Document aBEz5XNYo3K073q1g91GBMndN

Askarel (Aroclor-Chlorlnated Benzene Filled) Transformers Askarel, nonflammable dielectric and cooling medium for tranaformers, waa Introduced commercially in 1932 by General Electric Company under their trade name, "Pyranol." Subsequently, General Electric licensees have been using the same materials under their various trade names. In terms of Pyranol alone, since 1932 more than 33,000 Pyranol transformers ranging In size from 1-1/2 to 27,000 KVA and totaling more than 10,000,000 KVA have established an excellent operating record. These Installations Include current transformers and potential transformers. In 1938 the first Pyranol Induction voltage regulators were built and there are now more than 1230 units In service totaling more than 110,000 KVA These figures, applicable to Pyranol alone, are growing larger each year and are given as a gsneral Indication Of the scope of this business which acoounts for a large portion of our Aroclor sales. Miners} oil filled transformers. Including power and distribution types, account for about 75f' to 80$ of transformer construction. However, due to the flammability of mineral oil, these units are used outdoors mainly. While mineral oil transformers are the least expensive type, nevertheless, in confined areas (buildings, etc.) where it would be required to enclose this type of transformer In a vault (concrete, etc.) the overall Installation cost becomes much greater than the cost of using an Askarel or dry type transformer unit which In itself provides the safety required. Consequently, for many Industrial (factories) and commercial (office buildings) Installations, the Askarel and dry type trans formers are used rather than the mineral oil type transformers. In this type of Installation the voltage ranges used are met most adequately by the Askarel transformers. The KVA class range in which Askarel units are most commonly used is about 30 to 1,000 KVA. However, as mentioned previously, in some cases they have ranged up to 27,000 KVA. This then establishes the prlnolpal field of application for Askarel transformers and nerves to Indicate that their direct competition Is the dry type transformer and very seldom mineral oil transformers. Cost Is one of the most Important factors In selecting a transformer. A very general Idea of cost, but by no means conclusive one, may be gathered from the following generalization: 1. Sealed dry type transformers which require silicone (Class H) Insulation for ambient temperature operation at 180C. oost about 23$ more than open, dry type trans formers . MOhS 001916 2. Open, dry type units, which for operation at 55C. ambient temperature may use cotton, nylon, etc., Insulation, coat about the same as Askarel units. However, Tor SoC. ambient temperatures, glass and porcelain Insulation (Class B) is required and thece units are as much as 10#-15# higher In cost than Askarel types. 3. Askarel transformers which use cotton, paper, cardboard, et. (Class A Insulation) cost about 15 more than mineral oil transformers. Mineral oil units use oleo resinous varnish and Formvar Insulation. Because of this and other construction factors, It is not satisfactory to use Askarel in a transformer designed or specified for mineral oil use. On the other hand, mineral oil can be used usually In units Insulated to handle Askarel. Many factors In addition to Initial coat enter the selection of trans former construction. Also, prompt availability or supply and servic ing are considered. The following Is given as a typical analysis of Indoor transformers considering their various operating character istics as a basis for selecting the specific type best suited for the Job. ANALYSIS OF INDOOR TRANSFORMERS A modern trend lr. Indoor distribution Involves the transmission of power to centers of eleotrical load at voltages higher than utiliza tion levelB, then reducing the voltage through power-oenter trans formers . By carrying power through the plant at higher voltages, large savings result from copper reductions; voltage drop Is minimized, and the performance of motor and lighting Installations Is correspondingly Improved. The cost of power centers and secondary distribution trans formers Is more than offset by savings resulting from the elimination of long, heavy distribution runs. t. ijouo-von,s or lower, 4l60. and 2400. Secondary ..... ....... _ ___ .. ___ 4-wlre, with short conductor runs to Individual loads. Where the motor load is the governing factor, distribution at 480-volts lo usually more economical, with lighting ciroults at 2400.20-volts 3-wlrs supplied through stepdown transformers. However, where the lighting load Is the major