ABB REB670 Series Applications Manual page 198

Section 7
Current protection
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 107
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 107: Reverse power protection with underpower or overpower protection
7.8.3 Setting guidelines
GlobalBaseSel : Selects the global base value group used by the function to define IBase , UBase
SBase . Note that this function will only use IBase value.
and
Operation : With the parameter Operation the function can be set On / Off .
192
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
1MRK 505 370-UEN D
SEMOD172134-4 v7
Busbar protection REB670
Application manual