A review of lifetime assessment of transformers and the use of Dissolved Gas Analysis Master Thesis by Sabina Karlsson Master Thesis written at KTH School of Electrical Engineering, 2006/2007 Supervisor: Assistant Professor Lina Bertling, KTH School of Electrical Engineering Assistant Supervisor: PhD Student Tommie Linquist, KTH School of Electrical Engineering Examiner: Assistant Professor Lina Bertling, KTH School of Electrical Engineering XR-EE ETK 2007:005 Abstract The Reliability-Centred Asset Maintenance (RCAM) is a structured approach to determine maintenance strategies for electric power system which is developed at KTH School of Electrical Engineering. RCAM focus on reliability aspects of the system and one of the main steps in RCAM is to modeling the relationship between reliability and the eect of main- tenance for critical components within the system. The transformer has been identied as a critical component within a power system and in spring 2006 the Ph.D project Life time modeling and management of transformers was started within the RCAM group. The overall goal for the project is to develop a quantitative model for the lifetime distribution of a transformer with the nal goal to implement the developed model into a maintenance planning. Dissolved Gas Analysis (DGA) is a widely used technique to estimate the condition of oil-immersed transformers. Incipient faults within the transformer may be detected by analyzing the gases which are dissolved in the transformer-oil. The objective of this thesis is mainly to analyze available data from DGA, and investigate if this kind of data may be use- ful in quantitative modeling of the transformers reliability. One conclusion from this work is that the diculty in modeling the trans- formers reliability lies mainly in the limited availability of adequate data. Transformer is a reliable device and since the number of failures is crit- ical in mathematical modeling of a components reliability it becomes very dicult to determine such models. Another aspect is the diculty to draw conclusions about a transformer's condition only from the DGA results. Although there are standards available for this purpose the DGA interpretation should also be based on other information about the partic- ular transformer such as size, construction and operation circumstances. During this work no sources have been found from which the correlation between DGA data and probability for transformer failures could be es- timated. For this reasons the proposed failure rate function in this work is based on several subjective assumptions and has not been possible to verify. Sammanfattning Reliability-Centred Asset Maintenance (RCAM) är en strukturerad metod för planering av underhållsstrategier av elkraftssystem som uveck- lats vid Skolan för Elektro- och Systemteknik på KTH. RCAM fokuser- ar på tillförlitlighetsaspekter hos systemet och ett utav huvudstegen i metoden är att matematiskt modellera samband mellan tillförlitlighet och eekt av underhåll för systemkritiska komponenter. Transformatorn har blivit identierad som en systemkritisk komponent och under våren 2006 startades doktorandrojektet Life time modeling and management of transformers upp inom RCAM gruppen på KTH. Det övergripande målet för projektet är att ta fram en kvantitativ livslängds fördelning av en transformator med det slutgiltiga målet att implementera denna mod- ell i en underhållsplanering. Dissolved Gas Analysis (DGA) är en vida använd metod för att skatta konditionen hos oljefyllda transformatorer. Begynnande fel i en transfo- rator kan upptäckas genom analys av de gaser som nns lösta transfor- matoroljan. Syftet med detta examensarbete är i huvudsak att anlysera tillgänglig data från DGA och utvärdera om denna typ av data skulle kun- na användas som underlag för kvantitativ modellering av tillförlitligheten hos en transformator. En slutsats från detta arbete är att den stora svårigheten med att försöka sig på att kvantitativt modellera en tillförlitlighetsfunktion för en trans- formator ligger till stösta del i bristen av adekvat data. Transformatorn är en relativt tillförlitlig komponent och eftersom antalet felinträande är kritiskt vid denna typ av modellering så blir det svårt att ta fram en modell med kvalité. DGA i sig är ingen bestämd vetenskap och tolkn- ing av DGA data är osäker och måste även baseras på övrig information om transformatorn ifråga såsom storlek, konstruktion och driftförhållan- den. Under arbetets gång har ingen statistik påträats utifrån vilken man skulle kunna skatta den eventuella korrelationen mellan DGA data och sannolikhet för felinträande för transformatorn. Detta sammantaget led- er till att den föreslagna modellen i detta arbete är grundad på era subjektiva antagande och de har inte heller kunnat verierats. Contents 1 Introduction 1 1.1 Background . 1 1.2 Objective . 1 1.3 Approach . 1 1.4 Scope of the work . 1 2 General theory 2 2.1 Reliability Centred Maintenance (RCM) . 2 2.2 Reliability Centered Asset Management (RCAM) . 