On December 20, 2007, Wolf Creek was performing the monthly test of
EDG B. Prior to the operator placing his hand on the RAISE/LOWER handswitch, the load made a step increase from 4.2 MWE to 7.2 MWE and did not respond to lower signals from the control room handswitch. The
EDG was tripped shortly thereafter. Troubleshooting examined the digital reference units (
DRUs), the 2301A electronic engine governor, and the Woodward electromechanical governor mounted on the engine. Wolf Creek found the voltage readings on various terminals to be acceptable. However, resistance measurements across the two terminals that connect the electronic engine control unit and the electromechanical governor was expected to be 35 ohms and was found to have infinite resistance, indicating an open circuit. The cover plate of the governor was removed and one wire was found to be disconnected from its terminal. Wolf Creek has not determined the cause for this failure or its impact on safety systems. During interviews, Wolf Creek engineering stated that the electromechanical portion of the governor would fail and the
EDG mechanical portion of the governor would drive the
EDG to its high speed and maximum fuel setpoint. Because the
EDG was synchronized to the grid, the inspectors judged it reasonable that the
EDG would not cause the grid to increase in frequency but that the
EDG would carry an amount of load equivalent to the mechanical governor setpoint. The inspectors questioned the licensee on the impact if this failure occurred during a valid safety injection signal or
loss of offsite power. The inspectors found that such a frequency variation had not been previously analyzed. If the
EDG governor failed in the observed manor, the
EDG may have driven the bus and the connected safety equipment to a frequency that exceeds the
TS 3.8.1 limit and/or the currently analyzed limit. This governor failure is being tracked by Wolf Creek in Condition Report (CR) 2008-000088. Since additional inspection is needed to evaluate the failure mode and significance of the failure, this issue is being treated as an Unresolved Item (
URI)
05000482/2007005-01,
EDG B governor failure effect on supplied equipment.
