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Electrical characteristics of medium voltage
capacitors (continued)
PROTECTION DEVICES FOR "ALL-FILM" MV CAPACITORS
MEDIUM VOLTAGE RANGE
Protection using internal fuses
Due to the advantages they provide, internal fuses are
the most frequently used means of protecting "all-film"
MV capacitors.
In this technology, each elementary capacitance
forming the capacitor is protected by its own
internal fuse.
When there is a fault on an elementary capacitance,
the internal fuse eliminates the corresponding
capacitance and the continuity of service of the
capacitor is assured.
Given the large number of elementary capacitances
that make up the device, the loss of power resulting
from the first fault is negligible (less than 2%). The
external unbalance protection will only be activated if
there is a large number of "broken down" elementary
capacitances in one capacitor which will create an
unbalance. The operation of an internal fuse is activated:
- When the voltage of the capacitor reaches its
maximum value, and therefore the current reaches
its minimum value, the voltage difference at the
terminals of the "faulty" elementary capacitance
will trigger the blow-out of the corresponding fuse.
- When the current reaches it maximum value,
and therefore the voltage reaches its minimum
value, the flow of the energy stored in the parallel
operational capacitances in the "faulty" capacitance
will trigger the blow-out of the corresponding fuse.
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3
1
Internal view of an "all-film" MV
capacitor with internal fuses
1. Discharge resistor
2. Internal fuse
3. Elementary capacitance
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