consideration, distribution Is generally at 208Y/120-volts 4-wlre, with motor loads supplied at 480-volts through step-up . transformers. A variety of equipment is available to protect power center trans formers, such as hlgh-voltagc air-break switches in ratings to Interrupt full-load current or simply the exciting current. For added protection fueea may be protected by various combinations of main MGNS 061917 -3- and feeder breakers In metal-clad compartments, thermal magnetic breakers, safety plug-in units, fused plug-in units, or a variety of switching or Interrupting devices between the transformer and the individual load. The Impedances of the transformer Itself may be used to advantage when circuit protection is considered. Impedances In the range of 5.0jt to 5.5$ have been accepted as a compromise between better voltage regulation and limitation of fault currents. Impedance of this order limits short circuit stresses yet permits the burning clear of many faults that may occur In the secondary circuit. The transformer may be supplied with various accessories for its own protection. Temperature Indicators show the temperature of the windings under various loads. Thermal relays provide overload pro tection by automatically operating cooling fans, signal lights and alarms, or tripping the main oircult breaker If an excessive load or short circuit should occur. If it is desirable to prevent operation of the primary disconnect switch while the secondary breaker la closed. Interlocking devices may be used. Such precautions are Important for safety of personnel and for protection of the primary dlsoonneot switoh. Maximum eervlce life is obtained by Incorporating adequate protection of equipment Into the electrical power distribution system. Hazards to personnel are reduced to a minimum by enclosing busbar, wiring, switoh equipment and transformers, sheet metal enclosures, bus duct and conduit prevent contact with live parts, accidental short circuits, collection of dirt, dust and moisture, wear and darage. Enclosed electrical equipment Is safer, requires less maintenance and gives evidence of careful and orderly workmanship. Comparisons Between Liquid Filled and Dry Type Units Transformers using mineral oil as a dielectric and cooling medium have a serious disadvantage for Indoor use. The oil will support combus tion and serious fires and explosions may result from Internal arcing or Ignition from any external source. Earlier practice required a sturdy fireproof wall, usually of concrete, around the transformer when located in or near a building. Construction or the vtult alone was oostly and terminal connections away from the transformer were complicated. An added oonoem was provision for ventilation. With the development of the fire-proof, liquid dielectric, ..skarel (Aroclor-TCB mixture), and the dry type tranaformer, it became a safe practice to install these transformers Indoors without the need of expensive fireproof vaults. Both /.skarel filled and dry type trans formers have become very popular for Indoor use. Each has certain advantages and disadvantages. The liquid Askarel has a dielectric strength comparable to mineral oil ana thus may be Incorporated without difficulty Into designs to withstand the same 60 cycle and impulse dielectric testa as are standard for oil filled transformers. The following table lists ASA standard dielectric tests for liquid filled and dry type transformers: HONS 061918 -4- Type Transformer KVA Class 60 Cycle l Minute Test Full Wave Impulse Test Dry Askarel or Oil Filled 5 12 KV 6.7 19 KV 15 31 KV 5 19 KV 0.7 26 KV 15 34 KV 25 KV 35 KV 50 KV 60 KV 75 KV 95 KV It will be noted that dry type Impulse lev-els are roughly one-half as high as the level for liquid filled transformers. 60 oyole tests are also lower though very nearly equal for the 15 KV olass. However, Installed indoors where danger from lightning Is considerably reduced, the precautions necessary for surge protection are not as important as for the outdoor transformer. ( The dry type transformer must rely upon olearanoes in air for ltB Insulation strength. If an attempt were made to design the dry type transformer with Impulse levels equal to liquid filled trans formers, cost, weights, dimensions and losses would be condlderably Increased. The resultant loss of economy is not considered Justified Although It is not a great disadvantage, the power factor of Askars1 Is somewhat higher than that of mineral oil. Experience has shown that the power faotor of the complete Insulation structure of oil filled units Is in the range of 1 and 2. Aakarel filled transformers are found to vary between 3 aid 12JS. Askar*1 has a somewhat erratic behavior at