7110047 design of power transformers

8/3/2019 7110047 Design of Power Transformers

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EHV Power Transformers

J S Sastry

GM (EHV Transformers)

Vijai Electricals


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Topics to be covered What is a Transformer

Transformer principle

Basic design concepts Construction features

Materials used and improvements

Types of failures and diagnostics Challenges to Transformer Designers

Reactors


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IFYOU HAVE NO DOUBTS

YOU HAVE NOT

UNDERSTOOD ANYTHING.

- a Spanish proverb.


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What is a Transformer A transformer is a static equipment

which transformers power from onecircuit to another by stepping up or downthe primary voltage without any changein the frequency.

The two circuits may or may not beconnected but are magnetically coupled.


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F

eatures of Power Transformers Single Phase Three phase

Star or Delta connected Primary Star or Delta connected Secondary With or without Tertiary winding

Provided with Off-circuit tap switch orOn-load Tap Changer for voltageregulation


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Transformers in Network


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Transformer Operating Principle


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Operating PrinciplesEquivalent Circuit


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Specification Parametersfor Generator Transformer


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Specification

Parameters

for Network

Transformer


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Design Principles - Core


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Design Principles -Core

Higher the number of steps in cross section, better is spaceutilization and smaller is the core diameter.90 to 95 % utilization factor is optimal.

Core area (A) is determined by the Flux Density (B) which inturn depends on many factors - like loss capitalization andoverall design economics.

As the no load losses attract very high capitalization,

attempts are continuously made to reduce them.


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Design Principles - Core

Improved 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 are

available in which the specific core losses arelower than normal grades.

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


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Design PrinciplesWindings- L.V winding

L.V Windings in Transformers are either Spiral wound for for low current ratings

Helical Wound with radial cooling ducts

for higher ratings.The conductor used is paper insulated

rectangular copper (PICC)

For higher currents, transposed conductors are

used, to uniformly distribute the current acrossthe cross section of the wire of coil.


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Design Principles- L.V winding


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Design PrinciplesH.V Winding/1

HV winding is invariably uses PICC or CTC.

Type of winding used is

- Disc winding up to 132 kV- Layer winding or- Interleaved winding or- Rib shielded winding


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Cg

Cs = K Cg/Cs

Design PrinciplesImpulse Voltage Distribution


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Design PrinciplesImpulse Voltage Distribution

= 0

= 1 0

=5

X

V


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Windings - Interleaved winding


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Windings - Rib Shield Winding


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Design PrinciplesTertiary Winding/1

In Star-Star Connected Transformers and Auto-transformers, Tertiary Winding is used-

- to stabilize phase to phase voltages in case ofunbalanced load

- Suppressing third harmonic currents in earthedneutral

- reducing zero sequence reactance- for supplying auxiliary load or for connecting

capacitors.


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Design PrinciplesTertiary Winding/2

Tertiary is required to be designed for apower rating equal to one-third the ratedpower, it increases the cost of the

transformer by 10- 12 percent.Tertiary winding is known to fail due to

transferred surges and Short circuits

Present practice is to do away withtertiary up to 100 MVA for 3 phase 3limbed core transformers.


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Design PrinciplesCooling

Most Common Cooling Methods Symbol

Natural Circulation of Air and Oil ONAN

Natural Oil Circulation and forcedAir Circulation ONAF

Forced Oil and Forced Air Circulation OFAF

Forced Air Circulation and DirectedOil Flow through Windings ODAF


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Oil circulates due to gravity and thermal head.


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Loading and cooling 2

Effect of cooling improved by fans.


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Effect of cooling improved by pumps.


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By canalizing oil in the radial ducts, effect of cooling furtherimproves. Velocity of oil in windings increases several times.


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Design Basis - Capitalization

CBIP Capitalization Formula:

Capitalized Cost = Initial Cost (IC) + Capitalized{ No-load Loss (N) + Load Loss (L) + AuxiliaryLosses (A) }

Capitalized Cost = IC + Cn.N + Cl.L + Ca.A

Factors affecting Cn; Cl and & Ca

Rate ofInterestRate of Electrical EnergyLife of Transformer


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Improvements in Materials

Transposed Conductors

Transposed conductors (CTC) are used to improve current distribution in thecross section of the winding wire.Individual cable can be coated with epoxy so that the cured and finishedconductor is mechanically stronger and withstand short circuit forces better.


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Improvements in materials

Insulating Paper

Insulating Paper continues to be Cellulose

Kraft Paper (Temperature Class 135C)

Thermally upgraded paper ( Nomex , Aramid155) is used in hot spot regionsin transformers with space constraints, like

Traction Transformers.


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Improvements in Materials

Insulating Oils

Paraffin T.O.B.S Based transformer oil is used inalmost all Transformers in India. Naphtha basedtransformer oils are available.

Silicone Fluids are used in regions where firehazard is present- e.g., E.S.P Transformers.

Silicones are low viscosity fluids and require specialprocessing plant- besides new types of gaskets andcomponents.


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Improvements in MaterialsSealed Conservators


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PRESSURERELIEF

DEVICE


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SRBP

OIP

RI

P

BUSHI

NGS


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Poor Quality of Processes

Design orProduction or

Maintenance

results in


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Types of failures

Infant failures: Early life failures are the result oflatent or delivered defects.- Latent defects are abnormalities that cause failure,

depending on degree of abnormality and amount ofapplied stress.

- Delivered defects are those that escape test /inspection within the factory

- They are directly proportional to total defects inthe entire processes.


