11.2.2 Application M14952-3 v4Overfrequency protection function SAPTOF is applicable in all situations, wherereliable detection of high fundamental power system frequency is needed. Thepower system frequency, and rate of change of frequency, is a measure of theunbalance between the actual generation and the load demand. High fundamentalfrequency in a power system indicates that the available generation is too largecompared to the power demanded by the load connected to the power grid.SAPTOF detects such situations and provides an output signal, suitable forgenerator shedding, HVDC-set-point change and so on. SAPTOF is very sensitiveand accurate and can also be used to alert operators that frequency has slightlydeviated from the set-point, and that manual actions might be enough.11.2.3 Setting guidelines M14959-3 v7All the frequency and voltage magnitude conditions in the system where SAPTOFperforms its functions must be considered. The same also applies to the associatedequipment, its frequency and time characteristic.There are two application areas for SAPTOF:1. to protect equipment against damage due to high frequency, such asgenerators, and motors2. to protect a power system, or a part of a power system, against breakdown, byshedding generation, in over production situations.The overfrequency start value is set in Hz. All voltage magnitude related settingsare made as a percentage of a settable global base voltage parameter UBase. TheUBase value should be set as a primary phase-to-phase value.Some applications and related setting guidelines for the frequency level are givenbelow:Equipment protection, such as for motors and generatorsThe setting has to be well above the highest occurring "normal" frequency and wellbelow the highest acceptable frequency for the equipment.Power system protection, by generator sheddingThe setting must be above the highest occurring "normal" frequency and below thehighest acceptable frequency for power stations, or sensitive loads. The settinglevel, the number of levels and the distance between two levels (in time and/or infrequency) depend very much on the characteristics of the power system underconsideration. The size of the "largest loss of load" compared to "the size of thepower system" is a critical parameter. In large systems, the generator shedding canbe set at a fairly low frequency level, and the time delay is normally not critical. Insmaller systems the frequency START level has to be set at a higher value, and thetime delay must be rather short.1MRK 506 369-UEN B Section 11Frequency protectionLine distance protection REL670 2.2 IEC 561Application manual