higher voltages but applications up to and including 44,000 volts are In satisfactory ssrvioe. Askarel filled transformers require periodic sampling of the liquid to oheok the dielectric strength. Because of the high specific gravity of Askarel, moisture or free water will accumulate above ( the liquid level. Poroelaln tubes on H.V. leads to terminals are used to protect against failure over this low resistance moisture film. As the top is the region of highest temperature in the trans former tank, the Askarel may become saturated with moisture. The saturated Askarel .olroulates by thermal action and the moisture Is absorbed in the innermost regions of the insulation structure. Water vapor may form In the top of the tank If cooled sufficiently. This vapor deposited on porcelain or other parts above the liquid level will reduce effective spaclngs and creepage distances. Filtering at intervals Is necessary to remove any moisture and Impurities, often a special absorbing agent, such as activated clay. Is required to remove Impurities In solution as well as any moisture present. While arcing of the Astore1 transformer la a possibility and would Involve gassing If continued, a properly installed and protected Askarel unit has only a very remote chance for this ocourence. MGNS 061919 -5- Due to the high specifla gravity of Aalcarel - 13 lbs. par gallon - transformers using this liquid ara very heavy. Their weight is roughly 30 to 400 higher than a dry type transformer of equivalent rating. Such added weight may be a consideration when moving or installing the transformer on platforms or other levels. Floor spaoe required offers little advantage for either type. Cool ing tubes extend from the tank wall of Askarel filled units and add to the overall dimensions. In the same manner, dry type transformers require liberal insulation olearancee in air. The toy type trans former enolosure may be oonvenlently matched to the height of incoming primary and secondary swltohgear enclosures for uniform dimensions and pleasing appearanoe of the oomplete unit substation. Cause and Control of Vibration and Noise The main souroe of transformer noise is due to magnetostriction in the core. When an alternating magnetising current is applied, there are minute contractions and elongations of the core laminations. i Vibrations are set up in the oore and ooll supporting frames and trans mitted to the tank or enolosure. Askarel with its high speoiflo gravity adds considerable mass to the transformer assembly. The combination of internal parts and fluid dlelectrlo tends to make the oomplete interior a solid structure which dampens or attenuates the vibrations originating in the oore. Bnoloaures for the dry type transformer are made of lighter gauge sheet metal. The panels are in removable sections with ventilating openings. The enclosures are Intended only to protect the trans former from falling dust, dirt, water, rodents and damage from ordinary movement of men and equipment. It is essentially a light framework and of little value In reduolng the level of noise. A comparison of established sound levels for the two types of trans I formers are listed. Sound Level KVA 205^100 301- 500 501- 700 701-1000 Llauld Filled "5o uiclbels 5b Decibels 60 Decibels 62 Decibels 66 uecibeis 6b Decibels 70 Decibels 72 Decibels In a factory where the general ambient sound level is high, the noise from the transformer may not be noticed. In a school, office building, or hospital, the suae noise level may be objectionable. Vibrations may be transmitted through steel structural member* walls, or other parts of the building. It may be necessary to erect sound absorbing barriers, line the interior of rooms with sound absorbing material or mount the transformer on sound deadening supports to prevent vibra tions from being transmitted. KOfcS 061920 -6- The sound level of the transformsr varies In relation to the flux density at whioh the oore is operated. Any substantial reduction in noise level would require a transformer of special design. In suoh a special transformer, it must be expected that the oore loss would be low; cost, weight, copper loss and Impedance would be high. If price and performance Is of no consideration, it is possible to design a transformer of any desired noise level. In general, the Askarsl filled transformer is preferred for locations where excessive dirt or moisture Is present or when a minimum noise level is desired without added cost. For applications where reduced weight and a minimum of maintenance Is desired, the dry type trans former is preferred. For distribution to smaller individual loads, suoh as lighting oiroults, machine tools or special devloes, the