ABB Relion 670 Series Applications Manual page 204

Section 8
Current protection
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 of 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 primary side of the step-up transformer. Power may also come from a start-up service
transformer connected to the external network. One has to design the reverse power protection
so that it can detect reverse power independent of the flow of power to the power plant
auxiliaries.
Hydro turbines tolerate reverse power much better than steam turbines do. Only Kaplan turbine
and bulb turbines may suffer from reverse power. 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.
Ice and snow may block the intake when the outdoor temperature falls far below zero. Branches
and leaves may also block the trash gates. A complete blockage of the intake may cause
cavitations. The risk for damages to hydro turbines can justify reverse power protection in
unattended plants.
A hydro turbine that rotates in water with closed wicket gates will draw electric power from the
rest of the power system. This power will be about 10% of the rated power. If there is only air in
the hydro turbine, the power demand will fall to about 3%.
Diesel engines should have reverse power protection. The generator will take about 15% of its
rated power or more from the system. A stiff engine may require perhaps 25% of the rated power
to motor it. An engine that is well run in might need no more than 5%. It is necessary to obtain
information from the engine manufacturer and to measure the reverse power during
commissioning.
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