ELECTRICAL MAINTENANCE

Transformer Maintenance: The Cheapest Form of Insurance
Part 3: The Cycles of Maintenance

The frequency of transformer maintenance should be based on the factors previously discussed in this series of articles. Typically, most transformers require full maintenance on an annual basis. As an absolute minimum, each of your key transformers should be inspected over a three year period.

If you decide not to shut down your transformer annually then '6-Part Chemical and Dissolved Gas In Oil' samples should be collected annually.

The visual inspections should be completed on a monthly basis and the results of those inspections should be recorded.

When planning for a power shutdown, it is important to collect and analyze the oil samples well in advance of the planned shutdown date. The reasoning behind this is simple, if there are problems discovered in the oil samples it may be practical to take advantage of the planned power shutdown to make the necessary repairs and/or further investigations as needed to resolve the problem.

We have discussed a myriad of tests and inspections that can be completed on any given transformer. Not all of these tests have to be done on a regular basis since this would not be cost effective and some of the tests are used to help identify problems that may be indicated by other preliminary tests.

Therefore, in order to help shed some light on this topic we will go through the step-by-step process of transformer testing.

Tables 2-4 indicate the step-by-step process to transformer testing (see Part 2 of this article in last issue)..

Testing Standards
Every test that is described in this paper has a minimum standard of performance that is governed by several bodies such as:

• ANSI/IEEE
• NETA
• Manufacturers' Specifications
• Factory Test Results
• Commissioning Test Results
• Past Maintenance Records

However, when testing any electrical apparatus, it is commonly found that the minimum standards are usually met or exceeded. This indicates to the untrained maintainer that the equipment is sound and fit for service but this may not necessarily be the case.

Whenever maintenance testing is performed, it is essential to compare current test results with the results from previous years. This comparison can reveal trends that may be indicating potential problems even though the present year's results are acceptable in and of themselves.

Whenever you are comparing results it is critical to consider the rate of change of each result and if the results have been corrected to suit the ambient conditions. Virtually all electrical tests performed on the windings and insulation are affected by the ambient conditions and temperature variations. Each specific test has a method of correcting the results for this comparison. If the data is corrupted because it has not been properly corrected then it is useless to you.

Table 5 outlines typically acceptable test data for power transformers. Use these limits as a guideline when reviewing the data from your transformer test results.

Summary
The life expectancy of a power transformer is directly related to the condition of the insulation system.

If steps are taken to assess the risk factors that cause insulation decay and practices are put in place that reduce these risks then your transformer will provide power for its full life expectancy.

The key to achieving this goal is a comprehensive maintenance program designed to allow you to detect problems in their very early stages and then take corrective action without delay.

Keeping concise records, following-up on repairs, monitoring the operating parameters and conditions, and working safely are the keys to success.

References

1. Fifty Years: A Guide to Transformer Maintenance, J Kelly, 1981
2. Academy of Infrared Thermography
3. National Electrical Testing Associ-ation Substation Maintenance Specification 1981