Document 9eE4x9VkqQjZyJjKN4XkE003

Maintenance of Askarel-insulated Transformers A few simple precautions plus effective sealing against moisture will result in almost indefinite life and effectiveness for askarel and the equipment it insulates. Clean, dry askarels have excel lent dielectric strength ranging from 35 kv to 45 kv as measured with an 0.1" gap at 25C. By P.G. Benignus Functional Fluids Department Monsanto Chemical Company The Author . . . Mr. Benignus graduated from Illinois College in 1933 and received his MS degree from Washington University in 1934. Since 1947 he has specialized at Monsanto, market ing Aroclor (Monsanto's registered trade name) and related askarel di electrics. skarel insulating AFLUID, besides having an electrical breakdown strength higher than petroleum-based oils, is one of the most inert, chemically stable, heat-resistant, noncorrosive liquids in common use. It is fire proof and will not oxidize or sludge and has good heat-transfer properties. Askarel remains per fectly stable unless broken down by exposure to arcing. Arc-formed gas is not combustible but amount of gas formed by a strong, presistant arc may result in excessive pressure within a tank. Many askarel-insulated transformers are fitted with pressure-relief devices for this reason. Commonly-used transformer as karels are mixtures of chlorinated biphenyl and chlorinated benzene. A small amount of hydrogen chloride scavenger is added to the fluid to protect the transformer in sulation in event of arcing. Only two formulations are used regard less of trade names under which they are marketed by various transformer manufacturers. These two formulas are fully compatible with each other and can be used interchangeably or mixed without difficulty or adverse effects. The main enemy of askarel is water. If water gets into the transformer tank only a small portion (125 ppm) can dissolve 0 509384 TOWOLDMONOQ58917 Table I TYPICAL VALUES POUND IN ASKAREL FLUID UNDER VARIOUS CONDITIONS OF USE A New askarel B CD E Askarel in After normal properly* built operation; a new or re survey of 50 built units prior units, most 2-5 to use. yrs. old; some 10. Non-arced but slightly con taminated as karel in im properly* built units, new or slightly used. Same askarel as in Column D--after re fining. F Heavily arced askarel due to neglect and in ternal electrical malfunction. Inspection Check Points Color Light straw Light straw Light straw Can have for eign shades, blue, green, red cast. Foreign shades remain show ing extraction of oil soluble color. Black Clarity Clear, free from particles Clear, practi Clear, trace of Clear, slight cally free from particles amount of particles - particles Clear, free Contains car from particles bon particles Moisture 25C. 30 ppm 30 ppm 10-70 ppm all 20-100 ppm dissolved water all dissolved water 30 ppm Near 1 25 ppm saturation level --plus possible undissolved water Dielectric Strength 25c., o.i" gap 35 KV minimum 35-48 KV 35-45 KV 30-42 KV 40-45 KV 5-15 KV Volume Resis tivity 100C,, 500 Volts, DC., 0.1" gap 100 x 109 ohmcm. minimum usually 500-1500 40 x 10 ohmcm. minimum usually at least 100 x 10 15-400 x 109 ohm-cm. about 5 x 109 ohm-cm. at least 1500 x 10 ohm-cm. about 3 x 10 ohm-cm. Power Factor: 100C. 60 cy. 2-5% 25C. 60 cy. 0.05-0.1% 10-25% 0.5-2% 10 to near 80% 0.5 to near 10% 30 to 100% about 7 to over 15% 2-5% 0.1-0.2% 100% at least 25% *A properly built transformer is defined as one in which the materials of construction are electrically compatible with askarel. in the fluid. The rest floats on the top where it can cause trouble. Dielectric strength of askeral drops rapidly as concentration of mois ture approaches its saturation point. However, moisture-laden askarel can be restored to its orig inal quality by passing it through dry blotting paper in a filter press. Severe arcing also reduces di electric strength of askarels by breaking them down. The fluid turns black due to particles of carbon. It is not economical to reclaim heavily arced askerel and it should be discarded. Table I shows typical conditions of askarels after various usage. Only Simple Tests Required to Determine Condition Any askarel that passes visual inspection and the usual dielec tric-strength test can be considered adequate for continued use. It is generally considered good prac tice to check transformer askarel every six or nine months for clarity and color and to make a break down-strength test. Samples should always be taken from top of the liquid rather than from the bottom as is the case with transformer oil as water is lighter than askarel and floats on its surface. These simple checks should be supported by the usual periodic resistivity and power-factor tests on overall trans former insulation. New askarel has a minimum dielectric strength of 35 kv and