ABB Relion REC670 Applications Manual page 165

1MRK 511 358-UEN A
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%.
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 well 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 73
IED. The underpower IED 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 IED to trip if the active power from the generator is
less than about 2%. One should set the overpower IED to trip if the power flow from the
network to the generator is higher than 1%.
Operate
Line
Margin
Operating point
without
turbine torque
IEC06000315 V2 EN-US
Figure 73: Reverse power protection with underpower IED and overpower IED
Application manual
illustrates the reverse power protection with underpower IED and with overpower
Underpower IED
Q
Operate
Line
Margin
P
Operating point
without
turbine torque
Overpower IED
Q
P
IEC06000315-2-en.vsd
Section 7
Current protection
159