transformer short circuit performance -...

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


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


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


Data from Test Stations - TF#1

Transformers,> 25 MVA

1996: 8 tested1997: 6 tested1998: 12 tested1999: 15 tested


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.


Short-Circuit Calculations: steps

Fault CurrentsField DistributionForce DistributionMechanical StressesMechanical Strength

LV HV

(magnetic

circuit on left side)

2D-rotational symmetric


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


Typical Magnetic Field Plots


Components of Force(axial and radial)

Axial Forces

Radial Forces

LV(inward)

Radial Forces

HV(outward)


Radial forces (due to the axial field)

LV HV


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)


Examples of deformation- LV- Radial Buckling


Axial Forces (due to the radial field)

LV HV


Axial forces; deformation effects

Wc t starea

bE b CSA

LMTcrit

2 2

30

Rectangular Copper Conductor

Continuously Transposed Conductor


Clamping Arrangement

(A-leg) (B-leg) (C-leg)


If the winding clamping fails…..

(Winding support structure

had collapsed

)


Strength ?


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.


Copper Proof Stress


Conductor Profiles


Effect of bonding CTC


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


Manufacturing Ability

Winding Techniques

Processing

Dimensioning

Support Structures• Internal to winding• External to winding


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.


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.


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.


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


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


Results from TF#4Diagnostic & Monitoring

Techniques Reviewed– Winding Capacitances– Magnetising currents– Leakage reactance– Low Voltage Impulse– Frequency Response Analysis

Most Popular Development is FRA


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


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


Magnetising current

• Require only very simple equipment

• Easiest way of detecting any shorted turns

• Limited sensitivity for other types of faults


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


Impedance change example(from actual tests in short circuit testing station)

IEC : < 1 % change (coreform transformer)


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


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


Cigré Technical Brochure N° 209 – 2002

(for core-type and shell-type transformers)

http://www.e-cigre.org


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

transformer short circuit performance -...

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