disc wound transformers

7/28/2019 Disc Wound Transformers

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Dry-Type disc wound transformers inmedium voltage applications

Derek R. Foster I.Eng. MIIE

---------------------------------------------------------------------------------------------------------------------------------Medium voltage, dry-type transformers may have their high voltage windings constructed usingeither the layer winding technique or the disc winding technique. Both winding techniques providethe same result in terms of electrical performance parameters, i.e. turns ratio, impedance etc.However, the use of transformers employing disc wound high voltage windings can result inincreased reliability and therefore reduced downtime.---------------------------------------------------------------------------------------------------------------------------------

IntroductionThe basic purpose of a transformer is toconvert electricity at one voltage to

electricity at another voltage, either of higheror lower value. In order to achieve thisvoltage conversion, coils are wound on alaminated silicon steel core which provides apath for the magnetic flux. The coilscomprise a number of turns of conductor,either copper or aluminum, wound as twoelectrically separate windings, called theprimary winding and the secondary winding.The primary winding is connected to thesource of voltage while the secondarywinding is connected to the load. The ratioof primary to secondary turns is the same asthe required ratio of primary to secondaryvoltages. The turns of conductor forming theprimary and secondary windings must beinsulated from one another, while theprimary winding must be insulated from thesecondary winding and both the primary andsecondary windings must be insulated fromground. The insulation of turns and windingsis collectively called the insulation system ofthe transformer.The insulation system must be designed towithstand the effects of lightning strikes andswitching surges to which the transformer issubjected, in addition to the normal

operating voltages. A further requirement ofthe insulation system is that it mustwithstand the environmental conditions towhich it is exposed, such as moisture, dustetc. A variety of techniques and materialsare employed to achieve the necessaryperformance characteristics of the insulationsystem.

Layer windingFor low voltage, i.e. 600 Volt class windings,the winding technique used almost

exclusively is the layer winding technique,also sometimes called helical winding orbarrel winding.In this technique, the turns required for thewinding are wound in one or moreconcentric layers connected in series, withthe turns of each layer being wound side byside along the axial length of the coil untilthe layer is full. The conductors of thewinding are insulated and so between turnsthere will be a minimum of two thicknessesof insulation. Between each pair of layersthere will be layers of insulation materialand/or an air duct.

Fig.1Layer winding


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Low voltage windings will generally bewound top to bottom, bottom to top etc.using a continuous conductor, until all layersare complete. High voltage windings, i.e.above 600 Volt class, may be wound in thesame way, provided the voltage betweenlayers is not too great. To reduce the voltagestress between layers, high voltage windingsare often wound in only one direction, forexample, top to bottom. When the first layerof winding is complete, the windingconductor is laid across the completed layerfrom bottom to top and then the next layer iswound, again from top to bottom. In thisway, the voltage stress between layers ishalved. The conductor must, of course, haveadditional insulation where it crosses thewinding from bottom to top.

Fig.2 Transformer with layer wound coils

Disc windingIn the disc winding, the required number ofturns are wound in a number of horizontaldiscs spaced along the axial length of thecoil. The conductor is usually rectangular incross-section and the turns are wound in aradial direction, one on top of the other i.e.one turn per layer, until the required numberof turns per disc has been wound. Theconductor is then moved to the next discand the process repeated until all turns havebeen wound. There is an air space, or duct,between each pair of discs. The discwinding requires insulation only on theconductor itself, no additional insulation isrequired between layers, as in the layerwinding.

Fig.3Disc winding

The disc wound high voltage winding isusually wound in two halves, in order thatthe required voltage adjustment taps may bepositioned at the electrical center of thewinding. In this way the magnetic, oreffective length of the winding is maintained,irrespective of which tap is used, andtherefore the magnetic balance betweenprimary and secondary windings is always

close to its optimum. This is essential tomaintain the short circuit strength of thewinding, and reduces the axialelectromagnetic forces which arise when thewindings are not perfectly balanced.

Fig.4 Transformer with disc wound coils

Characteristics of Layer wound coilsAs stated previously, the layer wound coilrequires insulation between layers, in

addition to the conductor insulation. Thethickness of insulation required will dependupon the voltage stress between layers, andcomprises one or more thicknesses of theappropriate insulation material. In practice,due to the nature of the construction of alayer wound coil, the finished coil will haveseveral unavoidable small air pocketsbetween turns and between layers. Many ofthese air pockets will become filled with


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resin during vacuum pressure impregnationof the coil. However, it sometimes happensthat some air pockets remain and it is inthese air pockets that partial discharges canoccur, greatly increasing the possibility ofpremature aging of the insulation andeventual failure. Catastrophic failure canoccur within a few months of energization.Under short circuit conditions, theelectromagnetic forces developed causetransformer windings to attempt totelescope. At the same time the coil endblocking is trying to prevent movement. Theresult is often that the turns of the windinghave a tendency to slip over one another,causing turn-to turn failure, due to abrasionof the insulation as the turns rub together.A further disadvantage of the layer woundcoil is its poor impulse voltage distributionbetween the first few turns of the winding,

due to the high ground capacitance and thelow series capacitance.

