Common Failure Mechanism of a Transformer - Essay Example

Common Failure Mechanism of a Transformer Name of the Author 1. INTRODUCTION A transformer is an electrical device utilized for transfer of energy through electromagnetic induction between circuits. Similar to all other electrical devices, a transformer also experiences faults that result to failures. A transformer contains a wide events spectrum that include design defects, maintenance errors, voltage surges, sabotage, lighting strikes, rapid unanticipated insulator deterioration, structural damage, and lightning strikes can result to transformer explosion and fire, or many other problems. A simple transformer fault at distribution section can result to power black-out in an entire region, supported by that transformer [1]. Particularly, fire in oil-cooled transformer, which has a number of thousand litters of insulating combustible oil can lead to severe destruction of structural components of an adjacent power plant that include concrete wall and destroy or damage electrical components that include nearby circuit breakers, bus work, and transformers. Different issues can results to different faults in a transformer, and thus, different solutions may need to be employed to solve or detect different issues [2]. 2. TYPES OF TRANSFORMERS AND FAILURE RISK A transformer is referred to as a static device containing a single winding or coupled windings. It may contain magnetic core which is used to induce circuits’ mutual coupling. Transformers are utilized in electric power systems for power transfer through electromagnetic induction, at the same frequency between circuits, with altered values of current and voltages. Transformers are basically used to step up or step down voltage based on transformer position in electricity transmission line [3]. There are basically five voltages levels used for distribution and transmission of AC power. They include distribution voltage that ranges between 2.5 and 35 kV, ultra-high voltage at UHV, 1100 kV, medium voltage or sub-transmission voltage at MV, 34.5 to 115 kV, and high voltage at HV, 115 to 230 kV, and extra-high voltage at EHV, 345 to 765 kV. The MV, UHV, HV, and EHV equipment is mostly situated at electrical power stations or power plants in the electric grid. Transformers at distribution level are situated on poles in the distribution mesh, in service vaults, in buildings, or on outdoor pads [2]. There two forms of transformers which are mostly employed tor different voltage changing activities. They include liquid insulated transformers and dry type transformers. Dry transformers have gas or solid insulation material and they normally have lower fire hazard as compared to liquid insulated transformers, since the level of combustible material in these transformers is low. Liquid insulated transformer are categorized into two groups with regard to their vulnerability to fire hazard. The two categories include flammable liquid transformers and less flammable liquid transformers. Less flammable liquid transformers contains ester or silicone oil, which are anticipated to contain a great fire point of more than 300oC. This makes it hard to ignite. Flammable liquid transformer contains liquid that include mineral oil that is regarded to have high fire risk due to moderately lower fire point of between 100oC and 170oC and combustible liquid oil [2]. Although flammability have to be reduced, alternative insulating liquid is required, an aspect that results to toxicity issues. Thus, most of main transformers still use oil for insulation. However, liquid insulated transformers are normally employed in distribution system and installed outside, where the damage related with fire is minimal. The explosion or fire risk in a transformer is normally favoured by technical failure or abnormal circumstances. Thus, it is essential to plan and centre the efforts by defining priorities with main aim of enhancing the system reliability, to lower the failure hazard [4]. 3. OTHER POSSIBLE TRANSFORMER FAILURES AND THEIR CAUSES Faults might happen in various components and parts of transformer as a result of thermal, electrical or mechanical stress initiated by various conditions. Among the most common transformer failures include winding failure, bushing failure, tap changer failure, core failure, cooling system failure, tank failures, and protection system failure among others. Windings are used to step-up or step-down the transformer voltages. Windings endure mechanical, thermal and dielectric stress in this process. This stress initiates most of the faults that happen in the winding. It normally cause winding burn-out or breaking [5]. This fault has a priority number of between 6 and 30. Dielectric fault is initiated by breaking of insulator between turns, which cause winding turns’ flashover and then a short circuit. Bushes are high voltage insulating devices that are surrounded by oil. Brushing failure happens after a transformer is used for some period of time. Its priority number is between 24 and 48. It happens due to loosening conductors, and sudden increase in voltage, and poor maintenance. The tap changer is used to regulate the level of voltage to control the output. Any small fault in removing or adding secondary windings of a transformer can result to wrong output and thus, a fault. Its priority number lies between 28 and 52. This condition can be caused by lack of maintenance, old capacitors, and motor breakdown [6]. Transformers contains laminated steel centres in surrounded by windings in the middle that work to ensure magnetic flux concentration. Core fault impacts transformer windings. Core is usually laminated to lower eddy-current. This lamination can defect due to corrosion, old oil or poor maintenance. Core destruction result to thermal heat increase and thus, overheating. Core failure priority number is 6. The tank function in the transformer acts as used oil container. Tank faults is caused by environmental stress, sun radiation, high humidity, or corrosion leading to cracks or leakage in the walls of the tank [7]. This result to reduction of insulation and hence overheating. The priority number for this failure is 18. Protection system safeguard a transformer from faults by detection and resolving of faults. Unresolvable fault is isolated to ensure that it does not initiates destruction. Protection systems contains sudden pressure relays, pressure relief valve circuitry, surge protection, and buchholz protection. This is the main occurring failure with priority number lying between 22 and 64. Cooling system failure lowers the head created in transformers as a result of iron and copper losses. The system contains oil pumps, cooling fans and water-cooled heat exchangers. The cooling system failure results to overheating which creates a number of problems. The priority number for this issue lies between 26 and 48 [8]. 