# Transformer and VP is the phase-to-neutral voltage on

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Last updated: May 16, 2019

Transformer rating specified in (kVA)because copper loss of a transformer               depends on current and iron losson voltage. So, total transformer depends on volt ampere (VA) and not on phaseangle between voltage and current. It is independent of load power factor.That’s why rating of transformer in (kVA) and not in KW(kilowatt).                        5. Why transformerrating specified in KVA?The four basic configurations of a three-phase transformer,we can list the transformers secondary voltages and currents with respect to theprimary line voltage, VL and its primary line current IL as shown in the following table. Where:  n equalsthe transformers “turns ratio” (T.R.

) of the number of secondary windings NS, divided by the number of primary windings NP. (NS/NP) and VL is the line-to-line voltage with VP being the phase-to-neutral voltage.   Three phasetransformer line and voltage current.  Where again, VL isthe line-to-line voltage, and VP isthe phase-to-neutral                            voltage on either the primary or thesecondary side. Three phase voltage and current. (vi) Scott connection- Two transformersare used in this type of connection. One of the transformers has centre taps onboth primary and secondary windings. Scott- connection can also be used forthree phase to two phase conversion.

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The connection is made as shown in thefigure above. (v)  Opendelta connection- Open delta connection can be used when one of the transformersin ?-? bank is disabled and the service is to be continued until the faultytransformer is repaired or replaced. It can also be used for small three phaseloads where installation of full three transformer bank is un-necessary. Thetotal load carrying capacity of open delta connection is 57.

7% than that wouldbe for delta-delta connection.  Formulae showsdelta star turns ratio.  (iv) Delta-starconnection- The primary winding is connected indelta and the secondary winding is connected in star with neutral grounded.

This type of connection is mainly used in step-up transformer at the beginningof transmission line. The ratio of secondary to primary line voltage is ?3times the transformation ratio. There is 30° shift between the primary andsecondary line voltages.   Formulae shows star delta turns ratio. (iii) Star-delta connection-The primary winding is star-star (Y) connected with grounded neutral and thesecondary winding is delta connected. This connection is mainly used instep down transformer at the substation end of the transmission line.

Theratio of secondary to primary line voltage is 1/?3 times the transformationratio. There is 30° shift between the primary and secondary line voltages.  (ii) Delta-deltaconnection -This connection is generally used for large, low-voltagetransformers. Number of required phase/turns is relatively greater than thatfor star-star connection. The ratio of line voltages on theprimary and the secondary side is equal to the transformation ratio of thetransformers. This connection can be used even for unbalancedloading. Another advantage of this type of connection is that even if onetransformer is disable system can continue to operate in open delta connectionbut with reduced available capacity.   (i)   Star-starconnection is generally used for small, high-voltagetransformers.

Because of star connection, number of required turns/phase isreduced. Thus, the amount of insulation required is also reduced. Thus, theamount of insulation required is also reduced. This connection can be used onlyif the connected load is balanced. (vi)          Scottconnection(v)            opendelta(iv)          delta-star(iii)          star-delta(ii)           delta-delta(i)             star-starThewindings of primary and secondary can be connected in either a star or delta.

This provides a total of 4 possible connections for 3 phase transformer.Threephase transformer connection In a shell type three phasetransformer, three phases are more independent than they are in core type. Eachphase has its individual magnetic circuit. The construction of shell type threephase transformer is illustrated in the figure at right. The construction issimilar to that of three single phase shell type transformers kept on the topof each other.

The construction of a core type threephase transformer is as shown in the figure. The core consists of three legs orlimbs. As usual, the core is made up of thin laminated sheets to reduce eddy current losses.

Each limb has primary and secondary windings in cylindrical shape arrangedconcentrically.  Threephase ac system present in almost all major generation and distribution systemsin the world. It constructed from 3 single phase transformers. A three-phasetransformer is set up by three sets of primary and secondary windings, each setwound around one leg of an iron core assembly. Basically it looks like threesingle-phase transformers sharing a joined core as in figure.

Three phasetransformer core contains three sets of windings. Those sets of primary andsecondary windings will be connected in either ? or Y configurations to shapean entire unit. This winding can be associated together in differentcombinations. 3. Three phase transformer (operationand types)  So the saving in copper material depends on the valueof 1/K. The lower the value of K,more saving in copper material.   Savingof copper as step up transformer.

Therefore, the saving in copper material depends onthe value of K (voltage transformation ratio).The higher the value of K themore saving in copper material. The weight of winding material(copper) is proportionalto the length and area of across section of the conductor.Savingof copper in auto transformer as step down transformer.   2.

How copper is saving in autotransformer?       3.    Absent of galvanic isolation.2.     Used only in thelimited places where a slight variation of the output voltage from inputvoltage is required. If an auto transformer is used tosupply low voltage from a high voltage and there is a break in the secondarywinding, the full primary voltage comes across the secondary terminal which isdangerous to the operator and the equipment.

So the auto transformer should notbe used to for interconnecting high voltage and low voltage system.1.     The secondary winding is notinsulated from the primary winding. Disadvantages of auto transformer. 3.     A auto transformerhas higher efficiency than two winding transformer.

