1-Design Concepts and Specification - Betancourt

7/28/2019 1-Design Concepts and Specification - Betancourt

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Powering reliable solutions for you Proprietary information

Design Concepts and Specification

Enrique Betancourt R.

Powering reliable solutions for you

Transformers Technology and DiagnosticsSeminar

Prolec GE - WEIDMANN

May 2013 Monterrey, NL

Copyright Prolec GE Internacional


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Contents1. Fundamentals

2. Construction

3. Basic Requirements

4. Types of Transformers

5. Components and Performance Parameters

6. Key Design Stages

Copyright Prolec GE InternacionalE.Betancourt R.


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1. Fundamentals



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Definition and Principle of Operation

ELECTRICAL TRANSFORMER:

DEVICE WITH NO CONTINUOUSLY-MOVING PARTS, THAT BY

MEANS OF ELECTROMAGNETIC INDUCTION, TRANSFERS

ELECTRICAL ENERGY BETWEEN TWO CIRCUITS AT,

GENERALLY, DIFFERENT VOLTAGE BETWEEN TERMINALS.

a

c

N1V E

2

i1

v1 v2

i2

N1 N2

Magnetic

Field

(Flux) Electric

Current

(NxI)

WindingNo. 1

WindingNo. 2

Ferromagnetic

Core

Flux



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1-phase pad-mount

1-phase pole-mount

3-phase primarysubstation

xfmr

3-phase pad-mount (CPAD) network

secondary substationtransformer (SST)- liquid- vent dry

- cast coil

3-phasegeneratorstep-up

Network Autotransformer

500 kV20 kV

115 kV

13.2 kV

600-127 V

220-127 V

220-127 V

The Role of Transformers in the Grid



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2. Construction



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Basic Construction of a Power Transformer (Core type, >7.5MVA)

- Silicon steel laminations

- Stepped to fit round

section

- Vertical Legs, horizontal

Yokes

- Size impacts tank height

and length



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- Clamp and isolate the core

- Core grounded at a single point

- Cooling ducts to avoid hot spots

Core Insulation



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Winding Packages

- Each phase-package has

primary and secondary

windings

- Cylindrical shape provides

high mechanical strength

- Inner LV Wdg. outer HV Wdg.

- Oil enters cool at bottom and

leaves hot at top

- High strength rings axially

clamp the windings



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- Tie together the core laminations

- Provide support for coil

clamping and lead structures

- Large size units require

insulated clamps

- Must withstand handling,

shipping and short circuit forces

Frames and Tank Attachments



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- Provide safe dielectric

clearances for winding leads

- Must withstand shipping and

short circuit forces

- Provide support for NLTC andother components

Lead Structures



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Leads

- Rated for maximum operation

and test currents

- Insulation according to test

voltage and clearances

- Hot spot below winding hot spot

- Brazed or crimped joints

- For high currents, magnetic

clearances important



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Bushings

- Allow pass of HV leads

through grounded tank cover

or walls

- Most from procelain, some

polymeric

- LV solid, HV condenser type

- Connector area according to

maximum current

- Normally mounted in turrets

with current transformers

- Most oil-air type



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Tank Bottom


- Robust for lifting and

transportation

- Flat or with stiffeners

- Attachment points for

seismic forces

- Inner attachments for

core and coils


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Tank (w/o cover)

- Structural, low carbon steel

(mainly)

- Keeps core and coils oil

immersed, clean and free from

moisture

- Welds and gaskets must be

leak proof

- It withstands vacuum

processing, handling,

shipping, operating pressure

and seismic forces

- Reacts with magnetic leakage

flux to produce stray losses



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Transformer Assembly



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Tank Cover

- Welded to the walls

- Non-magnetic steel inserts

- Holds main and auxiliary

bushings, CTs and pressure

relays

- For conservator type, conveys

bubbles quickly to gas relay



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Tank Radiators

- Attached with valves and

flanges to the walls

- Heat exchangers made of

soft steel panels

- Fans improve heat transferrate

- Single or common headers

convey oil out of (top) and

back in (bottom) the tank



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Conservator Tank



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Bushings and Arresters



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3. Basic Requirements



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Normal Spec

Type (conventional 2W, 3W, Auto, GSU)