4. FAULT TESTING TECHNIQUES IN A TRANSFORMER There are various techniques employed to test failures in a transformer. One of the most applicable technique is Failure Modes and Effects Analysis (FMEA). This is an instrument employed to assess potential modes of failure, their causes and effects in a structured and systematic manner. Failure mode focuses on the manner in which a transformer or its components can fail while effects analysis focuses on establishing the failure consequences. The FMEA purpose is to take action to reduce or eliminate failures, starting with the highest-priority failures to the least. This analysis can either assume quantitative or qualitative techniques [9]. Reliability Cantered Asset Management (RCAM) is another technique employed to resolve failure in a transformer. This is done by use of quantitative technique. RCAM relates total maintenance costs and system reliability with preventive maintenance (PM). The association between effect and reliability of maintenance outgoing from components failure mechanism and failure causes is computed. This provide the most applicable solution to the identified problem [8]. HAZOP analysis is another technique that can be applied to establish the possible transformer failures risks and categorization of the risk [2]. This involves the use of a table to evaluate each input and the risk it may initiate in a transformer. The identified risks are categorized and prioritized to be able to identify the risks to be handled first. Other diagnostic techniques are employed where spreadsheets are used to identify the main identifiable problems and test are prescribed to establish the cause of the problems Some of these tests include oil test, turn ratio, insulating resistance, induced voltage and exciting current. The problem is diagnosed based on the test result. In this case, a single problem can be diagnosed using different tests [1]. Fault Tee Analysis is a top-down deductive technique aimed at evaluating the impacts of initiating events and faults on a complex system. However, it does not very effective in identifying all probable initiating faults [9]. 5. PROPOSED ACTION Among the proposed techniques, the most applicable analysis and diagnose technique is FMEA. FMEA is an essential procedure to assess and identify risks and risk consequences related with possible modes of transformer failure. It list modes of failure, possible cause each failure, failure effect, severity of the failure and propose a corrective measure that can be applied. The review of previous researches demonstrates that FMEA technique is utilized in some power systems fields that include motor drives, wind turbines, induced machines and solar modules [9]. FMEA is a bottom-up, inductive analysis technique that focuses on assessing the impact of single function or component failure on subsystems or equipment. This technique is excellent in thoroughly classification of initiating faults, and acknowledging their local impacts. FMEA has been tested through research and ascertained that the technique can easily been modified to evaluate modes of failure, effects and causes in the power transformer. After identifying all these effects it would be easier for an electrician to decide the best cause of action based on the priority of the fault established by FMEA. FMEA can also be used to enhance the process of maintenance practice whereby by detection of possible faults and cause, the technician can know what to be checked regularly to ensure proper functioning of a transformer [10]. REFERENCES [1] H. William and P. E. Bartley. (2003). “Failure analysis of transformer.” [On-line]. pp. 1-13. Available: http://www.imia.com/wp-content/uploads/2013/05/EP09_2003-FailureAnalysisofTransformers.pdf [2] H. P. Berg and N. Fritze. (2011). “Reliability of main transformers.” RT & A. [On-line]. 20(2), pp. 52-69. Available: http://gnedenko-forum.org/Journal/2011/012011/RTA_1_2011-07.pdf [3] M. Minhas, J. Reynders and P. de-Clerk. (1999). “Failure in power system transformers and appropriate monitoring techniques.” High Voltage Engineering Symposium. [On-line]. 22 – 27 August 1999, Conference publication no. 467, IEEE. [4] M. Mirzai, A. Gholami and A. Aminifar. (2006). “Failures analysis and reliability calculation for power transformers.” Journal of Electrical Systems. [On-line].2(2), pp. 1-12. Available: http://journal.esrgroups.org/jes/papers/2_1_1.pdf?acr_id=247 5] S. T. Jan, R. Afzal and A. Z. Khan. (2015). “Transformer failures, causes & impact.” International Conference Data Mining, Civil and Mechanical Engineering (ICDMCME’2015) Feb. 1-2, 2015 Bali (Indonesia). [On-line]. pp. 49-52. Available: http://iieng.org/siteadmin/upload/8693E0215039.pdf [6] V. V. Sokolov and A. K. Lokhanin. (n.d). “Internal insulation failure mechanisms of HV equipment under service condition.” BUENO&MAK. [Online]. pp. 1-7. Available: < http://buenomak.com.br/publicacoes/pdf/ISOLACAO-failure_mechanisms_of_hv_transformers.pdf> [7] Hattangadi. (1999). “Electrical fires and failures: A prevention and troubleshooting guide.” Tata McGraw-Hill Education. [On-line]. pp. 1-288. ISBN 0074631659, 9780074631652 [8] A. Franzen and S. Karlsson. (2007).”Failure modes and effects analysis of transformers.” KTH Electrical Engineering. [On-line]. pp. 1-24. Available: < http://www.generalpurposehosting.com/updates/TRITA-EE_2007_040.pdf> [9] M. Akbari, P. Khazaee, I. Sabetghadam and P. Karimifard. (2013). “Failure modes and effects analysis (FMEA) power transformers.” 28th International Power System Conference. [On-Line]. pp.1-7. Available: http://psc-ir.com/cd/2013/papers/1970.pdf [10] A. Shahsiah, R. C. Degeneff and J. K. Nelson (2007). “Modelling dynamic propagation of characteristic gases in power transformers oil-paper insulation.” IEEE Transactions on Dielectrics and Electrical Insulation. [On-line]. 14(30), pp. 710 – 717 Read More

