The Reliability in Action department features case studies submitted by our readers. To have your case study considered for inclusion in an upcoming issue, e-mail it to parnold@noria.com or mail it to Reliable Plant, P.O. Box 87, Fort Atkinson, WI 53538. If we publish your case study, we'll send you an official Reliable Plant T-shirt. This issue's case study was written by a predictive maintenance specialist at an electric company in the western United States:
In November 2005, a routine infrared survey located hot spots on isophase bus tube ducts. Initial thoughts led our team to believe that the heat was generated from corrosion-laden bolts. Therefore, recommendations to repair the hot spot dealt strictly with these bolts and the close surroundings. However, when initial recommendations failed to resolve the problem, the phenomenon and solution was in question. How could corrosion cause hot spots in these bolts and no hot spots in the many other bolts in the same area? A recent outage would reveal the root cause.
Hot spot 1 is displayed in photograph and infrared form.
Area, location and description: The troubled area bolts were located on the isophase bus ducts connecting the 180-megawatt generator to the 13.8-kilovolt main transformer, and also near the auxiliary transformer. The bolts connected bonding jumpers between ducts. A total of five troubled areas were found.
Findings: Upon routine infrared rounds, the following shots were observed (see photos on this and subsequent page).
Initial recommendations: We were consulted that this event was a "common anomaly in many isophase systems." Apparently, induction causes heating in iron bolts, washers or both. Also, circulating currents through grounded isophase bus duct and support beam systems can cause heating. The grounding strap normally carries the most heat and could continue heating to the melting point.
Hot spot 2 is displayed in photograph and infrared form.
The recommendation was, first of all, to change the bolt and washer to stainless non-ferrous hardware. The second option was to clean all grounding straps on the ducts. The idea was that these grounding straps were taking all of the current and the other "cold" straps were not.
This work was completed and a post-work infrared survey done. The survey found that the situation was unresolved. While the heating of the isophase bus was a common issue in industry, we were experiencing something quite different. The answer would be found when investigating recent work done on the ducts and transformers.
Hot spot 3 is displayed in photograph and infrared form.
Recent outage/action: As the history on this equipment was researched, we discovered that personnel working on the transformer had cut the shorting plate (see below) at the main transformer in order to replace the center bushing during the overhaul. Cutting the shorting plate changed the dynamics of the induced currents in the three phases and allowed circulating currents to build up. This caused the high temperatures in the grounding straps' bolts.
Post-repair condition: Post-temporary repair conditions were acceptable, and plans for permanent repairs were made. After the final repairs, heating in all areas was once again normal.
Hot spot 4 is displayed in photograph and infrared form.
Hot spot 5 is displayed in photograph and infrared form.
After the final repairs, heating in all of the areas was once again normal.
Another view showing that heating was once again normal.
Personnel working on the transformer had cut the shorting plate.