Load Rating (base and extended), MVA

Rated Voltages (HV, LV, TV), kV

Winding Connection (Y, Delta, Z)

Temperature Rise (65oC, 55oC)

Impedance (voltagedrop)

Ambient Temperature (30oC Avg., 40oC mx)

Core overexcitation 110% no load, 105% at full load

Tolerances according to ANSI-IEEE

Technical Requirements



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Core overexcitation

Overloads with/without loss of life

Impedances for more than two windings

Impedance swing within taps range

Extreme ambient temperatures (55oC-50oC)Short circuit with overvoltage

Corrosive operating ambient

Frequent short circuits, switching or lightning

Reduced tolerances (impedance, losses, ratio)

Special Requriements



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Total owningcost (initial cost + losses)

Cost of transportation and erection on site

Performance as specified

Service reliability

On time delivery on site

Cycle time for delivery of drawings

Criticals to Quality (CTQs)



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Balanced cost of losses (no-load and Load) vs. materials cost

Cooling (Normal and Emergency Load)

Exact estimation of hot spot temperatures

Limit thermal degradation of cellulose and oil

Limit thermal surface load (mW/mm2)

Avoid excessive gas generation

Overvoltage Endurance (Impulse, Switching)

Limit electrical stress in oil

Exact calculation of voltage distribution

Mechanical Withstand (Short Circuit, Vacuum, Shipping)

Exact calculation of forces and stresses

Estimation of impact strength of materials

Design Challenges



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4. Types of Transformers



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Number of PhasesThree PhaseSingle Phase

Type of CoreCore typeShell type

Main Cooling MediumOil (air/water)

Air (air/water)

Synthetic Fluid

Application in the Power SystemSubstation step downGenerator step-up

Autotransformador (inter tie)RegulatorDistributionIndustrials

Number of Main WindingsTwo windingMultiwinding (usually three)

Transformation RatioOff circuit Taps (10% Range)On Load Tap Changer (20% Range)No taps

b/2

bb/2 1

2

3

3

2

1

bb/2

b/2

Transformer Classifications


( Source: Dietrich, Transformatoren )


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X1 XoX3X2

H1 H3H2

Wye Connection Delta Connection

Three-Phase Winding Connections

Higher voltages

Neutral can be grounded

Higher currents

Capacitively referred toground



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Autotransformer Connection

Throughput power:

VH * IH = VX * IX

Converted power:

VX * IC

Converted / Throughput:

= 1 VX / VH

= NS / (NS + NC)

Lower cost than equivalent transformer

Same grounding H and L sides (galvanic coupling)

Y-Y Connection

Low impedance, high short circuit forces

Lower benefits as VX


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Three Phase Transformer Single Phase Transformer

in a 3-Phase Bank



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Fundamental Design Equation

Zi

i

N

Zi

i

R

N ddt

AC voltage excitation of a ferromagnetic core

V(t) = Vmax* sin(t) = Z*i(t) + N d(t)/dt y N (d(t)/dt)If Z*i(t)


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The ferromagnetic core builds a magnetic circuit

The flux is the same in every section of a single loop

N1

N2

N3

1

1

2

3

2

3

Three winding transformer

Open circuit voltages:

11

44.4 NfV

22

44.4 NfV

33

44.4 NfV

3

3

2

21

N

V

N

V

N

V

Turns Ratio

1



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Exciting Current of Power Transformers

Exciting Current

e Induced Voltage Magnetic Fluxi Exciting Currentim Magnetization Currentic Exciting Loss Current

e

i

im

ic

e e

Non sinusoidal waveshape



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Harmonics in Exciting Current

Phase shift between harmonics ina three phase system

Main harmonics in Exciting Current

I1m Fundamental frequency component(50/60 Hz).

