Section 9 Frequency protection9.1 Underfrequency protection SAPTUF (81)9.1.1 IdentificationFunction description IEC 61850identificationIEC 60617identificationANSI/IEEE C37.2device numberUnderfrequency protection SAPTUFf <SYMBOL-P V1 EN819.1.2 ApplicationUnderfrequency protection SAPTUF (81) is applicable in all situations, where reliabledetection of low fundamental power system frequency is needed. The power systemfrequency, and the rate of change of frequency, is a measure of the unbalance between theactual generation and the load demand. Low fundamental frequency in a power systemindicates that the available generation is too low to fully supply the power demanded bythe load connected to the power grid. SAPTUF (81) detects such situations and providesan output signal, suitable for load shedding, generator boosting, HVDC-set-point change,gas turbine start up and so on. Sometimes shunt reactors are automatically switched in dueto low frequency, in order to reduce the power system voltage and hence also reduce thevoltage dependent part of the load.SAPTUF (81) is very sensitive and accurate and is used to alert operators that frequencyhas slightly deviated from the set-point, and that manual actions might be enough. Theunderfrequency signal is also used for overexcitation detection. This is especiallyimportant for generator step-up transformers, which might be connected to the generatorbut disconnected from the grid, during a roll-out sequence. If the generator is stillenergized, the system will experience overexcitation, due to the low frequency.1MRK 506 334-UUS A Section 9Frequency protection269Application manual
