Every steam turbine is also provided with emergency governors which come into action under specific conditions. In general, an unplanned or emergency shutdown of a turbine is known as a “turbine trip”. The turbine trip signal initiates fast closure of all steam inlet valves (e.g. turbine stop valves – TSVs) to block steam flow through the turbine.
The turbine trip event is a standard postulated transient, which must be analyzed in the Safety Analysis Report (SAR) for nuclear power plants.
In a turbine trip event, a malfunction of a turbine or reactor system causes the turbine to trip off the line by abruptly stopping steam flow to the turbine. The common causes for a turbine trip are for example:
- the speed of the turbine shaft increases beyond specific value (e.g. 110%) – turbine overspeed
- balancing of the turbine is disturbed or due to high vibrations
- failure of the lubrication system
- low vacuum in the condenser
- manual emergency turbine trip
Following a turbine trip, the reactor is usually tripped directly from a signal derived from the system. On the other hand, the reactor protection system initiates a turbine trip signal whenever reactor trip occurs. Since there remains still energy in the nuclear steam supply system (NSSS), the automatic turbine bypass system will accommodate the excess steam generation.