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Lifetime modeling and management of transformers
KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. (Reliability Centered Asset Management (RCAM))ORCID iD: 0000-0002-5138-0272
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This work have studied and developed lifetime estimation methods for the power transformer and how these could be used for asset management purposes. It is researchperformed in the intersection of the fields of reliability theory, statistical analysis, and stochastic process theory applied to lifetime estimations and management of transformers.

The chosen approaches are the following. The thesis assess the effect of thermal stresses on the lifetime of the transformer. The effect of hotspot temperature is assessed with a loss of life measure. Within this study, improvements to the thermal model have been made.

The thesis moves on to an alternative method for lifetime estimation in which diagnostic measurements are forecasted using a stochastic process and iterative realizations of this stochastic process is used to estimate a probability distribution for the transformer.

The thesis moves on to study the loss of life measure from a system perspective by calculating the loss of life estimate from the load profiles of cold load pickup, increased electric vehicle penetration, and normal operation. These are applied in such a way that they can be used for asset management purposes.

Then, the thesis uses estimates of failure times with the aim to reach a probabilistic, dynamic capacity rating selection method which use a failure rate which is conditioned on the time-dependent load level.

Abstract [sv]

Det här arbetet har studerat och utvecklat metoder för estimering av transformatorlivslängden och hur dessa skulle kunna användas för hantering av tillgångar1. Detta är forskning framförd inom skärningspunkten mellan fälten: tillförlitlighetsteori, statistiskanalys och statistisk process teori. Detta har sedan blivit applicerat på hantering av transformatorer som tillgång.

En beskrivning av de valda ansatserna följer. Den här avandlingen skattar effekten av termisk påfrestning på livslängden av en transformator. Effekten av hotspot temperaturenestimeras med ett mått som skattar livslängdsförlusten för given drift. Inom den här studien så har förbättringar påden termiska modellen gjorts.

Avhandlingen går vidare till att utveckla en alternativ metod för livslängds estimering som nyttjar diagnostiska mätningar genom att prognosticera dessa med hjälp av en stokastisk process och genom iterativa realisationer av dessa, estimera sannolikhetsfördelningen för transformatorns livslängd.

Avhandlingen går vidare till att positionera livslängsförlustsmåttet i ett system genom att beräkna livslängdsförlusten under de olika driftförhållandena: kallastpåslag, ökad penetration av elektriska fordon, och normal drift. Dessa implementeras så att de kan användas med syftet att hantera tillgångar.

Slutligen så går avhandlingen vidare till att estimera feltider med syftet att välja en probabilistisk, dynamisk kapacitet som använder en felintensitet som är betingad på en tidsberoende belastningsnivå.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , xiii, 60 p.
Series
Trita-EE, ISSN 1653-5146 ; 2013:037
Keyword [en]
transformers, lifetime modeling, asset management, thermal modeling, lifetime estimation
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-129971ISBN: 978-91-7501-883-6 (print)OAI: oai:DiVA.org:kth-129971DiVA: diva2:653948
Public defence
2013-10-21, F3, Lindstedtsvägen 26 (Entreplan), KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20131007

Available from: 2013-10-07 Created: 2013-10-07 Last updated: 2013-10-07Bibliographically approved
List of papers
1. Benefits of Reliability Centred Asset Management
Open this publication in new window or tab >>Benefits of Reliability Centred Asset Management
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2013 (English)In: 22nd International Conference and Exhibition on Electricity Distribution (CIRED 2013), 2013, 0668- p.Conference paper, Published paper (Refereed)
Abstract [en]

This paper provides an overview of risk and reliability assessment techniques, some which are available for distribution system operators, and others that are in the process of development. The main contribution of this paper is showing the possibilities and benefits of detailed risk and reliability analysis. Six samples of findings from research developed over the last decade within the RCAM group (Reliability Centred Asset Management) at the Royal Institute of Technology, Stockholm Sweden, are presented. The research is directly associated with risk and asset management applied to power systems. The first three examples are within developed research, followed by three areas where great potential is seen: 1) The value of accurate thermal models of transformers; 2) The impact of tariff regulation on asset management decisions; 3) Detailed interruption studies; 4) Dynamic rating; 5) Combined risk and reliability analysis of primary equipment and control equipment; 6) Systematic diagnostic measures for asset management.

