Investigation of Lightning and Grounding Installation Practices That Resulted In A Turbine-Generator Failure Within An LNG Supplied Generation Plant

This paper presents a root cause analysis (RCA) of a lightning atmospheric event that caused the trip of a combined cycle power plant. It will concentrate on why the 138kV breaker for a 135 MVA steam generator (STG) opened under load during a lightning event. When the generator breaker opened, the steam turbine went into an overspeed condition. This resulted in the ejection of a 36-ton generator rotor. In contrast, the adjacent Gas Turbine Generator (GTG) continued to operate without incident. The GTG had proper electronic grounding, including connection to the ground grid and complete magnetic shielding of the entire control system. The RCA covered why the STG governor, Automatic Voltage Regulator (AVR) and control systems failed. It also addressed the installation and grounding design errors, antenna effects of grounding conductors, and electronic grounding of the generator controls. Other areas such as inadequate lightning protection, personnel step and touch potential safety design, transferred earth potentials, and the related LNG Gas supply were included. The reasons why the STG had an overspeed event will not be discussed in this paper since it was not part of the scope of the site RCA. Specific recommendations to prevent a re-occurrence will be presented and to improve the overall site performance unique to a coral-based rock location. Recommended improvements are in process.