ABB RES670 Applications Manual page 167

1MRK 511 407-UEN C
Phasor measurement unit RES670 2.2 IEC
Application manual
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 65 illustrates the reverse power protection with underpower protection and
with 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 synchronization may be higher. One should set the
underpower protection (reference angle set to 0) to trip if the active power from the
generator is less than about 2%. One should set the overpower protection
Section 8
Current protection
161