transformer overloading and assessment of loss-of-life for
TRANSCRIPT
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Transformer Overloading and Assessment of Loss-of-Life
for Liquid-Filled Transformers
P.K. Sen, PhD, PE, Fellow IEEE Professor of Engineering Colorado School of Mines
PSERC Project Webinar, April 19, 2011
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Presentation Outline
• Introduction and Motivations • Remaining Transformer (Insulation) Life,
Transformer Thermal Model, IEEE Loading Guides
• Modeling and Economic Evaluation • Simulation and Results • Conclusions
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• Oil-Filled Transformer - <500 kVA (distribution, OA)
- 500 kVA - 100 MVA (distribution power) (OA, OA/FA, OA/FA/FA)
- >100 MVA (transmission power) (OA/FA/FA and OA/FA/FOA)
Transformer Type and Nameplate Rating
• 65°C average winding temperature rise (thermally-upgraded paper insulation)
• 55°C average winding temperature rise (not thermally-upgraded paper, kraft)
• 30/40/50/56 MVA @55o / 65o C TR (OA/FA/FA)
@55°C TR 3
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• Thermal Model - Design Consideration (Losses & Cooling) - Temperature (Top & Bottom Oil, Winding Hottest-Spot Temp.)
- Insulation Degradation
- Estimation of Transformer Remaining Life from Its Insulation
• Economic Model - Total Owning Cost (TOC) - Revenue Requirement
- Other Models!!
Issues and Considerations
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Thermal Model • Design •Temperature • Insulation • Life
Economical Model Revenue Requirement
Optimization (Minimum Cost & Optimum Sizing) • New Design • Retrofit Design
- Replace now - Replace later - Add a second transf. - ??
Uncertainties & Probability Approach • Future load growth projection •Ambient temp. •Transformer Failure
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Loading Guide Standards
• IEEE Std. C57.91-1995 (USA) – Clause 7 (Classical Model) – Annex G (Detailed Model)
• IEC 354-1991 (EUROPE, CANADA and Others)
• IEEE & IEC Comparisons
IEC is somewhat similar to IEEE Clause 7
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• Long-term Failure Mode – Mechanical and chemical properties of insulation
deterioration – Dielectric strength reduction (approx. 10% over
the transformer life)
• Short-term Failure Mode – Bubble formation (gassing), temporally dielectric
strength reduction (30% impulse strength reduction @ 180 °C )
Transformer Failure Modes
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• The Tensile Strength (Mechanical) of cellulose insulation is used as a criteria for loss-of-life estimation.
• Recently, the Degree of Polymerization (DP)
(Chemical) has also become a popular measurement of insulation life.
• If a sample of insulation near hot spot location is
made available, the remaining life of the transformer could be approximately predicted based on specific criteria.
Remaining Transformer (Insulation) Life
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0
20
40
60
80
100
120
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2
Remaining Life (per unit)
% T
ensi
le S
tren
gth
The normalized remaining life by tensile strength method based on 20% remaining tensile strength (RTS) as end-of-life criteria, created by least square method
Remaining Life by Tensile Strength Method
97.05ln633.01Life Remaining TSR
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Remaining Life by DP Method
0
100
200
300
400
500
600
700
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 -0.2
Remaining Life (per unit)
Deg
ree
of
Po
lym
eriz
atio
n
The normalized remaining life by DP method based on remaining DP of 200 as end-of-life criteria, created by least square method
622ln881.01Life Remaining RDP
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IEEE Std. C 57.91-1995, Per-Unit Life
• Per-Unit Life
• Relative Aging Factor
• Equivalent Aging
• % Loss-of-Life
273
000,151810 x 80.9 HST
eL
273
000,15 39.164HST
AA edt
dLF
1
0
dtFT
F
T
AAEQA
100 × Life Insulation Normal
Tx F= EQA
• Normal Insulation Life
• Hottest-Spot Temperature (THS) 11
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THERMAL MODEL
Hottest-Spot Temperature (T ) HS
Hottest-spot temperature = ambient temperature (TA) + top oil rise over ambient + hot spot rise over top oil (DTG)
(DT TO)
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GTOA TTTT DDHS
Steady-State Equations
n
TORTOR
RKTT
DD
112
mGRG KTT 2DD
Thermal Model (IEEE Clause 7)
K = Per-Unit Loading (Actual Load/Rated (max) Load)
R = Loss Ratio (Load Loss @ Rated Load/No-Load Loss)
Exponent „n‟ and „m‟ depends on “Cooling” 13
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Thermal Model (IEEE Clause 7) Transient Equations
uTOTOTO
TO TTdt
Td,DD
D
n
TORuTOR
RKTT
DD
112
,
,uGGG
G TTdt
TdDD
D
m
uGRuG KTT )( 2, DD
Oil
Winding
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Modified Transient Equations
AuTOTOTO
TO TTTdt
TdDDD
D,
)(1
1)(1
1)( , sTs
sTs
sT A
TO
uTO
TO
TO D
D
D
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Thermal Block Diagram (IEEE Clause 7)
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• Loading cycle, Complex load growth
• Ambient temperature variation Thermal Model
Aging rate, %Loss-of-life
273
000,15383
000,15
HSTeRateAging
LifeNormal
telifeofloss
HST
100* %
273000,15
383000,15
D
Aging Rate and % Loss-of-Life
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• “Clause 7” thermal model: Simple, Requires minimum information, uses top-oil temperature
• “Annex G” thermal model: More complex, Requires
more information, Focus on duct oil temperature change, uses bottom-oil temperature
IEEE “Clause 7” & “Annex G” Comparisons
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IEEE Classical Model (Clause 7) IEEE Detailed Model (Annex G)
1. Top oil temperature rise at rated load2. Hot spot temperature rise over top oil at
rated load3. Loss ratio at rated load4. Winding time constant5. Oil time constant or6. Type of cooling system
1 Top oil temperature rise at rated load2 Hot spot temperature rise at rated load3 Average winding temperature rise at rated
