Abstract: In the load reducing process of unit No.11 of Wangting Power Plant, the highpressure governing valve closed suddenly, which caused that the water level in steam drum rose and the main fuel protection tripped (MFT) finally. It was determined by analysis that the reason caused the high pressure governing valve to be closed suddenly as following，the gap and the porthole of the MOOG valve were blocked by the pollutant in the EH oil system and the valve was malfunctioned. The governing features of the water level in the steam drum of drum boiler and the matters needing attention in the operation were put forward, which is helpful for avoiding unplanned outage and improving safety and reliability of the unit.

Key words: EH fire-resistant oil, high pressure governing valve, water level in steam drum, three impulses