Application - ABB 615 series Technical Manual

1MAC059074-MB A
4.8.2.5
615 series ANSI
Technical Manual

Application

32R/32O is used to provide protection against an excessive power flow in the set operating
direction. The main application is the protection of generators and turbines. It can also be
used in feeder protection applications, for example, the ring network.
32R/32O 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 32R/32O 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.
Whenever a low value of the reverse power setting is required, an
underpower protection should also be used in conjunction with 32R/32O.
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
Section 4
Protection functions
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