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Overfrequency and
underfrequency Protection
f>, f< (81H/81L)
Frequency protection is used for load sharing, loss of mains detection
and as a backup protection for over-speeding.
The frequency function measures the frequency from the two first
voltage inputs. At least one of these two inputs must have a voltage
connected to be able to measure the frequency. Whenever the
frequency crosses the user's pick-up setting of a particular stage, this
stage picks up and a start signal is issued. If the fault situation
remains on longer than the user's operation delay setting, a trip signal
is issued. For situations, where no voltage is present an adapted
frequency is used. See "Principles of Numerical Protection
Techniques" in Chapter 2.
Protection mode for f>< and f><>< stages
These two stages can be configured either for overfrequency or for
underfrequency.
Under voltage self blocking of underfrequency stages
The underfrequency stages are blocked when biggest of the three
line-to-line voltages is below the low voltage block limit setting.
With this common setting, LVBlk, all stages in underfrequency
mode are blocked, when the voltage drops below the given limit. The
idea is to avoid purposeless alarms, when the voltage is off.
Initial self blocking of underfrequency stages
When the biggest of the three line-to-line voltages has been below
the block limit, the underfrequency stages will be blocked until the
pick-up setting has been reached.
Four independent frequency stages
There are four separately adjustable frequency stages: f><, f><><,
f<, f<<. The two first stages can be configured for either
overfrequency or underfrequency usage. So totally four
underfrequency stages can be in use simultaneously. Using the
programmable stages even more can be implemented (refer to
"Programmable Stages (99)"). All the stages have definite operation
time delay (DT).
Setting groups
There are two settings groups available for each stage. Switching
between setting groups can be controlled by digital inputs, virtual
inputs (mimic display, communication, logic) and manually.
3-57
Protection Functions
857-UM001A-EN-P – July 2009
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