Transformer Short Circuit Performance
Tutorial of Cigre Working Group A2.19Convener: Jim Fyvie, Scotland, 2002
Updated by: Jos Veens, Netherlands, 2009
2Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Develop the WG Philosophy
How do we deal with different cultures.– Small, Medium, Large (Power Transformers)
How do we allow for manufacturing differentials.– Coreform: Disc, Spiral, Layer; Shellform: Pancake.
How do we deal with different safety margins.– Green-Orange-Red– Add a percentage
3Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Structure of WG12.19
Task Force #1• Service Conditions – Field Survey
Task Force #2• Design Calculation
Task Force #3• Design Review
Task Force #4• Diagnostic & Monitoring
4Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Results – TF#1Service Conditions – Field Survey
S/C Tests = 23 % failures in 3934 Tests• 905 failures on new units
Field Failures = 0.0123 % failures in 121460 Transformer years• 1 failure every 8000 transformer years
5Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Data from Test Stations - TF#1
Transformers,> 25 MVA
1996: 8 tested1997: 6 tested1998: 12 tested1999: 15 tested
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Results of Round Robin – TF#2Design Calculation
Made the design data of a large transformer(400 MVA, 220 kV) available for comparision.
Good agreement on Radial forces– The axial fields were very similar
Poor agreement on Axial forces.– The radial fields were different due to different assumptions made
on the outer boundaries.
7Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Short-Circuit Calculations: steps
Fault CurrentsField DistributionForce DistributionMechanical StressesMechanical Strength
LV HV
(magnetic
circuit on left side)
2D-rotational symmetric
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Fault Currents
I_fault_rms = V1 / Z ; (Z = Z_grid + Z_transformer) I_fault _rms = 100 / Z% x InI_fault_peak = I_fault_rms x peak factor (2.55/2.69/2.77)
Forces :: (I fault peak)^2
9Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Typical Magnetic Field Plots
10Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Components of Force(axial and radial)
Axial Forces
Radial Forces
LV(inward)
Radial Forces
HV(outward)
11Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Radial forces (due to the axial field)
LV HV
12Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Radial Forces
F Da
rad
2F F Drad
crit
E hR
2
24
critrE t ND
( ) ( )2
212
2
2
2 tblFrad
(pressure, forces on winding lead-out)
(Forces translatedInto pressure on copper)
13Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Examples of deformation- LV- Radial Buckling
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Axial Forces (due to the radial field)
LV HV
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Axial forces; deformation effects
Wc t starea
bE b CSA
LMTcrit
2 2
30
Rectangular Copper Conductor
Continuously Transposed Conductor
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Clamping Arrangement
(A-leg) (B-leg) (C-leg)
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If the winding clamping fails…..
(Winding support structure
had collapsed
)
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Strength ?
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Results of TF#3Design Review
Relational equations were agreed– This allows for different ‘manufacturer’s constants’ to be used.
Equations for Critical Stress were agreed– With the basic material strength and shape factors dependant on
construction and conductor type.
Design Review to be Interrogative rather than Comparative.
– Allows manufacturers to explain their philosophy.
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Copper Proof Stress
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Conductor Profiles
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Effect of bonding CTC
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Compilation of Maximum Stresses
Windings LV HV + Tap TapStress Strength Stress Strength Stress Strength
Compressive Stress on Stampings(MPa)Axial Bending Stress(MPa)Compressive Stress on Sticks(Mpa)Radial Buckling Stress(Mpa)Radial Bending Stress(MPa)Hoop Stress(MPa)Tilting Force(Tonnes)Winding End Axial Force Top/Bottom(Tonnes)Force on Lead Exits (Tonnes)Force on Lead Runs
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Manufacturing Ability
Winding Techniques
Processing
Dimensioning
Support Structures• Internal to winding• External to winding
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Winding Techniques
Vertical winding– Normally used for HV windings with all round
access for winders. Interleaved windings and shielded windings. Continuous disc windings may be wound out and spun back to keep tight.
Horizontal winding– Used for large LV windings with parallel CTC’s
winding supplied from a hanging catenary to keep tension and prevent caging, or under tension if strip. Windings must be free from gaps.
26Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Processing
Insulation must be consolidated, ie fully dried and shrunk.
No gaps within the windings and all major insulation fully oil impregnated with hot de-gassed oil.
No locked oil pockets and Cooling ducts the correct size.
27Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Dimensioning
The leakage field plots are carried out by using the design dimensions with some building tolerance.
The units must be manufactured within this tolerance or the calculations are meaningless.
‘A’, ‘d’ and Diameters must be correct and the windings must be balanced axially.
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Supporting Structures
The inner winding must have round radial support, else we cannot claim the higher buckling mode criteria.
The Copper must have the correct hardness.
The lead ends must be supported or the windings will twist and move all the supporting blocks.
The Clamps must tolerate the end forces and any pre-load on the windings, and allow for any lifting required.
LV HV
Core
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Check List of TF #3(by no means complete)
Are the fault currents correct
Is the field model suitable
Is the force program suitable
Is the construction suitable
Are the materials suitable
Are the strength values traceable
Is the failure risk acceptable
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Results from TF#4Diagnostic & Monitoring
Techniques Reviewed– Winding Capacitances– Magnetising currents– Leakage reactance– Low Voltage Impulse– Frequency Response Analysis
Most Popular Development is FRA
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Need for Detection Means
• Visual inspection (in the field) is impractical (access to inner windings).
• Significant winding shrinkage can occur with age.
• Winding displacement may take place leading to reduction of mechanical strength against future short circuits.
• Detection of those displacements provides early warning of impeding failures
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Winding Capacitance(s)
• Requires standard test equipment .
• Sensitivity depends on the type of fault
• More effective when it is possible to make separate measurements for each phase
LVHV
CoreC between LV and HV
C between LV and core
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Magnetising current
• Require only very simple equipment
• Easiest way of detecting any shorted turns
• Limited sensitivity for other types of faults
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Impedance
• Probably the most widely accepted method of detecting winding movement
• technique is simple and requires relatively standard equipment
• Impedance often differ significantly between phases
• most winding mechanical failures can be diagnosed by impedance measurements
• Provides a numerical criterion for test conclusion
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Impedance change example(from actual tests in short circuit testing station)
IEC : < 1 % change (coreform transformer)
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Frequency response analysis (FRA)Detection of change in a global quantity:• May be Winding capacitance.• or Magnetising current• or Impedance.
Shows in local change in signal propagation in:
• Low Voltage Impulse (LVI, not so repeatable)• Frequency Response Analysis (FRA)
range: 10 kHz-2 MHz; interpretation always together with results of the previous measurement types
37Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
WG 12.19 members:Convenor; Jim Fyvie – U.K.
Task Force Leaders;TF #1 Anders Lindroth – Sweden TF #2 Kees Spoorenberg – NetherlandsTF #3 Jim Fyvie – U.K. TF #4 John Lapworth – U.K.
Members;Serge Therry – Switzerland Anatoly Panibratetz – RussiaEndre Matthe – Hungary Rafael Gonzalez – SpainMasami Ikeda – Japan Wolfgang Knorr – GermanyWladislaw Pewca – Poland Horoshi Murakami – JapanEric Chemin – France Hasse Nordman – FinlandVolodimir Zaitsev – Ukraine Victor Lazarev – UkraineJoe Foldi – Canada Willi Felber – AustriaBenedikt Damm – Austria Bob Del Vecchio – USAGerard Robert – France Luis Cheim – BrasilChen Kui – China V Pitsuriya – ThailandTim Noonan – Ireland Walter Wasinger – Australia
38Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Cigré Technical Brochure N° 209 – 2002
(for core-type and shell-type transformers)
http://www.e-cigre.org
39Short-Circuit Performance of Power Transformers – Cigre Tutorial of WG A2.19
Thank you ; Questions ?
IEC Standard: 60076-5 Ed. 3.0 (2006-02)Title: Ability to withstand short circuit
Annex A (informative) Theoretical evaluation of the ability
to withstand the dynamic effects of short circuit