ABB RELION REX640 Technical Manual page 1068

Protection functions
turbines. It can also be used in feeder protection applications, for example, the ring
network.
DOPPDPR in the forward direction can be used to protect the generators or
motors from delivering or consuming excess power. For example, the generator
overpower protection can be used to shed a noncritical feeder load or to start
parallel generators. A synchronous motor may start consuming more reactive power
in case of loss of excitation, in which case the forward overpower protection is used
to detect such condition.
The DOPPDPR function has many applications when used as reverse power
protection. A generator in a power plant converts mechanical energy to electrical
energy. Sometimes the mechanical power from a prime mover may decrease to a
limit that it does not cover the internal losses. The synchronous generator becomes
a synchronous motor and starts importing power from the system. The effect of a
generator acting as a motor implies no risk to the machine but can cause damage
to the prime mover. The extent of the damage depends on the type of the prime
mover.
Steam turbines become overheated easily if the steam flow drops too low or if the
steam ceases to flow through the turbine. The break of a main steam pipe, damage
to 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.
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.
1068
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.
1MRS759142 F
REX640
Technical Manual