ABB RELION 670 Series Applications Manual page 445

1MRK 504 152-UEN B
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.
A hydro turbine that rotates in water with closed wicket gates will draw electric power from
the rest of the power system. This power will be about 10% of the rated power. If there is only
air in the hydro turbine, the power demand will fall to about 3%.
Diesel engines should have reverse power protection. The generator will take about 15% of its
rated power or more from the system. A stiff engine may require perhaps 25% of the rated
power to motor it. An engine that is good run in might need no more than 5%. It is necessary
to obtain information from the engine manufacturer and to measure the reverse power during
commissioning.
Gas turbines usually do not require reverse power protection.
Figure 266
overpower protection. The underpower protection gives a higher margin and should provide
better dependability. On the other hand, the risk for unwanted operation immediately after
Application manual
illustrates the reverse power protection with underpower protection and with
Section 8
Current protection
439