Transformer Failure Modes ABB 2013-04-16

© ABB Inc. 2013

Mustafa Lahloub, ABB INC April 16, 2013

ABB Red TIE SeriesTransformer Failure Modes

© ABB Inc. 2013

Transformer Failure Modes

AgendaPrimary 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

© ABB Inc. 2012

Transformer Failure Modes Core Form Transformer

© ABB Inc. 2012

Transformer Failure ModesStresses Acting on Power Transformers

Mechanical Stresses Between conductors, leads and windings due to

overcurrents or fault currents caused by short circuits and inrush currents

Thermal Stresses Due to local overheating, overload currents and leakage

fluxes when loading above nameplate ratings; malfunction of cooling equipment

Dielectric Stresses Due to system overvoltages, transient impulse conditions

or internal resonance of windings

© ABB Inc. 2012

The fault current is governed by:

Open-circuit voltage Source impedance Instant of fault onset

Displacement of current

Transformer Failure ModesMechanical Stresses in Power Transformers

© ABB Inc. 2012


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

© ABB Inc. 2012

Innerwinding

Outerwinding

Radial forces inwards compressive stress

Radial forces outwards tensile stress

Fmean

Transformer Failure ModesMechanical Stresses in Power TransformersEffect of the radial forces on windings

© ABB Inc. 2012

Innerwinding

Outerwinding


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

© ABB Inc. 2012

Axial short circuit forces accumulate towards winding mid-height

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

Transformer Failure ModesMechanical Stresses in Power TransformersEffect of the axial forces on windings

© ABB Inc. 2012

B B Fax Fax

B B Fax Fax

Axial imbalance will create extra axial forces

The forces tend to increase the imbalance

Transformer Failure ModesMechanical Stresses in Power Transformers – Axial

© ABB Inc. 2012

Failure mode Spiraling:Characteristic failure mode for inner and outer winding

Failure mode Buckling:Characteristic failure mode for inner winding

Transformer Failure Modes Mechanical Stresses in Power Transformers - Radial

© ABB Inc. 2012

Transformer Failure ModesMechanical Stresses in Power TransformersTwo examples showing buckling of inner windings

© ABB Inc. 2012

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

Transformer Failure Modes Mechanical Stresses in Power Transformers

© ABB Inc. 2012

Failure mode Conductor tilting

Failure mode Bending of cables

Failure mode Collapse of end support


© ABB Inc. 2012

Transformer Failure Modes Mechanical Stresses in Power Transformers

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

© ABB Inc. 2012

Transformer Failure ModesShort-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

© ABB Inc. 2012

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

© ABB Inc. 2012

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

© ABB Inc. 2012

Transformer Failure Modes Mechanical Risk: Short Circuit Forces & Stresses

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

HV Radial(Hoop)

HV Axial(tipping orcrushing)

LV Radial(Buckling)

LV Axial(tipping orcrushing)

LTCWindingRadial

(Buckling)

LTCWinding

Axial(tipping)

Design #1Design #2Design #3Design #4

Little Risk of Failure

Slight Risk of Failure

High Risk of FailureDes

ign

Mar

gin

© ABB Inc. 2012

Transformer Failure ModesThermal Stresses in Power Transformers

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

© ABB Inc. 2012

Winding hot spotTop oil rise

hot spot factor

Winding average rise

Copper over winding oil gradient

AmbientWinding

Temperature

Bottom oil

Copper over tank oil gradient

Transformer Failure ModesWinding Temperature Rise and HS Calculation

© ABB Inc. 2012

Transformer Failure Modes Thermal Risk: Intensive aging

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.

© ABB Inc. 2012

Transformer Failure Modes Cellulose Insulation

Cellulose Fiber Chain

© ABB Inc. 2012

Transformer Failure Modes Degree of Polymerization - DP

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.

© ABB Inc. 2012

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.

© ABB Inc. 2012

0.1

1.0

10.0

100.0

1000.0

10000.0

50 60 70 80 90 100 110 120 130 140 150

Temperature [oC]

Life

Exp

ecta

ncy

(yea

rs)

Dry & Clean (Insuldur)

Acidic Oil (Insuldur)

1% Water Content (Insuldur)

3-4% Water Content (Insuldur)

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:

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 ModesThermal Stresses in Power TransformersLife Expectancy Based on DP and Other Factors

© ABB Inc. 2012

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.

© ABB Inc. 2012

Transformer Failure Modes Dielectric Stresses in Power Transformers

Overvoltage integrity Overvoltages can be divided into two classes:

Continuous Transitory

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

© ABB Inc. 2012

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.

