1MAC309294-MB F Section 4Protection functionsRER620 223Technical Manual4.4.1.9 Technical dataTable 241: 81 Technical data4.4.2 Load shedding and restoration 81S4.4.2.1 Identification4.4.2.2 Function blockFigure 101: Function block4.4.2.3 FunctionalityThe load shedding and restoration function 81S is capable of performing load sheddingbased on underfrequency and the rate of change of the frequency. The load that is shedduring the frequency disturbance can be restored once the frequency has stabilized to thenormal level.The measured system frequency is compared to the set value to detect the underfrequencycondition. The measured rate of change of frequency (df/dt) is compared to the set valueto detect a high frequency reduction rate. The combination of the detected underfrequencyand the high df/dt is used for the activation of the load shedding. There is a definite timedelay between the detection of the underfrequency and high df/dt and the activation of 81S.Characteristic ValueOperation accuracy f>/f< ±10 mHzdf/dt ±100 mHz/s (in range |df/dt| < 5Hz/s)± 2.0% of the set value (in range5 Hz/s < |df/dt| < 15 Hz/s)Start time f>/f< < 80 msdf/dt < 120 msReset time < 150 msOperate time accuracy ±1.0% of the set value or ±30 msFunction description IEC 61850identificationIEC 60617identificationANSI/IEEE C37.2device numberLoad shedding and restoration LSHDPFRQ UFLS/R 81S