Series
IET Conference Publications, Volume 2013, Issue 615
Keyword
Diagnostic measures, Distribution systems, Management decisions, Primary equipments, Reliability assessments, Risk and reliability analysis, Royal Institute of Technology, Tariff regulation
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-123575 (URN)10.1049/cp.2013.0668 (DOI)2-s2.0-84897619482 (Scopus ID)978-184919732-8 (ISBN)
Conference
22nd International Conference and Exhibition on Electricity Distribution, CIRED 2013; Stockholm; Sweden; 10 June 2013 through 13 June 2013
Note

QC 20130718

Available from: 2013-06-12 Created: 2013-06-12 Last updated: 2015-10-02Bibliographically approved
2. Transformer hot-spot temperature estimation for short-time dynamic loading
Open this publication in new window or tab >>Transformer hot-spot temperature estimation for short-time dynamic loading
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2012 (English)In: Proceedings of 2012 IEEE International Conference on Condition Monitoring and Diagnosis, CMD 2012, IEEE , 2012, 217-220 p.Conference paper, Published paper (Refereed)
Abstract [en]

On the transformer, the effect of thermal stress is aging of the solid insulation. Excessive thermal stress could damage the solid insulation. To avoid this, transformer condition monitoring systems could use thermal models to forecast the operating temperatures during dynamic loading. There are several thermal models of varying complexity, including the thermal models stipulated by the IEC and IEEE standards. In this paper, the thermal model which is referred to by IEC Std. 60076-7 as the differential method, is modified, so it accounts for the oil viscosity dependence on temperature. The model is validated using hot-spot temperature measurements from a 40-MVA transformer, OFAF-cooled, 21/115 kV which is located in the subarctic climate region.

Place, publisher, year, edition, pages
IEEE, 2012
Keyword
hot-spot temperature, oil viscosity, thermal model, transformer, winding losses
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-119205 (URN)10.1109/CMD.2012.6416414 (DOI)2-s2.0-84874274826 (Scopus ID)978-146731020-8 (ISBN)
Conference
2012 IEEE International Conference on Condition Monitoring and Diagnosis, CMD 2012, 23 September 2012 through 27 September 2012, Bali
Note

QC 20130313

Available from: 2013-03-13 Created: 2013-03-08 Last updated: 2013-10-07Bibliographically approved
3. Investigation of transformer top-oil temperature considering external factors
Open this publication in new window or tab >>Investigation of transformer top-oil temperature considering external factors
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2012 (English)In: Condition Monitoring and Diagnosis (CMD), 2012 International Conference on, IEEE , 2012, 198-201 p.Conference paper, Published paper (Refereed)
Abstract [en]

Standard estimation of top-oil temperature uses a thermal model related to load changes and variation of ambient temperature. Attempts have been done to improve the accuracy of top-oil temperature calculations by introducing internal properties into the model i.e. oil viscosity and winding resistance. The interest of this paper is to investigate the effect of external factors on top-oil temperature by looking into the weather, i.e. wind velocity. The results are compared with measurements on a 63MVA-ONAF 55/140 kV transformer unit, which is operated in ONAN cooling mode. The unit is located in subarctic climate, and it is equipped with a monitoring system and a weather station.

Place, publisher, year, edition, pages
IEEE, 2012
Keyword
external factors, monitoring, power transformer, thermal model, top-oil temperature, wind velocity
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-119193 (URN)10.1109/CMD.2012.6416409 (DOI)2-s2.0-84874255028 (Scopus ID)978-146731020-8 (ISBN)
Conference
2012 IEEE International Conference on Condition Monitoring and Diagnosis, CMD 2012, 23 September 2012 through 27 September 2012, Bali
Note

QC 20130308

Available from: 2013-03-08 Created: 2013-03-08 Last updated: 2013-10-07Bibliographically approved
4. On Improvements of Power Transformer ConditionMonitoring by Considering Thermal Models
Open this publication in new window or tab >>On Improvements of Power Transformer ConditionMonitoring by Considering Thermal Models
2012 (English)In: Proceedings of the 12th International Conference on Probabilistic Methods Applied to Power Sysmens, 2012Conference paper, Published paper (Refereed)
Abstract [en]

This paper aims to improve condition monitoring of power transformers by concerning thermal modeling. The common loss of life estimate could be used as a condition estimate during condition monitoring but for this estimate to be reliable, an accurate thermal model for the hotspot temperature calculations is absolutely essential to develop.

Studies have shown a significant deviation between measured and calculated hotspot values. This deviation is particularly pronounced during transient load at severe environments. Temperature modeling during transient load conditions puts high requirements on the dynamic part of the model; hence focus has been given to this particular load type in this paper. Moreover, the Nordic climate have winters with temperatures as low as -20C. In order to apply these models in these settings, this paper is discussing this as well. As possible thermal model improvements, this paper identifies the ambient temperature, loss variation with temperature, and oil viscosity changes with temperature as features which need to be utilized in the thermal model. This paper describes the current research within these areas.

