TRANSFORMER CONSTRUCTION

1 TRANSFORMER CONSTRUCTION A TRANSFORMER consists of two windings coupled through a magnetic medium. The two windings work at different voltage level. The two windings of the TRANSFORMER are called High voltage winding andLow voltage winding. Both the windings are wound on a common core. One of the winding is connected to ac supply and it is called primary. The other winding is connected to load and it is called secondary. The TRANSFORMER is used to transfer electrical energy from high voltagewindingto low voltage winding or vice-versa through magnetic field. The CONSTRUCTION of transformers varies greatly, depending on theirapplications, winding voltage and current ratings and operating frequencies. The two major types of CONSTRUCTION of transformers (used in transmission anddistribution of electrical energy) are core type and shell type.

2 Depending on the application, these transformers can be classified asdistribution transformers and power transformers . The TRANSFORMER is extremely important asa component in many differenttypes of electric circuits, from small-signal electronic circuits to high voltagepower transmission systems. The most important function performed by transformers are, Changing voltage and current level in an electric system. Matching source and load impedances for maximum powertransfer in electronic and control circuitry. Electrical TYPE TRANSFORMER In core type TRANSFORMER , the magnetic core is built of laminations to form arectangular frame and the windings are arranged concentrically with eachother around the legs or limbs. The top and bottom horizontal portion of the core are called yoke.

3 The yokes connect the two limbs and have a cross sectional area equal to orgreater than that of limbs. Each limbcarries one half of primary and secondary. The two windings are closely coupled together to reduce the leakagereactance. The low voltage winding is wound near the core and high voltage winding iswound over low voltage winding away from core in order to reduce theamount of insulating materials TYPE TRANSFORMER In shell type transformers the windings are put around the central limb and theflux path is completed through two side limbs. The central limb carries total mutual flux while the side limbs forming a partof a parallel magnetic circuit carry half the total flux. The cross sectional area of the central limb is twice that of each side limbsCOMPARISON OF CORE & SHELL TYPE TRANSFORMERSCORE TYPESHELL TYPE1.

4 Easy in design and Has low mechanical strength due to non-bracing of Reduction of leakage reactance is noteasily The assembly can be easily dismantledfor repair Better heat dissipation from Has longer mean length of core andshorter mean length of coil turn. Hence bestsuited for EHV (Extra High Voltage) Comparatively High mechanical Reduction of leakage reactance is It cannot be easily dismantled Heat is not easily dissipated fromwindings since it is surrounded by It is not suitable for EHV (Extra HighVoltage) TRANSFORMER transformers up to 200kVA are used to step down distribution voltage to astandard service voltage or from transmission voltage to distribution voltageare known as distribution transformers . They are kept in operation all the 24 hours a day whether they are carryingany load or not.

5 The load on the distribution TRANSFORMER varies from time to time and thetransformer will be on no-load most of the time. Hence in distribution TRANSFORMER the copper loss (which depends on load)will be more when compared to core loss (which occurs as long as transformeris in operation). Hence distribution transformers are designed with less iron loss and designedto have the maximum efficiency at a load much lesser than full load. Also it should have good regulation to maintain the variation of supplyvoltage with in limits and so it is designed with small value of TRANSFORMER The transformers used in sub-stations and generating stations are called powertransformers. They have ratings above 200kVA. Usually a substation will have number oftransformers working in parallel. During heavy load periods all the transformers are put in operation and duringlight load periods some transformers are disconnected.

6 Therefore the power transformers should be designed to have maximumefficiency at or near full load. Power transformers are designed to have considerably greater leakagereactance that is permissible in distribution transformers in order to limit thefault current. In the case of power transformers inherent voltage regulation is less importantthan the current limiting effect of higher leakage EQUATION OF single PHASE TRANSFORMER The equation which relates the rated kVA output of a TRANSFORMER to the areaof core and window is called output equation. In transformers the output kVA depends on fluxdensity and ampere-turns. The flux density is related to core area and the ampere-turns is related towindow area. The simplified cross-section of core type and shell type single phasetransformers are shown in figures (4-1) and (4-2).

