Modern Transformer Maintenance, Life-extension Strategies

by Dennis Ledel Sr. and Chuck Baker, SD Myers

Transformer maintenance and life extension has been defined many ways in many markets. The perspective, however, is becoming more consistent and of universal concern. Common factors have driven the unification, including aging transformers, a higher than expected failure rate of electrical equipment used to replace older units (with increased installed capacities), and a steady decrease in transformer expertise.

When looking at a long- and short-term transformer management strategy, consider what is aged and considered modern; what is practical and realistic; and what is the end goal with a clear plan.

IEEE C57 Standards recommend that transformers rated above 500-kVA be electrically tested annually. In practice, these transformers are tested every three to five years, and some industries extend testing to six- and eight-year cycles.

IEEE C57 recommends that transformers rated above 500-kVA be electrically tested annually.

There are also concerns about failure rates of key components such as bushings, arrestors, de-energized tap changers and load-tap changers (LTCs). Their maintenance must be part of a plan. Failure in any of these components can reduce the useful life of a transformer.

Many factors in current procedures have changed. Older transformers were designed and built without computers that favor smaller, lighter and more compact designs. Older transformers are more robust because they hold excess capacity for overloads, and they’re failing for multiple reasons: deterioration of the cellulose insulation systems, contacts, bushings and fluid leaks, and they no longer meet customers’ capacity requirements.

Modern transformers typically are manufactured using cost-competitive designs, materials and fabrication, and they’re computer-designed to meet modern specifications. In some cases, tests such as heat runs and three-phase bolted fault current tests are simulated with computer-driven software packages. In other cases, no physical empirical testing is performed. Some application problems are associated with or compounded by engineers’ love affair with speed control and soft start on inductive motors with silicon controlled rectifiers and load commutated inverter systems common in industrial and commercial applications. These systems generate harmonic wave forms that older transformers are ill-equipped to handle. The end product is elevated operating temperatures that shorten the lives of the cellulose insulation systems and degrade the dielectric fluid. Modern transformers can be designed to accommodate distorted wave forms (K-factors), but the transformers will be smaller, lighter and contain less cellulose insulating materials. Few have modified their maintenance standards for these design changes.

Numerous transformer manufacturers do not dry the insulating materials to meet the IEEE C57 Standards prior to shipment. The standard recommends that paper insulation contain less than 0.5 percent moisture by dry weight. Field tests using the IEEE standards have discovered new and remanufactured transformers with moisture levels in the cellulose insulation systems at 1 to 1.4 percent. (This problem also will be revealed in an elevated CHL power factor of 0.5 to 1 percent). The additional moisture will:

1. Increase the losses in insulation systems with a reduced expected life of the transformer.

2. Elevate the acidity level of the dielectric fluid and further damage the cellulose paper.

3. Reduce dielectric characteristics of the fluid and cellulose insulation.

Short-term planning and cash flow considerations have taken precedence over long-term planning and the capital investment of transformers.

Specialists Retiring

Workers who fabricate, design and service power transformers-including electrical power engineers-are retiring without passing their years of hands-on knowledge to the younger generation. Tight budgets translate to no overlap of new employees and retiring employees and the interruption of continuous work required to maintain quality products. This global scenario adds to possible defects in manufacturing and services. The labor force committed to installing, assembling and commissioning transformers experiences the same labor and skill shortages as fabricators in the shops. Shortages include special skills in electrical testing, vacuum filling and fluid purifying, plus the professional support staff who test and design transformers. Electrical engineers typically prefer more glamorous careers in computer science vs. the power industries.

Electrical engineers typically prefer more glamorous careers in computer science vs. the power industry.

These factors place at risk the reliability of electrical power systems globally. The response should be the development of a comprehensive transformer maintenance program with strategies to manage these variable factors proactively. The alternative is frequent power outages and equipment failures with production losses, which most commercial, industrial and residential users will consider unacceptable.

Site Surveys, Data Management are Practical Components

Past standards and procedures likely will not meet future needs. A new approach is required to maintain reliability and continuity.

Site surveys and procedures for data management can accomplish much. Practical components that can be addressed immediately include:

