|Checking for potentially corrosive sulfur.|
by Richard Heywood, Doble Engineering Co.
A comprehensive health assessment of all power assets is critical to your organization’s long-term strategy and planning.
This is true whether you’re a utility company working hard to maintain asset reliability and employee safety standards while avoiding in-service failures, a power station working to avoid downtime or striving to meet compliance demands from regulators.
Risk always has been present in the power industry but is increasing steadily as large, critical assets age.
Complicating matters further is the complexity of evaluating and assessing that risk with new tools designed to test health and planned functionality, plus an increasing volume of available data and a shortage of experienced maintenance engineers.
For power companies, it’s time to think more strategically about how they’re assessing each transformer individually, as well as their entire population of transformers holistically.
The State of Critical Assets in the Industry
In several developed countries, many medium and large power transformers in operation are 30-plus years old, causing concern over their long-term reliability and performance.
Replacement of these units can have long lead times and large capital expenditures and needs to be done strategically.
To maintain the delivery of safe, reliable power with an aging asset fleet, a method of assessment is needed to identify transformers at high risk of failure and those that require further monitoring or maintenance to ensure continued reliability.
One of the industry leading methods is an asset health review (AHR) designed to give companies detailed assessments of the assets in their fleets and to alert the companies to the actions needed to maintain these assets in good operational order and minimize costly outages.
A comprehensive AHR informs those responsible for transformer maintenance practices and strategic plans, including replacement programs, repairs, upgrades and loading strategies.
Organizations that implement AHR programs often find their electrical systems are more robust and reliable than ever before.
This allows resources to be focused on where they are needed: the small percentage of the population that has serious faults or is in poor condition.
Part of what separates the best AHRs, though, is predictability: A first-class AHR provides organizations with a more accurate assessment of anticipated problems and potential failures and better scheduling of maintenance and long-term contingency planning.
|Corrosion on a B-phase series winding.|
The Right Data and an Outside Perspective
The first step to beginning any effective AHR is to acquire the critical background data. This includes:
- Loading and visual information;
- Data acquired from tests while the equipment is in service: oil disolved gas analysis (DGA), insulation resistance (IR) and partial discharge (PD); and
- Out-of-service test data, such as capacitance and power factor, sweep frequency response analysis (SFRA), winding resistance, dielectric frequency response (DFR), etc.
Many companies outsource some of their testing. Some companies perform routine testing in-house if they have the expertise and outsource the more specialized services. Other companies have operational staff but not the technical staff for any electrical testing of these assets. Once the data is acquired, it is beneficial to have a multidisciplinary team of chemists, field-testing engineers and transformer design engineers that can evaluate the data.
Some of the data interpretation is simple; other times it can be complex and require trained specialists with good field experience to provide valuable insights. Finding an experienced, independent partner company with no vested interest in the equipment under evaluation is often wise. It’s understandable to try to reduce costs by handling an AHR internally, but it can be a missed opportunity. An organization should carefully evaluate the cost vs. benefit of using external expert AHR services before making that decision.
An AHR must be treated like an audit. Here is why:
- There is the element of limited exposure to failure. Generally, power professionals see just one to two failures every several years, which is great but limits the experience organizations have in dealing with failure. That makes it even more important to hire an outside team that knows exactly what to look for.
- The right outside vendor has expertise in most, if not all, aspects of known failure causes, and there are many reasons assets go offline or fail.
Recently, a power producer with more than 600 transformers and more than 200 generator step-up units hired a vendor to perform a fleet AHR service.
Before contracting the AHR, the company had seen inconsistent test results but lacked the internal resources to fully investigate and identify the causes, much less come up with a solution.
With the AHR, the company confidently determined the health of all critical transformers, improved transformer fleet reliability and created a documented assessment to drive maintenance, long-term strategy and planning.
Asset health is important for generation, transmission, distribution and industrial customers.
Recently a generation company that had performed an AHR of its fleet and maintained it over a few years documented the improvement in its transformer fleet reliability and enhanced its documented assessment process to help drive maintenance, long-term strategy and planning.
The Ins and Outs
To complete a comprehensive AHR, an organization must invest in experts who understand every component of an asset.
