Bender LINETRAXX VMD461, Руководство пользователя

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54 VMD461_D00314_01_M_XXEN/10.2018
Frequency (81) (menu 3.3)
The VMD461 can consider three different over- and underfrequency limit val-
ues. A separate response delay
t
(off)
can be set for each limit value.
The limit values for f
(on)max
and f
(on)min
are also set here.
Separate switch-on limit values can be defined by means of the f
(on)…
param-
eters.
If the parameters are set to "off", the corresponding switch-off limit values are
also the switch-on limit values (no hysteresis). The measuring values may not
exceed these limit values to allow connection of the electrical installation.
df/dt (81R) (menu 3.4)
Islanding detection (df/dt) ( ROCOF )
The VMD461 uses a passive method for islanding detection (three-phase volt-
age and frequency monitoring).
The monitoring of the "Rate of Change of Frequency" (df/dt)
is an islanding detection function.
If a subnetwork is shut down by the energy provider it might still be supplied
by electrical installations contained in the subnetwork. Underfrequency and
overfrequency monitoring might not be sufficient to detect this dangerous,
uncontrollable state, since the generators try to keep the frequency at the
nominal frequency level.
Due to an unbalance between the generated energy and the consumed ener-
gy the frequency fluctuates around the nominal frequency. If this happens at
a certain minimum speed, it is a sign of islanding and the VMD461 shuts down
the electrical installation.
For the intended use, the following requirement must be
complied with when setting
several over- and undervolt-
age limit values :
[f>>>] > [f>>] > [f>] > [f
(on)max
] > [f
(on)min
] > [f<] > [f<<] >
[f<<<].
CAUTION