ABB REC670 Applications Manual page 215

1MRK 511 401-UEN A
Bay control REC670 2.2 IEC
Application manual
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 of 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 primary 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%.
Section 8
Current protection
209