ABB RELION 670 Series Applications Manual page 191

1MRK 505 337-UUS A
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 92
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%.
Underpower protection
Operate
Line
Margin
Operating point
without
turbine torque
IEC09000019 V2 EN-US
Figure 92: Reverse power protection with underpower or overpower protection
Application manual
illustrates the reverse power protection with underpower protection and with
Overpower protection
Q
Operate
Line
Margin
P
Operating point
without
turbine torque
Q
P
IEC09000019-2-en.vsd
Section 7
Current protection
185