reached (in addition to being monitored for exceeding or falling below the set
threshold). Monitoring for overfrequency is combined with monitoring for a
frequency increase; monitoring for underfrequency is combined with monitoring
for a frequency decrease. If both operate conditions are satisfied, a set timer
stage is started. The timer stage can be blocked by way of an appropriately
configured binary signal input.
3.27.6 Frequency Monitoring Combined with Mean Frequency Gradient
Monitoring (Δf/Δt)
The frequency gradient can differ for system disturbances in individual
substations and may vary over time due to power swings. Therefore it makes
sense to take the mean value of the frequency gradient into account for load-
shedding systems.
In this operating mode of over-/underfrequency protection, frequency monitoring
must be set for “underfrequency monitoring”.
Monitoring the mean value of the frequency gradient is started with the starting
of frequency monitoring. If the frequency decreases by the set value Δf within
the set time Δt, then the Δt/Δf monitoring function operates instantaneously and
generates a trip signal. If a frequency change does not lead to an operate
decision of the monitoring function, then the Δt/Δf monitoring function will be
blocked until the underfrequency monitoring function drops out. The trip signal
can be blocked by way of an appropriately configured binary signal input.
F5Z0128C
Start Δt Start Δt
Δt Δt
Δf
f<
Function blocked
Trip Δf/Δt
f
t
Fig. 3-139: Operation of frequency monitoring combined with Δf/Δt monitoring.
3 Operation
P634
P634/EN M/R-42-A // P634‑311‑653 3-187
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