IEEE Std C37.48-1997 covers in detail
the application guidelines for high-voltage external capacitor fuses.
The energy stored in the healthy
capacitors of one series group of parallel-connected capacitors will
discharge into the failed capacitor unit of that group and its fuse.
The fuse shall be able to interrupt the energy supplied by the
parallel group of capacitor units when they are charged to their peak
voltage.
If the capacitor bank design has an
available discharge energy higher than the capacitor units or
expulsion fuses can withstand, current-limiting fuses with adequate
energy rating should be considered. When ungrounded wye capacitor
banks are supplied in an enclosure, current-limiting fuses shall be
used to eliminate the arc products that occur with the use of an
expulsion fuse.
These arc products in the confined
enclosure could cause further evolution of the fault.
Current-limiting fuses may also be required on enclosed single-group
ungrounded wye banks that are designed with two bushing units. In
this design, the first bushing is used for the phase connection, the
second bushing used for the neutral connection, and the case
connected to ground.
This arrangement requires the capacitor
fuses to interrupt system fault current in the event of a failure of
the unit insulation near the phase bushing. NEMA CP1-1988 [B11]
suggests a parallel energy limit of 15 kJ (4650 kvar) for all film
dielectric capacitors.
Expulsion fuses are frequently applied
with higher parallel energy (to 30 kJ) (Mendis et al. [B9]). This
higher energy application is acceptable if the total available
discharge energy of the bank does not exceed the discharge energy
rating of the fuse or the capability of the faulted capacitor unit.
To determine proper fuse selection, the
capacitor unit case rupture curve shall be available from the
manufacturer.
Case rupture curves are different for
different capacitor unit constructions and designs. The total
clearing curve of the fuse or fuse link is then compared to the case
rupture curve; adequate protection is assured if the total clearing
curve of the fuse is to the left of and below the rupture curve of
the capacitor unit.
Other important considerations for
external fuse selection and operation include the following:
— Fuses should be designed and rated
for the externally fused capacitor bank application.
— Fuses should provide for the fast
isolation of a faulted capacitor unit.
— Voltage interruption capability of
the fuse shall be coordinated with the voltage withstand capability
of the capacitor unit.
— Fuses shall handle the transient
inrush and outrush current.
— Fuses shall be designed for the
current loadability requirements, including harmonics and adequate
allowance factors.
— Fuses shall be designed for the
inductive and capacitive current interruption capability.
— Fuse characteristics shall
coordinate between the different shunt bank protection schemes and
the characteristics of the fuses (that is, expulsion,
current-limiting, or a combination of both).
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