ABB Brazil JCM A-1 Power Products Division Power Transformers ABB Brazil Power Products Division Dr. José Carlos Mendes Transmission and Power Products Technology Mng Development and Engineering Mng Corporative Executive Engineer ABB Asea Brown Boveri Power Products Division Power Transformers São Paulo, SP - Brazil email: [email protected]tel: +55 11 2464 8410 cel: +55 11 9 8354 5358 High Voltage Power Transformers: Short-Circuit – Stress, Strength, Design, Testing, Advanced Technologies and Recommendations
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ABB Brazil JCM A-1Power Products DivisionPower Transformers
ABB BrazilPower Products Division
Dr. José Carlos MendesTransmission and Power Products Technology MngDevelopment and Engineering MngCorporative Executive Engineer
ABB Asea Brown BoveriPower Products DivisionPower TransformersSão Paulo, SP - Brazil
High Voltage Power Transformers:Short-Circuit – Stress, Strength, Design, Testing, Advanced Technologies and Recommendations
ABB Brazil JCM A-2Power Products DivisionPower Transformers
Content
introductionshort-circuit in power transformers
overviewfault types and severityreliability and failure ratesinternational standards
short-circuit performancethermal and mechanical capabilitiesshort-circuit current characterization and dynamic effectshort-circuit forces, failure modes, tap changers position, aging effectsresidual pressing forces
short-circuit and power transformer designdesign steps (fault current; SC forces; SC stresses; design criteria)experiences
short-circuit full testtest needs, results and failure ratesreference list
advanced materials and technologies conclusionsrecommendations
technical specification – Tender and Designperformance verification – Design Review and/or Full SC Test
Short-Circuit in Power Transformers
ABB Brazil JCM A-3Power Products DivisionPower Transformers
Guarulhos, São Paulo - BR
ABB Brazil Power Products Division
Power Transformers
Blumenau, SC - BR
PPTR Power Transformerspower transformers up to 765kVshunt reactors up to 765kVheavy current industrial transformers
service (Eng Solution, Factory and Site Repairs, Monitoring Systems, TrServices)insulation componentstransformer components (Bushings, Tap Changers, etc)
PPMV Medium Voltage, PPHV High Voltage
PPTR Distribution Transformers
ABB Brazil PP Division Power Transformers
ABB Brazil JCM A-4Power Products DivisionPower Transformers
Power Transformer ImportancePower Transformer ImportanceContext
Power Transformer
PT CT Capacitor
Power Transformer: 50%...70% Substation Cost
Phase-Shifter Transf
Circuit Breaker
HV Switche
Surge Arrester
ABB Brazil JCM A-5Power Products DivisionPower Transformers
SE Tijuco Preto - TerminalHVAC ITAIPU - São Paulo
765kV yard
345kV yard
550kV yard
345kV capacitor banks
765/345kVbanks Autos 1500MVA
765/550kVbanks Autos 1650MVA
Electric Power Existing Infra Structure AgingElectric Power Existing Infra Structure Aging
Scenarios
FURNAS SE Tijuco Preto São PauloITAIPU HVAC 765 kV Transmission System
ABB Brazil JCM A-6Power Products DivisionPower Transformers
High Voltage Power Transformers:Short Circuit - Overall
ABB Brazil JCM A-7Power Products DivisionPower Transformers
Economic environment can affect power transformers design and performance:there are now more strong temptations to save active material
there are now more strong temptations to go closer to mechanical limits
present tenders comparison process may be weak to compare short-circuit performance
Industry Standards to check the short-circuit mechanical strength and integrity of Power Transformers:
existing standards, as an example IEC60076-5 3rd Ed 2006-02, establishes recommendations as
a) Short-Circuit tests; or
b) Design Review evaluation/verification (IEC proposes typical allowable, critical stresses)
OverviewOverview
Short-Circuit in Power Transformers
ABB Brazil JCM A-8Power Products DivisionPower Transformers
Fault OccurrenceFault Occurrence
Utility experience
line faults statistics more frequent than substation faultshighest number of faults in systems rated up to 100kVsingle-phase faults are the most frequent >65% of all faults, typically due to lightning strokes
line faults 1ph, 2ph, 3ph not so severe if far from transformersubstation faults 1ph, 2ph, 3ph are severe cases, with highest fault current, not frequentWorst case:3 phases short-circuit caused by forgotten safety grounding devices after maintenance…
Short-Circuit in Power Transformers
ABB Brazil JCM A-9Power Products DivisionPower Transformers
Useful Life and ReliabilityUseful Life and Reliability
Short-Circuit in Power Transformers
time, years
Failu
re R
ate
infa
nt fa
ilure
s
agin
g fa
ilure
s
normal failures
2-6years
40-60yrs
Acceptable Failure Rate
new end of lifenormal life
life extension
70-80yrs
20-30yrs
ABB Brazil JCM A-10Power Products DivisionPower Transformers
ABB Brazil JCM A-11Power Products DivisionPower Transformers
Failure - Step Up Transformer 3 , 440MVA, 16.5-16.5/500kVFailure - Step Up Transformer 3 , 440MVA, 16.5-16.5/500kV
Short-Circuit in Power Transformers
46 kA
500 GVA
28kA
1.8 kA
GCB Failurephase-to-ground fault
LV1
LV2HV
network
ABB Brazil JCM A-12Power Products DivisionPower Transformers
Failure - Regulating Step-Down Transformer 3 , 60MVA, 230/13.8kV, OLTC at HV sideFailure - Regulating Step-Down Transformer 3 , 60MVA, 230/13.8kV, OLTC at HV side