Im3 3rd

harmonic componentIm5 5th harmonic component

3rd harmonic is dominant, and cancels in 3 phase systems.



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Core Type (3P)

Shop assembly of a shelltype core (1P)*

* Courtesy: Tramosa, Monterrey Repair Shop

Core and Shell Types



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20 12 30 32

20 12 30 32

FM

SPLIT

FM

COM

POUND

FM

SOLID

SM

SOLID

0

Core lamination

cuts:

Cycle?

Stock?

Losses?

Noise?

Scrap?

Hot spots?

Width?

Types of Cores



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Flux concentration in the

gaps at jointsStep lapped joints

A B C

Lower Yoke

Upper Yoke


Core Joints

5 0,5 mm

a) Low flux density

b) Higher flux density



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Core Loss

Magnetic flux concentration,

driving hot spot at the Tjoint

Magnetic flux distribution in a three

limb core

0.6 0.8 1 1.2 1.4 1.6 1.8 2T0

0.4

0.2

0.6

0.8

1

1.2

1.4

1.6

1.8

1.5 1.7

P

4

2

6

8

10

12

14

0

16

18

S 1.5= 1..17 VA/kg

P 1.5= 0..87 W / kg

P

S

S

VA / Kg


Excitation losses (NLL, Core loss)

B

W / Kg



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Components of Core LossHysteresis Loss:

H

B

B5

Br

Hc

Core Eddy-Current Loss:

The laminated core exhibits strongly reduced eddy current losses, because each path ha s lowinduced volta g e and high electrical resistance .

W = H dB [ w/m 3 ]

W/cycle =f

H dB

~ Area within hysteresis loop

Area = f (Bmax)Hysteresis Loss :

PH = Volume * f *c * Bmax

e

e = 1.5 . . . 2.0 Experimentalc, e material con stant s

Concentricloops

0 sin wt 0 sin wt

In each loop :

eind ~

d

dt

icirc. ~ ength

eind

l

Peddy~ ength

eind

l

2

Solid Core Lam ina ted Core

Cancellation of

circulating

currents in alaminated core.



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50

60

70

80

90

110

100

%

Material

Losses

Thickness

Standard

Hi-B

Laser

Future

Trend in core loss for cold laminated steels.

0.35 0.23 500.270.3 0.2mm 0.6 0.8 1.21.0 1.4 1.6 1.8 2.0T

0.03

0.01

0.02

0.05

0.1

0.2

0.3

0.5

2.0

1.0

Material

Losses

W/kg

Flux Density

1

2

Comparison Hi-B vs. Amorphous Metal1.- Hi-B silicon steel

2.- Amorphous metal


Core material losses



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Comparison HiB vs.

Conventional oriented

grain steel

Other key variables for the Core:

Mechanical strength and stability

Grounding

Clamping pressure

Volts per turn

Gaps at the joints

0.60.8 1 1.2 1.4

1.61.8

2

Tesla

0

0.4

0.2

0.6

0.8

1

1.2

1.4

1.6

1.8

1.5 1.7

W/kg


Excitation VA

Hi B

Conventional



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Types of Conductors

1) Rectangular conductor with paper insulation.

2) Twin conductor.

3) Continuously Transposed Conductor (CTC).

1)

3)

2)

1) 2) 3)

4) 5)

Layer and Disk Windings

1) Layer Wdg. (double layer).

2) Continuous Disk Wdg.

3) Sequential Disk Wdg.

4) Pair of continuous disk sections.

5) Pair of interleaved disk sections.


Conductors and Windings



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Helical (single, doble, triple)Layers

Tapping Winding

Barrel with taps

Multistart

Disks

Continuous

Intershield

Interleaved

Layers

Conventional

Special arrangements

Types of Windings



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( Source: Transformatoren, Dietrich )

Insulation Assemblies



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( Source: EHV Weidmann )

Field Plots for Insulation Design

HV Wdg.

A

HV Wdg.

B

LV Wdg.

A

Core



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Finite Element Mesh

Core



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Equipotential Lines

Core

LV Wdg.