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Types of failures

Mid life failures: These are results of

-F

reak system disturbances- Wrong specifications

- Poor maintenance


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Types of failures

Old age failures: These are results of

- Ageing of insulation system- Wear & tear


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DEFECT ELIMINATION

METHODS

FMEA

FTA


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TRANSFORMER

END FRAMES

CORE

INSULATION

WINDINGS

TANK

OIL

RAW MATERIAL

FABRICATION

FITTINGS

MAJOR ASSEMBLIES COMPONENT ELEMENT


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Windings

Terminals

Tank & DielctricFluid

OnloadTapchanger

Magnetic Circuit

OtherAccessories

COMPONENTS CAUSING FAILURE IN

SERVICE (29%)

(29%)

(13%)

(13%)

(11%)

(5%)


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Electrical

* Over-voltages* Part winding resonance* Partial Discharge* Arcing* Magnetizing in-rush current* Over-fluxing

MAIN FACTORS CAUSING STRESSES IN

THE WINDING


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Mechanical * Core Vibration* Force due to Short Circuit or Faults

Thermal * Winding Temperature* Core loss* Core Shorting

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

MAIN FACTORS CAUSING STRESS (continued)


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Hot Spot Temperature

Tan Delta and Capacitance Insulation Resistance

Moisture and Polarization Index

Partial Discharge Detection

Frequency Response Analysis

DIAGNOSTIC TOOLS FOR

TRANSFORMERS


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Dissolved Gas Analysis (DGA) Degree of Polymerization

Furfural in Oil

Particles/fibers in Oil

DIAGNOSTIC TOOLS FOR

TRANSFORMERS (continued)


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Challenges in Transformer

Design & Manufacturing

Structures design (tank etc.) To be designed for: Lifting & Jacking

Full or partial vacuumInternal PressureSeismic Load

Tests conducted: Leakage test

Vacuum testRadiography (if specified)DPT on load bearing items


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Short-circuit withstand capability

Adequate radial supports Use of pre-compressed press-board to minimize

shrinkage in service Proper stabilization of coils Use of glued conductors Springs or hydraulic dampers if required




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Stray losses :

Stray losses due to linkage of high magnitudeof flux with magnetic materials

Stray losses form a large part, more than 20%of total load losses

These may cause hot spots


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Stray losses control :

Measures for stray loss control

Use of laminated material By breaking the magnetic path

By providing non-magnetic shield

By providing parallel low reluctancemagnetic path




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High Voltage stresses

Design ofInsulation system to ensure withstand

capability for Lightning Impulse and Switching Surges.

Long duration high voltage system disturbances

Internal Partial Discharges




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This is done by - Choosing type of windings Calculation/plotting of impulse / switching

surges and long duration voltage stressdistribution

Provision of adequate major and minorinsulation by using angle rings, mouldedcomponents etc.

Corona shielding where required




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Reactors

Series ReactorsFor limiting fault currents

Smoothing ReactorsFor filtration of harmonics

Shunt ReactorsFor capacitive VARs compensation.


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Reactors

Series Reactors:Used for limiting fault currentsConnected in series with generators or feeders and

transmission lines.

Capability for short-time high currentsThey should have linear magnetic characteristics underfault conditions.

They are designed to withstand mechanical and thermaleffects of short circuits.

They have fully insulated windings since both endsshould be able to withstand the lightning impulsevoltages.


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Reactors

Smoothing Reactors: For filtration of harmonics

Dry type or oil filled Shunt Reactors:

Used for capacitive VARs compensation.

Should have constant impedance upto 1.3times rated voltage


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Inductance of a Coil

Inductance of a coil is calculated by theformula

Inductance = K x Mean dia of coil xSquare of number of turns. Factor K depends on the physical

dimensions of the coil Coil height Radial height


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General

considerations/characteristics The reactor should have linear characteristics

Ability to withstand short circuit

Fully insulated windings in case of Series Reactors

Capability for short-time high currents

Shunt reactors:

Single or three phase unit: Large rated reactors sometimes poses problems intesting. To over come this, 3 single phase units are most suitable. Further, inthe case ofbank, zero sequence impedance is equal to positive sequenceimpedance.

Requirement of zero sequence impedance is dependent on systemconsiderations. If this is 90 to 100% of positive phase impedance, corelessdesign with magnetically shielded 5-limbed gapped core reactor is mostsuitable. If this value is not very important or a value of about 50 to 60%, 3-limbe gapped core reactor may be used.


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Gapped core reactor: To achieve high impedance of reactor in a coretype construction, gaps having suitable size are inserted in themagnetic circuit. The required effective length of the magnetic shieldis mainly dependent on the dimension of the distributed gaps in the

iron core and is largely dependent on the winding height. The magneticfield of the gapped core is controlled by means of gaps. Comparedwith magnetically shielded coreless reactors, gapped core reactors canbe operated at higher flux density, the selection of which is dependentupon the requirement of linear characteristic. Conventional type,involute disc type and radially arranged moulded types are normallyemployed to make the core elements. In modern reactors, moulded

type radial core sections moulded in epoxy resin to prevent movementbetween individual laminations are commonly used. The radiallaminations prevent fringing flux from entering flat surfaces of coresteel, which would result in eddy current overheating and hot spots


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Constructional features

Oil immersed

Dry type

Coreless

Gapped core

With or without non-magnetic shields

With or without magnetic shields


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Reactor Construction

Coreless Construction: In a coreless reactor, there is no magnetic

material inside the coil Gapped Construction:

To achieve high impedance of reactor in acore type construction, gaps having suitable

size are inserted in the magnetic circuit. Core can be conventional or involute disc

type or radially arranged moulded type.


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7110047 design of power transformers

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