Transformer Failure Modes Abb 2013-04-16

Mustafa Lahloub, ABB INC April 16, 2013

ABB Red TIE Series Transformer Failure Modes © ABB Inc. 2013

Transformer Failure Modes Agenda Primary Causes of Transformer Failure  Balancing the “three leg stool”  Thermal degradation  Dielectric withstand  Mechanical performance  Causes of insulation system degradation  Identification of failure vulnerabilities – including key transformer components

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Transformer Failure Modes Core Form Transformer

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Transformer Failure Modes Stresses Acting on Power Transformers 

Mechanical Stresses 



Thermal Stresses 



Due to local overheating, overload currents and leakage fluxes when loading above nameplate ratings; malfunction of cooling equipment

Dielectric Stresses 

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Between conductors, leads and windings due to overcurrents or fault currents caused by short circuits and inrush currents

Due to system overvoltages, transient impulse conditions or internal resonance of windings

Transformer Failure Modes Mechanical Stresses in Power Transformers The fault current is governed by:

  

Displacement of current © ABB Inc. 2012

Open-circuit voltage Source impedance Instant of fault onset

Transformer Failure Modes Mechanical Stresses in Power Transformers 





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A short circuit gives rise to:  Mechanical forces  Temperature rise The transformer must be designed so that permanent damage does not take place Electromagnetic forces tend to increase the volume of high flux  Inner winding to reduced radius  Outer winding towards increased radius  Winding height reduction

Transformer Failure Modes Mechanical Stresses in Power Transformers Effect of the radial forces on windings

Fmean

Inner winding

Radial forces inwards  compressive stress

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Outer winding

Radial forces outwards  tensile stress

Transformer Failure Modes Mechanical Stresses in Power Transformers 

Radial forces result in:  Buckling for inner windings  Increased radius for outer windings  Spiraling of end turns in helical winding

Inner winding © ABB Inc. 2012

Outer winding

Transformer Failure Modes Mechanical Stresses in Power Transformers Effect of the axial forces on windings

The radial component of the leakage flux creates forces in axial direction

Axial short circuit forces accumulate towards winding mid-height © ABB Inc. 2012

Transformer Failure Modes Mechanical Stresses in Power Transformers – Axial B

B

Fax

Fax

Axial imbalance will create extra axial forces

B

B

Fax

Fax

The forces tend to increase the imbalance

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Transformer Failure Modes Mechanical Stresses in Power Transformers - Radial

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Failure mode Buckling:

Failure mode Spiraling:

Characteristic failure mode for inner winding

Characteristic failure mode for inner and outer winding

Transformer Failure Modes Mechanical Stresses in Power Transformers Two examples showing buckling of inner windings

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Transformer Failure Modes Mechanical Stresses in Power Transformers Axial force failure modes:  Collapse of winding end support  Tilting of winding conductors  Telescoping of windings  Bending of cables between spacers  Damage of conductor insulation

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Transformer Failure Modes Mechanical Stresses in Power Transformers Failure mode Collapse of end support

Failure mode Bending of cables Failure mode Conductor tilting

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Transformer Failure Modes Mechanical Stresses in Power Transformers 

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Axial forces cause:  Mechanical stress on insulation material  Risk for conductor tilting

Transformer Failure Modes Short-Circuit Failure

Unit Auxiliary Test Transformer Failure Internal High Speed Film Camera Footage

© ABB Inc. Originally taken by The General Electric Company at Pittsfield, Massachusetts

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Movies should be screened in the grey area as featured here, size proportion 4:3. No titles should be used.

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Transformer Failure Modes Risk: Short Circuit Forces & Stresses Through faults are often the cause of transformer failures  Many older designs have insufficient margin for today’s fault currents  Loose coils due to aging can cause failures  Normal aging can cause brittle insulation and increased failures  Even brief overloading may cause significant aging  Oxygen in the oil can double the aging rate  Moisture in the insulation increases aging rate 2-5 times depending on the amount of moisture

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Transformer Failure Modes Mechanical Risk: Short Circuit Forces & Stresses

Design Margin

Little Risk of Failure

Slight Risk of Failure

Design #1 Design #2 Design #3

High Risk of Failure

HV Radial HV Axial LV Radial LV Axial (Hoop) (tipping or (Buckling) (tipping or crushing) crushing)

Design #4

LTC Winding Radial (Buckling)

LTC Winding Axial (tipping)

