5-278 G30 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUALCONTROL ELEMENTS CHAPTER 5: SETTINGS5The 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 partof the system from the rest of the system, the effect is a reduction in frequency. If the control systems of the systemgenerators do not respond fast enough, the system can 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 tonormal, the load can be restored automatically or manually.UNDERFREQ 1 SOURCE — This setting is used to select the source for the signal to be measured. The element first checks fora live phase voltage available from the selected source. If voltage is not available, the element attempts to use a phasecurrent. If neither voltage nor current is available, the element does not operate, as it does not measure a parameterbelow the minimum voltage/current setting.UNDERFREQ 1 MIN VOLT/AMP — Selects the minimum per unit voltage or current level required to allow the underfrequencyelement to operate. This threshold is used to prevent an incorrect operation because there is no signal to measure.UNDERFREQ 1 PICKUP — Selects the level at which the underfrequency element is to pickup. For example, if the systemfrequency is 60 Hz and the load shedding is required at 59.5 Hz, the setting is 59.50 Hz.Figure 5-153: Underfrequency logic5.8.6 Overfrequency (ANSI 81O)SETTINGS CONTROL ELEMENTS OVERFREQUENCY OVERFREQUENCY 1(4)There are four overfrequency elements, numbered 1 through 4.A frequency calculation for a given source is made on the input of a voltage or current channel, depending on which isavailable. The channels are searched for the signal input in the following order: voltage channel A, auxiliary voltagechannel, current channel A, and ground current channel. The first available signal is used for frequency calculation. OVERFREQUENCY 1 OVERFREQ 1 FUNCTION:DisabledRange: Disabled, Enabled OVERFREQ 1 BLOCK:OffRange: FlexLogic operand OVERFREQ 1 SOURCE:SRC 1Range: SRC 1, SRC 2, SRC 3, SRC 4 OVERFREQ 1 PICKUP:60.50 HzRange: 20.00 to 65.00 Hz in steps of 0.01 OVERFREQ 1 PICKUPDELAY: 0.500 sRange: 0.000 to 65.535 s in steps of 0.001 OVERFREQ 1 RESETDELAY : 0.500 sRange: 0.000 to 65.535 s in steps of 0.001 OVERFREQ 1 TARGET:Self-resetRange: Self-reset, Latched, Disabled OVERFREQ 1 EVENTS:DisabledRange: Disabled, Enabled
