15.5 kV Capacitor Fuses (DNX15 and DNXC15)

Which banks put a 15 kV class fuse on each unit

The 15.5 kV class covers the units that sit between the common 7.2 kV class and the 19.92 kV units used at 34.5 kV. A full-range current-limiting table and an outdoor combination table both list 9960 V, 12470 V, 13280 V, 13800 V and 14400 V units at 15.5 kV, and an indoor combination table covers the whole span from 8320 V to 14400 V, which also takes in 11400 V units.

Two very different banks arrive here. A wye bank on a higher distribution voltage puts line-to-neutral voltage on each unit: ANSI C84.1 nominal voltages as adopted by one state regulator include 24940Y/14400 V and 22860Y/13200 V, and one capacitor maker's table pairs 9960 V units with 17.25 kV, 11400 V with 19.74 kV, 12470 V with 21.6 kV, 13280 V with 22.9 kV, 13800 V with 23.9 kV and 14400 V with 24.9 kV. A delta bank on a 12.47 kV or 13.8 kV system puts line-to-line voltage on each unit, so it uses 12470 V or 13800 V units and also needs a 15.5 kV fuse, even though a wye bank on the same feeder would sit in the 8.3 kV class.

8320 V units: flagged, not settled

On 14.4 kV wye systems the unit is often an 8320 V unit; one utility's standard uses exactly that on its 14.4 kV grounded wye system. The full-range and indoor combination tables put 8320 V units in the 15.5 kV class. The outdoor combination table from the same maker as the indoor table puts them at 8.3 kV. We do not resolve that here.

The arithmetic shows why the line falls where it does. 8320 V is already above 8300 V, and 1.1 x 8320 V is 9152 V for the 110% contingency that IEEE's 2005 interpretation of IEEE 18-2002 says fuses should withstand. A 15.5 kV fuse has ample margin over both; an 8.3 kV fuse depends on how its maker rates the class maximum. The current difference is real too: a 200 kvar, 8320 V unit at 32.5 A after the margin works out to DNXC15-35 here and DNXC8-40 on the 8.3 kV page. Send DENCO the unit voltage and kvar and ask DENCO engineering to state the class in the quote.

DNX15 and DNXC15 catalog numbers

DENCO's 15.5 kV capacitor fuses come in two designs. The DNX15, on catalog p.72, is clip style with threaded end studs available and is listed in three C ranges, the lowest running from 1C to 4.5C for very small currents. The DNXC15, on p.74, is a full-range current-limiting fuse for bus bar to lead mounting, listed in amperes up to 43 A.

DENCO's catalog says a DNX melts in 1000 seconds at 1.7 to 2.4 times its C number. That is DENCO's definition; no standard defines a C rating. The DNXC catalog adds that the series was tested at IPH to IEC 60282-1: 2009-10 and IEC 60282-2: 2008-04, quoted here as the catalog's wording.

Unit current at 15.5 kV class voltages

Divide unit kvar by unit kV for the current, then apply the 1.35 protective margin that published guides for current-limiting unit fuses use; one maker says it covers normal overvoltages, harmonics, capacitor tolerances and a 25 °C ambient. A rural utility bulletin gives the anchor point: a 100 kvar unit at 14.4 kV draws 6.94 A, half the current of the same unit at 7.2 kV. Each cell reads unit current / current x 1.35.

50 kvar is left out on purpose. One manufacturer's fusing table gives 50 kvar recommendations only for units up to 9960 V, because fuses small enough for those units have trouble withstanding the I2t of outrush.

Worked examples, including where DNXC15 runs out

A 300 kvar, 13800 V unit in a 13.8 kV delta bank draws 21.7 A, 29.3 A with the margin. At 25 °C DNXC15-30 covers it. In a 40 °C enclosure, DENCO's DNXC derating (catalog p.74: A_T = A_25 x [1 - 0.0065 x (T - 25)], a factor of 0.9025 at 40 °C) takes DNXC15-30 down to 27.1 A, short of 29.3 A, and DNXC15-35 to 31.6 A, which is enough.

A 400 kvar, 14400 V unit in a 24940Y/14400 V bank draws 27.8 A, 37.5 A with the margin. DNXC15-43 covers it at 25 °C and at 40 °C, where it carries 38.8 A. At 50 °C the factor is 0.8375 and DNXC15-43 carries 36.0 A, below 37.5 A. There is no larger DNXC15, so this goes to DENCO.