Figure 3. Results of the Short-Circuit Strength Design Analysis used in a Life Assessment Study © ABB Inc. 2012

Transformer Failure Modes Thermal Stresses in Power Transformers  

 



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Loading is primarily limited by highest permissible temperatures in the transformer, especially within the windings Temperature limits are based on:  Expected lifetime  The risk for oil vaporization Permissible temperatures are generally expressed as temperature rises above ambient Ambient temperature is in turn defined by current standards  24 hour ambient temperature average 30° C  Maximum ambient 40° C In accordance to Standards:  Winding temperature rise 65° K  Top oil temperature rise 65° K  Hot spot temperature rise 80° K

Transformer Failure Modes Winding Temperature Rise and HS Calculation Winding hot spot Top oil rise

hot spot factor

Winding average rise

Copper over tank oil gradient

Copper over winding oil gradient

Winding

Ambient

Bottom oil

Temperature © ABB Inc. 2012

Transformer Failure Modes Thermal Risk: Intensive aging

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Transformer Failure Modes Thermal Risk: Intensive aging

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Transformer Failure Modes Cellulose Insulation     

Cellulose insulation is a polymer of glucose molecules. The glucose molecules are joined together to form a long chain. These chains form the fiber used to make insulation. Natural chains may be up to 1400 elements long. Reduction of this Polymerization number occurs during manufacture of the insulation material and the transformer.

Cellulose Fiber Chain

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Transformer Failure Modes Degree of Polymerization - DP   

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Degree of polymerization is a measure of the number of intact chains in a cellulose fiber. It provides an indication of the ability of the transformer insulation to withstand mechanical force (due to through-faults, etc). New transformer insulation is about 1200 -1000 DP.

Transformer Failure Modes Factors affecting DP 

Chemical reactions cause de-polymerization (breaking of polymer chains):  Hydrolysis due to water. (Moisture in transformer)  Pyrolysis due to heat. (Hot spots, overloads,…etc.)  Oxidation due to Oxygen. (Oxygen in oil)  Acidity of the oil also accelerates this process.



Aging occurs at normal load and ambient temperature but it is accelerated by high insulation temperature, humidity and oxygen.



This reduces the insulation mechanical strength and the windings become more vulnerable to physical damage or dielectric failure during through-faults.



Windings hot spots are more affected than the insulation between the windings as the host spot areas age faster. Insulation between windings may however loose some dielectric strength due to absorbing moisture.

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Transformer Failure Modes Life Expectancy Based on DP and Other Factors It is assumed that the DP of transformer insulation is approx. 1,000 at the start of life and approx. 200 at the end of life. This graph shows the expected life of thermally upgraded insulation (Insuldur) under various conditions: 10000.0 Dry & Clean (Insuldur) Acidic Oil (Insuldur)

L ife E x p e c ta n c y (y e a rs )

1000.0

1% Water Content (Insuldur) 3-4% Water Content (Insuldur)

100.0

10.0

1.0

0.1 50

60

70

80

90

100

110

120

130

140

150

Temperature [o C]

For long insulation life expectancy, it is important to keep the insulation dry, keep acidity and oxygen concentration of oil low and provide good cooling for insulation © ABB Inc. 2012

Transformer Failure Modes Thermal Stresses in Power Transformers Life Expectancy Based on DP and Other Factors

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Transformer Failure Modes DP Measurement Method 

The DP is measured by viscosity measurements according an ASTM method after dissolving the paper samples in cupriethylene diamine solvent. 

Paper samples must be taken from enough different areas in a transformer in order to get a profile of deterioration of the cellulose



When combined with detailed design knowledge, measurements in one area of the transformer can give information on the condition of paper in inaccessible areas of the windings.

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Transformer Failure Modes Dielectric Stresses in Power Transformers Overvoltage integrity  Overvoltages can be divided into two classes:  Continuous  Transitory  

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Continuous overvoltage is related to the core and its magnetization (“normal” 50Hz or 60 Hz stresses) Transitory overvoltage refers to intermittent stresses placed on the insulation system, usually at much higher levels than the power frequency stresses

Transformer Failure Modes Dielectric Stresses in Power Transformers Transient Voltages  Lightning and switching impulse surges are called “Transients” because their duration is short.  The frequencies are much higher than the power frequency (60 Hz here) operation frequency.  Transient calculations are used to find the time dependent distribution of transient voltages, applied on the line terminals, over the windings.