External Short-Circuit at LV SideFailure in the HV Regulating Winding HVr
Short-Circuit in Power Transformers
ABB Brazil JCM A-13Power Products DivisionPower Transformers
IEC60076-5 3rd Ed 2006-02 Power Transformers Ability to Withstand Short-CircuitIEC60076-5 3rd Ed 2006-02 Power Transformers Ability to Withstand Short-Circuit
Characteristics:overall revisionincludes the alternative to prove the power transformer short-circuit withstand by calculation method based on similar transformer testinggives calculation guidance and criteria
Short-Circuit in Power Transformers
ABB Brazil JCM A-14Power Products DivisionPower Transformers
IEEE C57.12.00-2010 Specification and IEEE C57.12.90-2010 Test CodeIEEE C57.12.00-2010 Specification and IEEE C57.12.90-2010 Test Code
C57.12.90 section 12 – established proceduresfor the short-circuit test of oil immersed powertransformers, including approval criteria anddiagnostics methods
Short-Circuit in Power Transformers
ABB Brazil JCM A-15Power Products DivisionPower Transformers
High Voltage Power Transformers:Short Circuit – Thermal and Mechanical Capabilities
ABB Brazil JCM A-16Power Products DivisionPower Transformers
GeneralGeneral
External short-circuits events include:3-phase short-circuit2-phase isolated short-circuit2-phase to ground short-circuit1-phase to ground short-circuit
faults on any one set of terminals at a time.
Power Transformers must be designed and built to withstand without damages:mechanical stresses; andthermal stresses
produced by external short-circuits events.
Overcurrent:symmetric component (rms) of short-circuit fault currentasymmetric component (peak) of short-circuit fault current
Short-Circuit in Power Transformers
ABB Brazil JCM A-17Power Products DivisionPower Transformers
High Voltage Power Transformers:Short Circuit – Current and Dynamic Effect
ABB Brazil JCM A-18Power Products DivisionPower Transformers
Short-Circuit CurrentShort-Circuit Current
Under an external short-circuit event:
the first peak of the fault current over the transformer will increase to a multiple of the rated current.
The short-circuit fault current will depend on:
pre-fault open circuit voltagesource and transformer impedanceinstant of the fault onset (initial phase angle)
Short-Circuit in Power Transformers
R L
u(t)=Û.sen( t+ )
i(t) short-circuit
ABB Brazil JCM A-19Power Products DivisionPower Transformers
Short-Circuit CurrentShort-Circuit Current
Short-Circuit in Power Transformers
1st peak of the instantaneousshort-circuit current
RX
ABB Brazil JCM A-20Power Products DivisionPower Transformers
Short-Circuit CurrentShort-Circuit Current
Dynamic Effect
Short-Circuit in Power Transformers
= 0 deg R X F X/R Kx1,410,3 10 60 33 0 2,70
k. 2
= 90 deg
= 90 deg R X F X/R Kx1,410,3 10 60 33 90 1,39
k. 2
= 90 deg
ABB Brazil JCM A-21Power Products DivisionPower Transformers
High Voltage Power Transformers:Short Circuit – Electromagnetic Dynamic Forces
ABB Brazil JCM A-22Power Products DivisionPower Transformers
short-circuit pass through over current (Ik, rms and Imax, peak)short-circuit current Ik establishes a Leakage Magnetic Field Hk= f (Ik) Ampere Law
in a conductor with a short-circuit current Ik immersed in a leakage magnetic flux Hk is established a electromagnetic force Fk = f ( Ik x Hk ) Biot-Savat Law
means that the short-circuit Force is proportional to the square of the short-circuit-currentIk Hk = f (Ik) Fk = f (Ik ;Hk) Fk = f (Ik2)
Short-Circuit in Power Transformers
Short-Circuit ForcesShort-Circuit Forces
Dynamic Effect
in a transformer design the short-circuit current Dynamic Effect is considered:peak value of the asymmetric current (Imax, peak)instantaneous value of the voltage is at its zero value ( = 0 deg and = 90 deg)asymmetry factor k = f(R/X)peak factor kp = 2 . k
means that the short-circuit Dynamic Force is proportional to the square of the peak value of the asymmetric component of the short-circuit current
Fd = f (Imax2) Fd = f (( 2. k . Ik)2)Ik Ikmax = 2 . Ik Imax = k. 2 . Ik
ABB Brazil JCM A-23Power Products DivisionPower Transformers
I1
F1(I2)
B1(I2)
B2(I1)
F2(I1)
I2
1 2d
dldl
Electromagnetic Forces: Two (2) Single ConductorsElectromagnetic Forces: Two (2) Single Conductors
F
IB
dF dq v B i d B B d i sen
F I I Id
N1 20 1 2
2
v ddt
i dqdt B H
H d IAmpere Law
Bio-Savat
I
B
Short-Circuit in Power Transformers
ABB Brazil JCM A-24Power Products DivisionPower Transformers
Electromagnetic Forces: Two (2) Winding – Magnetic Stray Leakage FluxElectromagnetic Forces: Two (2) Winding – Magnetic Stray Leakage Flux
Short-Circuit in Power Transformers
A Two Windings Transformer:
2D plotmagnetic stray field lines
Radial directionAxial direction
the magnetic stray flux has components in axial and in radial direction
there are field components outside the windings
AXIALdirection
RADIALdirection
RADIALdirection
ABB Brazil JCM A-25Power Products DivisionPower Transformers
Electromagnetic Forces: Two (2) Windings Electromagnetic Forces: Two (2) Windings
Short-Circuit in Power Transformers
The direction of forces:is always directed perpendicular to the magnetic field lines
Forces usually are split into the two componentsRadial forcesAxial forces
Electromagnetic forces tend to:reduce radius of inner winding (compression)increase radius of outer winding (tensile)reduce winding height (compression)increase the main insulation duct increase existing un-symmetries.
