in a power system indicates that the generated power is too low to meet the demands of theload connected to the power grid.The underfrequency can occur as a result of the overload of generators operating in anisolated system. It can also occur as a result of a serious fault in the power system due tothe deficit of generation when compared to the load. This can happen due to a fault in thegrid system on the transmission lines that link two parts of the system. As a result, thesystem splits into two with one part having the excess load and the other part thecorresponding deficit.The frequency gradient is applicable in all the situations where the change of thefundamental power system voltage frequency should be detected reliably. The frequencygradient can be used for both increasing and decreasing the frequencies. This functionprovides an output signal suitable for load shedding, generator shedding, generatorboosting, set point change in sub-transmission DC systems and gas turbine startup. Thefrequency gradient is often used in combination with a low frequency signal, especially insmaller power systems where the loss of a large generator requires quick remedial actionsto secure the power system integrity. In such situations, the load shedding actions arerequired at a rather high frequency level. However, in combination with a large negativefrequency gradient, the underfrequency protection can be used at a high setting.4.6.1.6 SignalsTable 588:81 Input signalsName Type Default DescriptionF SIGNAL 0 Measured frequencydF/dt SIGNAL 0 Rate of change of frequencyBLOCK BOOLEAN 0=False Block signal for activating the blocking modeTable 589:81 Output signalsName Type DescriptionTRIP BOOLEAN TripOPR_OFRQ BOOLEAN Trip signal for overfrequencyOPR_UFRQ BOOLEAN Trip signal for underfrequencyOPR_FRG BOOLEAN Trip signal for frequency gradientPICKUP BOOLEAN PickupST_OFRQ BOOLEAN Pickup signal for overfrequencyST_UFRQ BOOLEAN Pickup signal for underfrequencyST_FRG BOOLEAN Pickup signal for frequency gradient1MAC059074-MB A Section 4Protection functions615 series ANSI 683Technical Manual