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Transformer Failure Modes Dielectric Stresses in Power Transformers Winding oscillation 1,0

0,1 0,2

1

0,3

3

0,4

2 4

0,5 0,6 0,7 0,8

h/H

0,9 1,0

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u

0

Winding

Winding length

Û 0,8

0,6

0,4

0

0,2

Voltage

Transformer Failure Modes Dielectric Stresses - Main Insulation Design 2 D Field Plot

2 D field plots can be used to check the design of the main insulation

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Transformer Failure Modes Analysis of Bushing Failure   

525 kV unit – assumed bushing failure Simulation showed electric stress was greatest on the paper insulation around the shield ring Used simulation to redesign insulation barriers FLC evaluation

CAD-model © ABB Inc. 2012

Field distribution over the barriers and HV-LV windings

Transformer Failure Modes

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

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Top transformer failures (78%) from Doble:  43% winding insulation  19% bushings  16% tap changers Other areas of concern:  Pollution, dust & debris affecting bushings & cooling systems  Cooling System inefficiency  COPS Tank elevation  Blocking or Wedging In 1998, Hartford Steam Boiler projected:  2% annual failure rate of existing installed base in 2008  5% annual failure rate of existing installed base by 2013

Transformer Failure Modes / Diagnostic Techniques Highly Effective On-line Actions are Best PROBLEMS

MECHANICAL

THERMAL

SERVICE CONDITIONS

PROVEN

OF THE EQUIPMENT[1]

EFFECTIVENESS[2]

OFF-S OFF-S OFF-S OFF-S OFF-S

M L H M/H H

GAS-IN-OIL ANALYSIS 6. Gas chromatography 7. Equivalent Hydrogen method

ON ON

H M

OIL-PAPER DETERIORATION 8. Liquid chromatography-DP method 9. Furan Analysis

ON ON

M/H M/H

HOTSPOT DETECTION 10. Invasive sensors 11. Infrared thermography

ON ON

L H

OIL ANALYSIS 12. Moisture, electric strength, resistivity, etc.

ON

M

OFF-S

L

ON ON

M/H M/H

OFF-S OFF-S

H H

DIAGNOSTIC TECHNIQUES

1. Excitation Current 2. Low-voltage impulse 3. Frequency response analysis 4. Leakage inductance measurement 5. Capacitance

13. Turns ratio DIELECTRIC

PD MEASUREMENT 14. Ultrasonic method 15. Electrical method 16. Power Factor and Capacitance 17. Dielectric Frequency Response

ABB Service Handbook for Transformers, Table 3-1, Page 72 [1] OFF-S = equipment out of service at site, OFF-L = equipment out of service in laboratory, ON = equipment in service [2] H=High, M=Medium, L=Low © ABB Inc. 2012

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Transformer Failure Modes Solutions to Common Problems Exist Upgrade and retrofit solutions to alleviate a number of know and unknown operating risks including:        

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Streaming Electrification Nitrogen Gas Bubble Evolution COPS System Elevation GE Mark II Clamping Shell Form Rewedging GE Type U Bushings Cooling Problems LTC Problems

Transformer Failure Modes Case #1 – Floating Shield between HV and LV 

FRA tests were performed on a 42-MVA transformer, 115/46 kV (delta-wye), to investigate high acetylene level in the DGA



End-to-end measurements on HV windings and capacitive interwinding tests between HV and LV showed a problem on phase B

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Transformer Failure Modes Case #1 – Floating Shield between HV and LV 

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The fault was a loose electric contact of the copper bonding braid on the aluminum shield strips which caused the strips to “float” electrically

Transformer Failure Modes Case #2 – Shorted Core Laminations  

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The measurements were performed on a three-phase transformer rated 250 MVA, 212 kV/ 110 kV/ 10.5 kV, before and after the repair of the core. The first core-related resonance is clearly modified by the fault: the shorted laminations caused a decrease in the core magnetizing inductance (increase in resonance frequency) and an increase in the eddy currents in the core (increased damping).

Transformer Failure Modes Case #2 – Shorted Core Laminations 

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The core fault is shown below

Transformer Failure Modes Case #3 – Shorted Turns  

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FRA responses of the series windings of a 140-MVA autotransformer (220/69 kV with tertiary winding). The fault was located on phase C of the tertiary winding. In this condition, the low-frequency measurement on the HV winding of the same phase was influenced because of the lower inductance due to the shorted turns on a winding of the same phase (increased first resonance frequency).