ABB Brazil JCM A-26Power Products DivisionPower Transformers
Force Components:F1 – single direction and constant forceF2 – single direction and exponentially damped forceF3 – un-damped alternated double power frequency forceF4 – damped alternated power frequency force
Short-Circuit in Power Transformers
Electromagnetic Forces and its Four (4) ComponentsElectromagnetic Forces and its Four (4) Components
F1 cont [kN]F2 expon [kN]F3 cos amort [kN]F4 cos [kN]Ftotal [kN]
time, sec
instantaneous Forceinstantaneous Forcepeak Forcepeak Force
ABB Brazil JCM A-27Power Products DivisionPower Transformers
Ba
comp. AXIAL
Br
comp. RADIAL
(I2)
I2I1
Br (I1)
Short-Circuit in Power Transformers
Electromagnetic Forces: Two (2) WindingElectromagnetic Forces: Two (2) Winding
IjBrj
BajFaj = f (Brj)
Frj = f (Baj)
Ii Bri
Bai
Fai = f (Bri)
Fri = f (Bai)
I1
Bn
I2 Zcarga
disp
F
IB AXIAL component
RADIAL component
load
leak
ABB Brazil JCM A-28Power Products DivisionPower Transformers
IjBrj
BajFaj = f (Brj)
Frj = f (Baj)
Ii Bri
Bai
Fai = f (Bri)
Fri = f (Bai)
F
IB
radrad
cond
FN A2
tensile stress and elongation of outer windingcompression stress and buckling of inner windingdeterioration of cellulose insulation
I1
Bn
I2 Zcarga
I k I2 2
2
k eRX1
current:
B N IHax 0
2 2
enr
F B2
N I D2rad
ax2 2
med
69.22100
55.22100
kMVATransf
kMVATransf
Short-Circuit in Power Transformers
Electromagnetic Forces in Winding: RadialElectromagnetic Forces in Winding: Radial
Fradial
load
ABB Brazil JCM A-29Power Products DivisionPower Transformers
I1
Bn
I2 Zcarga IjBrj
BajFaj = f (Brj)
Frj = f (Baj)
Ii Bri
Bai
Fai = f (Bri)
Fri = f (Bai)
F
IB
conductors tiltingwinding axial collapsetelescoping with axial displacement of conductorsaxial bending deflection of conductors between spacersinfluenced by winding asymmetriesdeterioration of cellulose insulation
lower upper winding end supportscore frame and winding pressing mechanical structure
Faxial
Short-Circuit in Power Transformers
Electromagnetic Forces in Winding: AxialElectromagnetic Forces in Winding: Axial
load
ABB Brazil JCM A-30Power Products DivisionPower Transformers
ABB Brazil JCM A-32Power Products DivisionPower Transformers
SPIRALING: tangential displacement of end turns of a Helical Winding
Radial Mechanical Force in a Helical Winding Exit:rotating reaction force of winding mass accelerationradial compression stress (bucking) of inner winding (N/mm2)radial tensile stress (elongation) of the outer winding (N/mm2)total cross section area of the winding exit (mm2)
Failure Modes:axial deformation and winding rotationmechanical instability of the winding
exit cross area: Ss [mm2]
s [N/mm2]
compression radial stress (buckling)
Fs = s . Ss [N]
InnerWinding
OuterWinding
Short-Circuit in Power Transformers
Radial Forces Failure Modes – Spiraling of Helical Winding ExitsRadial Forces Failure Modes – Spiraling of Helical Winding Exits
ABB Brazil JCM A-33Power Products DivisionPower Transformers
Axial Forces Failure Mode:axial collapse by excessive axial compressionwinding conductors mechanical instabilitytelescoping and axial displacement of conductorsconductors tiltingaxial bending of conductors and excessive deflectionsolid insulation rupture – spacers, end insulation, core clamps
Short-Circuit in Power Transformers
Axial Forces Failure Modes – Winding Conductors Compression and TiltingAxial Forces Failure Modes – Winding Conductors Compression and Tilting
ABB Brazil JCM A-34Power Products DivisionPower Transformers
Axial Collapse:axial collapse typical in layer windingsaxial displacement of winding turnsconductors elongationfree radial displacementrupture of conductors paper insulationturn-to-turn failure
Axial Bending:disc and helical type windingsexcessive axial bending of conductors damage and/or rupture of conductorsinsulation paper
Short-Circuit in Power Transformers
Axial Forces Failure Modes – Winding Compression and Axial CollapseAxial Forces Failure Modes – Winding Compression and Axial Collapse
ABB Brazil JCM A-35Power Products DivisionPower Transformers
Peripheral Displacement of Conductors and Supports:spiral compression of inner windingrupture of conductors insulation paperaxial misalignment of supports and mechanical instability
Distortion in Discontinuities:inadequate support in one direction leading to mechanical instability in the opposite direction
inadequate supportinadequate traction force and fixingspiraling of helical winding exits
Short-Circuit in Power Transformers
Axial and Radial Forces Failure Modes – Combined EffectsAxial and Radial Forces Failure Modes – Combined Effects