Transformer Failure Modes Dielectric Stresses in Power TransformersTransient Voltages

© ABB Inc. 2012

Winding

Win-ding

length

Voltage

Winding oscillation

Transformer Failure Modes Dielectric Stresses in Power Transformers

uÛ

1,0

0,8

0,6

0,4

0,200

0,10,2

0,30,4

0,50,6

0,70,8

0,91,0

h / H

4

2

3

1

© ABB Inc. 2012

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

2 D Field Plot

Transformer Failure Modes Dielectric Stresses - Main Insulation Design

© ABB Inc. 2012

Field distribution over the barriers andHV-LV windings

CAD-model

FLC evaluation

Transformer Failure ModesAnalysis 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

© ABB Inc. 2012

Transformer Failure Modes

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

© ABB Inc. 2012

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

PROBLEMS DIAGNOSTIC TECHNIQUESSERVICE CONDITIONS OF THE EQUIPMENT[1]

PROVEN EFFECTIVENESS[2]

MECHANICAL

1. Excitation Current2. Low-voltage impulse3. Frequency response analysis 4. Leakage inductance measurement 5. Capacitance

OFF-SOFF-SOFF-SOFF-SOFF-S

MLH

M/HH

THERMAL

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

ONON

HM

OIL-PAPER DETERIORATION 8. Liquid chromatography-DP method9. Furan Analysis

ONON

M/HM/H

HOTSPOT DETECTION 10. Invasive sensors11. Infrared thermography

ONON

LH

DIELECTRIC

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

13. Turns ratio OFF-S L

PD MEASUREMENT14. Ultrasonic method 15. Electrical method

ONON

M/HM/H

16. Power Factor and Capacitance 17. Dielectric Frequency Response

OFF-SOFF-S

HH

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

Transformer Failure ModesSolutions to Common Problems Exist

Upgrade and retrofit solutions to alleviate a number of know and unknown operating risks including: Streaming Electrification Nitrogen Gas Bubble Evolution COPS System Elevation GE Mark II Clamping Shell Form Rewedging GE Type U Bushings Cooling Problems LTC Problems

© ABB Inc. 2012

Transformer Failure ModesCase #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

© ABB Inc. 2012

Transformer Failure ModesCase #1 – Floating Shield between HV and LV

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

© ABB Inc. 2012

Transformer Failure ModesCase #2 – Shorted Core Laminations

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).

© ABB Inc. 2012

Transformer Failure ModesCase #3 – Shorted Turns

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).

© ABB Inc. 2012

Transformer Failure ModesFRA 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

© ABB Inc. 2012

Transformer Failure ModesFRA 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

© ABB Inc. 2012

Transformer Failure ModesFRA Diagnostic Example – Hoop Buckling

Hoop buckling is produced within a transformer winding due to excessive compressive forces during a fault.

© ABB Inc. 2012

Transformer Failure ModesFRA Diagnostic Example – Clamping Failure

A clamping failure may be produced within a transformer winding due to bulk winding movement.

© ABB Inc. 2012

Transformer Failure ModesDielectric Frequency Response Testing

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

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:

HiLoHi

Lo

Hi

Lo

Ground

Hi

Lo

Ground

© ABB Inc. 2012

0.001

0.010

0.100

1.000

1 1 8 3 5

Tan

D

Aged Oil, 0.5%Moisture

Good Oil 1.3%Moisture

PF =. 00324

.001 .01 .1 1 10 100 1000

Frequency, Hz

Transformer Failure ModesDFR Testing – Distinguishing Between Aged Oil and Moisture

60

© ABB Inc. 2012

0.001

0.010

0.100

1.000

1 1 8 3 5

Tan

D

Aged Oil, 0.5%MoistureGood Oil 1.3%MoisturePF =. 00324

Measured DR0.7% Moisture

.001 .01 .1 1 10 100 1000

Frequency, Hz

Transformer Failure ModesDFR Analysis – Fitting the Right Dielectric Parameters

60

© ABB Inc. 2012

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

Transformer

.01 .10 1 10 100 1000

Frequency, Hz

XV to Ground

XV to Ground after Repair

Transformer Failure ModesDFR Example – High Core Ground Resistance

© ABB Inc. 2012

.01 .10 1 10 100 1000

Frequency, Hz

Dielectric Response Fingerprint Function caused by Chemical Contamination of the Windings

Transformer Failure ModesDFR Signature Example – Chemical Contamination

© ABB Inc. 2012

.01 .10 1 10 100 1000

Frequency, Hz

Normal Moisture(.7%)

High Moisture(1.7%)

Dielectric Response Fingerprint Function Showing the Effect of High Moisture

Transformer Failure ModesDFR Example – Effect of High Insulation Moisture

© ABB Inc. 2012

Surface Moisture in Paper Estimated Only From Moisture in Oil Against Volume Moisture From DFR

Volume Moisture in Paper

Xfrmr #Temp(oC) Type Constr. Oil Cond

(pS/m)Moist by Oil

Sat (%wt)Moist. by DR

(%wt)

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

Transformer Failure ModesDFR Moisture Analysis versus Moisture Equilibrium Method

© ABB Inc. 2012

Loading Limits Based On Moisture Content

Hottest Spot Temperature(oC)

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%

Transformer Failure ModesDFR Analysis – Moistures and Loading Capability

Transformer Failure Modes ABB 2013-04-16

Documents

Transcript of Transformer Failure Modes ABB 2013-04-16