2% of rated power. Even if the turbine rotates in vacuum, it will soon become overheated anddamaged. 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 minutesdepending on the type of turbine. A high-pressure turbine with small and thin blades willbecome overheated more easily than a low-pressure turbine with long and heavy blades. Theconditions vary from turbine to turbine and it is necessary to ask the turbine manufacturer ineach case.Power to the power plant auxiliaries may come from a station service transformer connectedto the primary side of the step-up transformer. Power may also come from a start-up servicetransformer connected to the external network. One has to design the reverse powerprotection so that it can detect reverse power independent of the flow of power to the powerplant auxiliaries.Hydro turbines tolerate reverse power much better than steam turbines do. Only Kaplanturbine and bulb turbines may suffer from reverse power. There is a risk that the turbinerunner moves axially and touches stationary parts. They are not always strong enough towithstand 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 maycause cavitations. The risk for damages to hydro turbines can justify reverse power protectionin unattended plants.A hydro turbine that rotates in water with closed wicket gates will draw electric power fromthe rest of the power system. This power will be about 10% of the rated power. If there is onlyair 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 itsrated power or more from the system. A stiff engine may require perhaps 25% of the ratedpower to motor it. An engine that is well run in might need no more than 5%. It is necessary toobtain information from the engine manufacturer and to measure the reverse power duringcommissioning.Gas turbines usually do not require reverse power protection.Figure 63 illustrates the reverse power protection with underpower IED and with overpowerIED. The underpower IED gives a higher margin and should provide better dependability. Onthe other hand, the risk for unwanted operation immediately after synchronization may behigher. One should set the underpower IED to trip if the active power from the generator isless than about 2%. One should set the overpower IED to trip if the power flow from thenetwork to the generator is higher than 1%.Underpower IED Overpower IEDQ QP POperating pointwithoutturbine torqueMargin MarginOperateLine OperateLineOperating pointwithoutturbine torqueIEC06000315-2-en.vsdIEC06000315 V2 EN-USFigure 63: Reverse power protection with underpower IED and overpower IEDSection 7 1MRK 504 158-UEN ACurrent protection132Application manual
