Transformer failure Reduction Techniques

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Transformer Failure Reduction Techniques page | 1 Dept of EEESIR CRR COE 2015-16 CHAPTER I INTRODUCTION Transformers transfers electrical energy between circuits completely insulated from each other.Duringthistransferofenergyvoltagelevelmaybesteppedup/down(dependingonthe tums)withconsequentdecrease/increasecurrentsfromprimarytosecondarywindings,thus maintaining the power transferred constant. Therefore when voltage is stepped up the current is stepped down. This makes it possible to use very high voltages for transmission lines resulting in alowercurrent.Highervoltageandlowercurrentreducetheconductorsize,transmissionline losses and improves the voltage regulation as well. Transformershave made possible economic delivery of electric power over long distances. In our country the electrical energy is usually generated at 11 or 15.75 or 2l kv, stepped up to l32;220;400 or 765 kV with the help of step up transformers for transmission and then stepped down the voltage to ll kV for feeding distributing transformers stepping down the voltage further to 400/230 volts for the consumer uses. PRINCIPLE OF TRANSFORMER: The physical basis of the transformer involves the Faradays laws of electro magnetism in whichanalternatingfluxinducesvoltageinthecoilandLenzLawwhichstatesthattheeffect produced will oppose the cause. It consists of two inductive coils which are electrically separated butmagneticallylinkedthroughapathoflowreluctancebymutualinduction.Ifonecoilis connected to the source of alternating voltage an alternatingflux is set up in the laminated core mostofwhichislinkedwiththeothercoilproducesmutuallyinducedemf.Ifthesecondcoil circuitisclosed,acurrentflowsinitandsoelectricenergyistransferredfromtheprimaryto secondary coil. Transformer Failure Reduction Techniques page | 2 Dept of EEESIR CRR COE 2015-16 CHAPTER ll CONSTRUCTION and PARTS OF TRANSFORMER 2(a) Transformer construction: The transformer is simple in construction and consists of magnetic circuit linking with two windingsknownasprimaryandsecondarywindings.Besidesmagneticcircuitandwindingsit consistsofasuitablecontainerfortheassembledcoreandwindingssuchasatank,asuitable medium for insulating the core and the windings from its container such as transformer oil, suitable bushings (either of porcelain , oil filled or condenser type) for insulating and bringing out terminals of the windings from the transformer tank, temperature gauge for measurement of temperature of hotoilorhottestspottemperatureandoilgaugetoindicatetheoillevelinsidethetank. Transformers are also provided with the conservator tank in order to slow down deterioration of oil and keep the main tank full of oil. Fig.2.1: Cut View of a Power Transformer 2(b) Parts of a transformer:2.1. Transformer tank:Small capacity tanks are fabricated from welded sheet steel, while larger ones are assembled from plain boiler plates or cast aluminum parts, usually mounted on a shallow fabricated steel base. The lids of these transformer tanks can be of cast iron, a water proof gasket being used at the joints. Forcoolingpurpose,coolingtubesareweldedwiththetank,butincaseofradiators.Separate radiators are individually welded and then bolted-on to the transformer. A tank must withstand the stresses developed by jacking and lifting and shall be no longer than necessary to accommodate the core, windings and internal connections with appropriate electrical clearance. Transformer Failure Reduction Techniques page | 3 Dept of EEESIR CRR COE 2015-16 2.2. Core Assembly: CoreAssemblyformsthemagneticcircuitlinkingthetwowindingsofthetransformer. Those parts of the, magnetic circuit which carry the transformer windings, are called the limbs or legs, and those parts which connect the legs and serve for closing the magnetic circuit are termed yokes. The core material used in its construction should satisfy: Maximum flux is created with minimum magnetizing current Minimum core loss. Theuseofsteelinmagneticcircuitintroducesironorcorelossbutensuresahigh permeability of the magnetic circuit. Because of the high permeability the magnitude of exciting current necessary to create the required flux in the core is small. The presence of steel core causes 100% of the magnetic flux created by the primary o be linked with secondary. The magnetic frame ofthetransformerisbuiltupoflaminatedhotrolledorcoldrolledorientedsteelconsistingof 3.5% silicon. The higher content of silicon increases the resistivity of the core, thereby reducing the eddy current core loss. As the flux in the core is pulsating one ,it becomes necessary that the transformer cores are laminatedandthelaminationsshouldbeinsulatedandmadeasthinaspossibleinorderto minimize the eddy current loss. 2.3. Winding Assembly: The most important features that the windings of a transformer should possess are: Transformer Failure Reduction Techniques page | 4 Dept of EEESIR CRR COE 2015-16 a)Thewindingshouldbeeconomicalbothasregardsinitialcost,withaviewtothemarket availability of copper b) The efficiency of the transformer in Service c) The heating conditions of the windings should meet standard requirements d) The winding should be mechanically stable in respect to the forces appearing when sudden short circuit of the transformer occurs e) The winding should have the necessary electrical strength in respect to the over voltages. Transformer windings are made of solid or stranded copper or aluminum strip conductors. Heavy current capacity needs conductors of large cross section to reduce eddy current losses in the conductors, several small wires or parallel straps are preferred to one large strap. This gives rise tounequalreactancecomponentstheconductorwhichcanbeeliminatedbytranspositionof conductors. 2.4 Bushings: Bushings are incorporated to bring the Extra High voltage winding terminations through thecoverofthetransformertank.Varietyofbushingsusedforvariousvoltageclasseswere mentioned below: Voltage Type of bushing Class