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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
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Contents1. Fundamentals
2. Construction
3. Basic Requirements
4. Types of Transformers
5. Components and Performance Parameters
6. Key Design Stages
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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
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- 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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Transformer Assembly
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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
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( 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
( Source: Dietrich, Transformatoren )
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
( Source: Dietrich, Transformatoren )
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
( Source: Dietrich, Transformatoren )
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
( Source: Dietrich, Transformatoren )
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.
( Source: Dietrich, Transformatoren )
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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( Source: EHV Weidmann )
Finite Element Mesh
Core
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( Source: EHV Weidmann )
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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( 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.
( Source: Dietrich, Transformatoren )
P.U. ImpulseVoltage
Relative winding length
Lightning impulse overvoltage
1.0 P.U.Impulse
Voltage
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Reactance between windings
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( Source: Karsei, Kereny, Kiss, Large Power Transformers )
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Winding arrangements
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( Source: Karsai, Kereny, Kiss,
Large Power Transformers )
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Equivalent Circuit
( Source: Karsai, Kereny, Kiss,
Large Power Transformers )