The next step in the selection process is to verify that the capacitor bank fuse ampere rating and speed characteristic, determined as described in Unit 3, will protect the individual capacitor units in the bank against case rupture. This is done by comparing the total clearing time-current characteristic curve of the fuse with the case-rupture curve appropriate for the capacitor units employed. Case-rupture curves are published by the various capacitor manufacturers and illustrate the probability of case rupture for various time and current relationships. A case rupture is generally defined as any opening of the faulted capacitor unit’s case — from a cracked seam or bushing seal to a violent bursting of the case.

In comparing the total clearing curve of the capacitor bank fuse with the appropriate case-rupture curve, you will notice that they intersect at some high value of current, and that they may or may not also intersect at some lower value of current, depending on the capacitor bank connection, its configuration, and on the particular type of capacitor units employed (i.e., paper-film or all-film). Because capacitor-unit case-rupture can result from low-magnitude faults persisting for an extended period of time, as well as from high magnitude faults, the low-current and high-current intersections should be evaluated separately.

Low-current faults.

The presence (or absence) of a low-current intersection between the total clearing curve of the capacitor bank fuse and the case-rupture curve has largely been ignored in the past when selecting a bank fuse for other than ungrounded-wye connected capacitor banks, for which the faulted capacitor-unit current is limited to three times the actual capacitor bank current. For ungrounded-wye connected banks, it was usually recommended that the capacitor bank fuse simply be selected to clear three-times normal phase current in 300 seconds or less, and that a fuse so selected should thereby prevent the faulted capacitor unit’s case from rupturing. Analysis of field experience, however, has shown that capacitor-unit case ruptures can indeed occur under low-fault conditions, regardless of the capacitor bank connection — including ungrounded-wye connected capacitor banks protected by a fuse link selected using the method described Part III. Accordingly, the presence (or absence) of a low-current intersection between the total clearing curve of the capacitor bank fuse and the case-rupture curve should be evaluated for each capacitor bank fusing application.

When comparing the total clearing curve of the capacitor bank fuse with the case-rupture curve in the low-current region, it is helpful to remember that if the capacitor bank is wye connected with multiple capacitor units in parallel in each phase or is delta connected, the faulted phase current and the faulted capacitor-unit current are not equal until the last series group of packs is shorted. As a consequence, the total clearing time of the capacitor bank fuse (which responds to faulted phase current) would be compared to the time permitted on the capacitor unit’s case-rupture curve for the faulted capacitor-unit current. This is illustrated in Figure 2 in Part III, based on capacitor units having a total of six series groups of packs with the faulted unit having four series groups shorted. As a general rule, capacitor unit case rupture will be avoided if, for all number of series groups of packs shorted, the total clearing time of the fuse is less than the time permitted on the case-rupture curve, at their respective current values.

Because the total number of series groups of packs in high-voltage capacitor units is not published and varies from manufacturer to manufacturer, the results illustrated in Figure 2 in Part III should be considered somewhat tentative until the same comparison can be made for other possible numbers of series groups of packs. As noted in previous articles, it is always good practice to evaluate various numbers of series groups of packs, based on the assumption that the voltage across any individual series group is between the generally accepted values of 1 kV and 2.5 kV.

If the total clearing time of the capacitor bank fuse is greater than the time indicated on the case-rupture curve, for one or more numbers of series groups of packs shorted, then there is a possibility that the faulted capacitor unit’s case may rupture. The actual probability of case rupture, however, cannot be determined in absolute terms for a number of reasons. Of particular significance is the fact that there is no way to know how long it will take for the evolving series-group failure process to advance from one shorted series group of packs to the next. As described at the beginning of Part I, paper-film dielectric capacitor units tend to fail quickly, usually resulting in sufficient phase-current escalation to operate the bank fuse before the faulted unit’s case actually ruptures. Series-group failures in all-film dielectric capacitor units, however, tend to develop over a much longer period of time, with the result that the faulted unit’s case can simply rupture due to the increase in internal temperature and pressure resulting from the increased current flow. For these reasons, evaluating the probability of case rupture when selecting a capacitor bank fuse requires careful consideration. You may wish to consult the capacitor unit manufacturer for guidance.

High-current in grounded-wye and delta-connected capacitor banks.

Although most capacitor-unit ruptures in these banks begin as slowly evolving series group failures, as described above, there are a number of conditions which can occur wherein the faulted capacitor unit will be exposed to extremely high currents — on the order of the available fault-current level:

one or more series groups of packs may fault directly to the capacitor unit’s case (either alone or by first promoting pack failure in adjacent series groups);
the capacitor unit’s case may bulge as a result of the gas generated by previously shorted series groups of packs, such that the dielectric fluid level drops below the bushing terminal connections, resulting in an internal flashover; and
the evolving series-group failure process may continue to the point where the last series group of packs is shorted before the capacitor bank fuse operates.

Figure 1. Total clearing curve of bank fuse
will intersect case rupture curve at high
value of current and may or may not
intersect at a lower value of current.

For any of the conditions described above, the faulted phase current and the faulted capacitor-unit current will be the same, and will equal the available phase-to-ground fault-current level in a grounded-wye connected capacitor bank, and the available phase-to-phase fault-current level in a delta-connected capacitor bank. Therefore, to protect the capacitor units in these banks against case rupture, they should be applied only where the maximum phase-to-ground and phase-to-phase fault-current levels, respectively, are lower than the fault-current value at which the total clearing time-current characteristic curve of the bank fuse and the case-rupture curve intersect (see Figure 1).

If the available fault-current level is greater than the maximum fault-current level for capacitor bank protection (for a properly selected capacitor bank fuse), the capacitor bank, as configured, cannot be adequately protected by fuse cutouts. For this situation, you may wish to consider either one or both of the following alternatives: (1) use larger individual capacitor units in the bank, or units of a different construction type or from a different manufacturer, which may yield a higher maximum fault-current value for capacitor bank protection; or (2) use partial-range (backup) current-limiting fuses in series with the fuse cutouts. More expensive full-range current-limiting fuses may also be considered for the capacitor bank fuse. The use of full range current-limiting fuses has some significant drawbacks, however, since their time-current characteristics normally require that such a large ampere rating be used that they provide virtually no protection against evolving series-group failure. In addition, if current-limiting fuses are employed (either partial range or full range) the available fault current must be of sufficient magnitude to melt the current-limiting fuse in one-half cycle or less for the fuse to be effective.

The fifth and final article in this series will describe how to select the capacitor bank fuse to withstand the transient outrush currents that occur when a nearby capacitor bank is energized (commonly referred to as back-to-back switching) or when there is a system disturbance such as a nearby fault. Go to Unit 5.