A 400 kvar, 12470 V unit in a 12.47 kV delta bank needs 43.3 A even at 25 °C, a fraction above DNXC15-43. Do not round it down. DENCO can look at the DNX15 50C to 140C range or another layout.

Parallel energy in 15 kV class substation groups

Units in this class often sit in externally fused substation banks with several units in parallel per series group. When one unit fails, the healthy units in its group discharge into it through its fuse. One maker's fuse guide calculates that stored energy at 1.1 times crest of rated voltage, which gives 3.19 joules per kvar.

Worked example: a group of twelve 300 kvar, 13800 V units. A failed unit sees the other eleven, 3,300 kvar, which is 11 x 300 x 3.19 = 10,527 J at that maker's basis. One relay maker's design rule for externally fused banks is to use more than 10 units in parallel so that one open fuse leaves no more than 10% overvoltage on the rest, and a regional transmission guideline suggests keeping effective parallel kvar under 4,650 kvar (15,000 watt-seconds) so expulsion fuse energy limits are not exceeded. DENCO does not publish a parallel energy figure for the DNXC, so give DENCO the group size and unit kvar.

Metal-enclosed and pole banks at 15 kV class

In metal-enclosed equipment, one bank maker's manual says 15 kV class capacitors are normally connected ungrounded wye. On a 13.8 kV system that gives 7960 V units and an 8.3 kV fuse; the same enclosure wired delta would need 13800 V units and a 15.5 kV fuse, so the connection drawing decides the page you are on. Ungrounded wye also limits fault current at a failed unit: one manufacturer's guide says faults in ungrounded banks are capacitively limited, typically to less than two or three times bank load current, while delta faults are limited only by the system.

On poles, one capacitor maker builds grounded wye banks for 24,940 V systems at 125 kV BIL, and a rural utility bulletin tabulates group fusing for 300 to 900 kvar banks at 24,900/14,400 V. One maker catalogs 11400 V to 14400 V units at 95 kV BIL and also at 125 and 150 kV BIL, so include the unit data sheet with your request.

Group fusing figures at 14.4 kV, for comparison

Pole banks on 24.9 kV and 14.4 kV systems are often group fused rather than unit fused, and the published group figures are a useful cross-check on scale. A rural utility bulletin's T link table for 24,900/14,400 V grounded wye banks lists 300 kvar at 6.9 A on an 8T link, 450 kvar at 10.4 A on a 10T, 600 kvar at 13.9 A on a 15T and 900 kvar at 20.8 A on a 20T. The same bulletin says group protection should have a nominal rating of 135% to 140% of the capacitor line current.

One city utility's protection standard for its 14.4 kV grounded wye system with 8.32 kV units lists 300 kvar at 10.8 A on a 30K link, 1,200 kvar at 43.3 A on a 65K and 2,700 kvar at 97.4 A on a 100K, and does not allow fusing larger than 100K because of the capacitor tank rupture curve. Those are expulsion link figures for whole-phase groups. A DNXC15 is a unit fuse that carries one unit's current, so the numbers are not interchangeable, but they show why higher-voltage banks reach the same kvar with much smaller fuses than the 8.3 kV class.

What to send DENCO for a 15.5 kV quote

• Unit voltage and kvar; say so clearly if the units are 8320 V.
• System voltage and connection, including whether a wye bank is grounded.
• Units in parallel per group and number of series groups.
• Available fault current at the bank.
• Maximum ambient at the fuse, for DNXC derating.
• DNX15 clip or DNXC15 bus bar to lead mounting, indicator option, quantity.
• Any fuse part number being replaced, for DENCO engineering to confirm a replacement.

Common mistakes at 15.5 kV

• Fusing a 12.47 kV or 13.8 kV delta bank in the 8.3 kV class. Delta units carry line-to-line voltage.
• Treating 8320 V as settled either way. Published tables split; get DENCO's answer in writing.
• Rounding 43.3 A down to DNXC15-43. If the arithmetic exceeds the top rating, ask DENCO.
• Forgetting derating in a hot enclosure. At 50 °C DNXC15-43 carries 36.0 A.
• Ignoring parallel energy in large groups. A fuse that suits the current may still face more stored energy than it should.
• Specifying 50 kvar units at 12470 V and above without asking about outrush I2t.

Frequently asked questions