ABB RELION REX640 Technical Manual page 972

Section 4
Protection functions
966
one or more blades in the steam turbine or an inadvertent closing of the main stop
valves are typical causes for the low steam flow. The steam turbines of turbo
generators can be protected during a low steam flow with the overpower protection
operating in reverse direction. Hydroturbines tolerate reverse power much better than
steam turbines do. 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.
A hydroturbine that rotates in water with the closed wicket gates draws about 10% of
the rated power from the rest of the power system if the intake is blocked due to ice,
snow, branches or leaves. A complete blockage of the intake may cause cavitations.
If there is only air in the hydroturbine, the power demand drops to about 3%. The risk
of damages to the hydroturbines can justify the reverse operation of the overpower
protection in unattended plants.
Whenever a low value of the reverse power setting is required, an
underpower protection should also be used in conjunction with
DOPPDPR. The limit depends on the CT and VT accuracy.
Diesel engines should have overpower protection in reverse direction. The generator
takes about 15% or more of its rated power from the system. A stiff engine may require
25% of the rated power to motor it. A well run engine may need no more than 5%. It
is necessary to obtain information from the engine manufacturer and to measure the
reverse power during commissioning.
Reverse overpower can also act as an alternative for an under excitation protection in
case of small generators. If the field excitation is reduced, the generator may start
importing the reactive power, making the generator run as an asynchronous generator.
A synchronous generator is not designed to work asynchronously and may become
damaged due to heating in the damper windings or heating in the rotor due to slip
frequency current.
When operated in reverse power direction, DOPPDPR can be used as an alarm if the
power flowing from the industry is feeding the grid, which may not be desired as per
the rules and regulations of the utility owning the grid.
1MRS759142 C
REX640
Technical Manual