ABB REB670 Series Applications Manual page 197

1MRK 505 370-UEN D
7.8 Directional underpower protection GUPPDUP
7.8.1 Identification
Function description
Directional underpower protection
7.8.2 Application
The task of a generator in a power plant is to convert mechanical energy available as a torque
on a rotating shaft to electric energy.
Sometimes, the mechanical power from a prime mover may decrease so much that it does not
cover bearing losses and ventilation losses. Then, the synchronous generator becomes a
synchronous motor and starts to take electric power from the rest of the power system. This
operating state, where individual synchronous machines operate as motors, implies no risk for
the machine itself. If the generator under consideration is very large and if it consumes lots of
electric power, it may be desirable to disconnect it to ease the task for the rest of the power
system.
Often, the motoring condition may imply that the turbine is in a very dangerous state. The task
of the reverse power protection is to protect the turbine and not to protect the generator
itself.
Steam turbines easily become overheated if the steam flow becomes too low or if the steam
ceases to flow through the turbine. Therefore, turbo-generators should have reverse power
protection. There are several contingencies that may cause reverse power: break of a main
steam pipe, damage to one or more blades in the steam turbine or inadvertent closing of the
main stop valves. In the last case, it is highly desirable to have a reliable reverse power
protection. It may prevent damage to an otherwise undamaged plant.
During the routine shutdown of many thermal power units, the reverse power protection gives
the tripping impulse to the generator breaker (the unit breaker). By doing so, one prevents the
disconnection of the unit before the mechanical power has become zero. Earlier disconnection
would cause an acceleration of the turbine generator at all routine shutdowns. This should
have caused overspeed and high centrifugal stresses.
When the steam ceases to flow through a turbine, the cooling of the turbine blades will
disappear. Now, it is not possible to remove all heat generated by the windage losses. Instead,
the heat will increase the temperature in the steam turbine and especially of the blades. When
a steam turbine rotates without steam supply, the electric power consumption will be about
2% of rated power. Even if the turbine rotates in vacuum, it will soon become overheated and
damaged. The turbine overheats within minutes if the turbine loses the vacuum.
The critical time to overheating a steam turbine varies from about 0.5 to 30 minutes
depending on the type of turbine. A high-pressure turbine with small and thin blades will
become overheated more easily than a low-pressure turbine with long and heavy blades. The
conditions vary from turbine to turbine and it is necessary to ask the turbine manufacturer in
each case.
Busbar protection REB670
Application manual
IEC 61850 IEC 60617
identification identification
GUPPDUP
P <
2
SYMBOL-LL V2 EN-US
Section 7
Current protection
SEMOD156693-1 v4
SEMOD158941-2 v4
ANSI/IEEE C37.2
device number
37
SEMOD151283-4 v5
191