A capacitor bank fuse (typically a fuse cutout and a fuse link) must be selected for the voltage rating, the available fault current, and the continuous current carrying requirements of the capacitor bank on which it is to be applied. Since there are a number of voltage, short-circuit interrupting, and maximum ampere ratings available, you should choose the fuse cutout that will meet both your present and future requirements. In addition, from the wide variety of ampere ratings and speeds available, you should select the fuse link ampere rating and speed characteristic that provides the optimum protection for the system as well as for the capacitor units in the bank.

Voltage rating.
In general, the maximum voltage rating of the capacitor bank fuse should equal or exceed the maximum line-to-line operating voltage level of the system. In the case of grounded-wye connected capacitor banks on solidly-grounded-neutral (multi-grounded) systems, however, the maximum voltage rating of the fuse need only equal the maximum system line-to-neutral voltage level, provided that the BIL rating and the leakage distance to ground of the cutout mounting are sufficient for the application.
Short-circuit interrupting rating.
The symmetrical short-circuit interrupting rating of the capacitor bank fuse should equal or exceed the maximum available fault current at the capacitor bank location. When determining the interrupting rating of the fuse cutout, it may be desirable to consider the X/R ratio of the system at the capacitor bank location, since fuse cutouts may have higher-than-nominal symmetrical interrupting ratings for those applications where the X/R ratio is less than the value of 8 or 12 (depending on the fuse cutout voltage rating and interrupting current rating) specified by ANSI Standards.
Ampere rating and speed characteristic.
The ampere rating and speed characteristic of the capacitor bank fuse should be selected to:
1. Withstand the normal transient inrush current associated with energizing an isolated capacitor bank;
2. Carry continuously the highest anticipated capacitor bank current, including any increases above the nominal bank current due to capacitor-unit manufacturing tolerances, harmonic currents, or system operating voltage levels higher than the nominal voltage rating of the capacitor bank;
3. Operate as promptly as possible in response to an evolving capacitor-unit failure;
4. Protect the individual capacitor units in the bank against case rupture in accordance with applicable case-rupture curves; and
5. Withstand the transient outrush current from the capacitor bank that results when a nearby capacitor bank is energized or when a fault occurs nearby.

These principles are examined in greater detail in this and future articles.

Withstand Energizing Inrush Currents

When a single capacitor bank is energized, there will be a transient inrush of charging current which the capacitor bank fuse must be capable of withstanding without operating or without sustaining damage to its fusible element. The magnitude and frequency of this charging current depend upon the total inductance and capacitance of the circuit, as well as the magnitude of the source voltage at the instant the capacitor bank is energized. When evaluating transient inrush currents, it is generally assumed that the bank is energized at a voltage peak, thereby producing the maximum inrush current value. While the resistance of the circuit determines the rate at which the transient inrush current decays — and hence its I²t — it has only a negligible effect on the initial magnitude and frequency of the inrush current. To determine whether the capacitor bank fuse will withstand these transient inrush currents, a comparison must be made between the high-frequency surge-withstand I²t capability of the fuse and I²t of the inrush current.

In making such a comparison, however, it should be noted that the high-frequency surge-withstand I²t capability of the capacitor bank fuse is not the same as its minimum melting I²t derived from the published minimum melting time-current characteristic curve, which is based on a frequency of 60 Hz. Transient inrush currents have frequencies much higher than 60 Hz, and these high frequencies result in non-uniform current distribution in the fusible element (skin effect), plus mechanical stresses resulting from the increased electromagnetic forces involved. High-frequency surge-withstand I²t capability values for distribution fuse links have been the subject of a number of papers in the literature. In general, these values are determined through multiple applications of high-frequency transient currents, such as a distribution fuse link might experience when a switched capacitor bank is energized repeatedly. Available data also indicates that a fuse link's high-frequency surge-withstand I²t capability decreases as the frequency increases. Moreover, this withstand capability is dependent on the fuse link’s fusible-element material and construction. For example, the range of high-frequency surge-withstand I²t capability values for S&C Positrol Fuse Links employing silver or silver-copper eutectic fusible elements is from 15% to 60% of the 60-Hz minimum melting I²t value. Similarly, the range of high frequency surge-withstand I²t capability values for S&C Positrol Fuse Links employing nickel-chrome or silver-tin fusible elements is from 15% to 35% of the 60-Hz minimum melting I²t value. In either case, the values cited above are appropriate for repetitive applications of transient currents with frequencies through 8 kHz, which is well above those normally encountered when energizing a single capacitor bank.

From published technical papers, the following equation can be derived for the I²t of the high-frequency transient inrush current:

Equation 1:

in which E is the peak value of the line-to-ground voltage in volts when the capacitor bank is energized, C is the equivalent capacitance of the oscillatory circuit in farads, and L is the inductance of the oscillatory circuit in henrys. The constant k in Equation 1 is equal to 3.7 and represents an inrush-current damping factor of 0.81 — the ratio of two consecutive opposite-polarity current amplitudes, with the smaller amplitude in the numerator. Use of a 0.81 damping factor is considered appropriate for overhead distribution systems, since the resistance of the circuit typically limits the magnitude of the first peak of the transient current to 90 percent or less of the peak of the un-damped inrush current.

Because of the impedance of the system between the source and the capacitor bank, the I²t of the energizing inrush current is typically limited to a value well below the high-frequency surge-withstand I²t capability of the capacitor bank fuse. For example, consider the energization of a single 1200-kVAR grounded-wye connected capacitor bank rated 13.8 kV three-phase, with two 200-kVAR capacitor units per phase. For an available fault-current level of 5000 amperes RMS symmetrical (a representative value for distribution pole-top capacitor bank fusing applications), the I²t of the transient inrush current, calculated using Equation 1, will be on the order of 630 ampere-squared seconds. By comparison, the unpreloaded high-frequency surge withstand I²t capability of the capacitor bank fuse will range from 5,550 to over 15,700 ampere-squared seconds, depending on the ampere rating, speed characteristic, and element material of the particular fuse link employed. Thus, it is generally accepted that transient inrush currents associated with energizing isolated capacitor banks are not responsible for the nuisance melting of capacitor bank fuses.

The next article will explain how to select a capacitor bank fuse to accommodate the anticipated capacitor bank current, and to achieve prompt operation in response to an evolving capacitor-unit failure. Go to Unit 3.