3 2.3 Failure Modes and Eects Analysis (FMEA) . 3 2.4 Maintenance . 4 3 Transformer 5 3.1 Transformer construction . 5 3.2 Transformer failure modes and mechanisms . 7 3.3 Maintenance activities for transformer . 9 3.4 Condition monitoring techniques . 9 4 Dissolved Gas Analysis (DGA) 13 4.1 DGA . 13 4.2 DGA procedure . 13 4.3 Interpretation of Dissolved Gas Analysis . 14 4.4 Methods of interpretation (IEC, IEEE) . 14 5 Mathematical models 23 5.1 Reliability data . 23 5.2 Reliability and Failure rate functions . 23 5.3 Models to estimate the lifetime of transformer . 25 6 Ranking methods to prioritize investment and maintenance ac- tivities for transformers 28 6.1 Illustration of a ranking method to assess lifetime status of trans- formers . 28 6.2 An extension of the ranking method . 29 7 Case study SwePol link HVDC station 33 7.1 Introduction . 33 7.2 Input data . 33 7.3 Analysis of gas data . 34 7.4 Conclusions of the data analysis . 41 3 8 A proposed failure rate modeling with the aid of gas rate data 43 8.1 Proposed failure rate model . 43 8.2 Data requirements to verify the model . 46 8.3 Illustrative examples of the proposed model . 46 8.4 Conclusion . 48 9 Conclusion 49 Abbreviations CH4 Methane C2H2 Acetylene C2H4 Ethylene C2H6 Ethane CO Carbon Monoxide CO2 Carbon Dioxide CM Corrective Maintenance DGA Dissolved Gas-in-oil Analysis FMEA Failure Modes and Eect Analysis FSI Functionally signicant item GSU Generator Step-Up Transformers HVDC High-voltage direct current IEC International Electrotechnical Commission IEEE Institute of Electrical and Engineers N/A Not available O2 Oxygen OLTC On-Load Tap Changer PM Preventive Maintenance ppm parts per million RCAM Reliability Centered Asset Management RCM Reliability Centered Maintenance TDGC Total Dissolved Gas Concentration TRAFO Transformer 1 Introduction 1.1 Background The power transformer is a critical and capital intensive asset within a power system. The economic consequences of a transformer failure can be large due to the cost of property damage, repairs and the costs due to transmission service interruption. In spring 2006 the Ph.D project, Life time modeling and manage- ment of transformers, was started within the RCAM group at KTH [1]. The overall goal for the project is to develop a quantitative probabilistic model for the lifetime of a transformer component. The nal goal will be to implement the developed model into a maintenance planning related to ongoing project on RCAM. This thesis will examine one of the areas that may contributes to the long-term goal of the project. 1.2 Objective Failure probability estimation is required in maintenance asset management. The main objective of this work is to analyze available condition monitoring data, and investigate if such kind of data sources can be useful in lifetime esti- mation of transformers. 1.3 Approach In order to learn more about transformers in general, and to investigate what kind of statistical data that could possible be obtained, interviews with trans- former experts and study visits to ABB transformer manufacturer in Ludvika and to SwePollink HVDC station in Karlshamn have been carried out. Litera- ture studies have been made to get an overview of what have been made in the eld of life time estimation of transformer. Especially the interpretation guide- lines [2][3] of Dissolved Gas-in-oil Analysis results have been further studied to get a deeper understanding of the use and procedure of this technique. To gain understanding of potential transformer failures and their causes, the method to perform Failure Mode and Eect Analysis in [4] underlie the way in which a FMEA- scheme for transformer was performed. 1.4 Scope of the work Delimitation for this work is to investigate if condition monitoring data (DGA) may be useful in reliability modeling of transformer. 1 2 General theory The following sections will give an overview of the general concept of Reliability Centred Maintenance and an extension of it. The concept Failure Modes and Eects Analysis and dierent maintenance denitions will also be reviewed. 2.1 Reliability Centred Maintenance (RCM) RCM is a technique that is used to develop cost-eective maintenance plans and criteria so the operational capability of equipment is achieved, restored, or maintained. The main objective of RCM is to reduce the maintenance cost by focusing on the most important functions of the system. There are several dierent formulations of RCM processes in the literature. According to [5] an RCM analysis basically provides answers to the following questions 1. What are the functions and associated performance standards of the equip- ment in its present operating context? 2. In what ways can it fail to full ll its functions? 3. What is the cause of each functional failure? 4. What happens when each failure occurs? 5. In what way does each failure matter? 6. What can be done to prevent each failure? 7. What should be done if a suitable preventive task cannot be found? The RCM analysis may be carried out as a sequence of activities or steps.