How a Remotely Located Power Plant Successfully Repaired a Leaking Transformer
Aging oil-filled transformers often develop leaks from rust, corrosion, welding defects, and worn gaskets. During the warranty period, repairing these leaks is typically the responsibility of the transformer manufacturer (OEM). However, once the warranty expires, the costs of fixing transformer oil leaks fall on the asset owner. For remote locations, these challenges and costs are magnified due to logistical constraints and limited access to experienced repair personnel, repair equipment, or reliable technologies. This article reviews how a remote power utility in South Asia overcame these obstacles with the help of a technical service partner and an innovative transformer leak repair system.
The Challenge of a Transformer Oil Leak
A 70 MVA generating transformer (GT) at a remote hydroelectric power plant in Bhutan was experiencing oil leakage from the main tank gasket. With the warranty expired, the Japanese transformer OEM could no longer provide repair and maintenance services. The plant manager faced the challenge of finding cost-effective resources to address the leak repair exacerbated by the plant’s remote location and the transformer’s unique Bell Tank design. These factors required an alternative approach before a comprehensive maintenance and repair plan could be established.
Diagram 1: Comparing types of power transformer tank designs
Understanding the Situation in Bhutan
Bhutan is a mountainous country bordered by China to the north and India to the south and west. While the plant’s remote location compounded logistical challenges of getting repair technicians to the plant, the transformer’s unique Bell Tank design presented additional technical hurdles. Leakage from the Bell Tank gasket meant the standard solution of replacing the gasket would be time-consuming and expensive. Additionally, the uncertain availability of a replacement gasket due to supply chain issues made the process of scheduling the repair during a planned shutdown nearly impossible. Without experienced maintenance personnel nearby, the plant manager sought solutions beyond Bhutan’s borders.
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Calling on a Specialist to Repair the Leaking Transformer
In anticipation of the GT warranty expiration, the hydro-plant manager had begun planning for potential repairs to the transformer. To this end, the manager and technical personnel in Bhutan had attended an online presentation by Bhagyodaya Sales & Services (BSS) in India and its technical support team, LeakXpert, on the different strategies and materials used to repair transformer oil leaks. With this presentation in mind, the plant manager contacted LeakXpert to discuss the services they could provide for Bell Tank gasket leakage in Bhutan.
With every transformer leak scenario being unique, LeakXpert needed to evaluate the situation in Bhutan so they could provide the best solution. They asked the asset owner to provide photos of the leak areas to start the process of devising the most effective repair approach.
After assessing the GT gasket leakage photos, LeakXpert confirmed that the leakages could be repaired. One option would be the traditional process of replacing the main gasket. The other option would be to seal the leakage on the outside of the transformer using a leak repair sealant. The award of this repair contract would be granted through a bidding process, so the development of an efficient repair process was a critical step.
Why the Traditional Approach of Gasket Replacement was Rejected
Unlike other transformer designs, the Bhutan hydroelectric plant GT design is inverted so the gasket is located at the bottom perimeter of the tank. To replace the main gasket on these types of transformers, the entire tank and active part of the transformer must be lifted off the bottom tank flange. This becomes a more complicated and involved replacement process, which requires heavy-lifting equipment and cannot be done when the transformer is energized. The traditional process of replacing a faulty gasket in a transformer with this specific Bell Tank design involves:
1. De-energizing the transformer
2. Draining the oil into auxiliary storage tanks
3. Reconditioning the oil during gasket replacement to remove moisture and atmospheric gases to which the oil was exposed
4. Using heavy-lifting equipment to access the gasket at the bottom of the tank
5. Installing a new gasket
6. Reassembly to secure the tank to the newly gasketed base
7. Filling the transformer tank with reconditioned oil in step 3.
8. Re-energizing the transformer
Due to the challenges of replacing the gasket in this transformer, using a repair sealant designed for power transformer leaks was determined to be the safest, most effective, and cost-efficient solution. This would require sealing the 18-meter perimeter of the Bell Tank cover and the 132 pairs of the tank flange nuts and bolts.
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Using a power transformer repair sealant would also provide additional opportunities to reduce the time and cost of the repair. For example, the repair of the gasket leak would not require removing, storing, or reconditioning the oil while the sealing is being done. This would eliminate time-consuming oil reconditioning and refilling the tank. Plus, the repair could be done without shutting down the transformer. The only downside to this recommendation is that the oil would still actively leak during the sealing process. To overcome this challenge, the selection of a durable sealant system, whose sealing capacity would not be compromised in the presence of oil, was required.