maximum moisture content of 30 ppm. If periodic dielectric-strength tests indicate a downward trend or if value has fallen to 22 kv or below as determined by ASTM test D877, moisture content should be determined using Karl Fischer method (ASTM D1533). If mois ture content has increased to 70 or 80 ppm, the askarel should be dried using a filter press. New askarel is clear and light straw in color. It normally darkens somewhat after long use. But 0b09385 there is a distinct difference be tween this darkening and the black color that results when askarel has been arced heavily. In addition, arced askarel contains carbon par ticles that can be seen in the fluid. Dielectric-breakdown tests and observations of color are adequate tests for such conditions. Arced askarel should never be reused. After the transformer has been drained, the system should be inspected for evidence of mois ture (rusting) and for mechanical defects. The transformer manu facturer should be consulted about any unusual conditions that are found. It will be necessary to clean the unit thoroughly. Usually this is done with a soft nylonbristle brush using new askarel as the cleaning fluid. After the trans former has been cleaned and re paired, it should be flushed and filled with new askarel. Quite a different condition is revealed if the askarel turns red, blue, or green. This indicates con tamination from construction ma terials such as from certain types of gaskets, pressboard, core bind ings, coatings, or adhesives not compatible with the insulating fluid. Such solutions usually lower resistivity and increase power fac tor of the askarel. While such conditions are not known to have an adverse effect on dielectric strength of the askarel itself, they should be reported to the trans former manufacturer. Reconditioning Askarel Moisture and undissolved solids can be removed effectively from askarel by filtering through dry blotter paper in a filter press. Most operators prefer portable presses that can be taken directly to the job. The filter paper must be thoroughly dried immediately be fore use. For best results the paper must be spread for maximum sur face exposure in an oven in which hot air is circulated at 110C for six to eight hours. The transformer should be kept under moderate load until just be fore filtering the askarel. This will keep the fluid and coils above atmospheric temperature to pre vent condensation of moisture from the air. Askarel should not be filtered when relative humidity is 75% or higher. Fluid tempera ture of 55 to 60C is ideal for filtering because at this range the paper is most effective in picking up moisture and there will not be excessive evaporation of askarel. Processing should start im mediately after de-energizing the transformer., Askarel should be drawn from near top of the liquid level, passed through the filter, and fed back through the bottom transformer outlet. Half a dozen passes through dry blotter paper will reduce amount of water in askarel as high as 120 ppm to 10 ppm. Dielectric breakdown strength will increase as moisture is removed. Filtering should be continued until a value of 35 kv or higher is attained. Power-factor tests on askarel fluid have no particular signifi cance in revealing its insulation value. Power factors of askarels vary widely from values of 2% to as much as 100% at 100C and 60 cycles. Use history indi cates that askarel power factors will normally be much higher than power factors of mineral oils with no adverse effects so long as breakdown strength remains high. Power factor and volume resis tivity of reclaimable askarel can be further improved after filtration by treatment with conditioned attapulgiis clay or Fuller's earth. The clay or earth should be dried Askarel-insulated transformers require little attention except to keep moisture from entering case. TOWOLDMONOQ58919 and activated by heating for 12 hours in a shallow steel tray at 300 to 350F immediately prior to use. Amount of earth to be used should be from 0.1 to 0.2% by weight of askarel. Askarel weighs about 13 pounds per gallon. The earth must be deposited evenly throughout the filter. This can be done by dividing the earth into thirds. Each third, in turn, should be mixed with a small quantity of clean askarel in a clean container and the mixture pumped through the filter. Hot askarel (but not over 55-60C) should be passed through the earth-coated filter and recirculated until the fluid is clear and tests show that electrical properties are fully restored. Only insignificant loss of scav engers in askarels occurs when the fluid is refined by normal treat ment with 0.1 to 0.2% by weight of earth. Installing a container of anhy drous alumina or activated clay in the transformer circulating sys tem is another method of reduc ing power factor and of increasing