Overspeed trip devices are provided to protect prime movers such as steam turbines, gas turbines, hydroelectric turbines, wind turbines and internal combustion engines from overspeeding in the event of load rejection coincident with failure of the speed controller. Severe overspeed conditions result in catastrophic destruction of the prime mover resulting in extensive property damage and business interruption loss. To prevent such an event, it is vital that overspeed trip devices are tested to original equipment manufacturer (OEM) specifications annually, for base-loaded units and at pre-startup of standby units.
All prime movers have overspeed protection systems that detect when operating speed is exceeded by the designated OEM specified percentage. In this scenario, prime movers will initiate the closure of valves to isolate all potential energy sources, steam for steam turbines, gas for combustion turbines, wicket gates for hydroelectric turbines and fuel supply valves for internal combustion engines. For wind turbines, an electric control system to change the blade pitch, and yaw control supplemented with mechanical brakes is utilized.
Overspeed occurs in three circumstances:
- Loss of load
- Failure of the control system (Governor)
- Failure of the overspeed protection system
There are two primary types of overspeed protection systems: Mechanical/Hydraulic and Electronic.
Mechanical overspeed trip protection has a spring-loaded bolt that is held in a recessed position attached to the rotating shaft of the turbine. As the turbine shaft spins, centrifugal force is asserted on the bolt. The spring-loaded bolt is designed to stay in place at the designed revolutions per minute (rpm) of the turbine shaft. If the turbine shaft exceeds the predetermined designed overspeed, the centrifugal force on the bolt exceeds the mechanical force maintained by the spring, causing the latch to tilt the pivot and shut down the energy source to the turbine. Mechanical trip devices have been in use for decades and largely phased out by Electronic systems.
Mechanical trip systems are subject to wear and are not always accurate, with a variance below or above the 108% to 112% overspeed generally recommended by OEM. Therefore, Mechanical systems must be diligently maintained and tested. Mechanical overspeed trip is tested by removing the load from the prime mover. In the case of a driven generator, the generator breaker is opened. For some drives, the drive coupling must also be uncoupled. The speed of the prime mover is increased often by bypassing or changing the limits on the governor, the trip speeds are recorded and adjustments are made if required to meet OEM specifications.
Electronic systems differ by manufacturer, however, their working principles are similar.
Systems are comprised of several sensors that are mounted on a speed wheel spur gear type of wheel. The sensors measure the passage of teeth of the rotating speed wheel spur gear. The overspeed digital logic controller determines the shaft rpm based on designed ratio of the rotating spur gear. If the rpm exceeds the preset rpm of the shaft, logic controllers signal the trip relay to de-energize, and the machine goes through the trip procedure.
There are multiple methods to test electronic overspeed trip systems. This can be accomplished by changing the overspeed trip set point to achieve OEM-specified trip speed. Newer electronic overspeed trip devices are installed with control cards that permit a simulated self-test. This is most common for gas turbines.
Electronic overspeed protection supersedes the reliability of mechanical trip system accuracy and response times. Per API 670, overspeed reaction time should be less than 50 msec. Also, electronic governor and electronic overspeed protection systems must be installed in strict accordance with API 612 and API 670. Consequently, during inspections, overspeed systems should be revived. Mechanical overspeed trip systems should be replaced with electronic speed trip systems, or an Electronic system should be installed in addition to the already installed mechanical system for greater reliability and accuracy.
