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August 2011

Asset Health Reviews

Transformer Population or Group Study 

By Paul Griffin, Vice President of Consulting and Testing Services

It is important to know the general condition of the transformers you own or operate in order to maintain system reliability. Since you can’t ask transformers how they feel, the next best thing is to test them. As in any health review of a population, the first phase is to screen for problems using indicator tests that can detect a wide range of potential issues, including problems specific to that population.

Byproducts form as transformers' insulating materials deteriorate through overheating or discharge activity. These byproducts serve as problem indicators that, fortunately, can be detected using oil samples. 

It has been well established that oil tests can be a basis for sound condition assessment. Testing options include: 

Dissolved gases in oil
Detects a wide range of faults in oil and other dielectric liquids and paper insulation

Furans
Detects faults involving paper insulation and general aging of the paper and similar materials

Dielectric breakdown voltage and water content
For the ability to withstand voltage

Oil aging tests

Special tests
Such as those for corrosive sulfur

Today condition based assessment of many energized tap changers (often referred to as load tap changers or LTCs) is possible because oil testing can detect the deterioration process early. As deterioration worsens maintenance can be scheduled at the proper time. The same tests that are performed on oil samples from the main tank of the transformer can be applied to oils from LTCs with different diagnostics. Doble has found that model specific information can be used to refine the LTC condition evaluation process. An LTC ranking can then be determined outlining priorities for testing and maintenance.

There are other new field tests such as RFI surveys that can be used to screen for partial discharge activity for in-service transformers and other apparatus. Another in-service tool is infrared analysis, which can be used to detect connection and other problems.

Remember that after making measurements it is just as important to review the data and come to a conclusion or determine ranking. Many utility companies will review incoming data and in some cases perform a periodic review of the population. In practice, both are necessary. A quick review is needed to respond to changes that require urgent attention or further investigation. Reviewing data of the group or population takes this a step further. It is a good opportunity further explore differences or trends that might not be as apparent when reviewed individually.

This is also a good time to refine your relative ranking for testing and maintenance plans. Typically, this type of population review identifies 80% of units as needing a routine test program with no further intervention required. In the remaining 20% there will be a group that requires more frequent attention to determine when further testing or remedial action is required. There should be less than 2% that require some form of immediate attention.

These screening options are very helpful for determining what other tools might be necessary for further investigation into abnormal behaviors. However, these tools do not detect all problems. For example, after a through fault a transformer might have mechanical damage that does not cause deterioration of the insulation, yet the transformer is quite vulnerable to failure at the next though faults. Tests such as Sweep Frequency Response Analysis (SFRA) or leakage reactance are much better suited to detect these problems. Also, bushings are best monitored using electrical tests such as power factor and capacitance. A good maintenance program requires all of these tests.

Screening tests are useful and frequently applied since they do not require an outage. The review of individual units and population data will maximize the benefits of these programs. 

Paul Griffin is Vice President of Consulting and Testing Services at Doble Engineering Company. He has been with Doble since 1979 and has published over 50 technical papers pertaining to testing of electrical insulating materials and laboratory diagnostics. He is a member of numerous industry committees and working groups and received  the ASTM Committee D-27 Charlie Johnson Award and the ASTM Award of Merit.