for an electrical apparatus judged as non-PCB, the solid porous electrical insulation being impregnated with the PCB-containing coolant, characterized in that it comprises: a) draining the PCB-containing refrigerant from the tank to remove a major portion of the PCB-containing refrigerant contained therein; b) filling the tank with a temporary dielectric coolant liquid such as halogenated aromatic and aliphatic hydrocarbons or mixtures thereof which are miscible with PCBs and have sufficiently low viscosity to circulate in the tank and penetrate into spaces in the porous solid electrical insulation, and is capable of being easily separated from the PCB; c) electrically operating said electrical induction apparatus and continuing the electrical operation for a period of time sufficient to elute PCB contained in the PCB-containing coolant impregnated in the porous solid insulation to said temporary dielectric coolant; d) then draining said temporary dielectric coolant liquid containing eluted PCB from the tank; e) repeating the cycles of steps (b), (c) and (d) when the elution rate of PCBs to the temporary dielectric coolant exceeds 0.55 ppm PCB per day, calculated on the weight of said permanent dielectric coolant; and f) filling the tank with a substantially PCB-free permanent coolant selected from high-boiling high-viscosity silicone oils, synthetic ester fluids, poly-a-olefin oils and hydrocarbon oils to thereby reclassify said electrical equipment to non-PCB status. 2. Method according to claim 1, characterized in that the temporary dielectric coolant liquid is trichlorobenzene, tetrachlorobenzene or mixtures thereof and that the permanent coolant is a dielectric silicone oil coolant. 3. Method according to claim 1, characterized in that, when carrying out step (d) in the preceding cycle and step (b) in the subsequent cycle, the temporary coolant is drained from the top of the tank while freshly cooled temporary dielectric coolant liquid is fed to the bottom of the tank and while electrical operation of the apparatus is continued. 4. Method according to claim 1, characterized in that steps (d) and (f) are carried out by feeding said PCB-free permanent coolant to the bottom of the tank while removing the temporary dielectric coolant liquid in the tank from the top thereof and while electrical operation of the apparatus is continued . 5. Method according to claim 1, characterized in that the tank is equipped with thermal insulation to raise the temperature of the temporary dielectric coolant liquid contained therein during each step (c) during electrical operation of the electrical induction apparatus. 6. Method according to claim 1, characterized in that the temporary dielectric coolant liquid in the tank is heated in step (c) while the electrical induction apparatus works electrically. 7. Method according to claim 1, characterized in that the temporary dielectric coolant liquid in step (c) is removed from the tank, heated and returned to the tank while maintaining sufficient temporary dielectric fluid in the tank and allowing the electrical induction apparatus to work. 8. Method according to claim 1, characterized in that the temporary dielectric liquid is more volatile than said PCB and is separated from said containing PCB by distilling off the temporary dielectric coolant liquid. 9. Method according to claim 1, characterized in that the temporary dielectric coolant liquid containing PCB eluted from the solid insulation is drawn off from the tank as a split stream during electrical operation of the electric induction apparatus while adding fresh temporary dielectric coolant liquid in an amount essentially equivalent to the amount of PCB-containing temporary dielectric fluid that is drawn off in the split current. 10. Method according to claim 1, characterized in that the tank is flushed for PCBs after step (a) and before step (b) with the temporary dielectric coolant liquid trichlorobenzene, a mixture of trichlorobenzene and tetrachlorobenzene or 1,2,4-trichlorobenzene. 11. Method According to claims 1 to 10, characterized in that the permanent coolant used to fill the tank is a silicone oil. 12. Method according to claims 1 to 10, characterized in that the permanent coolant used in step (f) is a silicone oil with the formula: (CH3)3Si0[(CH3)2Si0]nSi(CH3)3 where n is a number with such a value that it gives a viscosity of approx. 50 cSt at 25°C.