3.4 Overspeed trip
The overspeed trip is the final protection against catastrophic failure of the turbine following a load rejection, causing the unit to become disconnected from the power system. Additionally, an overspeed may be caused by a governor failure on an unsynchronised turbine causing an excessive steam demand.
An excess approaching 100% would cause a rapid acceleration of over 10% speed per second. In documented cases where this has occurred, the centrifugal stress limits of the rotating parts have been exceeded and a major rupture has taken place with components penetrating the casing. Speeds as high as 180% have been recorded.
The manufacturer normally carries out a works overspeed proof test at 120%, well below the design limit at which failure could occur. The setpoint of the overspeed trip is such that this speed would never be exceeded even at the maximum acceleration rate of the turbine. Because of the delay time associated with the mechanisms, the finite response time of the valve relays and the stored energy in the form of steam and water within the turbine, it is logical to split the 20% range of 'available' overspeed equally between the governor and the overspeed trip. The stop valves (operated by the overspeed trip) and the governing valves will therefore have similar requirements for their maximum closure rates. The overspeed trip is set in the range of 110-111% speed. This recognises normal governor action over a ± 1% speed range.
Overspeed is detected by means of a pair of spring-loaded trip bolts mounted in an extension to the turbine shaft at the HP end of the machine. A typical arrangement is shown in Fig 2.44. One trip bolt assembly is used for each trip channel and each assembly is provided with independent on-load test facilities. The bolt has its centre of gravity a short distance from the axis of rotation and, at low speeds, is held retracted by a spring. At 10% overspeed, the out of balance force overcomes the spring force, causing the bolt to extend beyond the shaft where it trips the static trip lever and releases the latch to trip the turbine. Adjustment to the tripping speed is carried out when the turbine is stationary and access to the adjusting plug is possible.
The overspeed trip can be tested without actually overspeeding or taking the set off-load. This is done by selecting either the 'front' or 'rear' system for testing, thus automatically isolating the associated emergency trip valve. An interlocked movement of the trip test lever and trip test valve then injects a supply of lubricating oil under pressure into the rotating turbine shaft, through porting which directs it to the over-speed bolt under test. This then flies out and trips its emergency trip valve via the lever and trip plunger. Following release of the test pressure, these items are reset and the second bolt selected for test.