Electrical Digest 11

7/29/2019 Electrical Digest 11

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LECTRICAL POWER SYSTEMENGlNEERlNG &TECHNICAL FIELD SPEClALlSTSlTERNATlONAL FORENSIC ENGINEERS

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ARC FURNACE TRANSFORMER FAILURESThe electrical and mechanical duties imposed on

transformers used in electrical arc furnace installations cabe very exacting. The techniques of the operation and thunusual loads they produce are so severe, particularly onthe transformer, that special design requirements andlater, the maintenance of the units is essential. Recentexamples of arc furnace transformer failures illustratethat it is possible in many instances for a forensicengineer to investigate and determine causes of failure,provided sufficient evidence is left.

An arc hrnace is primarily used for convertingscrap metal pieces to reusable material. The scrap metalis loaded into a refractory lined bowl and subjected to anintense electric arc generated between moveable elec-trodes and the scrap material. Until the scrap material isreduced to a molten mass, the electrical loads imposed othe supply are severe and erratic. The melting process catypically require up to 100,000 Amps of electric current

overheatingon secon+ p 0 p ~ide. which will have superimposed on it, harmonic variationwhich are wildly fluctuating, causing very unusual load

patterns in the supplying transformer. This means that the arc furnace must have a supplyintransformer which is custom designed to meet erratic loads for what will effectively be theequivalent of many short circuits.

The rough behaviour produced by a vast succession or snort circuits can be made eveworse by mistreatment by the operator. For instance, when closing down the furnace itshould be normal practice to raise the elec-trodes first. Some operators will simply tripthe control circuit breaker resulting in consid-erable mechanical stress on the transformers.In fact, many problems in arc furnace trans-formers can be traced to mechanical failures.Some of the failure analysis work on arcfurnace transformers done by Brosz andAssociates illustrate such problems as well asdifficulties associated with overheating.(Please see over) " Close-tcp view of a defective 10,000RW bus connectionNote: Feeler gauge between mating surfaces.64 Bullock Drive, Markham, Ontario, L3P 3P2, Tel: (905) 472-6660 (24 HRS.) Fax: (905) 472-6665266 Elmwood Avenue , Buffalo, New York, 1422 2, Tel: (716)855-1515


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CASE IOn a particular 60,000 Amp, 750V, 50Hz arc furnace unit the high current main

connections to the furnace were found to have gaps between the connections.Severe burning and pitting found at 10,000 Amp rated connections was evidence ofextreme heat. A feeler gauge could be pushed between the connection surfaces. The heat

necessary to produce such pitting andburning had to be extremely high. Thisheat would be transferred to the transform-er windings producing higher than normaltemperatures in the winding and oil. Thiswith the eccentric duty of the systemcombined to cause hot spots within theunit. Hot spots may lead to the deteriorationof the insulation around the windings andsludging and the production of othercontamination in the insulating oil.

Insulation failure, whether in solidsor liquids will lead eventually to totalfailure and the loss of the unit. The lesson

Close-up view of deteriorated/overheated 10,000AMPwater-cooled connection.here is that all current connections must have maximum surface contact and run cool. Keeping highly fluctuating and sometimes severe overcurrent conditions within acceptable tempeture limits will make insulation preservation easier. Installation checks to ensure good

contacts added to knowledgeable preventivemaintenance work, should aim for theserequirements.CASE II

In another case, where the furnacetransformer was designed using a shell formconfiguration, failure was due to the inability tocontrol short circuit forces. This is unusual sinshell form designs are always believed to bemechanically stronger than core form designs.

