Increasing Transformer Reliability Using Insulating Oil Analysis

Executive summary

Paul Boman, Hartford Steam Boiler

Transformer breakdowns can result in work stoppages and property losses with damage from power outage or fire. Using insulating oil analysis is an effective way to increase the reliability of transformers and other equipment at a reasonable cost. This testing technique has a proven track record of success and can be more effective and less disruptive than other available methods. Insulating oil samples can usually be taken without removing the equipment from service.

Identifying the Problem

Hartford Steam Boiler’s Transformer Oil Gas Analyst Program (TOGA®) uses insulating oil samples from operating transformers, oil-filled circuit breakers and other equipment to analyze for signs of abnormal operation. 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.

Scheduling and Follow-Up

Key points for any successful maintenance program are performing maintenance at the right time and the follow-up on needed action. These two items are specific focal points of the TOGA program that 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 needed actions do not fall through the cracks.

Risk Levels for Testing Frequencies

Knowing the limitations of any program is necessary to establish proper risk levels for testing frequencies with the right type of test. The TOGA program can successfully detect about a third of transformer failure modes, which is confirmed by research described in “Economic Issues 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.

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 its health and can give an indication of the condition's severity. Trending the oil sample results can give indications of changing conditions that require attention or allow the ability to plan for an outage at a later date. This very important point not only affects maintenance planning, but more importantly, production schedules.

A Cost-Effective Maintenance Program

Transformer failure rates from a non-TOGA testing program show a failure rate of 30 out of every 1,000 transformers. The TOGA testing program has a transformer failure rate of less than one (.64) of every 1,000 transformers. These results show that this type of maintenance program dramatically improves reliability at a reasonable cost.

Footnotes

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

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

About the Author

Paul Boman has worked in several positions for Hartford Steam Boiler since 1988. In 1999, he became TOGA® (Transformer Oil Gas Analyst) Program Director, and was responsible for the redesign of the program. TOGA is an Internet-based analytic program that performs asset management services for more than $520 million of equipment. Paul has a M.B.A. degree and is a member of the Institute of Electrical and Electronics Engineers (IEEE).

The full article

Increasing Transformer Reliability Using Insulating Oil Analysis

Introduction

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. But moving closer to production facilities, a large reduction of redundancy can be felt. On-site reliability issues can result in work stoppages but also include property casualty losses with damage from fire.  

A Useful Evaluation Tool

One of the most effective ways to protect a facility from transformer reliability issues is Hartford Steam Boiler’s Transformer Oil Gas Analyst Program, TOGA®. The TOGA program 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 which points toward a particular type of problem or condition. A condition can also develop from the equipment application so it is important to be knowledgeable about where the equipment is located.

Scheduling and Follow-Up

Key points for any successful maintenance program are performing maintenance at the right time and the follow-up on needed action. These two items are specific focal points of the TOGA program 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 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 there are exposed, energized electrical conductors near the sample point vicinity. Some fluid tests are sensitive to the weather and taking the sample during high humidity conditions like during a storm may affect the test results. Many TOGA customers also 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 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

Knowing the limitations of any program is necessary to establish proper risk levels for testing frequencies for the right type of test. The TOGA program can successfully detect about a third of transformer failure modes, which is confirmed by research described in “Economic Issues 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 (below) is a partial list of conditions that transformer oil testing can help identify. Placing emphasis on recommendation follow-up and consistent test result trending has established high reliability standards.

Table 1: Conditions Transformer Oil Testing Can Help ID 

 togachart  

Understanding these test results can be difficult because of the many variables from equipment types, sampling inconsistencies and laboratory reporting methods. Computer generated analysis can be generic and it 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 its health and can give an indication of the condition's severity. Trending the oil sample results can give indications of changing conditions that require attention or allow the ability to plan for an outage at a later date. This very important point not only affects maintenance planning, but 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 de-energize 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 TOGA story. The cooperative transformer was a sole supplier to a winter-only business and 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 that he addressed 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 presented to the Original Equipment Manufacturer (OEM) and the transformer was removed from service to investigate the problem. The OEM discovered the no-load tap changer contacts were the source of the problem and repairs were completed under the original warranty.

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

Case 4 - TOGA Recommendation Points Towards 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 expected equipment useful life. A follow-up sample was requested by TOGA that prompted a closer inspection of the transformer and a broken electrical bushing was found. A damaged bushing has an increased risk of flashover possibly causing a transformer failure. The bushing was replaced and the transformer returned to service.

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

Summary

Using insulating oil analysis is a cost-effective maintenance practice with a proven track record of increasing system reliability and saving owner resources. Insulating oil samples are normally taken without removing the equipment from service. Transformer failure rates from a non-TOGA testing program show a failure rate of 30 out of every 1,000 transformers. The TOGA testing program has a transformer failure rate of less than one (.64) of every 1,000 transformers. These results show that this type of maintenance program dramatically improves reliability at a reasonable cost.2

Footnotes

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

[2]Paul Boman, address, 1st International Conference on Insulation Condition Monitoring of Electric Plant, Wuhan University, Wuhan, China, Sept. 25, 2000.

About the Author

Paul Boman has worked in several positions for Hartford Steam Boiler since 1988. In 1999, he became TOGA® (Transformer Oil Gas Analyst) Program Director, and was responsible for the redesign of the program. TOGA is an Internet-based analytic program that performs asset management services for more than $520 million of equipment. Paul has a M.B.A. degree and is a member of the Institute of Electrical and Electronics Engineers (IEEE).

back to top

Disclaimer statement:

All recommendations are general guidelines and are not intended to be exhaustive or complete, nor are they designed to replace information or instructions from the manufacturer of your equipment. Contact your equipment service representative or manufacturer with specific questions.

back to top