O&M, Plant Optimization, Retrofits & Upgrades

Downtime Avoided

Issue 1 and Volume 121.

How One Plant Identified a Big Problem and Made Timely Repairs

A power generating station located in Southwest Kansas was preparing to come back online from a scheduled six-week outage. During startup testing of the isolated phase bus duct, ‘B’ phase experienced a failed megger test. Following the failed megger, the plant performed high potential testing, which also failed.

The operators had neither the time nor expertise to identify what was causing the issue, much less fix it. The plant was scheduled to go back online, and they simply were not ready. Any extended delay would not only be an inconvenience, but a failure that would cost hundreds of thousands in lost revenue, not including the cost of emergency response repairs. One of the power plant’s units needed to be assessed and returned to working order as soon as possible.

The plant needed help. They needed expertise from a company that not only possessed the necessary skill set and knowledge to address such an issue, but also had the flexibility to be on site the very next day. The plant could not afford to lose potential revenue from an extended outage.

A call was immediately placed to Powell Electrical Systems, Delta Unibus Division to take advantage of its Repair Replacement and Refurbishment bus program. As a key piece of the program, SE Energy was dispatched within hours to the site to arrive the next day, Friday at 7:30 a.m. Although the power generating station’s bus was not a Powell designed bus, SE Energy was well equipped to addres the issue. After it was determined that the problem was more extensive than anticipated, SE Energy mobilized additional crew members. Poised and ready in standby mode, the SE Energy crew arrived on Saturday at 7:30 a.m.

Assessment

After assessing the test conditions and work completed by the owner, the step was to inspect all the insulators on the B Phase. Once removed, the gaskets were inspected and discovered that many had been twisted during insulation, causing damage to the gasket.

Although the interior of the B phase was clean and dry, evidence of water ingress could be observed at the bottom of several insulators. It was discovered that a broken insulator with visible tracking was located on the bottom of one of the insulators. Once the hardware was removed for re-use the insulator was turned over to the plant’s owner. Once this broken insulator was removed, a 2.5kV megger was completed on A, B and C phase. Consequently, the B phase megger readings had improved to the point where they were now in line with both A and C.

Indication of water ingress was additionally found at the top of one of the insulators. The culprit in this situation was that the bolt holes had rusted, leading to additional corrosion. Finally, an inspection of the outer shell was conducted in order to check for cracks or other issues, and none were identified. The team now knew what the root causes of the problems were, and they would lay out a game plan to solve them.

Solutions

The gaskets discovered were either wiped clean and re-installed or replaced completely. The nature of the gaskets was determined to be a very likely source of the water ingress found at the bottom of one of the insulator and at the top of the other. SE Energy recommended installing additional drains at the low point in the bus in the area of the first broken insulator on all phases. In order to clean up the water ingress evidence and establish a baseline for future inspections, SE Energy vacuumed and hand-wiped the B phase at the point where the water ingress had been previously detected in order to establish a baseline for future inspections. Additionally, a visual inspection was conducted on the outdoor bellows which revealed that they were in fact near the end of their functional life.

Testing

A visual inspection of the indoor bellow was completed. This bellow was painted and, just as the outdoor bellow, this one was also nearing the end of its functional life. The bus was closed up, and preparations began for high potential testing. SE Energy cleaned up the work area, separated tools and closed out activities with the owner before demobilizing.

Preparation for testing was made by flagging off affected areas, verifying isolation and notifying other contractors working on the combustion turbine and generator. SE Energy conducted high potential testing. A, B and C phases were found within acceptable limits, and with similar results from the owner acceptance testing when the bus was originally installed. Test conditions were very favorable, 75°F with 75 percent relative humidity. After a successful test, the bus was released for return service, allowing the plant to keep its downtime to an absolute minimum, saving hundreds of thousands in both lost revenue and emergency service.

Recommendations

SE Energy addressed the immediate issues the plant was having which caused the failed high potential testing in order to safely return the unit to service. However, additional corrective actions would need to be taken during the next scheduled outage prevent further failures. The first recommendation was to replace all the insulator gaskets and stainless steel mounting hardware. Additionally, the power plant operators were instructed to inspect and hand wipe all the remaining insulators and the inside of the Isophase shell. New low point drains should be fabricated and installed at specific locations where the problems were detected. The rubber expansion bellows would need to be replaced in-kind along with the door gaskets and hardware both at the GSU and the generator.