CYG PRS-7367 - 81 URE Application Scope; 81 URE Settings

Protection Functions
110 PRS-7367
 Restoring frequency element
After the 81URE_Op output is activated with a duration time of 81URE_Valid_T and the frequency
has returned to a value higher than the set value of 81URE_Fr_Str, the timer and 81URE_Str
signal are activated. However, if the minimum voltage magnitude is below the 81URE_Vol_Blk set
value or the difference between the measured frequency and the rated frequency exceeds 20 Hz,
the operate timer and 81URE_Str signal are deactivated.
Where:
81URE_Fr_Str is the restore frequency setting value of the 81URE.
 Restore timer element
Once activated, the 81URE_Op output is activated after the set 81URE_T. The time characteristic
is according to DT. If the frequency drops below the set value of 81URE_Fr_Rstore before
81URE_Op is activated, the 81URE_Op will reset with a time delay of 30 ms.
Where:
81URE_T is the restore time setting of the 81URE.
The 81URE_Op output remains active for 100 ms.
Activating the 81URE_Blk intput deactivated the restoring operation. If the restoring command is
cancelled, the 81URE_Op output can be activated only when the next load-shedding operation is
detected, that is, when the 81URE_Op output is activated.
The activation of the 81URE_Blk deactivates the 81URE_Op output.
3.20.3 81URE 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. 81URE detects such situations and provides an output signal suitable for load
shedding. When the system recovers from the disturbance after the action of the under frequency
protection, the operating frequency recovers to nominal frequency. The load that was shed during
the disturbance can be restored.
3.20.4 81URE Settings
Table 3.20-3 81URE settings
No.
Name
Values(Range)
Unit
Step
Default
Description
1
81URE_Fr_
Str
30.00-50.00 at Fn=50;
40.00-60.00 at Fn=60
Hz
0.01
48.80
Start frequency setting value, this
value should be larger than
81URSi_Fr_Str (i=1,2,3,4)
2
81URE_Op_
T
0.04 - 200.00
s
0.01
0.04
Operate time delay