ABB REL650 Applications Manual page 248

Section 7
Current protection
242
Often, the motoring condition may imply that the turbine is in a very dangerous state. The
task of the reverse power protection is to protect the turbine and not to protect the
generator itself.
Steam turbines easily become overheated if the steam flow becomes too low or if the
steam ceases to flow through the turbine. Therefore, turbo-generators should have reverse
power protection. There are several contingencies that may cause reverse power: break of
a main steam pipe, damage to one or more blades in the steam turbine or inadvertent
closing of the main stop valves. In the last case, it is highly desirable to have a reliable
reverse power protection. It may prevent damage to an otherwise undamaged plant.
During the routine shutdown of many thermal power units, the reverse power protection
gives the tripping impulse to the generator breaker (the unit breaker). By doing so, one
prevents the disconnection of the unit before the mechanical power has become zero.
Earlier disconnection would cause an acceleration of the turbine generator at all routine
shutdowns. This should have caused overspeed and high centrifugal stresses.
When the steam ceases to flow through a turbine, the cooling of the turbine blades will
disappear. Now, it is not possible to remove all heat generated by the windage losses.
Instead, the heat will increase the temperature in the steam turbine and especially of the
blades. When a steam turbine rotates without steam supply, the electric power
consumption will be about 2% of rated power. Even if the turbine rotates in vacuum, it will
soon become overheated and damaged. The turbine overheats within minutes if the
turbine loses the vacuum.
The critical time to overheating a steam turbine varies from about 0.5 to 30 minutes
depending on the type of turbine. A high-pressure turbine with small and thin blades will
become overheated more easily than a low-pressure turbine with long and heavy blades.
The conditions vary from turbine to turbine and it is necessary to ask the turbine
manufacturer in each case.
Power to the power plant auxiliaries may come from a station service transformer
connected to the secondary side of the step-up transformer. Power may also come from a
start-up service transformer connected to the external network. One has to design the
reverse power protection so that it can detect reverse power independent of the flow of
power to the power plant auxiliaries.
Hydro turbines tolerate reverse power much better than steam turbines do. Only Kaplan
turbine and bulb turbines may suffer from reverse power. There is a risk that the turbine
runner moves axially and touches stationary parts. They are not always strong enough to
withstand the associated stresses.
Ice and snow may block the intake when the outdoor temperature falls far below zero.
Branches and leaves may also block the trash gates. A complete blockage of the intake
may cause cavitations. The risk for damages to hydro turbines can justify reverse power
protection in unattended plants.
1MRK 506 334-UUS A
Application manual