Furthermore, presented in a concluding discussion, the appropriate approach for these models are seen to be either the thermal-electrical analogy or the soft modeling techniques.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-129981 (URN)
Conference
PMAPS 2012 in Turkey
Note

QC 20131007

Available from: 2013-10-07 Created: 2013-10-07 Last updated: 2013-10-07Bibliographically approved
5. A method for evaluating the impact of electric vehicle charging on transformer hotspot temperature
Open this publication in new window or tab >>A method for evaluating the impact of electric vehicle charging on transformer hotspot temperature
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2011 (English)In: 2011 2nd IEEE PES International Conference and Exhibition on Innovative Smart Grid Technologies (ISGT Europe), 2011Conference paper, Published paper (Refereed)
Abstract [en]

The expected increasing market share of electric vehicles is a response to the combination of new technological developments, governmental financial control, and an attitude shift of residents to a more environmentally friendly lifestyle. The expected capacity required for charging, imposes changes in the load to the already existing components in the electric power grid. In order to continue managing these existing assets efficiently during this load change, it is important to evaluate the impact imposed by the battery charging.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-91293 (URN)10.1109/ISGTEurope.2011.6162755 (DOI)2-s2.0-84860751696 (Scopus ID)
Conference
2nd IEEE PES International Conference and Exhibition on Innovative Smart Grid Technologies (ISGT Europe), 2011. Manchester, UK . Dec 5 – 7, 2011
Note
QC 20120329Available from: 2012-03-12 Created: 2012-03-12 Last updated: 2014-09-24Bibliographically approved
6. Frequency Response Analysis (FRA) in the Service of Reliability Analysis of Power Transformers
Open this publication in new window or tab >>Frequency Response Analysis (FRA) in the Service of Reliability Analysis of Power Transformers
2011 (English)Conference paper, Published paper (Refereed)
Abstract [en]

This paper aims to improve condition monitoring of power transformers by concerning thermal modeling. The common loss of life estimate could be used as a condition estimate during condition monitoring but for this estimate to be reliable, an accurate thermal model for the hotspot temperature calculations is absolutely essential to develop.

Studies have shown a significant deviation between measured and calculated hotspot values. This deviation is particularly pronounced during transient load at severe environments. Temperature modeling during transient load conditions puts high requirements on the dynamic part of the model; hence focus has been given to this particular load type in this paper. Moreover, the Nordic climate have winters with temperatures as low as -20C. In order to apply these models in these settings, this paper is discussing this as well. As possible thermal model improvements, this paper identifies the ambient temperature, loss variation with temperature, and oil viscosity changes with temperature as features which need to be utilized in the thermal model. This paper describes the current research within these areas.

Furthermore, presented in a concluding discussion, the appropriate approach for these models are seen to be either the thermal-electrical analogy or the soft modeling techniques.

Keyword
FRA, Transformer diagnostic, reliability analysis, degradation modelling
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-129984 (URN)
Conference
CMDM 2012
Note

QC 20131007

Available from: 2013-10-07 Created: 2013-10-07 Last updated: 2013-10-07Bibliographically approved
7. Prerequisites for Transformer Lifetime ModelingTowards a Better Understanding
Open this publication in new window or tab >>Prerequisites for Transformer Lifetime ModelingTowards a Better Understanding
2010 (English)In: 2010 IEEE 11th International Conference on Probabilistic Methods Applied to Power Systems, PMAPS 2010, IEEE , 2010, 460-463 p.Conference paper, Published paper (Refereed)
Abstract [en]

The transmission transformer represent a significant asset in the electrical network. The transformer is expensive to manufacture and it is costly to replace. The cost of the transformer replacement is approximately 4 million EURO which is larger than the average component replacement activity. Therefore it is desired to make the replacement both timely and smooth to reduce unnecessary costs. Life time modeling is a tool for achieving such cost efficient replacements. This paper highlights the prerequisites for the transformer lifetime modeling. The lack of statistical failure data and a method for an overall condition assessment are identified as main issues. Furthermore, this paper presents one approach for transformer lifetime modeling based on Bayesian statistics which combines the expert opinion with the failure data in order to model the failure rate. This paper also suggests that the subjective information from the expert can be evaluated by the use of the analytic hierarchy process to achieve quality assurance. The overall objective of this paper is to present the research within the RCAM research group of transformer lifetime modeling and to enhance the understanding of the topic.

