As discussed earlier, to achieve prompt isolation of a failing capacitor unit, and thus minimize the probability of case rupture, the smallest practicable capacitor bank fuse ampere rating should be selected. A capacitor bank fuse so selected, however, may nuisance-melt when exposed to the transient outrush currents which occur under the following situations: (1) when a nearby distribution capacitor bank is energized repeatedly (commonly referred to as back-to-back switching); and (2) when there is a system disturbance such as a nearby fault.

These two situations are analyzed individually in this article for grounded-wye connected capacitor banks, using a technique similar to that described in Part II for evaluating energizing inrush currents. It should further be noted that the duty imposed on bank fuses applied on ungrounded-wye and delta-connected capacitor banks is equal to (in one phase) or less than that associated with grounded-wye connected banks. Accordingly, the procedures and concepts which follow are appropriate for all capacitor bank connections.

Capacitor bank outrush upon energizing a nearby capacitor bank (back-to-back switching). When a capacitor bank is energized in parallel with an already energized capacitor bank, the capacitors in the energized bank will discharge, through the preloaded bank fuse, into the newly energized bank. While the inrush current from the system is limited by the impedance (primarily inductive reactance) between the source and newly energized bank, the high-frequency transient outrush current from the first (already energized) bank is dependent only upon the surge impedance of the discharge path. Thus, it is a function of the equivalent capacitance of the two banks, the total inductance of the discharge path (i.e., the inductance of the conductors between the two banks and the inductance of the capacitor banks themselves) and, as noted earlier, the magnitude of the voltage at the instant the second bank is energized.

If the high-frequency surge-withstand I²t capability of the capacitor bank fuse is known, along with the equivalent capacitance of the circuit, and the line-to-ground voltage, the minimum equivalent circuit inductance L required between the two capacitor banks to avoid nuisance operation of the fuse protecting the first bank can be calculated using Equation 1, but expressed in the following form:

Equation 1:

Then, for a specific conductor size and configuration, with a known inductance per foot, the corresponding minimum line length between the two capacitor banks can be calculated.

Consider the two 600-kVar, 13.8 kV capacitor banks shown in Figure 1. Using Equation 1, it can be shown that the two capacitor banks can be located as close as 69 feet from each other, and not result in a nuisance operation of the fuse protecting the first bank, when the second bank is energized. Since overhead distribution capacitor banks are generally separated by considerably more than 69 feet of conductor — two or more pole-spans of conductor being typical — nuisance operation of capacitor bank fuses due to switching of distribution pole-top capacitor banks is not considered to be a major problem. It is, nevertheless, a factor that should be considered for each application.

Figure 1. Two grounded-wye connected capacitor banks.

If a capacitor bank is energized by a switching device that is not multiple-pre-strike-free, or de-energized by a switching device that is not re-strike-free, the voltage E in Equation 1 can be two (or more) times that normally encountered when energizing a discharged capacitor bank. This results in a 4-times higher I²t value and a 16-times greater minimum required distance between the two capacitor banks. Therefore, where close capacitor bank spacing is required, it is very important that only multiple-pre-strike-free and re-strike-free switches be applied as capacitor bank switches and that re-strike-free operation be maintained by proper servicing.

Capacitor bank outrush into a nearby fault. When a fault occurs on the system near an overhead distribution capacitor bank, the capacitor bank will discharge through the preloaded bank fuse into the fault. For this situation, the I²t of the outrush current from the capacitor bank is dependent upon the equivalent impedance of the circuit conductors between the capacitor bank and the fault, and the magnitude of the voltage at the instant the fault occurs. For the purpose of this discussion, a line-to-line fault will be assumed since the result is a higher outrush I²t through the bank fuse than would be obtained based on a line-to-ground fault.

The minimum required distance between the capacitor bank and the fault to avoid nuisance operation of the bank fuse can be calculated using Equation 1. Note, however, that when calculating the distance to a line-to-line fault, the line-to-line voltage must be used rather than the line-to-ground, which was used to calculate the distance between two adjacent capacitor banks. Also, the distance calculated is based on the total loop inductance, which includes in this case, the inductance of the return conductor. Hence, the distance to the fault is actually one-half of the total distance calculated.

Consider the 600-kVar, 13.8 kV capacitor bank shown in Figure 2. Using Equation 1, it can be shown that a line-to-line fault can occur as close as 166 feet from the capacitor bank, and not result in a nuisance operation of the capacitor bank fuse.

Figure 2. A grounded-wye connected capacitor bank and a nearby phase-to-phase fault.

Because it is impossible to predict the location and frequency of occurrence of line faults a certain probability of nuisance operation must be accepted when fusing overhead distribution capacitor banks. This risk is actually fairly small, since line faults are relatively infrequent, and those that do occur must be within a certain distance from the capacitor bank to result in sufficient I²t to operate the bank fuse.