Mustafa Lahlo ub, A BB INC April 16 16,, 2013 2013
ABB Red TIE S eries Trransformer F ailure Modes T ©ABB Inc. 2013
Tra Tr ansf sfo ormer F ailure Modes Agenda P rimary rimary Causes of Tran Transform sformer er Failu Failure re Balancing the “three leg stool” The Thermal degradation Dielectric Dielectric withs withstand Mechanical performance Causes of insulat insulation ion syst system em degradat degradation ion Identificat dentification ion of failure failure vulner vulnerabilit abilities ies – including including key transformer components
©ABB Inc. 2013
Tra Tr ansf sfo ormer F ailure Modes Agenda P rimary rimary Causes of Tran Transform sformer er Failu Failure re Balancing the “three leg stool” The Thermal degradation Dielectric Dielectric withs withstand Mechanical performance Causes of insulat insulation ion syst system em degradat degradation ion Identificat dentification ion of failure failure vulner vulnerabilit abilities ies – including including key transformer components
©ABB Inc. 2013
Transf Tra sfo ormer F ailure Modes Core Form Transformer
©ABB Inc. 2012
Transf Tra sfo ormer F ailure Modes Stresses Acting on Power Transformers
Mechanical Stresses
The Thermal Str Stress sse es
Due to local overheating, overload currents and leakage fluxes when loading above nameplate ratings; malfunction of cooling equipment
Dielectric Dielectric St S tresses
©ABB Inc. 2012
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
Transf Tra sfo ormer F ailure Modes Mechanical Stresses in Power Transformers The The fault current is governed by:
Displacem Displacemen entt of current ©ABB Inc. 2012
Open-circuit voltage Source impedance Instant of fault onset
Transformer Failure Modes Mechanical 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
Transformer Failure Modes Mechanical Stresses in Power Transformers Effect of the radial forces on windings
F mean
Inner winding
Radial forces inwards compressive stress
©ABB Inc. 2012
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
F ax
F ax
Axial imbalance will create extra axial forces
B
B
F ax
F ax
The forces tend to increase the imbalance
©ABB Inc. 2012
Transformer Failure Modes Mechanical Stresses in Power Transformers - Radial
©ABB Inc. 2012
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
©ABB Inc. 2012
Transformer Failure Modes Mechanical Stresses in Power Transformers Axial force failure modes:
Collapse of winding end support
Tilting of winding conductors
Telescoping of windings
©ABB Inc. 2012
Bending of cables between spacers
Damage of conductor insulation
Transformer Failure Modes Mechanical Stresses in Power Transformers Failure mode Collapse of end suppor t
Failure mode Bending of cables Failure mode Conductor tilting
©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 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
©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 Little Risk of Failure
n i g r a M n g i s e D
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
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
Transformer Failure Modes Winding Temperature Rise and HS Calculation Winding hot spo t Top o il ris e
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
©ABB Inc. 2012
Transformer Failure Modes Thermal Risk: Intensive aging
©ABB Inc. 2012
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
©ABB Inc. 2012
Transformer Failure Modes Degree of Polymerization - DP
©ABB Inc. 2012
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.
©ABB Inc. 2012
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) 1000.0 ) s r a e y ( y c n a t c e p x E e f i L
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
©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.
©ABB Inc. 2012
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.
Transformer Failure Modes Dielectric Stresses in Power Transformers Overvoltage integrity Overvoltages can be divided into two classes: Continuous Transitory
©ABB Inc. 2012
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
©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 Transformers Winding oscillation 0 0
Voltage 0 ,2
0 ,4
0 , 6
0 ,1
0 ,2
Winding
Winding length
0 , 3
3 0 ,4
0 , 5
0 , 6
0 ,7
0 , 8
0
h , 9 / H 1 , 0
©ABB Inc. 2012
1
2 4
Û u
0 , 8
1 , 0
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
©ABB Inc. 2012
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
CAD-model ©ABB Inc. 2012
FLC evaluatio n
Field di strib ution over the barriers and HV-LV windings
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
MECHANICAL
THERMAL
DIAGNOSTIC TECHNIQUES
SERVICE CONDITIONS
[1] OF THE EQUIPMENT
1. Excitatio n Current 2. Low-voltage impulse 3. Frequency respon se analysi s 4. Leakage indu ctance m easurement 5. Capacit ance
EFFECTIVENESS
OFF-S OFF-S OFF-S OFF-S OFF-S
M L H M/H H
GAS-IN-OIL ANALYSIS 6. Gas chr omatogr aphy 7. Equivalent Hydrog en method
ON ON
H M
OIL-PAPER DETERIORATION 8. Liquid chromatography-DP method 9. Furan Analysi s
ON ON
M/H M/H
HOTSPOT DETECTION 10. Invasive sensor s 11. Infrared thermogr aphy
ON ON
L H
OIL ANAL YSIS 12. Moisture, electric strength, resistivity, etc.
ON
M
OFF-S
L
ON ON
M/H M/H
OFF-S OFF-S
H H
13. Turns ratio DIELECTRIC
PROVEN
PD MEASUREMENT 14. Ultraso nic meth od 15. Electr ical metho d 16. Power Factor and Capacitance 17. Dielectri c Frequency Respon se
AB B Ser vi ce Han db ook for Tran sf or mers, 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
[2]
©ABB Inc. 2012
Transformer Failure Modes Solutions to Common Problems Exist Upgrade and retrofit solutions to alleviate a number of know and unknown operating risks including:
©ABB Inc. 2012
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
©ABB Inc. 2012
Transformer Failure Modes Case #1 – Floating Shield between HV and LV
©ABB Inc. 2012
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
©ABB Inc. 2012
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
©ABB Inc. 2012
The core fault is shown below
Transformer Failure Modes Case #3 – Shorted Turns
©ABB Inc. 2012
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
©ABB Inc. 2012
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
©ABB Inc. 2012
Transformer Failure Modes FRA 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 Modes FRA Diagnostic Example – Hoop Buckling
©ABB Inc. 2012
Transformer Failure Modes FRA Diagnostic Example – Clamping Failure A clamping failure may be produced within a transformer winding due to bulk winding movement.
©ABB Inc. 2012
Transformer Failure Modes Dielectric Frequency Response Testing Hi
The DFR test is a series of power factor measurements at multiple frequencies. It prov ides more information about the dielectric behavior of the insulation system. The method be used to di agnose the following conditions in transformers:
©ABB Inc. 2012
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
0.100
D n a T
P F =. 00324
0.010
0.001
1
.001
1
.01
8
.1
3
1
Frequ enc y, 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
0.100
D n a T
Measured DR 0.7% Moisture 0.010
0.001
1
.001
1
.01
8
.1
3
1
Frequ enc y, 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
©ABB Inc. 2012
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)
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
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 Typ e
Overload Level with 40°C Ambien t
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 Spo t o
Temperature( C)
©ABB Inc. 2012