resistivity of askarels. However, this method is seldom used as these factors are not generally considered important for askarel transformers. Preventive Maintenance Adequate seals against entrance of moisture are the prime safe guards against deterioration of askarel. Modern askarel trans formers accomplish this with welded construction wherever pos sible. Modern practice also in cludes silicone or Viton gaskets properly applied in grooves or held by concentric steel rings or stops as seals for hand-hole, switch, and terminal-compartment covers. These modern sealing arrange ments should be specified if max imum trouble-free operation is to be obtained. However, it is pos sible to improve sealing of many transformers not completely mod ernized. Silicone gaskets as original equipment or for replacement pur poses should be from 5/16 inch to 1/2 inch thick and generally rectangular in cross-section. DowCorning No. 50 Silastic or equival ent low-compression-set material is not deteriorated by askarel fluid or vapor. It resists weathering, and is thermally stable and flexible at all operating temperatures. No cement is required and a gasket made of this material can be re moved with ease and reused. Ample room should be allowed in the groove for the 20 to 25% compression that should be al lowed. Silicone swells slightly in the presence of askarel which con tributes to tightness of the seal. When not possible to weld small-size connections such as bushing or radiator flanges, in strument connections, etc., effec tive seals can be made with Flexitallic stainless-steel rings. Sur faces must be machined and par allel. Filler between stainless laminations of the concentric rings should be either silicone or Viton. It is equally important to pro vide effective bushing seals. One satisfactory type is made with rolled-on flanges and two ring seals rolled into a depression in the porcelain. The bushings should be sealed with silicone rings held under compression. Metal-to-glass or metal-to-porcelain sealed bush ings are also very satisfactory. If for any reason the preceding bushings cannot be used, a porce lain or glass bushing with a sili cone or Viton gasket retained in a groove makes an adequate sub stitute. The gasket can be either rectangular or circular in cross section and usually is 14 inch to 3/a inch thick. Improving Older Transformers It is possible to improve opera tion of even considerably older askarel transformers. Many aska rel units installed in the early 1930's used cork-nitrile rubber combinations or straight nitrile rubber for sealing main tank covers, handholes, switch and terminal chambers, relief dia phragms, etc. Leaky or deterio rated gaskets on suqh older units should be replaced and the units made moisture tight. Where straight nitrile rubber garcets were used, it is probable that grooves were provided to retain the nitrile rubber to protect it against exces sive compression and to minimize contact with askarel. It is easy to replace such gaskets with sili cone or Viton seals. It is not possible to use silicone or Viton gaskets if the transformer is equipped with flat flanges and the usual bolting. Holes are re quired in gasket material to fit over these flange bolts or studs. Re placement gaskets in these cases will probably have to be a corknitrile combination type such as Armstrong Cork Co. NC 757 or equivalent. This gasket can be cut from a single sheet or can be made up by scarfing strips of the material and joining them together. The joint should be cemented with dewaxed orange shellac or with an epoxy cement. Excess cement should not be allowed to reach interior of the transformer. After gasket and cover have been installed and bolted tightly, outside edge of the gasket should be coated thoroughly with an epoxy cement to increase weather resistance. There are a number of epoxy cements readily available and especially suited for this ap plication. Note -- such cements and epoxy coatings are not neces sary where silicone or Viton gas kets are used. Temporary repairs can be made on leaky cork or cork-nitrile gas kets when it is not convenient to take a transformer out of service by painting over the leaky area with epoxy cement. Further help toward eliminating moisture leaks in old transformers can be obtained by maintaining a positive nitrogen pressure within the transformer tank. Nitrogen can be introduced at two or three pounds above atmospheric pres sure to prevent ingress of moisture. A compound pressure gage that can read pressures above and be low atmospheric can be used to monitor this nitrogen pressure. Leaks causing any sudden pres sure drop can be located with a soap solution and repaired with epoxy cement after which nitrogen pressure can be built up again. Reprinted From August 1964 TRANSMISSh & DISTRIBUTION ObO<)3d7