Figure 3.19-2 Functional module diagram
The fundamental frequency is compared to the 81URS_Fr_Str setting. If the frequency is lower
than the set 81URS_Fr_Str and no block signal is activated, the operate timer and 81URS_Str
signal are activated. However, if the voltage magnitude is below the 81URS_Vol_Blk set value, the
difference between the measured frequency and the rated frequency exceeds 20 Hz or
slip-frequency exceeds 81URS_Dfdt_Blk when 81URS_Dfdt_Blk_Ena = 1, the operate timer and
81URS_Str signal are deactivated.
Where:
81URS_Fr_Str is the frequency setting of the 81URS.
The measured frequency is available in the monitored data view.
The time characteristic is according to DT. When the operation timer reaches the value set by
81URS_Op_T, the 81URS_Op output is activated. If the fault disappears before the module
operates, the operation reset delay time is 30ms.
The activation of the 81URS_Blk input resets the timer and deactivates the 81URS_Op and
81URS_Str outputs.
Note:
The reset value is the startup setting +0.02Hz
3.19.3 81URS Application Scope
81URS is applicable in all situations where a reliable detection of a low fundamental power system
voltage frequency is needed. A low fundamental frequency in a power system indicates that the
power generated is too low to meet the power demanded by the load connected to the power grid.
The frequency dips rapidly, resulting in load shedding either by the load shedding relays or by the
operator action. 81URS detects such situations and provides an output signal suitable for load
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