Trip signal issuing requires that the rate-of-change of frequency conditioncontinues for at least the user set time delay, tDelay. If the START condition, withrespect to the measured frequency ceases during the delay time, and is not fulfilledagain within a user defined reset time, tReset, the START output is reset, after thatthe defined reset time has elapsed. Here it should be noted that after leaving thehysteresis area, the START condition must be fulfilled again and it is not sufficientfor the signal to only return back into the hysteresis area.The RESTORE output of SAPFRC is set, after a time delay equal to the setting oftDelay, when the measured frequency has returned to the level corresponding toRestoreFreq, after an issue of the TRIP output signal. If tRestore is set to 0.000 sthe restore functionality is disabled, and no output will be given. The restorefunctionality is only active for lowering frequency conditions and the restoresequence is disabled if a new negative frequency gradient is detected during therestore period, defined by the settings RestoreFreq and tRestore.10.3.6.3 Blocking M14970-23 v6Rate-of-change frequency protection (SAPFRC) can be partially or totally blocked,by binary input signals or by parameter settings, where:BLOCK: blocks the START and TRIP outputsBLKTRIP: blocks the TRIP outputBLKREST: blocks the RESTORE outputIf the measured voltage level decreases below the setting of IntBlockLevel, both theSTART and the TRIP outputs are blocked.10.3.6.4 Design M14970-34 v7Rate-of-change frequency protection (SAPFRC) measuring element continuouslymeasures the frequency of the selected voltage and compares it to the settingStartFreqGrad. The frequency signal is filtered to avoid transients due to powersystem switchings and faults. The time integrator operates with a definite delaytime. When the frequency has returned back to the setting of RestoreFreq, theRESTORE output is issued after the time delay tRestore, if the TRIP signal hasearlier been issued. The sign of the setting StartFreqGrad is essential, and controlsif the function is used for raising or lowering frequency conditions. The design ofSAPFRC is schematically described in figure 132.Section 10 1MRK 505 394-UEN AFrequency protection280 Line differential protection RED650 2.2 IECTechnical manual