Place, publisher, year, edition, pages
IEEE, 2010
Keyword
Bayesian statistics, Component replacement, Condition assessments, Cost-efficient, Electrical networks, Expert opinion, Failure data, Failure rate, Life-times, Research groups, Statistical failure data, Subjective information, Transformer replacement
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-129982 (URN)10.1109/PMAPS.2010.5528966 (DOI)2-s2.0-77956442320 (Scopus ID)978-142445723-6 (ISBN)
Conference
2010 IEEE 11th International Conference on Probabilistic Methods Applied to Power Systems, PMAPS 2010; Singapore; Singapore; 14 June 2010 through 17 June 2010
Note

QC 20131007

Available from: 2013-10-07 Created: 2013-10-07 Last updated: 2014-08-26Bibliographically approved
8. On optimal transformer capacity rating
Open this publication in new window or tab >>On optimal transformer capacity rating
(English)Manuscript (preprint) (Other academic)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-129985 (URN)
Note

QS 2013

Available from: 2013-10-07 Created: 2013-10-07 Last updated: 2013-10-07Bibliographically approved
9. Influence of Ambient Temperature on Transformer Overloading During Cold Load Pickup
Open this publication in new window or tab >>Influence of Ambient Temperature on Transformer Overloading During Cold Load Pickup
Show others...
2013 (English)In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 28, no 1, 153-161 p.Article in journal (Refereed) Published
Abstract [en]

This paper proposes a method to investigate the socioeconomical aspects of transformer overloading during a cold load pickup (CLPU) in residential areas. The method uses customer damage functions to estimate the cost for their power interruption and a deterioration model to estimate the cost for transformer wear due to the CLPU. A thermodynamic model is implemented to estimate the peak and the duration of cold residential load. A stochastic differential equation is used to capture the volatility of the load and to estimate the probability for transformer overloading. In a numerical example, an optimal cold load pickup for a two-area system is demonstrated where transformer overloading is allowed. In this example, an ambient temperature threshold is identified, where transformer overloading is socioeconomically beneficial.

Keyword
Cold load pickup (CLPU), restoration, transformer aging, transformer loading
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-117646 (URN)10.1109/TPWRD.2012.2219079 (DOI)000312889500018 ()2-s2.0-84871718818 (Scopus ID)
Funder
StandUp
Note

QC 20150625

Available from: 2013-02-04 Created: 2013-02-01 Last updated: 2017-12-06Bibliographically approved
10. Modeling Impact of Cold Load Pickup on Transformer Aging Using Ornstein-Uhlenbeck Process
Open this publication in new window or tab >>Modeling Impact of Cold Load Pickup on Transformer Aging Using Ornstein-Uhlenbeck Process
2012 (English)In: IEEE Transactions on Power Delivery, ISSN 0885-8977, E-ISSN 1937-4208, Vol. 27, no 2, 590-595 p.Article in journal (Refereed) Published
Abstract [en]

Thermostatically controlled devices, such as air conditioners, heaters, and heat pumps may cause cold load pickup (CLPU) problems after a prolonged blackout. This causes an increased load on the power components in the electrical grid. The result is unpredictable aging and increased risk of failure. Quantifying this risk is crucial for efficient asset management for cost-intensive components such as the transformer. This paper presents a new approach to model the loading profile of a CLPU using stochastic differential equations. The realization of the loading profile is used to determine the aging of a transformer. Two models for the deterioration of transformer solid insulation represent the loss of life due to the CLPU. A comparison between two models for the aging of the solid insulation in the transformer is made in a case study. Due to the stochastic behavior of the load, there is a probability for loading the transformer above the recommended ratings, and this probability is estimated with Monte Carlo simulations.

Keyword
Cold load pickup, restoration, transformers
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-95113 (URN)10.1109/TPWRD.2012.2185521 (DOI)000302503900018 ()2-s2.0-84859763581 (Scopus ID)
Funder
StandUp
Note

QC 20120524

Available from: 2012-05-24 Created: 2012-05-14 Last updated: 2017-12-07Bibliographically approved
11. Reliability screening for identifying critical power transformers in real size transmission system
Open this publication in new window or tab >>Reliability screening for identifying critical power transformers in real size transmission system
(English)Manuscript (preprint) (Other academic)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-129988 (URN)
Note

QS 2013

Available from: 2013-10-07 Created: 2013-10-07 Last updated: 2013-10-07Bibliographically approved

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  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
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