transformer failure modes abb 2013-04-16
Post on 02-Jan-2016
360 Views
Preview:
DESCRIPTION
TRANSCRIPT
© 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
Transformer Failure ModesMechanical Stresses in Power Transformers
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
Transformer Failure ModesMechanical Stresses in Power Transformers
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
Transformer Failure ModesMechanical Stresses in Power Transformers
© 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
© 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
© 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 #2 – Shorted Core Laminations
The core fault is shown below
© 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 – Hoop Buckling
© 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
© ABB Inc. 2012
top related