Although flammability have to be reduced, alternative insulating liquid is required, an aspect that results to toxicity issues. Thus, most of main transformers still use oil for insulation. However, liquid insulated transformers are normally employed in distribution system and installed outside, where the damage related with fire is minimal. The explosion or fire risk in a transformer is normally favoured by technical failure or abnormal circumstances. Thus, it is essential to plan and centre the efforts by defining priorities with main aim of enhancing the system reliability, to lower the failure hazard [4]. 3. OTHER POSSIBLE TRANSFORMER FAILURES AND THEIR CAUSES Faults might happen in various components and parts of transformer as a result of thermal, electrical or mechanical stress initiated by various conditions.

Among the most common transformer failures include winding failure, bushing failure, tap changer failure, core failure, cooling system failure, tank failures, and protection system failure among others. Windings are used to step-up or step-down the transformer voltages. Windings endure mechanical, thermal and dielectric stress in this process. This stress initiates most of the faults that happen in the winding. It normally cause winding burn-out or breaking [5]. This fault has a priority number of between 6 and 30.

Dielectric fault is initiated by breaking of insulator between turns, which cause winding turns’ flashover and then a short circuit. Bushes are high voltage insulating devices that are surrounded by oil. Brushing failure happens after a transformer is used for some period of time. Its priority number is between 24 and 48. It happens due to loosening conductors, and sudden increase in voltage, and poor maintenance. The tap changer is used to regulate the level of voltage to control the output.

Any small fault in removing or adding secondary windings of a transformer can result to wrong output and thus, a fault. Its priority number lies between 28 and 52. This condition can be caused by lack of maintenance, old capacitors, and motor breakdown [6]. Transformers contains laminated steel centres in surrounded by windings in the middle that work to ensure magnetic flux concentration. Core fault impacts transformer windings. Core is usually laminated to lower eddy-current. This lamination can defect due to corrosion, old oil or poor maintenance.

Core destruction result to thermal heat increase and thus, overheating. Core failure priority number is 6. The tank function in the transformer acts as used oil container. Tank faults is caused by environmental stress, sun radiation, high humidity, or corrosion leading to cracks or leakage in the walls of the tank [7]. This result to reduction of insulation and hence overheating. The priority number for this failure is 18. Protection system safeguard a transformer from faults by detection and resolving of faults.

Unresolvable fault is isolated to ensure that it does not initiates destruction. Protection systems contains sudden pressure relays, pressure relief valve circuitry, surge protection, and buchholz protection. This is the main occurring failure with priority number lying between 22 and 64. Cooling system failure lowers the head created in transformers as a result of iron and copper losses. The system contains oil pumps, cooling fans and water-cooled heat exchangers. The cooling system failure results to overheating which creates a number of problems.

The priority number for this issue lies between 26 and 48 [8]. 4. FAULT TESTING TECHNIQUES IN A TRANSFORMER There are various techniques employed to test failures in a transformer. One of the most applicable technique is Failure Modes and Effects Analysis (FMEA). This is an instrument employed to assess potential modes of failure, their causes and effects in a structured and systematic manner. Failure mode focuses on the manner in which a transformer or its components can fail while effects analysis focuses on establishing the failure consequences.

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