ease of mounting and simplicity of oonneotlons Is an advantage of dry type transformers. Developments in Dry Type Transformers Originally dry type transformers wars Insulated with class A organic 1 materials suoh as ootton, paper, fiber, fullerboard, and insulation board all of which were readily combustible. As the temperature rise of the windings was limited to 550, substantial ventilation was required with many ducts in the coll. Expanded metal was generally used to enolose and partially protect the oolls. Current densities in the oonduotors were low. With the development of Class B Insulation permitting a temperature rise of 00C, It was possible to operate the coll oonduotors at higher ourrent deneltles. It was possible to reduce the area of ooll exposed for ventilation and sheet metal enclosures In general replaoed expanded metal. The class B dry type transformer is smaller and lighter than the older class A types. It is explosion-proof and sssentially fireproof. The Insulation aonslsta of porcelain, mica, asbestos, fibreglass l and similar inorganic materials with an organic binder. After treat ment in a speolal varnish, the colls are baked to give a glossy, tough finish highly resistant to moisture and penetration by collec tions of dirt and dust. Usually the only maintenance required is cleaning off the accumulations of dust. Such maintenance is fairly simple as the panels are easily removed for access to the internal parts. Excessive moisture or condensation on the colls must be guarded against and. If present, a speolal drying out proceas is required. The ventilated dry type transformer la not adapted to locations where water could enter or submerge the enclosure. A more recent transformer development has bean the hermetically sealed dry type transformer. The oonstruotlon consists of s large smooth tank enclosing the oore and oolls with glass sealed bushings and welded Joints to completely seal the Interior from the atmosphere. Without benefit of ventilation through the core and ooll structure, all heat losses must be dissipated by radiation and natural convection of gas flow from core and coll to tank. The operating HONS 061921 temperature for a praotlcal design le Increased beyond the permissible fJOC rise for olass B insulation. Development of class H Insulation has made possible the sealed dry type transformer with higher operating temperature. Class H insulation, of whloh the silicones are most oommon, permits operation at l8oC, a limit whloh has been tentatively and probably conserva tively established. Silicones have chemical compositions similar to glass, mloa, and quartz. An atmosphere of dry nitrogen gas Is usually sealed Into the tank at low pressure. This Inert atmosphere excludes oxygen which would aooelerate aging and deterioration of Insulation especially at higher temperatures. By sealing, the effeots of humidity, water vapor, dirt, dust, and other contaminants are eliminated. Maintenance for the sealed unit Is reduced to a minimum. Only perlodlo oheoks of gas pressure and occasional cleaning Is required. The tank le smooth without openings or radiator tubes, etc., which simplifies cleaning and maintaining a good appearanoe. Should the transformer be de-energlzed for long periods, no precautions need be taken to prevent condensation on the colls nor Is a dryout prooeas required before returning to service. Sound levels are considerably reduoed over ventilated dry types of equivalent rating. The confined Internal gas space and rigid welded tank reduoes the Intensity of noise from the core and coll assembly. Height and dimensions are oomparable to ventilated dry types and considerably lower than Askarel. 'The sealed dry type transformer Is adaptsd to any use or location where either oil filled, Askarel filled, or ventilated dry type transformer of 15 KV olass and below are used In network distribu tion systems. They are particularly suited for use In underground vaults where flooding regularly ooours. Indoors In dirty locations, or any contaminated atmosphere where corrosive aotlon is a hazard. The sealed units may be looated In the open or In busy areas without danger to personnel. The cost of class H Silicone Insulation materials at present Is much higher than for class B Insulation. Costs are being steadily reduoed through Improved manufacturing methods and greater volume, but present oosts of sealed dry type transformers are 20 to 25# higher than equivalent Askarel filled or open dry type transformers. To date, no competent data Indicate that the sealed dry type at its higher oost will oompete with the Askarel type transformer other than In special applications. P. 0. Benlgnue July 22, 1953 MOSS 061922