Chapter 11 - Frequency Protection Functions
Restoration
Frequency
Shedding
Frequency
Trip
Underfrequency
Element
Off
Complete
Holding
Timer
Off
Complete
Restoration
Timer
Off
On
Stage 1 Restore Start
Off
On
Stage 1 Restore Enable
Off
V00854
Figure 134: Load restoration with short deviation into holding band
If the system frequency remains in the holding band for too long it is likely that other system frequency problems
are occurring and it would be prudent to reset the restoration timer for that stage. For this reason, as soon as the
system frequency is measured to be within the holding band, the "Holding Timer" is initiated. If the system
frequency doesn't leave the holding band before the holding timer setting has been exceeded, the load restoration
time delay for that stage is immediately reset.
Note:
The holding timer has a common setting for all stages of load restoration.
An example of the case when the time in the holding band is excessive is shown below.
252
System Frequency
Underfrequency Trip
Underfrequency
Stage (n) Load
Element Pick -up
Shedding
Time less than
Holding Timer Setting
System Frequency Partial
Decay Restoration Timer
Suspended
System Frequency
Recovery Starts
Restoration Time Starts
Holding Band
Restoration Time
System Frequency
Completes Stage
Recovery Continues
(n) Load
Restoration Timer
Restoration
Resumes Timing
Time
P14xEd1-TM-EN-1
P14x