Transformer Failure Modes FRA Diagnostic Example – More Shorted Turns Shorted turns in transformers are produced by turn-to-turn faults and may have the following characteristics:  Adjacent turns lose paper and braze/weld together  They result in a solid loop around the core

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Transformer Failure Modes FRA Diagnostic Example – Axial Collapse Axial winding collapse is likely to have the following characteristics:     

Produced within a transformer winding due to excessive axial forces during a fault Windings shift relative to each other Gassing may result Transformer integrity is compromised Failure likely to be catastrophic if transformer continues in service

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Transformer Failure Modes FRA Diagnostic Example – Hoop Buckling Hoop buckling is produced within a transformer winding due to excessive compressive forces during a fault.

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Transformer Failure Modes FRA Diagnostic Example – Hoop Buckling

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Transformer Failure Modes FRA Diagnostic Example – Clamping Failure A clamping failure may be produced within a transformer winding due to bulk winding movement.

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Transformer Failure Modes Dielectric Frequency Response Testing Hi

The DFR test is a series of power factor measurements at multiple frequencies. It provides more information about the dielectric behavior of the insulation system. The method be used to diagnose the following conditions in transformers:     

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Lo

Ground

Moisture in the cellulose insulation High oil conductivity due to aging or overheating of the oil Chemical contamination of cellulose insulation Carbon tracking in cellulose High resistance in the magnetic core steel circuit

Hi Lo

Transformer Failure Modes DFR Testing – Distinguishing Between Aged Oil and Moisture 1.000

Aged Oil, 0.5% Moisture Good Oil 1.3% Moisture

Tan D

0.100

PF =. 00324

0.010

0.001

1

.001

1

.01

8

.1

3

1

Frequency, Hz © ABB Inc. 2012

5

10

60 100

1000

Transformer Failure Modes DFR Analysis – Fitting the Right Dielectric Parameters 1.000

Aged Oil, 0.5% Moisture Good Oil 1.3% Moisture PF =. 00324

Tan D

0.100

Measured DR 0.7% Moisture 0.010

0.001

1

.001

1

.01

8

.1

3

1

Frequency, Hz © ABB Inc. 2012

5

10

60 100 1000

Transformer Failure Modes DFR Example – High Core Ground Resistance XV to Ground

XV to Ground after Repair

.01

.10

1

10

100

1000

Frequency, Hz

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Dielectric Response Fingerprint Function caused by a High Core to Ground Resistance in Auxiliary Transformer

Transformer Failure Modes DFR Signature Example – Chemical Contamination

.01

.10

1

10

100

Frequency, Hz

Dielectric Response Fingerprint Function caused by Chemical Contamination of the Windings © ABB Inc. 2012

1000

Transformer Failure Modes DFR Example – Effect of High Insulation Moisture

Normal Moisture(.7%)

High Moisture(1.7%)

.01

.10

1

10

100

1000

Frequency, Hz

Dielectric Response Fingerprint Function Showing the Effect of High Moisture © ABB Inc. 2012

Transformer Failure Modes DFR Moisture Analysis versus Moisture Equilibrium Method Volume Moisture in Paper Xfrmr #

Temp (o C)

Type

Constr.

Oil Cond (pS/m)

1

23

GSU

Core

0.381

2.5

0.9

2

28

GSU

Core

0.492

1.8

0.9

3

23

GSU

Core

0.412

1.4

0.9

4

23

GSU

Core

1.34

2.8

0.7

5

13

3-wdg

Shell

1.5

*

1.2

6

27

Auto

Core

3

3.5

2

7

27

Auto

Shell

0.3

3.3

1

Moist by Oil Moist. by DR Sat (%wt) (%wt)

Surface Moisture in Paper Estimated Only From Moisture in Oil Against Volume Moisture From DFR © ABB Inc. 2012

Transformer Failure Modes DFR Analysis – Moistures and Loading Capability Loading Limits Based On Moisture Content Cellulose Moisture (% )

Overload Type

Overload Level with 40°C Ambient

120

3.5

Normal Loading

0%

130

2.4

Planned O/L Beyond N/P

6%

140

1.7

Long Time Emergency (1-3 mo.)

12%

180

0.8

Short-Time Emergency (½ -2hr)

40%

Hottest Spot o

Temperature( C)

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© ABB Inc. 2012