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What is a TransformerA transformer is a static equipmentwhich transformers power from onecircuit to another by stepping up ordownthe primary voltage without any

changein the frequency.The two circuits may or may not beconnected but are magneticallycoupled.

Features of PowerTransformersSingle PhaseThree phase

Star or Delta connected PrimaryStar or Delta connected SecondaryWith or without Tertiary winding

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Provided with Off-circuit tap switchor

On-load Tap Changer for voltageregulation

Design Principles -CoreImproved manufacturing techniques likecore building with 2-lamination packets,

step-lap joints, v-notched laminations,bolt-less cores are used.Hi- core steels like M0H, ZDKH, etc areavailable in which the specific core losses

arelower than normal grades.Amorphous steels drastically reduce no-loadlosses, but are now being used only indistribution transformers.

Design Principles -CoreImproved manufacturing techniques like

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core building with 2-lamination packets,step-

lap joints, v-notched laminations,bolt-less cores are used.Hi- core steels like M0H, ZDKH, etc areavailable in which the specific core lossesarelower than normal grades.

Amorphous steels drastically reduce no-loadlosses, but are now being used only indistribution transformers.

L.V Windings in Transformers are either

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Spiral wound for for low current ratings

Design Principles

Windings- L.V windingHelical Wound with radial coolingductsfor higher ratings.

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The conductor used is paper insulatedrectangular copper (PICC)

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For higher currents, transposedconductors areused, to uniformly distributethe current acrossthe cross section of thewire of coil.

Design Principles

H.V Winding/1HV winding is invariably uses PICC orCTC.Type of winding used is

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- Disc winding up to 132 kV- Layer winding

or- Interleaved windingor- Rib shielded winding

g

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Cs = K Cg/Cs

Design Principles

Impulse Voltage DistributionDesign PrinciplesTertiary Winding/1In Star-Star Connected Transformers and

Auto-transformers, Tertiary Winding is used-- to stabilize phase to phase voltages incase of

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unbalanced load- Suppressing third harmonic currents in

earthedneutral- reducing zero sequence reactance- for supplying auxiliary load or forconnectingcapacitors.

Design PrinciplesTertiary Winding/2Tertiary is required to be designed

for a

power rating equal to one-third theratedpower, it increases the cost of thetransformer by 10- 12 percent.Tertiary winding is known to fail due

totransferred surges and Short circuitsPresent practice is to do away with

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tertiary up to 100 MVA for 3 phase 3limbed core transformers.

sulating Paper continues to be CelluloseKraft Paper (Temperature Class 135C)

Improvements in materialsInsulating Paper

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Thermally upgraded paper ( Nomex ,Aramid155) is used in hot spot regionsin transformers with space constraints,likeTraction Transformers.* Core Vibration

*Force due to Short Circuitor Faults

Thermal

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* Winding Temperature* Core loss* CoreShorting

*Malfunctioning of Cooling System* Hot Spot (Local overheat)* Arcing

DIAGNOSTIC TOOLS FORTRANSFORMERS

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(continue

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d)Ch