What caused the instant shutdown of a Utah Power Plant?

Power Plant Generator Core Failure Analysis

In the fall of 2000, a coal fired power plant in Utah suddenly shut down, leaving operators scramling for answers. A tripped circuit on a generator, paired with sparks observed arcing from the machine, pointed to a problem with this unit. But it took detailed analysis from Exponent to pinpoint the cause and provide a roadmap for remediation.

The Challenge

The Hunter Power Plant in Castle Dale, Utah, is a coal-fired power plant that converts energy into bulk electrical power and distributes it to consumers throughout the electrical grid of Emery County. On November 24, 2000, the power plant experienced a sudden shutdown due to an issue in the stator core of the generator.

At the time of failure, the generator was operating within its rated capacity. When sparks and heavy arching were observed, the unit tripped automatically, shutting the system down. Exponent's electrical engineers were retained to conduct a thorough post-incident inspection of the generator, which revealed a series failure of the stator core.

Exponent's Multidisciplinary Solution

A stator core is made up of layers of laminate and coated on each side with a layer of electrical insulation to prevent current flow between them. In this case, the stacked laminated cylindrical structure was nearly 19 feet long and more than 8 feet across and had begun to partially melt. Generator insulation lamination reduces the magnetically induced circulating currents within the generator core that could create core losses and even thermal damage within the core.

Upon investigation, Exponent's multidisciplinary team of mechanical engineers, electrical engineers and thermal engineers determined that three narrow wormhole-like tunnels had melted along the lengthwise axis of the stator of the generator, resulting in 200 pounds of molten metal flowing out over the end of the stator core.

It was also determined that a small, inter-laminar short had grown into a major melt zone, triggering two other melt areas caused by intense over-fluxing of the magnetic circuit in the stator core. Exponent determined that the heat generated by the magnetically induced current within the generator core was high enough to melt the steel laminations. The heat was generated as a result of the currents flowing inside the generator core, which was due to compromised insulation of the core laminations.

Exponent's Impact

To prevent future incidents, Exponent recommended testing the integrity of the electrical insulation between the generator core laminations during scheduled maintenance using methods such as the stator core loop test. This would catch degraded lamination insulation in the early stages and prevent catastrophic core failure.