02_report_trans - F E A T U R E A R T I C L E Review of...

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Review of Condition Assessment of Power Transformers in Service Key Words: Transformer insulation, condition assessment, failure statistics, oil testing, dissolved gas analysis, partial discharge (PD), power factor, dielectric spectroscopy, recovery voltage, winding movement detection T ransformers are required throughout modern in- terconnected power systems. The size of these transformers ranges from as low as a few kVA to over a few hundred MVA, with replacement costs ranging from a few hundred dollars to millions of dollars. Power transformers are usually very reliable, with a 20-35 year design life. In practice, the life of a transformer can be as long as 60 years with appropriate maintenance. How- ever, the in-service failure of a transformer is potentially dangerous to utility personnel through explosions and fire, potentially damaging to the environment through oil leakage, is costly to repair or replace, and may result in significant loss of revenue. In a large public power utility, the number of transformers in the subtransmission and transmission network (excluding the lower-voltage dis- tribution network) can be from a few hundred to over one thousand (69 kV to 500 kV). As transformers age, their internal condition degrades, which increases the risk of failure. Failures are usually trig- gered by severe conditions, such as lightning strikes, switch- ing transients, short-circuits, or other incidents. When the transformer is new, it has sufficient electrical and mechani- cal strength to withstand unusual system conditions. As transformers age, their insulation strength can degrade to the point that they cannot withstand system events such as short-circuit faults or transient overvoltages. To prevent these failures and to maintain transformers in good operating condition is a very important issue for utilities. Traditionally, routine preventative maintenance programs combined with regular testing were used. With deregulation, it has become increasingly necessary to reduce maintenance costs and equipment inventories. This has led to reductions in routine maintenance. The need to reduce costs has also resulted in reductions in spare transformer ca- pacity and increases in average loading. There is also a trend in the industry to move from traditional time-based mainte- nance programs to condition-based maintenance. These changes occur at a time when the average age of the trans- formers in service is increasing and approaching the end of nominal design life. The change to condition-based maintenance has resulted in the reduction, or even elimination, of routine time-based maintenance. Instead of doing maintenance at a regular in- terval, maintenance is only carried out if the condition of the equipment requires it. Hence, there is an increasing need for better nonintrusive diagnostic and monitoring tools to assess the internal condition of the transformers. If there is a problem, the transformer can then be repaired or replaced before it fails.
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02_report_trans - F E A T U R E A R T I C L E Review of...

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