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158
Fuse failure supervision (FFRW)
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Different protection functions within the REO 517 protection, control and monitoring
terminals operate on the basis of the measured voltage in the relay point. Examples are:
distance protection function, undervoltage measuring function and voltage check for
the weak infeed logic.
These functions can operate unnecessarily if a fault occurs in the secondary circuits be-
tween the voltage instrument transformers and the terminal.
It is possible to use different measures to prevent such unwanted operations. Miniature
circuit breakers in the voltage measuring circuits, located as close as possible to the
voltage instrument transformers, are one of them. Separate fuse-failure monitoring re-
lays or elements within the protection and monitoring devices are another possibilities.
These solutions are combined to get the best possible effect in the fuse failure supervi-
sion function of REx 5xx terminals.
The fuse-failure supervision function, FFRW is intended for single- and two-phase sys-
tems.
The fuse-failure supervision function integrated in the REO 517 works as follows:
•
On the basis of external binary signals from the miniature circuit breaker or from
the line disconnector. In the first case all voltage dependent functions are affected.
However, in the second case the impedance measuring function is not affected.
•
based on changes: a large change in the voltage without a corresponding large
change in the current.
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The current and voltage measuring elements within one of the built-in digital signal pro-
cessors continuously measure the currents and voltages in all phases and calculate:
The change of current ∆ I/ ∆ t
•
The change of voltage ∆ U/ ∆ t
•
comparing them with their respective set values ∆ I< and ∆ U>.
The function becomes active one and a half period after the voltage exceeds the set op-
erate value U<.
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