ABB Relion 670 Series Applications Manual page 199

1MRK 511 401-UUS F
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 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 good 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.
Gas turbines usually do not require reverse power protection.
Figure 90
overpower protection. The underpower protection gives a higher margin and should provide
better dependability. On the other hand, the risk for unwanted operation immediately after
synchronization may be higher. One should set the underpower protection (reference angle set to
0) to trip if the active power from the generator is less than about 2%. One should set the
overpower protection (reference angle set to 180) to trip if the power flow from the network to
the generator is higher than 1%.
Bay control REC670
Application manual
illustrates the reverse power protection with underpower protection and with
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Section 8
Current protection
193