In discussions with the plant manager regarding the best path forward, LeakXpert detailed the experiences they had in the field with other sealants used ostensibly to repair transformer oil leaks. They described how many of these sealants had a reputation for poor performance because many of them were designed for other applications like home plumbing or automotive repair. These sealants did not contain the robust technical characteristics needed for effective transformer leak repair and contained components that were incompatible with high-voltage power generation and transmission operations. However, LeakXpert acknowledged that there was one unique sealant solution specifically formulated to withstand the operational conditions of transformers—Polywater® PowerPatch®.
Polywater® PowerPatch®: An Innovative Transformer Leak Repair Sealant
PowerPatch is a permanent leak repair system with two main components: a strong, fast-curing putty used to seal active leaks and a durable resin sealant. LeakXpert had worked with PowerPatch for over eight years, successfully completing hundreds of transformer repairs across India. Throughout that time, PowerPatch was used exclusively by LeakXpert to repair actively leaking transformers, including systems with Bell Tank gasket leaks.
The use of PowerPatch for transformer oil leak repairs has many advantages over ill-suited sealants. The most important capabilities of PowerPatch for the gasket leak in Bhutan were the ability to:
• Maintain adhesion on active oil leaks during the repair process.
• Maintain adhesion under stresses caused by vibration and mechanical expansion and contraction of the transformer during operation.
• Eliminate the potential of partial discharge inside the transformer due to its dielectric properties.
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Implementing PowerPatch for Transformer Repair
The proposal to use Polywater PowerPatch was accepted, and the LeakXpert team traveled from India to the hydro plant in Bhutan to address the GT leakage. Upon physical inspection, the team identified 25-30 active leaks through nut and bolt pairs and gasket areas, as well as 8-10 meters of non-leaking tank perimeter.
An often-overlooked source of oil leakage in transformers is the migration of oil through the tank bolt threads, which occurs because the threaded connection provides a natural pathway for oil to escape under pressure. This can become a significant issue, particularly after sealing the gasket seam, as oil pressure increases and seeks alternative escape routes. Once the gasket seam is sealed, oil pressure increases, causing the oil to migrate through any path of low resistance. The bolt threads provide such a path. To prevent such leakage, the nuts and bolts of the oil tank also required sealing.
To thoroughly repair the transformer leaks, the process was divided into three phases:
1. Surface Preparation
Thorough preparation of the transformer surfaces was critical for ensuring a successful seal. First, the removal of paint, grime, oil, and incipient corrosion was needed to maximize the adhesion of the sealant to the steel transformer surface. Then, surfaces where PowerPatch would be applied were abraded. This increased the surface area to which PowerPatch could adhere, making the final seal much stronger. The LeakXpert team of six took a half day to complete the surface preparation.
2. Seal Non-Leaking Areas
The next stage of the repair process involved sealing 8-10 meters of non-leaking areas of the GT. The reason for this was to create a barrier that prevented oil migration to other areas of the gasket seam and to the Bell Tank nuts and bolts once the active leaks were sealed. These areas did not require the use of both PowerPatch sealing components. Only the permanent PowerPatch resin material was used, which saved time and materials. The LeakXpert team completed the sealing of the non-active areas in 1.5 days.
3. Seal Active Leaks
The last step of the repair process involved stopping the active leaks and applying the final seal. First , a solvent wipe was used to remove oil from the abraded surfaces. Then, the fast-curing PowerPatch putty was applied to the active leaks. This arrested the flow of oil, creating a short interval of time for the LeakXpert team to clean and prepare the leak surface areas for a final seal of the permanent PowerPatch resin. This step in the process took the LeakXpert team three and a half days. In the end, PowerPatch created a strong, continuous seal along the entire perimeter of the main tank gasket seam.
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The Strength of PowerPatch: Maximizing the Operational Life of Transformers
In total, the LeakXpert team successfully completed the repair in five days, with a sixth day allocated for monitoring. The plant manager continued to monitor the repair over the next month to confirm its effectiveness. With the seal demonstrating no signs of leakage, the plant manager approved the repair and specified PowerPatch for any future leak repair needs.
“(The transformer) is still performing at various loads successfully. It has completed one season and still performs well without any problems. We will consider using PowerPatch on other equipment in our plant.” — Mr. Lobzang, Bhutan Hydro Plant Manager
The Bhutan hydro plant transformer repair highlighted the unmatched versatility of Polywater PowerPatch, showcasing its effectiveness in collaboration with the skilled LeakXpert team. This innovative solution ensured a durable repair while meeting the complex demands of a remote transformer site. This approach maximized the performance and lifespan of the generating transformer while minimizing repair time and costs. By addressing the unique challenges of the Bell Tank gasket leaks, PowerPatch provided a reliable, cost-effective solution for remote transformer in-field maintenance.
