INCREASING TRANSFORMER RELIABILITY USING INSULATING OIL ANALYSIS

BY PAUL BOMAN

Transformers are found at all levels of the industrial infrastructure, from the power generating station to the power end-user facility. Redundant supplies or multiple supply routes increase power supply reliability. A large reduction of redundancy can be felt as you move closer to production facilities. On-site reliability issues can result in work stoppages and can include property casualty losses with fire damage.

A USEFUL EVALUATION TOOL

The Hartford Steam Boiler’s Transformer Oil Gas Analyst Program, TOGAT, is an effective way to protect a facility from transformer reliability issues. It uses insulating oil samples from operating transformers, oil-filled circuit breakers, and other equipment to analyze for signs of abnormal operation.

The physical and gassing properties of insulating oil are well understood, and they are useful for evaluating equipment condition. As the insulating material or fluid is exposed to energy that is normally in the form of heat, gases are formed. Depending on the type and chemical formation of the insulating fluid, these gases can create a picture through interpretation that points toward a particular type of problem or condition. A condition can also develop from the equipment application, so it is important to know where the equipment is located.


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SCHEDULING AND FOLLOW-UP

Keys to any successful maintenance program are performing maintenance at the right time and following up on the action needed. The TOGA program specifically focuses on these points to help reduce or eliminate transformer reliability issues from resource- draining forced outages. TOGA uses a combination of an automatic sample-ordering program with maintenance action follow-up to ensure the needed actions do not fall through the cracks of a changing maintenance departmental structure.

TOGA samples can be taken during most conditions without removing the equipment from service, but there are some exceptions. Electrical safety procedures must be followed, especially if exposed, energized electrical conductors are near the sample point vicinity. Some fluid tests are sensitive to the weather, and taking the sample during high humidity conditions, such as during a storm, may affect the test results. Many TOGA customers plan their sample schedules based on the regional weather and peak loading conditions.

Industry standards recommend routine annual Dissolved Gas Analysis (DGA), moisture analysis, and a set of screen tests that check the insulating oil for signs of problems. Sometimes these recommendations are specific to an equipment application such as electricity generation or a manufacturing process. Transformers that have a critical application will normally have a more frequent sampling schedule. Some transformers have certain design characteristics that warrant increased testing frequencies for insulating oil moisture content. One example is water-cooled heat exchangers.

RISK LEVELS FOR TESTING FREQUENCIES

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Knowing program limitations is necessary for establishing proper risk levels for testing frequencies for the right types of test. The TOGA program can successfully detect about a third of transformer failure modes, which is confirmed by research described in “Economic Is-sues Concerning Loss Capitalisation and Monitoring”1 from the 2001 CIGRÉ SC 12 Colloquium Proceedings. TOGA is also able to identify another third of insulating oil-filled equipment problems on a limited basis. Table 1 is a partial list of conditions that transformer oil testing can help identify. Emphasizing recommended follow-up and consistent test result trending has established high reliability standards.

Understanding these test results can be difficult because of the many variables associated with equipment types, sampling inconsistencies, and laboratory reporting methods. Computer-generated analysis can be generic and can cause false alerts if misunderstood. The TOGA program provides the technical guidance for interpreting test results.

USING ROUTINE SAMPLES FOR PROBLEM ISOLATION

Insulating oil sample results are viewed as a single set of data for a snapshot indication of the equipment’s condition. Adding historical information forms a more complete picture of the equipment’s condition and can indicate the severity of the problem. Trending the oil sample results can indicate changing conditions that need attention or facilitate planning for an outage at a later date. This very important point afects not only maintenance planning but also, more importantly, production schedules.

CASE 1: DGA HELPS KEEP UNIT ON-LINE UNTIL SCHEDULED OUTAGE

An electrical cooperative found a distribution transformer producing combustible gases at an unacceptable rate. The distribution transformer had a high winter seasonal load that could not be curtailed to deenergize the transformer. The transformer would be out of service for several months depending on the type of problem found. TOGA samples were taken on a frequency that allowed the transformer to remain in-service with a high degree of confidence during the winter peaking season without incident. The transformer was then repaired and available for the following year’s winter load.

Ensuring high reliability was key to the success of this story. The cooperative transformer was a sole supplier to a winter-only business; a forced outage would have meant a complete shutdown of the customer’s operations.

CASE 2: SAFETY CONSIDERATIONS

A state-owned college had transformers scattered throughout its campus, close to areas occupied by students and inside buildings. The maintenance supervisor saw a safety risk and addressed it with the TOGA program. Periodic oil samples were used to identify insulating oil and transformer loading issues.

CASE 3: NEW TRANSFORMER INSTALLATION

A transformer had been in operation for almost two years when a routine TOGA report indicated an abnormal gassing pattern. Several TOGA samples were taken to verify the concerns and establish a trend. The TOGA recommendations were then presented to the original equipment manufacturer (OEM). The transformer was removed from service to investigate the problem. The OEM identified the no-load tap changer contacts as the source of the problem; repairs were completed under the original warranty.

The TOGA program recommends additional samples for new transformer installations when the equipment is first energized and after the equipment has been electrically loaded. Additional testing should also be considered after an increase in the normal loading profile.

CASE 4: TOGA RECOMMENDATION POINTS TOWARD A LARGER PROBLEM

TOGA identified elevated moisture content on a grain elevator distribution transformer. Elevated insulating oil moisture content can lower the insulating system dielectric strength and allow flashover that can damage a transformer. Moisture can also accelerate paper-insulation aging rates that will reduce the equipment’s expected useful life. TOGA requested a follow-up sample, which prompted a closer inspection of the transformer. A broken electrical bushing was found. A damaged bushing poses the increased risk of flashover that could cause a transformer failure. The bushing was replaced, and the transformer was returned to service.

Insulating oil analysis is an extremely useful and cost-effective tool for evaluating equipment health and operation. TOGA routinely recommends other testing or monitoring to decrease the possible causes of abnormal sample results. Sometimes, additional insulating oil analysis, such as checking for furanic compound levels or trace metals found in the insulating oil, is recommended to eliminate possible causes of the abnormal test results.

Insulating oil analysis has a proven track record of increasing system reliability and saving owner resources. Insulating oil samples usually are taken without removing the equipment from service. Transformer failure rates from a non-TOGA testing program show a failure rate of 30 of every 1,000 transformers, compared with TOGA’s rate of less than one (.64) of every 1,000 transformers. These results show that this type of maintenance program dramatically im-proves reliability at a reasonable cost.2

References

1. Breckenridge, Tom; Tom Harrison; John Lapworth; Elizabeth MacKenzie; and Simon White, “Economic Issues Concerning Loss Capitalisation and Monitoring,” Proceeding of CIGRÉ SC 12 Colloquium, University of Dublin, June 18-20, 2001, 7.

2. Boman, Paul, 1st International Conference on Insulation Condition Monitoring Electric Plant, Wuhan (China) University, Sept. 25, 2000.

PAUL BOMAN has worked in several positions for Hartford Steam Boiler since 1988. In 1999, he became TOGA program director with responsibility for redesign of the TOGA program. TOGA is an Internet-based analytic program that performs asset management services for more than $520 million of equipment. He has an MBA degree and is a member of the Institute of Electrical and Electronics Engineers (IEEE).

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