load4 Bottom oil temperature rise at rated load5 Losses data from test report6. Weight of Core & coil7. Weight of tank & fittings8. Gallons of fluid9. Type of cooling system10. Type of cooling fluid11. Type of winding material12. Winding time constant13. Location of hot spot14. Per unit eddy current losses at hot spot location
“Clause 7” and “Annex G” Data Requirements
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H o t s p o t
To p o il
L o a d in g
Te m p e r a tu r e Pr o f ile Plo ts ( IEEE A n n e x G Th e r m a l M o d e l)
T im e ( h r )
1 21 086420
Tem
pera
ture
(C
)
1 2 0
1 0 0
8 0
6 0
4 0
2 0
0
Loading (Per unit)
5
4 .5
4
3 .5
3
2 .5
2
1 .5
1
0 .5
0
IEEE Annex G & Clause 7 Thermal Model (Step Load Response)
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Economic Evaluation (Total Owning Cost)
Buying a New Transformer
Simplified Practice,
TOC = IPP + A * NLL + B * LL
• Size (MVA) • A & B Factors
Owner Specifies: • Initial purchase price (IPP) • Losses (Winding & Core)
Manufacturer Specifies:
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Revenue Requirement • Daily load curves • Daily ambient temp. variations • Overloading & end-of-life • Accurate energy loss • Transformer replacement • Refined & flexible method • Detailed economic analysis
Applicable for Investor Owned Utilities
(IOU)
Economic Evaluation (Revenue Requirement)
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OPTIMIZATION • NEW DESIGN • RETROFIT APPLICATIONS
THERMAL MODEL
ECONOMIC MODEL
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Daily load curve, Complex load growth
Ambient temperature variation
Economic data - discount rate, tax, - energy cost, etc.
Transformer Data
Thermal Model
Economic Model
Life Expectancy
TOC
Optimization (New Procurement)
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Existing system
Estimate Remaining Life
Remaining Life Expectancy
Historical load and ambient temp. data
Present load Load growth Load shape Ambient temp. Economic data etc.
Replace with new transformer now
Add a 2nd transformer
Overload existing transf. and replace with new one later
Optimization (Retrofit Design)
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Probability Tree Structure
ProbabilityDistribution
Path Levelized Probability Revenue Requirement
1 $134,000 0.0016
81 $101,000 0.0016
Probability
Equivalent LevelizedRevenue Requirement (ERR)
Utility’sFinancial
Model
Debt RatioEquity ReturnBorrowed Money RateTax Rate
Depreciation MethodBook Life
HistoricalData
Transformer ThermalModel
Transformer DataSize, Losses, Cost
New TransformerSizing Criteria
Cost of Transformer Cost of Losses
End-of-Life
Load
Ambient Temperature andLoad Profiles
Economic Data
Energy and Demand ChargeEscalation Rate
Estimated Remaining Life
Random Failure ModelThermal Data, Electrical Losses
Cost of Failure
Complete Method Block Diagram
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SIMULATION CASE STUDIES
AND RESULT
• Case #1 - New Design • Case #2 - Retrofit Applications
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• Buy new transformer for a new project
• Initial peak load = 20 MVA
• Load growth in first 4 years (Probability)
- High growth = 2.00%
- Moderate growth = 1.75%
- Low growth = 1.00%
• Load growth after the 4th year is 2.00%
Case #1: New Design Example Data
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Hot SpotTop Oil
Hot Spot and Top Oil Temperature
Year203020282026202420222020201820162014201220102008200620042002
Tem
pera
ture
(Deg
ree
C)
160
150
140
130
120
110
100
90
80
70
60
50
40
Case #1: Hot-Spot & Top-Oil Temperature Plots
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• Existing transformer rating = 18 MVA
• In service for 25 years
• Estimated remaining life = 0.25 pu.
• Initial peak load = 20 MVA
• Load growth in first 4 year (Probability) - High growth = 2.00% - Moderate growth = 1.75% - Low growth = 1.00%
• Load growth after 4th year is 2.00%
• Replace with new unit now or continue overload ..and replace later ?
Case #2: Retrofit Design Example Data
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• Graphic interface, Load, Save, Print, and Copy • Transient Loading Analysis (Daily load) • Life Cycle Study (with load growth from table) • Optimize Transformer Size
• Delay Replacement or Replace now Analysis
Windows Based Computer Program
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Screen Shot (1), Main Menu
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Screen Shot (2), Annex G Input
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Screen Shot (3), Load Profile Input
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Screen Shot (4), Load Growth Input
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Screen Shot (5), Temperatures Plot
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Screen Shot (6), Life Cycle Charts
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Screen Shot (8), Economic Data Input
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• Developed an Optimization Model for the
“Best Utilization” of a transformer: – Transformer replacement strategy (retrofit
design) – Optimum transformer sizing (new design)
• Computer program is written to facilitate long and complex calculations
• Windows based computer program
• Asset management tools
Conclusions
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• New programming is required for faster optimization computation
• The different depreciation method (economic
model) shall be explored
• Loss-of-life and hottest-spot calculation shall be updated when more real data is made available
• Other financial model shall be explored
• Sensitivity analysis on Loss Ratio (R)
Future Work
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Now publicly available on the PSERC Website
PSERC Project Report: Transformer Overloading and Assessment of Loss-of-Life
for Liquid-Filled Transformers
Project Leader: P.K. Sen Professor of Engineering Colorado School of Mines
Golden, CO 80401 [email protected], (303) 384-2020