ABB Brazil JCM A-36Power Products DivisionPower Transformers
Tap Changers (on-load OLTC and no-load DETC) Position
magnetic stray flux distribution and intensity is depending on OLTC and/or DETC position
changing OLTC and/or DETC position changes short-circuit force amplitude
power transformer must be short-circuit designed for the most critical OLTC and/or DETC position (maximum force)
while in operation, transformer may operates under a more favorable OLTC and/or DETC position, reducing short-circuit forces and stresses
Short-Circuit in Power Transformers
Short-Circuit ForcesShort-Circuit Forces
ABB Brazil JCM A-37Power Products DivisionPower Transformers
Auto Transformer with OLTC at MV side and separate Regulating Winding
Leakage Flux500/253 kV - max
Leakage Flux500/207 kV - mín
Short-Circuit in Power Transformers
Short-Circuit ForcesShort-Circuit Forces
ABB Brazil JCM A-38Power Products DivisionPower Transformers
Forces case 500/253 KV
Steady state short circuit winding currentterminal H1: 2866 ASteady state short circuit winding currentterminal X1: 6886 A
Winding Force on Force on Compressive Strand stress top yoke bottom yoke force (max) (max) (kN) (kN) (kN) (MN/m2) ======= ============= ============= ============= ============= TERCREGUL 15.2 9.9 16.0 -8.3MT 0.0 188.9 1535.2 -46.7AT 0.0 70.7 2319.1 98.6
Forces case 500/207 KV
Steady state short circuit winding currentterminal H1: 2734 ASteady state short circuit winding currentterminal X1: 5376 A
Winding Force on Force on Compressive Strand stress top yoke bottom yoke force (max) (max) (kN) (kN) (kN) (MN/m2) ======= ============= ============= ============= ============= TERC 0.0 0.0 0.0 0.0REGUL 0.0 1.7 184.2 -9.3MT 0.0 81.4 1795.7 -46.5AT 0.0 78.6 1734.0 92.7
Leakage Flux - 500/253 kV - max
Leakage Flux - 500/207 kV - mín
Short-Circuit in Power Transformers
Auto Transformer with OLTC at MV side and separate Regulating Winding
Short-Circuit ForcesShort-Circuit Forces
ABB Brazil JCM A-39Power Products DivisionPower Transformers
Transformer with DETC in HV Winding with Taps in the Winding
Leakage Flux550/13.8 kV - max
Leakage Flux500/13.8 kV - min
Short-Circuit in Power Transformers
Short-Circuit ForcesShort-Circuit Forces
ABB Brazil JCM A-40Power Products DivisionPower Transformers
Forces case 550 / 13.8 KV
Steady state short circuit winding currentterminal H1: 3211 ASteady state short circuit winding currentterminal X1: 73954 A
Winding Force on Force on Compressive Strand stress top yoke bottom yoke force (max) (max) (kN) (kN) (kN) (MN/m2) ======= ============= ============= ============= ============= BT 0.0 128.4 3351 -90.1 AT 0.0 906.1 1608.9 113.9
Leakage Flux - 550/13.8 kV - max
Forces case 500 / 13.8 KV
Steady state short circuit winding currentterminal H1: 3457 ASteady state short circuit winding currentterminal X1: 72365 A
Winding Force on Force on Compressive Strand stress top yoke bottom yoke force (max) (max) (kN) (kN) (kN) (MN/m2) ======= ============= ============= ============= ============= BT 0.0 137.7 2578.8 -88.6 AT 51.7 947.5 3477.6 123.0
Leakage Flux - 500/13.8 kV - min
Short-Circuit in Power Transformers
Transformer with DETC in HV Winding with Taps
Short-Circuit ForcesShort-Circuit Forces
ABB Brazil JCM A-41Power Products DivisionPower Transformers
Auto Transformer SC Phase-to-Ground at MV Side and SC Three-Phase at TV Side
Auto Transformer 1Ph Short-Circuit at MV Side
500/230/13.8 kV
Auto Transformer 3Ph Short-Circuit at TV Side
500/230/13.8 kV
Short-Circuit in Power Transformers
Short-Circuit ForcesShort-Circuit Forces
ABB Brazil JCM A-42Power Products DivisionPower Transformers
CURTO CIRCUITO TRIFASICO NO TERCIARIO Y1
Steady state short circuit winding currentterminal H1: 106 ASteady state short circuit winding currentterminal X1: 1588 ASteady state short circuit winding currentterminal Y1: -35040 A
Winding Force on Force on Compressive Strand stress top yoke bottom yoke force (max) (max) (kN) (kN) (kN) (MN/m2) ======= ============= ============= ============= ============= TERC 17.9 0.0 1193.0 -42.1REGUL 0.0 0.0 0.0 0.0MT 0.0 34.9 229.9 15.3AT 0.0 1.4 22.2 0.1
Auto Transf. 3Ph SC at TV 500/230/13.8 kV
CURTO CIRCUITO MONOFASICO EM X1
Steady state short circuit winding currentterminal H1: 2640 ASteady state short circuit winding currentterminal X1: -6768 ASteady state short circuit winding currentterminal Y1: 19790 A
