Life extension of oil filled transformers and shunt reactors

Over recent decades, the life extension strategy has changed from time-based to condition-based and risk-based maintenance. As transformer diagnostic techniques improved in recent years, operators have been looking for techniques, measures, and methods to extend the life span of installed transformers and reactors. Working Group (WG) A2.55 has produced TB 887 as a guideline for approaching transformer life extension projects.  The Australian member of the WG was Paul Guy.

In some circumstances, it may be debatable whether a performed intervention can be considered a life extension. The following definition has been applied to help clarify.

Life extension is defined as a set of major interventions on a transformer, beyond “normal” maintenance and repair, to remedy its problems, restore its condition and postpone a predicted end of functional, economic, or reliable life. It is applicable but not limited to aged transformers with or without defects or faults, functional or failed.

There are important questions that are typically asked by the person in charge of proposed life extension measures and are related to:

• Fleet management strategies
• Financial and regulatory considerations
• Screening for possible candidates for life extension
• Methods and good practice of life extension
• Decision making - business case of life extension.

Fleet management strategies

The TB shows an approach to adding life extension as an alternative to the classic existing “repair or replace” strategy. In this way, the replacement wave of an aging transformer fleet can be optimized by limiting and spreading the yearly number of units to be replaced.

Financial and regulatory considerations

Having a clear understanding of the expenditure rules and regulations to apply for all kinds of intervention is important when considering the impact of an investment on the financial income statement and revenues. Relevant costs to be considered during a life extension process are:

• Costs of efficiency and losses
• Cost due to reduction of expected life span
• Costs of failures and business interruption
• Non-quantifiable costs

Screening of possible candidates for life extension

Once the company strategy is in place and all regulatory and financial considerations are clear, the questions will be focused on the best candidate. A special focus is made on the aging mechanism of the insulation system as the functional end of life is commonly linked to the mechanical condition of the cellulose insulation. Understanding the role of moisture, temperature and oxygen as main ageing accelerating factors for all components is crucial to evaluate the effectiveness and potential success of a life extension measure. The TB shows how to analyse common failures and degradation by applying existing diagnostic technics and helps structure the process from an observation to a technical conclusion.

Methods and good practices of life extension

The TB shows the most relevant life extension methods, their technical benefit, related costs, and complexity. Major ageing mechanisms and life span reduction are mainly related to the insulation system and the components. The focus is, therefore, on the following options:

• Oil treatment processes
• Active part drying processes and re-clamping
• Restoration of sealings and rust treatment
• Change and upgrade of components
• Complete refurbishment, including winding exchange

Decision making - business cases of life extension

The brochure assists asset managers in evaluating general life extension alternatives and strategies. The brochure presents a techno-economic analysis of some life extension alternatives, namely:

• Cellulose insulation drying
• Sealing the oil preservation system
• Enhanced cooling
• Enhanced reliability through replacement or refurbishment of components
• Mid-life refurbishment

By addressing the typical set of questions, this TB will support interested readers in covering all major steps when applying a methodology for life extension measures on their transformers and reactors. In addition, the multitude of referenced technical documents should enable the readers to get all the required knowledge and allow them to dive further into specific technical or economical topics if desired.

The brochure is free to members and €300 to non-members.