A hydro turbine that rotates in water with closed wicket gates will draw electric powerfrom the rest of the power system. This power will be about 10% of the rated power. Ifthere 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 therated power to motor it. An engine that is well run in might need no more than 5%. It isnecessary to obtain information from the engine manufacturer and to measure the reversepower during commissioning.Gas turbines usually do not require reverse power protection.Figure 285 illustrates the reverse power protection with underpower IED and withoverpower IED. The underpower IED gives a higher margin and should provide betterdependability. On the other hand, the risk for unwanted operation immediately aftersynchronization may be higher. One should set the underpower IED to trip if the activepower from the generator is less than about 2%. One should set the overpower IED to tripif the power flow from the network to the generator is higher than 1%.Underpower IED Overpower IEDQ QP POperating pointwithoutturbine torqueMargin MarginOperateLine OperateLineOperating pointwithoutturbine torqueIEC06000315-2-en.vsdIEC06000315 V2 ENFigure 285: Reverse power protection with underpower IED and overpower IED9.11.3 Setting guidelinesGlobalBaseSel: Selects the global base value group used by the function to define IBase,VBase and SBase as applicable.Operation: With the parameter Operation the function can be set Enabled/Disabled.Mode: The voltage and current used for the power measurement. The setting possibilitiesare shown in table 41.Section 9 1MRK 506 369-UUS -Current protection556 Line distance protection REL670 2.2 ANSIApplication manual
