Hitachi Relion 670 Series Applications Manual page 154

Section 8
Current protection
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.
Gas turbines usually do not require reverse power protection.
Figure 72
underpower IED 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 IED to trip if the active power from the generator is less than about 2%. One should set the
overpower IED to trip if the power flow from the network to the generator is higher than 1%.
Operate
Line
Margin
Operating point
without
turbine torque
IEC06000315 V2 EN-US
Figure 72:
148
illustrates the reverse power protection with underpower IED and with overpower IED. The
Underpower IED
Q
Operate
Line
Margin
P
Operating point
without
turbine torque
Reverse power protection with underpower IED and overpower IED
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Overpower IED
Q
P
IEC06000315-2-en.vsd
1MRK511407-UUS Rev. N
Phasor measurement unit RES670
Application manual