Winding Force on Force on Compressive Strand stress top yoke bottom yoke force (max) (max) (kN) (kN) (kN) (MN/m2) ======= ============= ============= ============= ============= TERC 195.3 164.3 208.8 -13.9REGUL 0.0 0.0 0.0 0.0MT 0.0 246.5 2313.5 -39.2AT 0.0 31.5 1483.8 86.0
Auto Transf. 1Ph SC at MV 500/230/13.8 kV
Short-Circuit in Power Transformers
Auto Transformer SC Phase-to-Ground at MV Side and SC Three-Phase at TV Side
Short-Circuit ForcesShort-Circuit Forces
ABB Brazil JCM A-43Power Products DivisionPower Transformers
High Voltage Power Transformers:Short Circuit – Aging and Deterioration Effects
ABB Brazil JCM A-44Power Products DivisionPower Transformers
Solid Insulation Aging
Short-Circuit in Power Transformers
cellulosemoleculecellulosemolecule
O2 chemical bond
ABB Brazil JCM A-45Power Products DivisionPower Transformers
Insulation PaperPolymerization Degree DP
Insulation PaperPolymerization Degree DP
New Paper - DP 1050 ... 1300tensile strength of a new paperHalf Life Paper - DP 380 ... 450residual tensile strength of a half-life paperis about 50% of the one of a new paperEnd of Life Paper - DP 150 ... 200residual tensile strength of a half-life paperis about 25% of the one of a new paper
New Paper - DP 1050 ... 1300tensile strength of a new paperHalf Life Paper - DP 380 ... 450residual tensile strength of a half-life paperis about 50% of the one of a new paperEnd of Life Paper - DP 150 ... 200residual tensile strength of a half-life paperis about 25% of the one of a new paper
Dielectric Strength at end-of-life is reduced just about ~ 10% of its original new paper value
Dielectric Strength at end-of-life is reduced just about ~ 10% of its original new paper value
Short-Circuit in Power Transformers
Solid Insulation Aging and Mechanical Strength Reduction
ABB Brazil JCM A-46Power Products DivisionPower Transformers
High Voltage Power Transformers:Short Circuit – Winding Residual Pressing Forces
ABB Brazil JCM A-47Power Products DivisionPower Transformers
psp
psp
Cobre
Psp
Papel
F
KpaMps
Kpe
Kco
Kco
Kpi
Kps
Mpi
Mco
Mpa
Mpeh
h
Visco-Elastic Model
K E Sh
N mmmj
G hE S
mm Nmj
h FK
mmmj
Material Elasticity: ENomex > 4xECelulose
axial stiffness (Gmj) depending on temperature, characteristics and dimensions of:insulation material inside the windingsinsulation material outside in the windings distances
ratios between applied force and winding system elastic-inertial response:large range of variationsdepending on of design and forces distributionwinding amplification (resonances) and reduction (damping) of local forces
ABB Brazil JCM A-48Power Products DivisionPower Transformers
Benefits of Residual Pressing Forces: increase mechanical stiffness of the windingsincrease mechanical natural frequencies of the windingsreduction of short-circuit oscillatory forces in the insulationreduction of the axial displacement between windings
Modern Manufacturing Processes:drying process of the windingswinding mechanical stabilization of the windingswinding dimension stabilization of the windingsActive Part drying under a Vapor Phase processfinal pressing of the windings
guarantee and maintain the insulation mechanical stabilitymaintain over time the residual pressing force
Short-Circuit in Power Transformers
Residual Pressing Forces and Short-Circuit StrengthResidual Pressing Forces and Short-Circuit Strength
ABB Brazil JCM A-49Power Products DivisionPower Transformers
High Voltage Power Transformers:Short Circuit – Design Transformers to Withstand Short-Circuit Forces
ABB Brazil JCM A-50Power Products DivisionPower Transformers
Short-Circuit in Power Transformers
Designing Transformers to Withstand Short-Circuit ForcesDesigning Transformers to Withstand Short-Circuit Forces
Step 1 – Short-Circuit Fault Currents Calculation
Covering all external fault modes: three phase fault on HV, LV and Tertiary sideline-to-line to line fault on HV, LV and Tertiary sideline-to-ground fault on HV and LV side
Boundaries Conditions to Consider• the network impedance in line with the Standard or with the Specification• the pre-fault open-circuit voltage (Standard or Specification• the different tap changer positions• that impedances are subject of tolerances• that current limiting reactors may show saturation effects
ABB Brazil JCM A-51Power Products DivisionPower Transformers
Short-Circuit in Power Transformers
Designing Transformers to Withstand Short-Circuit ForcesDesigning Transformers to Withstand Short-Circuit Forces
Step 2 – Short-Circuit Forces and Stresses Calculation
Radial forces and• copper stresses in all windings
Spiraling forces and• copper stresses on the winding exits