This is because ofless ohm loss and core loss due to reduction of transformer material.2.     Auto transformer has better voltageregulation as voltage drop in resistance and reactance of the singlewinding is less.1.

For an case, if transformation ratio = 2, the size of the auto transformer would beapproximately half of the corresponding size of two winding transformer (50%).For transformation ratio say 20 however the size would be 95 %. The saving incost of the material is of course not in the same ratio. The saving of cost isappreciable when the ratio of transformer is low, that is lower than 2. Thusauto transformer is smaller in size and cheaper.Advantagesof auto transformerExampleof step down transformer.

The picture shows that number of primary higher thannumber of secondary coil.  B.   Step Down Transformer- A transformerin which Np >Ns is called a step down transformer. A step down transformeris a transformer which converts high alternating voltage to low alternatingvoltage.Exampleof step up transformer. The picture shows that Ns>Np.

A.   Step Up Transformer –At the point whenthe number of secondary coil is higher than the primary coil, it will be calledas step up transformer. It converts low alternative voltage to high alternativevoltage.

The currents ofthe primary and secondary windings are steaming on the opposite directions, sothe total current flowing through the basic part of the winding is equivalent tothe difference between the current on the low-voltage winding and the current on the high-voltage winding. Thetwo windings should have the same winding sense in order for an autotransformerto work properly. Theoperation of the transformer and common transformer is the same, then the connectionamongst input and output voltages and input andoutput currents and the ratio of number of turnsbetween the primary and the secondary winding is the same.

Principleof working operation.  Thisis done by comparing the amount of copper saved in the winding. If the ratio’n’ characterized as the ratio of the lower voltage to the higher voltage, atthat point it can be demonstrated that the saving in copper is: n100%Comparedto conventional double wound transformers of the same VA rating,autotransformers are much cheaper. When choosing upon using an autotransformerit is common to contrast with that of an equivalent double wound type.Inany case, the regulation, leakage inductance and physical size of anautotransformer for a given VA or Kva rating are less than for a double woundtransformer.Alongthese lines, the part of the windings passes on the contrast between the twocurrents cab be made from a smaller conductor size, since the currents are muchsmaller saving on the cost of an equivalent double wound transformer.

An autotransformer is used principally for theadjustments of line voltages to either change its value or to keep itconsistent. If the voltage modification is by a small amount, either up ordown, at that point the transformer ratio is small as Vp and Vs are almostequal. Currents Ip and Is, are also nearly equivalent.

Auto transformer terminalmarkings. The standard procedure for denoting an auto transformerwinding is to name it with capital letters. Essentially the normal unbiasedassociation is set apart as N or (n). For the secondary tapping’s, suffixnumbers are utilized for all tapping points along the auto transformersecondary winding.

These number generally begin at number1 and continue inascending order for all tapping points as shown.  Autotransformer with multiple tappingpointsThe autotransformer canalso be constructed with more than one single tapping point. Auto-transformerscan be utilized to provide different voltage points along its winding orincrease its supply voltage concerning its supply voltage VP as shown.Atthe point when the essential current Ip, is moving through the single windingtoward the arrow as shown up, the secondary current, Is, streams in theopposite direction. Along these lines, in the part of the winding thatgenerates the secondary voltage, Vs the present streaming out of the winding isthe difference of Ip and Is.  The type with programmedvoltage alteration can be utilized as programmed voltage regulator, to keep upan steady voltage at the clients administration during a wide range of line andload conditions.

Another application is a lighting dimmer that doesn’t createEMI typical of most thyristor dimmers.The yield voltage alteration canbe manual or automatic. The manual kind is relevant just for moderately lowvoltage and it known as variable AC transformer.

These are regularly utilizedas a part of repair look for testing gadgets under various voltages or toreproduce irregular line voltages. A constant variable turns ofratio can be observed by uncovering some part of the winding coils and makingthe secondary connection through a sliding brush. It is allowing for verysmooth control of output voltage. Discrete voltage represented by actual numberof turns does not effects the output voltage.

The voltage can be smoothlyvaried between turns as the brush has a relatively high resistance (comparedwith a metal contact) and the actual output voltage is a function of therelative area of brush in contact with adjacent windings. The relatively highresistance of the brush also prevents it from acting as a short circuited turnwhen it contacts two adjacent turns. Typically, the primary connection connectsto only a part of the winding allowing the output voltage to be varied smoothlyfrom zero to above the input voltage and thus allowing the device to be usedfor testing electrical equipment at the limits of its specified voltage range.

A variable autotransformer·       Construction: Auto transformer is a device that with only one winding wound on a laminatedcore. An auto transformer is similar to a two winding transformer but differ inthe way the primary and secondary winding are interrelated. Both primary sideand secondary side is common to a part of the winding. On load condition, apart of the load current is obtained directly from the supply and the remainingpart is obtained by transformer action. An Auto transformer works as a voltage regulator.·       Introduction:1)    AutoTransformer

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