Application - ABB REC670 Applications Manual

1MRK 511 401-UEN A
8.12.2
Bay control REC670 2.2 IEC
Application manual

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.
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
Section 8
Current protection
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