ABB Relion REC670 Applications Manual page 160

Section 7
Current protection
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 70
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 (reference angle set to 180) to trip if the power flow from the network
to the generator is higher than 1%.
154
illustrates the reverse power protection with underpower protection and with
1MRK 511 358-UEN A
Application manual