A

HV Wdg.

A

HV Wdg.

B



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DESIGN CONSIDERATIONS

Materials

Dielectric and magnetic clearances

Short circuit and transportation forces

Assembly process (factory and field)

Temporary assembly elements

Operating temperature and heat run

overloads

Magnetic balance

Current share in parallel circuits

Lead structures



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DESIGN CONSIDERATIONS

Structural steel

Low temperature operation

Seismic withstand

Shipping lugs

Welds for accessories

Location of accessories

Non magnetic inserts

Shipping detachablecomponents

Gas collecting pipework

Gaskets

Main Tank



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Main materials

Insulating FluidPaper and pressboard

Synthetic paper andpressboard

Wood (natural and synthetic)

High strength plastics

Glue and adhesives

Enamels

Requirements

Dielectric withstandMechanical strength

Temperature index

Process resistance (VP,vacuum, oil)

Long-term chemical stability

Insulating materials

Courtesy of Weidmann Electrical Technology



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6. Key Design Stages

Cooling

Mechanical forcesDielectric stress



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Natural circulation of oil through the windings:

Natural air cooling

ONAN

Forced Air Circulation

ONAF

Transformer Cooling Circuit



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Forced oil circulation ODAF, OFAF

ODAF Forced flow through the windings

OFAF Forced flow into the tank, only

Pump

Fans

Radiators

Transformer cooling circuit



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Cooling of WindingsTemp

Wdg.

Depth

Gradient

Cu-OilOilFlow

DOF Washerdeflectores

BarrierBarrier

Spacer

Radial duct

Axial Duct

Conductors

OilConductor

Axial Cut View Section (top)

View

Tamb HSTTOT

Cooling of windings



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60Magnetic Field and Resultant Forces

( Fuente: Dietrich, Transformatoren )

Short circuit withstand

Fa FaLV HV

LV HV

N



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Outer Winding Inner Winding

Center

Line

Top view of phase

package

Lateral section view of

phase package

Force vs. time

X

CoreX

FR

ISC

FA

SC forces in core type transformers



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Spacers

Conductors

Conductors

Axial Forces

Spacers

Axial effect of SC forces



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Short circuit endurance is fundamental for reliable operation

SC currents in power systems grow with interconnection

Standardized tests guarantee SC endurance under controlled conditions,

but, for how many more years? How many short circuit events?

Most insulating materials undergo degradation after long service

DGA and conventional testing do not guarantee timely detection of

mechanical weakness, to avoid catastrophic failures good, proven

design and manufacturing practice is a MUST

New techniques, as on line mechanical vibration monitoring, and off-line

SFRA promise better ability to detect incipient degradation

Short circuit strength and reliability



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Hydraulic

Jacks clampthe windings

Isostatic winding sizing



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Types of Disk Windings

Continuous Disk

Winding

c1 c1

c1

c1

c1

c1

c2

c2

c2

c2

c1

c1

c1

c1

Interleaved

Windings

C1 is high

Impulse withstand



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Impulso deTensin

Cg

Cg

Cg

Cs

Cs

Cs

V1

V2

V3

V

t

V1 > V2 > V3 . . . .

Lightning

Impulse

Non linear voltage distribution



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H

h

CORE WINDING

t

V

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,00

0,2

0,4

0,6

0,8

1,0

db

ca

h/H

InitialVoltage Distribution in Disk Windings

a.- Continuous disk winding.

b.- Interleaved disk winding.

c. - Partially interleaved winding.

d.- Final voltage distribution.


P.U. ImpulseVoltage

Relative winding length

Lightning impulse overvoltage

1.0 P.U.Impulse

Voltage



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Reactance between windings


( Source: Karsei, Kereny, Kiss, Large Power Transformers )


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Winding arrangements


( Source: Karsai, Kereny, Kiss,

Large Power Transformers )


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Equivalent Circuit

( Source: Karsai, Kereny, Kiss,

Large Power Transformers )

Documents

1-Design Concepts and Specification - Betancourt