Section 10 Frequency protection10.1 Underfrequency protection SAPTUF (81) IP15746-1 v310.1.1 IdentificationM14865-1 v5Function description IEC 61850identificationIEC 60617identificationANSI/IEEE C37.2device numberUnderfrequency protection SAPTUFf <SYMBOL-P V1 EN-US8110.1.2 Application M13350-3 v4Underfrequency protection SAPTUF (81) is applicable in all situations, where reliable detection oflow fundamental power system frequency is needed. The power system frequency, and the rate ofchange of frequency, is a measure of the unbalance between the actual generation and the loaddemand. Low fundamental frequency in a power system indicates that the available generation istoo low to fully supply the power demanded by the load connected to the power grid. SAPTUF (81)detects such situations and provides an output signal, suitable for load shedding, generatorboosting, HVDC-set-point change, gas turbine start up and so on. Sometimes shunt reactors areautomatically switched in due to low frequency, in order to reduce the power system voltage andhence also reduce the voltage dependent part of the load.SAPTUF (81) is very sensitive and accurate and is used to alert operators that frequency hasslightly deviated from the set-point, and that manual actions might be enough. Theunderfrequency signal is also used for overexcitation detection. This is especially important forgenerator step-up transformers, which might be connected to the generator but disconnectedfrom the grid, during a roll-out sequence. If the generator is still energized, the system willexperience overexcitation, due to the low frequency.10.1.3 Setting guidelines M13355-3 v7All the frequency and voltage magnitude conditions in the system where SAPTUF (81) performs itsfunctions should be considered. The same also applies to the associated equipment, its frequencyand time characteristic.There are two specific application areas for SAPTUF (81):1. to protect equipment against damage due to low frequency, such as generators,transformers, and motors. Overexcitation is also related to low frequency2. to protect a power system, or a part of a power system, against breakdown, by shedding load,in generation deficit situations.1MRK 511 364-UUS A Section 10Frequency protection157Application manual