7 The low voltage winding is placed nearer to the core in order to reduce theinsulation requirement. The space inside the core is called window and it is the space available foraccommodating the primary and secondary winding. The window area is shared between the windingand their insulations. The induced emf in a TRANSFORMER , Emf per turn, The window in single phase TRANSFORMER contains one primary and onesecondary winding. The window space factor Kw is the ratio of conductor area in window to totalarea of window. Conductor area in window, The current density is same in both the density, Areaof cross- section of primary conductor, Area of cross- section of secondary conductor, If we neglect magnetizing mmfthen primary ampere turns is equal tosecondary ampere turns. Therefore ampere turns, Total copper area in window,Ac = Copper area ofprimary winding + Copper area of secondary winding = (Number of primaryturns x area of cross-section of primar yconductor) + (Number of secondary turns x area of cross- section ofsecondary conductor) On equating the above equations,we get, ThereforeAmpere turns, The kVA rating of single phase TRANSFORMER is given by, On substituting for E and ATfrom equations we get,The aboveequation is the output equation of single phase EQUATION OF THREE PHASE TRANSFORMER The simplified cross-section of core type three phase TRANSFORMER is shown infigure.

8 The cross-section has three limbs and two windows. Each limb carries the low voltage and high voltage winding of a phase. The induced emfper phase, Emfperturn, In case of three phase TRANSFORMER , each window has two primary and twosecondarywindings. The window spacefactor K is the ratio of conductor area in window to totalarea of window, ThereforeConductorarea in the window, The current densityis same in both the , I = Primary current per phase = Secondary current per phase Area of cross- section of primary conductor, Area of cross- section of secondary conductor, If weneglect magnetizing mmf then primary ampere turns per phase is equalto secondary ampere turns per phase. Total copper area in window, Ac = (2 x Number of primary turns x area ofcross-section of primary conductor) + ( 2 x Number of secondary turns x areaof cross- section of secondary conductor) On equating we get, The kVA rating of three phase TRANSFORMER is given by, On substituting for E and AT from equations we get, Theaboveequation is the output equation of three phase PER TURN The TRANSFORMER design starts with selection of an appropriate value for emfper turn.

9 Hence an equation for emf per turn can be developed byrelating output kVA,magnetic and electric loading. In transformers , the ratio of specific magnetic and electric loading is specifiedrather than actual value of specific loadings. Let, ratio of specific magnetic and electric loading be, The volt-ampere perphase of a TRANSFORMER is given by the product of voltageand current per phase. Considering the primary voltage and current per phase we can write, We know that Emf per turn, On substituting for mfrom equation we get, From the above equation we can say that the emf per turn is directlyproportional to K. The value of K depends on the type, service condition and method ofconstruction of TRANSFORMER . The value ofK for different types of transformers is listed in table typeKSingle phase shell typeSingle phase core typeThree phase shell typeThree phase core type,distribution transformerThree phase core type,power transformer1 0 to 1 6_to 0 7 DESIGN OF CORES For core type TRANSFORMER the cross-section may be rectangular, square orstepped.

10 When circular coils are required for distribution and power transformers , thesquare and stepped cores are used. For shell type TRANSFORMER the cross-section may be rectangular. When rectangular cores are used the coils are also rectangular in shape. The rectangular core is suitable for small and low voltage transformers . In core type TRANSFORMER with rectangular cores, the ratio of depth to width ofthe core is to 2. In shell type transformers with rectangular cores the width of the central limbis 2 to 3 times the depth of the core. The figure shows the cross-section of TRANSFORMER cores. In square cores the diameter of the circumscribing circle is larger than thediameter of stepped cores of same area of cross-section. Thus when stepped cores are used the length of mean turn of winding isreduced with consequent reduction in both cost of copper and copper loss.