2.9 Overfrequency and
underfrequency Protection f>, f<
(81H/81L)
2 Protection functions
Technical description
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VM55.EN005
2.9.
Overfrequency and underfrequency
Protection f>, f< (81H/81L)
Enable_f VS_f_Over
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
chapter 1.2.
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 under frequency 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 (chapter
2.12). 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.
