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Sequential Tripping Upgrades for Steam Turbine Plants

By: Ira Rubin, POWER Engineers

ABSTRACT

Steam turbines are susceptible to overspeed damage from a trip event which includes generator and turbine tripping. Damage to steam turbines can be severe due to overspeed, possibly causing turbine blade damage and a loss of the generating unit. Improved generator protective relaying to allow the steam turbine to be tripped first, followed by tripping the generator, is called sequential tripping, and is preferable. Simultaneous tripping is required if a generator electrical fault is detected, however, the majority of tripping events are due to events related to the turbine, such as lube oil or vibration anomalies, bearing problems, and steam pressure and temperature conditions that migrate out of acceptable ranges. Overspeed as a result of simultaneous tripping could also cause turbine damage, but quickly tripping the generator takes priority to minimize generator damage.

Sequential tripping is performed by closing the steam stop valves with the generator still electrically connected. Without the steam supply to the turbine, shaft power to the generator will quickly decay. A reverse power protective relay (Device 32) is applied to detect the reverse flow of real power that occurs when the generator begins to function as a motor. When the reverse power relay actuates, the generator circuit breaker and exciter are tripped, completing the actions of a sequential trip. Using sequential tripping allows the energy within the turbine to sufficiently decay, thus reducing the possibility of overspeed and turbine damage.

The application of new generator protection allows programmable logic to be incorporated, resulting in ease of setting up both tripping schemes. Older generating plants constructed during the 1980’s and 1990’s using electro-mechanical relays may not provide the protection available from today’s digital programmable relays. Generator protection upgrades can be undertaken and installed during a scheduled plant outage.

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