Axial compression forces and • bending stresses in the cables• compression stresses on the papers of the winding conductors• critical tilting stresses of the winding conductors
Axial forces on the end supports and • compression and bending stresses in the mechanical support structure
Remark:The highest fault current will not always lead to the highest forces in a winding. Superimposed fields of other windings may create higher stresses!
ABB Brazil JCM A-52Power Products DivisionPower Transformers
Designing Transformers to Withstand Short-Circuit ForcesDesigning Transformers to Withstand Short-Circuit Forces
Short-Circuit in Power Transformers
Design to Radial Forces all windings are radially self supporting
inner windings are subject to “free buckling”
a dynamic phenomenonno radial supportstrength is determined by Cu hardness (yield point) and conductor geometry
outer windings are subject to tensionstrength is determined by Cu hardness
spiraling on windings exitsstrength is determined by Cu hardness
Design to Axial Forces axial forces are calculated by FEMconsidering
axial displacement due to workshop toleranceaxial displacement due to Layer or Helical winding pitch when applicable
windings are dimensioned for maximum compression forces
dynamic effects are considered by dynamic factors on the forces
winding ends are dimensioned for maximum unbalance forces anda part of the maximum compression force (“Bounce Back”) windings are radially self supporting
Step 3 – Design Criteria
ABB Brazil JCM A-53Power Products DivisionPower Transformers
Short-Circuit in Power Transformers
Designing Transformers to Withstand Short-Circuit ForcesDesigning Transformers to Withstand Short-Circuit Forces
Step 4 – Design Practices
Design practices focus on axial ampere-turn balancing optimization of all windings
provide safe fastening of the winding exitsprovide safe fastening of the connection leads
ABB Brazil JCM A-54Power Products DivisionPower Transformers
Short-Circuit in Power Transformers
Designing Transformers to Withstand Short-Circuit ForcesDesigning Transformers to Withstand Short-Circuit Forces
Simplifications are applied during design:
the forces alternate with the square of the current. During design only the peak force is calculated and considered as a static force. A dynamic factor may be used to consider the dynamic effects
core and Windings are a 3D arrangement. The routine field calculations are ran with 2 D programs
layer and helical windings have a pitch creating variable displacements. It depends on the manufacturer’s rules which displacement is considered
forces calculation on the winding exits is based on a simplified model
mechanical withstand limits of helical windings against spiraling forces are based on limited number of experiments.
ABB Brazil JCM A-55Power Products DivisionPower Transformers
Short-Circuit in Power Transformers
Designing Transformers to Withstand Short-Circuit ForcesDesigning Transformers to Withstand Short-Circuit Forces
Improvements achieved over the last 20 years:
computer programs run fault current and force calculations for all fault conditions and all winding connections automatically
3D magnetic tools have been applied for R&D work
epoxy bonding of winding conductors has become available and the application became a standard practice
hard and very hard drawn copper is applied more frequentlypre-compressed pressboard is exclusively used
there is more focus on the short circuit strength during the design stage
more experience has been gained in the field and by testing.
ABB Brazil JCM A-56Power Products DivisionPower Transformers
Short-Circuit in Power Transformers
Designing Transformers to Withstand Short-Circuit ForcesDesigning Transformers to Withstand Short-Circuit Forces
Remarkable overall observations:
designing for short-circuit strength has been of increased demand over last 20-30years
past power transformer short-circuit performance was not high with frequent failures by: winding radial bucklingwinding axial collapsingbroken press rings and end supports
design philosophy has changedmore experience has been gained, mainly after performing tests,calculation tools were improvedthere are better design rulesthere are better materials to strengthen windings and winding supports
power transformer short-circuit performance has increased over the last decadesno winding radial bucklingno winding axial collapsingno winding axial conductors tilting indication
in a transformer designed during that period.
