that the turbine runner moves axially and touches stationary parts. They are notalways strong enough to withstand the associated stresses.Ice and snow may block the intake when the outdoor temperature falls far belowzero. Branches and leaves may also block the trash gates. A complete blockage ofthe intake may cause cavitations. The risk for damages to hydro turbines can justifyreverse power protection in unattended plants.A hydro turbine that rotates in water with closed wicket gates will draw electricpower from the rest of the power system. This power will be about 10% of therated power. If there is only air in the hydro turbine, the power demand will fall toabout 3%.Diesel engines should have reverse power protection. The generator will take about15% of its rated power or more from the system. A stiff engine may requireperhaps 25% of the rated power to motor it. An engine that is well run in mightneed no more than 5%. It is necessary to obtain information from the enginemanufacturer and to measure the reverse power during commissioning.Gas turbines usually do not require reverse power protection.Figure 68 illustrates the reverse power protection with underpower IED and withoverpower IED. The underpower IED gives a higher margin and should providebetter dependability. On the other hand, the risk for unwanted operationimmediately after synchronization may be higher. One should set the underpowerIED to trip if the active power from the generator is less than about 2%. Oneshould set the overpower IED to trip if the power flow from the network to thegenerator is higher than 1%.Underpower IED Overpower IEDQ QP POperating pointwithoutturbine torqueMargin MarginOperateLine OperateLineOperating pointwithoutturbine torqueIEC06000315-2-en.vsdIEC06000315 V2 EN-USFigure 68: Reverse power protection with underpower IED and overpowerIED8.7.3 Setting guidelines SEMOD172150-4 v7GlobalBaseSel: Selects the global base value group used by the function to defineIBase, UBase and SBase as applicable.1MRK 511 407-UEN C Section 8Current protectionPhasor measurement unit RES670 2.2 IEC 167Application manual
