ABB REL670 2.2 IEC Applications Manual page 522

Section 9
Current protection
516
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 280 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
(reference angle set to 180) to trip if the power flow from the network to the
generator is higher than 1%.
Underpower protection
Q
Operate
Line
Margin
P
Operating point
without
turbine torque
IEC09000019 V2 EN-US
Figure 280: Reverse power protection with underpower or overpower
protection
Overpower protection
Q
Operate
Line
Margin
Operating point
without
turbine torque
IEC09000019-2-en.vsd
1MRK 506 369-UEN B
P
Line distance protection REL670 2.2 IEC
Application manual