• Inspecting thoroughly each time at the unit-scheduling inspections if necessary-and looking carefully for fluid leaks. Transformers with dielectric fluid leaks will have high moisture content in the fluid. Transformer gaskets and seals last some 25 years, requiring replacement once during a transformer’s lifetime. Fluid leaks not addressed quickly eventually will gain the attention of the Environmental Protection Agency. Repair leaks and aggressively manage moisture in a transformer’s insulation systems. High-moisture content in the dielectric fluid will damage the Kraft cellulose paper permanently. After the paper has been damaged, there will be loss of tensile strength that can never be restored to its original condition.
• Recording all gauge readings and observing the trends.
• Reviewing the equipment grounds, cooling fan operations, bushings, arrestors, animal guards, etc.
• Extracting an accurate fluid sample of the dielectric fluid and analyzing: standard screen for trending acids, oxidation and contaminants; moisture analysis; dissolved gas analysis; and furan for age trending and end-of-life projection. Trend these results for key actions to be taken.
• Using a small handheld thermal temperature gun to determine fluid levels, case temperature on the LTCs and transformers, bushings, arrestors and all connections. This simple task should be performed during any routine inspection program.
• Ensuring the substation yard is dry and free of debris, including vegetation. Also look for signs of animal intrusions. Our furry friends can damage insulating systems-they seem to prefer PVC insulation over ethylene propylene rubber.
• Ensuring a desiccant designed to allow the transformer to breathe through clean, dry air (if the unit uses one) has been maintained and is operable. These units are inexpensive and critical to maintain a steady stream of clean, dry air to the headspace in transformers. Many modern, maintenance-free units contain an electric heater designed to provide the transformer’s headspace with clean, dry air at all times. Consider using this equipment.
• Confirming the condition and operation of the nitrogen blanket interair systems.

After nonintrusive, energized tests and inspections have been completed, a transformer can be removed from service for de-energized electrical testing. This data will help determine transformer health and any remedial action required to extend its life. The final test report with recommendations will be used to communicate problems on key units. This data will be used to determine whether fluid processing is required, what type of fluid dehydration and reclamation with Fuller’s earth or if an internal inspection is needed that might include adjusting the blocking or repairing the de-energized tap changer.

The following electrical tests will provide some of this information:

1. Doble/power dissipation testing

2. Excitation/winding resistance/transformer turn-radio (TTR)/Megger testing (Perform the winding resistance and TTR tests on all no-load tap changer positions.)

3. Power factor testing of the bushings

4. Hot collars testing of the bushings and arrestors

5. Measuring the leakage current of the arrestors

6. Sweep frequency response analysis (SFRA)

7. Partial discharge electric testing

8. Leakage reactance testing

9. Frequency domain testing

For units that carry a critical load, online dielectric fluid monitoring can be conducted. Some quality units on the market continuously monitor the gases, dissolved gas analysis DGA and moisture content with saturation levels. Every action in a transformer will migrate quickly to the dielectric fluid in the form of gases, moisture or both. This includes thermal stress, dielectric failure and partial discharge. This data may be sent to any computer instantaneously to provide alarm points or a trip. These systems provide timely, accurate information to implement decision-making. The power industry has been slow to use modern electronic systems fully, but this is rapidly changing.

Long-term Solutions Develop Long-term Programs, Strategies

Although the problems are increasing, options for energized and de-energized maintenance exist. Develop or refine your long-term maintenance strategies.

Every person or organization responsible for transformer maintenance, life assessment or both should consider proactively developing or implementing a program that accomplishes these key objectives to ensure the safe, continuous operation of their electrical systems.

Everyone responsible for transformer maintenance, life assessment or both should consider a program that accomplishes key objectives to ensure the safe, continuous operation of their electrical systems.

Planning includes:

1. Impact. Understand the short- and long-term, direct and indirect impact and costs of unplanned power outages. List the impact and costs on all key units and prioritize investments and standards based on this. To aid in prioritizing investments and actions, know the estimated remaining useful life of each key transformer, a.k.a. condition assessment.

2. Condition. Understand the condition of transformers through intrusive and nonintrusive testing including site surveys, data collection (including fluid and electrical test data) and additional testing as required up to and including an internal inspection. This results in estimating the remaining useful life of a unit and detailing what services should be performed today, addressing any problems discovered. (At this point, you have a clear picture of the potential impact of a failure knowing the condition of key units calculating probability. From this, develop a detailed, short-term plan for today and for long-term eventual failure.)

The short-term plan ensures all critical units’ problems are corrected by limiting the damage to the units and employing testing standards with frequency and allowable levels for each transformer.

Develop a plan for the eventual failure of every critical transformer in your area of responsibility and control. Review your impact assessment and map who, what, why, when and where you will go when that failure occurs. Much can be done to minimize the outage impact if conducted prior to the eventual failure.

This approach and description applies to all electrical systems from the 1,000-kVA distribution- class transformers serving a critical load in a production line to a 1,000-MVA GSU in service providing power for multiple customers.

The question is: How do you practically begin this process from your current inherited or traditional methods? Be realistic. The design parameters of transformers have changed dramatically faster than maintenance standards and procedures during the past 20 years, and that is creating a problem.

Transformers are tightening in design specs, aging with the labor force, and the comprehensive management plans must be customized to your dependence on these critical power units.

The chronology of the recommendations listed begins with the simplest and most cost-effective, nonintrusive methods to maintain a reliable electrical power system. A well-planned strategy and condition assessment with nonintrusive testing should provide all the base information required to determine the health and reliability of your electrical systems.

Dennis Ledel Sr. is a trainer at SD Myers. Reach him at [email protected].

Chuck Baker is director of transformer field services at SD Myers. Reach him at [email protected].