Shell form transformer windings areconsidered easier to clamp and hold down durinshort circuit behaviour, but in this case the interclamps loosened and did not hold and typical shcircuit winding failures were found in the windi(see photo). Of course, the failure could havebeen due to extreme conditions within the genearc furnace circuit such as inadequate reactancebut, nevertheless, the evidence in this casesuggested deficient clamping pressures. Often,with continuous but erratic mechanical pummelShellform furnace transformer with loose core such as during the operation of the arc furnace,and shorted high voltage turns. clamps can become loose and certainly release t


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BROSZ AND ASSOCIATESELECTRICAL POWER SYSTEM ENGINEERING8 ECHNICAL FIELD SPECIALISTSINTERNATIONAL FORENSIC ENGINEERS

original design pressures. Also windingstend to shiR and shrink with age and thecombination of this and tough mechanicalpounding can add to the changing anddeteriorating clamping pressures. Thesolution - to ensure during any mainte-nance inspection of the transformer thatthe clamping pressures are adequate andadjusted if necessary.CASE Ill

A hrther problem was found inanother transformer with loose extra-flexible primary leads. Again, insufficientconsideration of the mechanical forces setup by the tough conditions experienced byarc hrnace transformers was found to bethe reason why the flexible lead insulation

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became chaffed and worn, resulting in a Broken high voltage coil blocking. Due to short circuitshort circuit fault. forces.

SOME SOLUTIONSThe solutions to preventing some arc furnace transformer failures will be found by

considering what is happening to cause failure. It must be accepted that the loading of thehrnace and the initial melt will set up abnormal conditions such as overvoltage, overcurrentsand harmonic distortion and these should be provided for in the general design. However,there is no escaping the fact that the result will still be mechanical pounding, high but fluctuat-ing currents and voltage distortions. The basics requirements then are to guard against this byensuring that currents are kept within design limits and to minimum proportions by ensuringgood contact surfaces and tight connections. If possible, monitor the heat generated by the

Shell type furnace transformer. High voltag e winding , turn-to-turn failure.

high current bars with thermal imagingsystems or heat sensitive devices nowavailable. Include, or check, therelaying system which protects againstextreme overcurrents and make suresuch protection is adequate to givenotice of early problems.

Internal inspection of thetransformer should be carried out atleast once a year by independentexperts in conjunction with themanufacturer. Certainly the first yearof operation is important because thatwill be time when the greatest rate ofchanges such as relaxed clampingpressure may occur. In any internal


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inspection, all clamping should bechecked and if needed, retightened.All leads and connections should besecured and every effort made tomake sure nothing is loose. Thewindings should be checked forinsulation deterioration wheneverpossible.

In shell form units this ismore difficult because of the particular construction, however, it may stibe possible to make an inspectionusing boroscopes and similar deviceFailure of extra flexible, high voltage conn ections. As with all transformers, thebest method to check on their health

is to check the condition of the insulating oil. The severe conditions experienced by furnacetransformers can lead to oil contamination much quicker than conventional units. Evenevidence of severe gassing will be found. Therefore, regular and more frequent oil analysischecks are recommended - at least quarterly. All the usual ASTM oil tests should be doneand also regular dissolved gas in oil analysis be taken. This technique is probably the bestreliability tool we have at the moment. In addition to measuring the degree of polymerizatioof the paper insulation, this measures the chemical byproducts of the insulating paper degradation. Inaddition, it is suggested that infrared therrnographicreadings be taken of the tank and the connections togive a clear understanding of the extent of externalheating. Internal temperature measurements onenergized windings can also be done using a special-ized technique.

Another problem which is sometimes over-looked is the location of the arc furnace transformer.The melting operation is not clean and if the trans-former is located in an area where it can collect dirtand grime, especially on the bushings, then flashoversand insulator breakdowns can occur. Ideally, thesetransformers should be located in a clean, sometimesair-conditioned and pressurized vault.

Failures in arc furnace transformers areassociated with high PD (property damage), high BI

' (business interruption) losses and large insurancedeductibles. Professional independent inspection anddiagnostics, taken on an ongoing basis, can go a longway towards reducing failures and eliminatingunnecessary repair costs.64 Bullock Drive, Markham, Ontario, L3P 3P2, Tel: (905)266 Elmwood Avenue, Buffalo, New York, 14222, Tel: (7

Internal connection failure on 18MVAtransformer.

472-6660 (24 HRS.) Fax: (905) 472-666516)855-1515

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Electrical Digest 11