ELECTRICAL MAINTENANCE

Transformers: Responding to the Baby Boom

In my transformer consulting business, I often draw an analogy between transformers and the human condition. Transformers, like people, have a relatively long life span and longevity is related to genetics, work environment, maintenance and the unpredictable negative occurrences that impact our lives.

Just like human population trends, transformers have also seen peaks and valleys in installation responding to the ebb and flow of energy needs. Figure 1 tracks the sales of transformers about 20 MVA and above, by a major Canadian manufacturer and is typical of the Canadian market as a whole.

The discussion of how long a transformer will last is at least a 2 or 3 beer conversation, but if we draw a line in the sand and say life expectancy is 35-40 years, then it is apparent that we have a substantial "aging population" that must be prepared for.

Factors Affecting Longevity Genetics (Original Equipment Manufacturer)
Although this is a sensitive subject for obvious reasons, there have always been "low cost" suppliers and "brand name" suppliers. Even though all transformers should be built to a national Standard (CSA, ANSI/IEEE, IEC), there is significant leeway to allow manufacturers to find varying cost alternatives in design and material.

Even within a "brand name" supplier's history, there will be glitches (after all, transformers are made by people). Also, all suppliers are subject to the almost inevitable increase in risk factors as transformer size (MVA) and voltage class have grown over the years. This article deals primarily with mature transformers, but even today it is important to ask suppliers you don't know for experience lists.

Application (What a Transformer Does for a Living)
Transformers are applied in many situations under varying conditions. A utility will employ power transformers in generation and transmission roles. Transformers connected to generators will typically be operated at or near full load continuously. If there is a slowdown, the transformer may drop to zero load and then may be required to go to full load very quickly. Transmission transformers are typically applied in pairs with each transformer carrying about half full load (this is changing of course). The transmission load will vary with time of day, season and with increased or decreased general load conditions.

Transformers used by industry have a myriad of applications, too many to cover in this article, but they are generally worked harder than in a utility application. In addition to potential stressful applications, they are also subjected to difficult environments of temperature and contamination that can have a negative impact on life span.

Transformers, somewhat unlike people, age significantly slower if they are not loaded to capacity. Age is measured not only in years of service but is also moderated by the load carried.

Unusual Occurrences
This category of stress would include lightning strikes, short circuits, temporary excessive overloading and mechanical damage. This data may be hard to track but can be significant as the impact tends to be cumulative. An older transformer may fail because of a single (the last straw) lightning strike or short circuit but in reality, the insulation system has been stressed and weakened over time by a succession of events.

Maintenance and Testing
My point in the context of this article is that maintenance/testing methods and cycles should be based on the importance of the equipment to the system or process, the application, and the relative age of the equipment. I would suggest a different monitoring approach for a 5-year-old, 30 MVA transmission transformer than the approach used for a 300 MVA, 25-year-old generator transformer or a 15 MVA, 35-year-old industrial application transformer.

When looking at the field test results, which would include Power Factor, Insulation Resistance, Standard Oil sample results, Dissolved Gas in Oil sample results and Furan analysis, we should not expect comparable results for the transformer examples given above.

Failure statistics predicting end of life are not available for the Canadian population of transformers. It is generally believed that "bathtub" curve (Figure 2) is representative of transformer failure trends. In the first few years, the raised failure rate is due to design and application failures, followed by a period of low stable failure rates for the majority of the equipment life. Approaching End of Life, the failure rate ramps upwards.

Moral of the Story
There is a large installed base of transformers built in the late 1950's and 1960's that are aging, some gracefully, some not. This equipment forms the core of many utility and industrial systems. This older equipment should have increased surveillance as it approaches end of life.

When reviewing field test records it is always best to compare results from similar transformers. This is not always possible with a small sample of installed equipment. Efforts should be made to assemble and share maintenance/test data to allow comparison and hopefully spot negative trends in segments of the transformer population.

Transformer users and maintainers must assess their older transformers installed during the construction boom periods to ensure that these transformers do not end their lives with a BOOM!

John van Kooy is the owner and technical principal of van Kooy Transformer Consulting Services Inc. www.vankooy.com. ET