ABB Brazil JCM A-57Power Products DivisionPower Transformers
High Voltage Power Transformers:Short Circuit – Short-Circuit Full Tests
ABB Brazil JCM A-58Power Products DivisionPower Transformers
Short-Circuit in Power Transformers
Aspects to Consider
Technical aspects:mandatory for distribution transformers ( 500kVA)test may be not possible for large units (>400 MVA). Only few laboratories are able to test units rated over 100MVA.transformer application (normal substation, GSU, interconnections between two systems (phase-shifter or HVDC), etc..)
Economical aspects:Testing costs (including transport costs) and associated risks (project delays in case of a failure);Evaluation of revenue losses, mainly for bulk power applications like interconnections, generating power stations, large industrial plants, etc.;Large number of units to be ordered e.g. distribution transformers
Composite evaluation of all technical and economical aspects:Costs of performing a short-circuit test series shall then be compared with the costs, inconveniences and risk of having a failure in service.
Short-Circuit TestShort-Circuit Test
ABB Brazil JCM A-59Power Products DivisionPower Transformers
Short-Circuit in Power Transformers
Designing Transformers to Withstand Short-Circuit ForcesDesigning Transformers to Withstand Short-Circuit Forces
Benefit and risk of testingtest results are an important feedback and a valuable source for improvements. Due to the high cost and the limited number of power test facilities the number of tests will remain limited
submitting a transformer to a short-circuit test is always connected to high expenses for the purchaser. In case of a failure the expenses become very high for purchaser and manufacturer
a short-circuit test therefore involves a risk for both parties
Why do transformers fail under a short-circuit testingdue to design weakness because:
• the rules did not fully cover the case• there are effects which have not been considered
due to shortcomings during manufacturingdue to transportation issues or poor test preparations
ABB Brazil JCM A-60Power Products DivisionPower Transformers
Short-Circuit in Power Transformers
Short-Circuit Test PerformanceShort-Circuit Test Performance
KEMA Netherlands Power Lab:77 large transformers (25 - 440 MVA, 20 -500 kV), 11 years of testing31% failure rate during tests (mostly because an unacceptable increase in reactance)
CIGRE SC12 1998:worldwide survey with high-power 12 labs12% average failure rate during tests (rated power > 40 and < 100MVA)42% average failure rate during tests (rated power > 100MVA)
ABB 1997-2013:59 TrafoStar power transformers tested (18 units with voltage 400kV or above)10% average failure rate during
KEMA'S TEST EXPERIENCES WITH SHORT-CIRCUIT WITHSTAND CAPABILITY OF LARGE POWER TRANSFORMERS : R. P. P. Smeets, L. H. te Paske, P.P. Leufkens - KEMA T&D Testing Services, Arnhem, the Netherlands
ABB Brazil JCM A-61Power Products DivisionPower Transformers
Tested and Proved Strength:
short-circuit failures reduction of power transformers199 units tested and approved in 45 yrs period 1968-2013
94 units tested and approved in 16 yrs period 1997-201310....1070MVA, 69...420kV, 1ph & 3ph, NLTC & OLTC
1 regulating down 40MVA unit manufactured by ABB Braziltest power labs KEMA, CESI and IREQ
Short-Circuit in Power Transformers
Short-Circuit Test PerformanceShort-Circuit Test Performance
ABB Brazil JCM A-62Power Products DivisionPower Transformers
Short-Circuit in Power Transformers
Short-Circuit Test PerformanceShort-Circuit Test Performance
1968-2013:199 units testedup to 420kVstep-up transf up to 775MVAauto transf up to 360MVA
1968-2013:199 units testedup to 420kVstep-up transf up to 775MVAauto transf up to 360MVA
1998 South America:to CADAFE VE1 unit from ABB Brazil36MVA 3Ph, 115/13.8kVtested at CESI IT
1998 South America:to CADAFE VE1 unit from ABB Brazil36MVA 3Ph, 115/13.8kVtested at CESI IT
1997-2013 TrafoStar:94 units tested59 TrafoStar & 35 SPT18 units at 400kV or above
1997-2013 TrafoStar:94 units tested59 TrafoStar & 35 SPT18 units at 400kV or above
ABB Brazil JCM A-63Power Products DivisionPower Transformers
High Voltage Power Transformers:Short Circuit – Advanced Materials and Technologies
ABB Brazil JCM A-64Power Products DivisionPower Transformers
Insulating Oil Impregnated
Spacer
Dynamic Modulus of Elasticity - Ratio, puDynamic Stress
Short-Circuit R&D and Advanced MaterialsShort-Circuit R&D and Advanced Materials
ABB Brazil JCM A-65Power Products DivisionPower Transformers
Paper Insulated Conductor Impact Test:
Stress Condition:impact height is increased to the rupture of the insulation paper
Short-Circuit in Power Transformers
Short-Circuit R&D and Advanced MaterialsShort-Circuit R&D and Advanced Materials
ABB Brazil JCM A-66Power Products DivisionPower Transformers
Advanced Simulation CapabilityDynamic Short-Circuit Calculation Response of a Helical Winding
