5-224 G30 Generator Protection System GE Multilin5.7 CONTROL ELEMENTS 5 SETTINGS55.7.5 UNDERFREQUENCY (ANSI 81U)PATH: SETTINGS CONTROL ELEMENTS UNDERFREQUENCY UNDERFREQUENCY 1(6)There are six identical underfrequency elements, numbered from 1 through 6.The steady-state frequency of a power system is a certain indicator of the existing balance between the generated powerand the load. Whenever this balance is disrupted through the loss of an important generating unit or the isolation of part ofthe system from the rest of the system, the effect will be a reduction in frequency. If the control systems of the system gen-erators do not respond fast enough, the system may collapse. A reliable method to quickly restore the balance betweenload and generation is to automatically disconnect selected loads, based on the actual system frequency. This technique,called “load-shedding”, maintains system integrity and minimize widespread outages. After the frequency returns to normal,the load may be automatically or manually restored.The UNDERFREQ 1 SOURCE setting is used to select the source for the signal to be measured. The element first checks for alive phase voltage available from the selected source. If voltage is not available, the element attempts to use a phase cur-rent. If neither voltage nor current is available, the element will not operate, as it will not measure a parameter below theminimum voltage/current setting.The UNDERFREQ 1 MIN VOLT/AMP setting selects the minimum per unit voltage or current level required to allow the underfre-quency element to operate. This threshold is used to prevent an incorrect operation because there is no signal to measure.This UNDERFREQ 1 PICKUP setting is used to select the level at which the underfrequency element is to pickup. For example,if the system frequency is 60 Hz and the load shedding is required at 59.5 Hz, the setting will be 59.50 Hz.Figure 5–104: UNDERFREQUENCY SCHEME LOGIC UNDERFREQUENCY 1UNDFREQ 1 FUNCTION:DisabledRange: Disabled, EnabledMESSAGE UNDERFREQ 1 BLOCK:OffRange: FlexLogic operandMESSAGE UNDERFREQ 1 SOURCE:SRC 1Range: SRC 1, SRC 2, SRC 3, SRC 4MESSAGE UNDERFREQ 1 MINVOLT/AMP: 0.10 puRange: 0.10 to 1.25 pu in steps of 0.01MESSAGE UNDERFREQ 1 PICKUP:59.50 HzRange: 20.00 to 65.00 Hz in steps of 0.01MESSAGE UNDERFREQ 1 PICKUPDELAY: 2.000 sRange: 0.000 to 65.535 s in steps of 0.001MESSAGE UNDERFREQ 1 RESETDELAY : 2.000 sRange: 0.000 to 65.535 s in steps of 0.001MESSAGE UNDERFREQ 1 TARGET:Self-resetRange: Self-reset, Latched, DisabledMESSAGE UNDERFREQ 1 EVENTS:DisabledRange: Disabled, Enabled827079A9.CDRFLEXLOGIC OPERANDSUNDERFREQ 1 FUNCTION:UNDERFREQ 1 BLOCK:UNDERFREQ 1 SOURCE:UNDERFREQ 1MIN VOLT / AMP:UNDERFREQ 1PICKUP :UNDERFREQ 1 DPOUNDERFREQ 1 OPUNDERFREQ 1 PKPRUNMinANDSETTING SETTINGUNDERFREQ 1RESET DELAY :UNDERFREQ 1PICKUP DELAY :SETTINGEnabled=1ACTUAL VALUEStPKPtRSTSETTINGSETTING SETTINGOff = 0VOLT / AMP LevelFrequency0 < f PICKUP<<
