Each phase of each filter step should be protected by fuses. Fuses should be current limiting, rated for the available fault current at the fuse location. Fuses should be UL-listed Class J or T, CSA-rated HRC 1, or equivalent.
The current rating of the fuses should be a minimum of two times the capacitor current calculated from its rated reactive power and its rated voltage. The voltage rating of the fuses should be greater than or equal to the system voltage.
Fuses internal to the capacitor should not be accepted as the primary means of filter protection. In a harmonic filter assembly containing more than one capacitor per reactor, a single set of fuses, one per phase, should be provided.
The current rating of the fuses should be at least equal the total filter current including all harmonics, with margin (typically 35%) to cover contingency conditions. Fuses should be located after the harmonic filter main lugs or main disconnect and before the contactor/ reactor/capacitor assemblies.
In addition to fusing, some harmonic filters may be protected by devices that detect phase loss or thermal overload and trip the step/unit off line. Such devices are not meant to replace fuses.
The fuses should be rated for the following conditions:
a) Maximum system voltage
b) Maximum continuous filter current, including fundamental and harmonics
c) Interrupting rating, equal to or greater than the available short-circuit current at the fuse location
d) Sized to limit the fault current to a level consistent with the capabilities of the harmonic filter components
e) Sized to withstand inrush currents when the harmonic filter is energized
Circuit breakers may be used in place of fuses to provide the primary means of overcurrent protection (i.e., thermal/magnetic trip). Alternately, circuit breakers or molded case switches may serve in addition to fuses to provide a primary means of disconnect (i.e., manual switch) or to provide overload (i.e., thermal trip) and/ or short-circuit (i.e., magnetic trip) protection for the complete harmonic filter.
Circuit breakers should be rated for the following conditions:
a) Maximum system voltage
b) The harmonic filter current spectrum, including the fundamental and harmonics (Note that this current spectrum should be determined based on the maximum system operating voltage and maximum positive capacitor tolerance.
The heating of the circuit breaker may be greater at higher frequencies than at the fundamental frequency because of eddy currents and the skin effect. It is not sufficient to specify only the rms value of the circuit breaker current.)
c) Interrupting rating, equal to or greater than the available short-circuit current at the harmonic filter
d) Sized to limit the fault current to a level consistent with the capabilities of the harmonic filter
e) Sized to have sufficient short-time current rating to withstand inrush currents when the harmonic filter is energizedf) The number and the frequency of switching operations