CAPACITOR FUSING APPLICATION GUIDE RULES
Basic points to consider in fusing a capacitor in electric system network
Basic points to consider in fusing a capacitor in electric system network
1) Purpose of fusing:
a. to isolate faulted bank from system
b. to protect against bursting
c. to give indication
d. to allow manual switching (fuse control)
e. to isolate faulted capacitor from bank
2) Recommended rating:
a. The continuous-current capability of the fuse should be at least 165 percent of the normal capacitor-bank (for delta and floating wye banks the factor may be reduced to 150 percent if necessary).
b. The total clearing characteristics of the fuse link must be coordinated with the capacitor "case bursting" curves.
3) Tests have shown that expulsion fuse links will not satisfactorily protect against violent rupture where the fault current through the capacitor is greater than 5000 amperes.
4) The capacitor bank may be connected in a floating wye to limit short-circuit current to less than 5000 amperes.
5) Inrush - for a single bank, the inrush current is always less than the short-circuit value at the bank location.
6) Inrush - for parallel banks, the inrush current is always much greater than for a single bank.
7) Expulsion fuses offer the following advantages:
a. they are inexpensive and easily replaced.
b. offers a positive indication of operation.
8) Current limiting fuses are used where:
a. a high available short circuit exceeds the expulsion or non-vented fuse rating.
b. a current limiting fuse is needed to limit the high energy discharge from adjacent parallel capacitors effectively.
c. a non-venting fuse is needed in an enclosure.
9) The fuse link rating should be such that the link will melt in 300 seconds at 240 to 350 percent of normal load current.
10) The fuse link rating should be such that it melts in one second at not over 220 amperes and in .015 seconds at not over 1700 amperes.
11) The fuse rating must be chosen through the use of melting time-current characteristics curves, because fuse links of the same rating, but of different types and makes have a wide variation in the melting time at 300 seconds and at high currents.
12) Safe zone – usually greater damage than a slight swelling.
a. Zone 1 - suitable for locations where case rupture/or fluid leakage would present no hazard.
b. Zone 2 - suitable for locations which have been chosen after careful consideration of possible consequences associated with violent case ruptures.
c. Hazardous zone - unsafe for most applications. The case will often rupture with sufficient violence to damage adjacent units.
13) Manufacturers normally recommend that the group fuse size be limited by the 50% probability curve or the upper boundary of Zone 1.
14) Short circuit current in an open wye bank is limited to approximately 3 times normal current.
15) Current limiting fuses can be used for delta or grounded wye banks provided there is sufficient short circuit current to melt the fuse within ½ cycle.
a. to isolate faulted bank from system
b. to protect against bursting
c. to give indication
d. to allow manual switching (fuse control)
e. to isolate faulted capacitor from bank
2) Recommended rating:
a. The continuous-current capability of the fuse should be at least 165 percent of the normal capacitor-bank (for delta and floating wye banks the factor may be reduced to 150 percent if necessary).
b. The total clearing characteristics of the fuse link must be coordinated with the capacitor "case bursting" curves.
3) Tests have shown that expulsion fuse links will not satisfactorily protect against violent rupture where the fault current through the capacitor is greater than 5000 amperes.
4) The capacitor bank may be connected in a floating wye to limit short-circuit current to less than 5000 amperes.
5) Inrush - for a single bank, the inrush current is always less than the short-circuit value at the bank location.
6) Inrush - for parallel banks, the inrush current is always much greater than for a single bank.
7) Expulsion fuses offer the following advantages:
a. they are inexpensive and easily replaced.
b. offers a positive indication of operation.
8) Current limiting fuses are used where:
a. a high available short circuit exceeds the expulsion or non-vented fuse rating.
b. a current limiting fuse is needed to limit the high energy discharge from adjacent parallel capacitors effectively.
c. a non-venting fuse is needed in an enclosure.
9) The fuse link rating should be such that the link will melt in 300 seconds at 240 to 350 percent of normal load current.
10) The fuse link rating should be such that it melts in one second at not over 220 amperes and in .015 seconds at not over 1700 amperes.
11) The fuse rating must be chosen through the use of melting time-current characteristics curves, because fuse links of the same rating, but of different types and makes have a wide variation in the melting time at 300 seconds and at high currents.
12) Safe zone – usually greater damage than a slight swelling.
a. Zone 1 - suitable for locations where case rupture/or fluid leakage would present no hazard.
b. Zone 2 - suitable for locations which have been chosen after careful consideration of possible consequences associated with violent case ruptures.
c. Hazardous zone - unsafe for most applications. The case will often rupture with sufficient violence to damage adjacent units.
13) Manufacturers normally recommend that the group fuse size be limited by the 50% probability curve or the upper boundary of Zone 1.
14) Short circuit current in an open wye bank is limited to approximately 3 times normal current.
15) Current limiting fuses can be used for delta or grounded wye banks provided there is sufficient short circuit current to melt the fuse within ½ cycle.
source: ABB Inc. HARD TO FIND INFORMATION ABOUT DISTRIBUTION SYSTEMS
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