Look for a partner with broad expertise in chemistry, online and offline electrical testing and transformer design and manufacturing experience.
This is not a one-person job but a group effort that requires a multidisciplinary team for effective execution.
In 2007, a European utility experienced an unexpected transformer failure.
A thorough and detailed review of the asset by an outside vendor found the failure to be the result of overheating, which resulted in sulfur corrosion (deposition of conductive copper sulfide in the paper) and severe solid insulation degradation.
The problem was in all three series windings, resulting in an inter-turn fault on C phase.
The overheating was caused by an incorrectly positioned oil flow guide: a design error; and incorrect cooler control settings: an installation error.
Because the organization owned 25 sister transformers just like the one that failed, the company and vendor decided it was necessary to review the remaining transformers across four categories: electrical, oil, PD and infrared thermography results.
The troubling conclusions required immediate action. Five other highly loaded autotransformers had oil containing excessive potentially corrosive sulfur, and four at the same site represented risk to operation of the network and were provisionally scheduled for replacement.
Had the organization not run all those tests across its assets as suggested by the AHR findings, it’s highly possible it would not have had the information to plan for the replacement of the four at-risk transformers.
The team that determined the issues with these transformers included chemists and field-testing and design engineers, each of whom brought his or her expertise to understanding the cause of the problem.
Many testing specialists in specific areas can look at single aspects of asset health.
There are a limited number of vendors, however, with the expertise and resources to put all the components together and understand how each piece works with and affects others.
Power organizations should look for one vendor that has various skills and experience to cover all aspects that must be considered: oil testing and evaluation, electrical data and design and loading issues and environmental and maintenance factors. How these pieces fit together bring value to a review.
|Failed C-phase winding.|
Building a Strategy to Last
An effective AHR program should allow an organization to do two things:
- Address immediate issues, focusing maintenance where it is needed, and
- Plan with purchases of new units, spares and replacements.
Using an evaluation methodology that categorizes the severity, urgency, type of issues and condition of assets is fundamental to this process.
A systematic approach is important to having a consistent evaluation scheme.
Metrics and algorithms assess and combine information into standard scales and color codes that can be used to allow staff to quickly identify specific issues, actions required and the overall health of the asset population.
Management and technical staff then can respond to problems that arise based on their urgency.
The resulting effectiveness is then captured and made visible in the updated fleet overview.
It’s important to have a methodology that works for the entire organization, management and responders.
Anyone should be able to pick up an AHR report and get a sense of what works and what does not.
Sometimes results are worse than anticipated and expose risks and issues that must be addressed right away.
Other times, fears are assuaged and budgets can be reallocated.
Either way, the information affects the bottom line and day-to-day operation and maintenance at electric power organizations.
An AHR is continuous, not a one-time event, and it’s built on a trusting, collaborative relationship between the asset owner company and the evaluating vendor.
Even when an AHR has been completed, the maintenance and monitoring of assets is not over.
Organizations must employ long-term strategies to monitor asset health on an ongoing basis with readable, actionable results that the entire team can absorb.
Moving forward with an AHR can be overwhelming, but there are a few straightforward internal steps any organization can take to help streamline the process.
- Pick a leader. Determine the best person internally to lead the initiative, which will include coordinating with the vendor and ensuring your team consistently is bringing them the information and data needed.
- Get on the same wavelength. Make sure your team is aligned in goals, desired findings and dedicated resources.
- Search for a vendor. Once your team has chosen someone to lead the project and knows the team is aligned in expectations and goals, it’s time to begin the vendor search. Remember, you’re looking for an independent partner with broad expertise in chemistry, online and offline electrical testing and transformer design and manufacturing experience.
Finding mistakes and issues is hard when you’re looking in the mirror.
Getting valuable information and creating a road map for improvement requires fresh eyes, along with a team of experts armed with the most advanced technology and the experience to know when something isn’t as safe as it appears.
Richard Heywood is the general manager of operations, consulting and testing services at Doble Engineering Co. in the U.K., where he specializes in high-voltage testing of equipment in power stations and substations. He also is involved with several CIGRE working groups that cover solid insulation, aging and markers. Heywood graduated from Surrey University with a doctorate in thermal aging of transformer paper.
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