Short-Circuit in Power Transformers
Short-Circuit R&D and Advanced SimulationsShort-Circuit R&D and Advanced Simulations
Axial Compressive Force on Spacers [N]Axial Compressive Force on Spacers [N] Axial Vibration Ampliyude [m]Axial Vibration Ampliyude [m]
ABB Brazil JCM A-67Power Products DivisionPower Transformers
Simulation, Analysis and Design Capability:
complete set of design rulesdesign criteria based on short-circuit teststechnology experience from ASEA, BBC, GE, ANSALDO2D & 3D advanced simulation toolsadequate material selection
Short-Circuit in Power Transformers
Short-Circuit R&D and Advanced DesignShort-Circuit R&D and Advanced Design
ABB Brazil JCM A-68Power Products DivisionPower Transformers
ESCELSA – 3Ph, 300MVA, 230/138-138/13.8kV
Nomex®
Short-Circuit in Power Transformers
Advanced Materials and Application
Short-Circuit R&D and Advanced ApplicationsShort-Circuit R&D and Advanced Applications
ABB Brazil JCM A-69Power Products Division
Power Transformers
High Voltage Power Transformers:Short Circuit – Conclusions and Recommendations
ABB Brazil JCM A-70Power Products DivisionPower Transformers
Short-Circuit in Power Transformers
ConclusionsConclusions
Short-Circuit withstand capability
short-circuit failures represent only a few percentage of total transformer failures but do generallyresult in catastrophic failures
power transformer ability to withstand the dynamic effects of a short-circuit can be verified by:a full short-circuit test at a certified power lab; ora Design Review and detailed theoretic evaluation supported by Standard as IEC60076-5 3rd Ed 2006-02.ABB is applying high values of asymmetry factor to calculate maximum peak of asymmetric short circuit current
(asymmetry factor k . 2 = 1.9 . 2 = 2.69 resulting maximum worst case Dynamic Peak of the SC current)ABB is applying 2D and 3D advanced FEM simulation tools to calculate Short-Circuit forces and stressesABB is applying advanced material with controlled Short-Circuit stressesABB has high demanding manufacturing control on winding related dimensions and processes
power transformer short-circuit full test:is the only method to guaranty the ability of a transformer to withstand a short-circuitthere are large differences in the number of Short-Circuit test failures among manufacturersnot all manufacturers submitted small-medium-large size transformers to a Short-Circuit full testare expensive and a technical, economical and risk evaluation should be performed before requiring a testadequate diagnostic methods, active part inspection and final type and routine tests at 100% of the rated levelsare needed to insure that a short-circuit test did not damage the transformer.ABB has tested 199 units 1968-2013 (94 units 1997-2013) with small failure rate compared to others MnfABB applying continuously tests feedback and experience to improve transformers design and reliability
ABB Brazil JCM A-71Power Products DivisionPower Transformers
TENDER Step:
to include new IEC60076-5 3rd Ed 2006-2 in tenderdocumentation as a Standard for short-circuit strengthverification of power transformers
always to require manufacturer’s stresses compared withtheir allowed or critical values, deviations to IEC60076-53rd Ed 2006-2 Standard to be commented
Design Reviews to be required and specified
mention in the Transformer Specification that customerconsiders the rights to ask for SC tests one month afterthe order signature.
RecommendationRecommendation
Short-Circuit in Power Transformers
ABB Brazil JCM A-72Power Products DivisionPower Transformers
RecommendationRecommendation
DESIGN Step:
short-circuit strength evaluation as per IEC60076-5 3rd Ed 2006-2 Standard
theoretical evaluation of a power transformer ability to withstand the dynamic effect of a short circuit either by:
Design Review where forces and stresses are compared with a short-circuit tested Reference Transformer from the manufacturerDesign Review by checking actual design stresses against the manufacturer design rules for short-circuit stressesStress limit overall guidance:a) stress shall not exceed manufacturers
allowable stresses; or b) stress shall not exceed 80% of the critical
material stress value; or c) stresses shall be compared to the stress
guidance in the new IEC60076-5 3rd Ed 2006-2 Standard.
Short-Circuit in Power Transformers
ABB Brazil JCM A-73Power Products DivisionPower Transformers
Short-Circuit Test:
important GSU Generator Step Up transformersimportant Auxiliary units in Power Plants
key feeding transformers at Power Plants Substations or huge load centersstrategic Intertie Transformers - 3 winding system transformers (Tertiary), Autos power transformers with helical windingspower transformers with axial split winding connections
one unit out from a Large Group of transformers with same/similar design
transformers connected to networks known for many faults and high fault currents