Transformerwinding

Series Groundcapacitance capacitance

Fig. 5Equivalent circuit for

Impulse voltage distribution

A transformer winding forms a complexnetwork of resistance, inductance andcapacitance. As far as the impulse voltagedistribution is concerned, the resistance canbe ignored and at the instant of applicationof the impulse wave, when very highfrequencies are predominant, the inductiveelements become effectively infiniteimpedances. The whole structure thereforereduces to a capacitive network (see fig.5).Each turn of a transformer winding isinsulated with a dielectric material and canbe thought of as one plate of a multiple platecapacitor. In addition, the combination ofdielectric material and air between each turnand ground forms further capacitiveelements.

Characteristics of Disc wound coilsThe major advantage of the disc wound coil

lies in its open construction and relative lackof insulation. For a 15kV class transformeremploying a disc wound primary winding,the number of discs will typically be in therange 36 to 48, resulting in a relatively lowvoltage per disc. Since each disc isseparated from the next by an air space, thevoltage stress between discs can easily behandled by the combination of conductorinsulation and air, no additional insulationbeing necessary. Each disc comprises anumber of turns with each turn occupyingone layer, i.e. one turn per layer: the voltage

stress between layers is therefore the sameas the voltage stress between turns andagain, can easily be handled by theconductor insulation. The turns of each disc,being wound tightly together provide almostno possibility of air pockets being presentwithin the disc. Due to the open constructionof the discs, any small air pockets whichmay be present are readily filled with resinduring vacuum pressure impregnation of thecoil. A properly designed and manufactureddry-type transformer disc winding thereforedisplays very low values of partial discharge,

typically in the range 10 to 20 picocoulombs.Unlike the layer wound coil, the disc woundcoil provides good impulse voltagedistribution, due to its inherently low value ofground capacitance and high seriescapacitance.The disc wound coil also displays excellentshort circuit strength. Each disc by itself ismechanically very strong and the completeassembly of discs are held very securely in


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place. While the electromagnetic forcesresulting from a short circuit result in atendency, for the windings to telescope, thehigh voltage turns usually remain intactrelative to each other. Instead, the completedisc has a tendency to distort as anassembly, with all the turns distorting by thesame amount. The transformer can oftencontinue to function, despite the distortion,until a convenient time arises for repair.

Losses/heatThe flow of electric current through the turnsof a transformer winding causes powerlosses which manifest themselves in theform of heat. These losses are called loadlosses and are proportional to the square ofthe current. Obviously, it is necessary todissipate this heat, to prevent overheating ofthe transformer, and in a dry-type

transformer, this is achieved by the use ofair spaces, or ducts, within the winding. Thelayer wound coil relies on vertical air ductsbetween layers and between windings, forcooling. Cool air enters the air ducts at thebottom of the coil and by natural convection,rises through the ducts, collecting heat on itsway, then exits the coil at the top. It isessential for proper operation of thetransformer that these air ducts are keptclear at all times. The insulation requiredbetween the layers of a layer wound coil hasa tendency to thermally lag the winding,

impeding the dissipation of heat. The greaterthe operating voltage of the winding, thegreater is the amount of insulation requiredand the greater is the lagging effect of theinsulation. Some radiation also takes placefrom the outer surfaces of the coils.

The open nature of the disc wound coilgreatly improves the transfer of heat fromthe winding to the surrounding air. Thethermal lagging effect of insulation isremoved and the multiple horizontal airspaces between discs provide a largesurface area for cooling by both radiationand convection.

ConclusionsThe combination of layer wound low voltagewinding, disc wound high voltage winding,NOMEX insulation and vacuum pressureimpregnation of the windings with asolventless epoxy resin, results in a veryreliable transformer with a long lifeexpectancy. Transformers constructed inthis way will be relatively free from partialdischarge and will provide excellent impulsestrength and short circuit strength, vital

requirements for reliable operation in themost demanding of applications.

Derek Foster is Engineering Manager at

Olsun Electrics Corporation.

Tel: (815) 678-2421

e-mail: [email protected]

disc wound transformers

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