8.3.3 Setting guidelinesThe parameters for Rate-of-change frequency protection SAPFRC (81) are set via thelocal HMI or or through the Protection and Control Manager (PCM600).All the frequency and voltage magnitude conditions in the system where SAPFRC (81)performs its functions should be considered. The same also applies to the associatedequipment, its frequency and time characteristic.There are two specific application areas for SAPFRC (81):1. to protect equipment against damage due to high or too low frequency, such asgenerators, transformers, and motors2. to protect a power system, or a part of a power system, against breakdown byshedding load or generation, in situations where load and generation are not inbalance.SAPFRC (81) is normally used together with an overfrequency or underfrequencyfunction, in small power systems, where a single event can cause a large imbalancebetween load and generation. In such situations load or generation shedding has to takeplace very quickly, and there might not be enough time to wait until the frequency signalhas reached an abnormal value. Actions are therefore taken at a frequency level closer tothe primary nominal level, if the rate-of-change frequency is large (with respect to sign).SAPFRC (81)PICKUP value is set in Hz/s. All voltage magnitude related settings aremade as a percentage of a settable base voltage, which normally is set to the primarynominal voltage level (phase-phase) of the power system or the high voltage equipmentunder consideration.SAPFRC (81) is not instantaneous, since the function needs some time to supply a stablevalue. It is recommended to have a time delay long enough to take care of signal noise.However, the time, rate-of-change frequency and frequency steps between differentactions might be critical, and sometimes a rather short operation time is required, forexample, down to 70 ms.Smaller industrial systems might experience rate-of-change frequency as large as 5 Hz/s,due to a single event. Even large power systems may form small islands with a largeimbalance between load and generation, when severe faults (or combinations of faults) arecleared - up to 3 Hz/s has been experienced when a small island was isolated from a largesystem. For more "normal" severe disturbances in large power systems, rate-of-change offrequency is much less, most often just a fraction of 1.0 Hz/s.1MRK 511 286-UUS A Section 8Frequency protection169Application manual
