CHAPTER 4: SETPOINTS PROTECTION869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–255Overfrequency (81O) The 869 relay provides two identical Overfrequency (OVERFREQ) elements per protectiongroup, or a total of 12 elements.A significant overfrequency condition, likely caused by a breaker opening anddisconnecting load from a particular generation location, can be detected and used toquickly ramp the turbine speed back to normal. If this is not done, the over speed can leadto a turbine trip, which would then subsequently require a turbine start up before restoringthe system. If the overfrequency turbine ramp down is successful, the system restorationcan be much quicker. The overfrequency monitoring feature of the relay can be used forthis purpose at a generating location.The Overfrequency feature is inhibited from operating unless the magnitude of the positivesequence or auxiliary voltage rises above a threshold. When the supply source isenergized, the overfrequency delay timer is allowed to start timing only when thethreshold is exceeded and the frequency is above the programmed Pickup level. In thesame way, when an overfrequency condition starts the overfrequency delay timer and thevoltage falls below the threshold before the timer has expired, the element resets withoutoperating.The Overfrequency element may be set as an instantaneous element with no time delay,or as a definite time delayed element. The Overfrequency element has a fixed minimumoperating threshold to prevent undesired operation during periods of unavailable voltage.The input voltages are the three phase-to-phase voltages from delta connected VTs (PTs),three phase-to-ground voltages from wye connected VTs (PTs), or single phase auxiliaryvoltage.The settings of this function are applied to each source to produce Pickup and Operateflags. The Overfrequency Pickup flag is asserted when the measured frequency of thespecified source is above the PKP value and the voltage is above the threshold. TheOverfrequency Operate flag is asserted if the element stays picked up for the time definedby the Pickup time delay. The element drops from Pickup without operation if themeasured frequency decreases below 0.03 Hz of the Pickup value and stays dropped outfor the defined time delay before the time for operation is reached.The minimum operating voltage is set as a threshold below which the element is blocked.Path: Setpoints > Protection > Group 1(6) > Frequency > Overfrequency 1(X)FUNCTIONRange: Disabled, Trip, Alarm, Latched Alarm, ConfigurableDefault: DisabledPICKUPRange: 20.00 to 65.00 Hz in steps of 0.01 HzDefault: 60.5 HzPICKUP DELAYRange: 0.000 to 6000.000 s in steps of 0.001 sDefault: 2.000 sDROPOUT DELAYRange: 0.000 to 6000.000 s in steps of 0.001 sDefault: 2.000 sMINIMUM VOLTAGERange: 0.000 to 1.250 x VT in steps of 0.001 x VTDefault: 0.700 x VTThe setting sets the minimum voltage for Overfrequency element operation specifiedper times VT.
