23 kV Capacitor Fuses (DNX23 and DNXC23)

From 34.5 kV systems and series groups to 23 kV fuses

The 23 kV class is the top DNXC voltage and the usual class for the highest-voltage distribution capacitor units. A full-range current-limiting table, an outdoor combination table and an indoor combination table all assign 19920 V and 21600 V units to 23.0 kV fuses. The outdoor combination table also lists 22800 V units at 23.0 kV.

The direct route is a wye bank on a 34.5 kV system. ANSI C84.1 nominal voltages, as adopted by one state regulator, include 34500 V and 34500Y/19920 V, and one capacitor maker's table pairs a 19920 V unit with a 34.5 kV wye system. One maker's pole racks for single-phase units are built for systems from 2,400 V to 34,500 V, so 19.92 kV pole banks are a catalog item.

The other route is the substation. Banks at transmission voltages are built from parallel groups connected in series to reach the system voltage, and one relay maker notes that choosing the highest available unit voltage results in the fewest series groups and the greatest sensitivity for unbalance protection. One relay maker's worked example is a 138 kV ungrounded single wye bank with four series groups of fourteen 21.6 kV units, where each unit runs at 138 / (1.732 x 4) = 19.92 kV in normal service. A regional transmission guideline describes 230 kV banks with 200 kvar, 19.92 kV units.

Units between the tables

One capacitor maker catalogs standard unit voltages of 15125 V, 19920 V, 20800 V, 21600 V, 22130 V and 22800 V, all at 150 kV BIL with a single bushing. The published fuse tables cover only some of them. 15125 V sits above the 14400 V top of the 15.5 kV lists and below the 19920 V start of the 23 kV lists. 20800 V and 22130 V appear in none of the three tables, and 22800 V appears in one.

The voltage arithmetic is worth having before you call. IEEE's 2005 interpretation of IEEE 18-2002 says fuses should withstand the 110% contingency that units ride through. 1.1 x 19920 V is 21,912 V. 1.1 x 21600 V is 23,760 V, and 1.1 x 22800 V is 25,080 V, both above 23,000 V. One maker's indoor table lists its 23.0 kV class with a rated maximum of 25.3 kV, 110% of the class figure. For 15125 V units, 1.1 x 15125 V is 16,638 V, above 15.5 kV. These are numbers, not answers: DENCO engineering states the class for any unit not in the tables.

DNX23 and DNXC23 catalog numbers

DENCO's DNX23, catalog p.72, is a clip-style current-limiting capacitor fuse with threaded end studs available, listed in two C ranges. Its DNXC23, catalog p.74, is a full-range current-limiting capacitor fuse for bus bar to lead mounting with three ampere ratings. The DNXC catalog states the series was tested at IPH to IEC 60282-1: 2009-10 and IEC 60282-2: 2008-04; that is the catalog's wording, quoted as such.

The DNX C number is defined in DENCO's catalog by melting: 1000 seconds at 1.7 to 2.4 times the C number. No standard defines C ratings for capacitor fuses.

Unit current for 19920 V, 21600 V and 22800 V units

Current per unit is kvar divided by unit kV. Published guides for current-limiting unit fuses then apply a 1.35 protective margin, which one maker ties to normal overvoltages, harmonics, capacitor tolerances and a 25 °C ambient. At these voltages the currents are small: a 200 kvar, 19920 V unit draws about 10 A.

The table runs to 600 kvar because capacitor makers catalog units that large at these voltages; one maker lists standard and heavy duty units from 100 to 600 kvar. Each cell reads unit current / current x 1.35.

Worked examples with only three DNXC23 ratings

With 12, 18 and 25 A to choose from, small changes in kvar or ambient move the answer a whole size, or off the list. DENCO's DNXC catalog p.74 derates above 25 °C: A_T = A_25 x [1 - 0.0065 x (T - 25)], where T is the maximum ambient the fuse will see. The factor is 0.9025 at 40 °C and 0.805 at 55 °C, so DNXC23-12 carries 10.8 A and 9.7 A, DNXC23-18 carries 16.2 A and 14.5 A, and DNXC23-25 carries 22.6 A and 20.1 A.

200 kvar, 19920 V: 13.6 A required. DNXC23-12 is too small at any ambient. DNXC23-18 covers it at 25 °C, at 40 °C and at 55 °C (14.5 A).

300 kvar, 19920 V: 20.3 A required. DNXC23-25 covers it at 25 °C and at 40 °C (22.6 A). At 55 °C it carries 20.1 A, just short, and there is no larger DNXC23. Take it to DENCO.

400 kvar, 21600 V: the arithmetic lands exactly on 25.0 A. DNXC23-25 matches only at a 25 °C ambient; any derating puts it below. A 400 kvar, 19920 V unit needs 27.1 A, beyond the DNXC23 list at any temperature, which points the conversation at the DNX23 50C to 140C range or another arrangement.

Substation banks: margins and stored energy

Not every user applies 1.35. A regional transmission guideline multiplies overvoltage 1.1, tolerance 1.15 and harmonics 1.1 for switching devices, then recommends 1.25 rather than the 1.35 sum, and builds its unit fuse table on 1.25. A relay maker's coordination procedure likewise calls for a fusing ratio of 1.25 or less for individual unit fuses. If your company standard uses a different margin, tell DENCO which one.

Stored energy grows with the group. The same transmission guideline notes that in a 230 kV bank with eighteen 200 kvar units per group, a failed unit and its fuse see the energy of the other 17 units, approximately 3.15 MVAR. It suggests generally keeping effective parallel capacitance under 4,650 kvar, about 15,000 watt-seconds, to stay within expulsion fuse energy withstand and keep can rupture probability low. DENCO does not publish a DNXC parallel energy limit; send the group size.

Group size also drives unbalance protection. In the relay maker's 138 kV example, with 14 units per group the bank can tolerate two failed units in a group but not three before healthy units exceed their permissible overvoltage. Unit fuses remove the failed units; the unbalance relay counts them.

The order of operations matters as much as the ratings. A relay maker's handbook, citing IEEE C37.99-2000, says unbalance protection should coordinate with the individual unit fuses so that the fuses isolate a faulty unit before the protection trips the whole bank, with the alarm level chosen to pick up the first blown fuse. One unbalance relay maker factory-sets a 10-second lockout delay so unit fuses can respond to evolving faults and the failed units can be identified by eye. Ask DENCO for the DNX23 or DNXC23 time-current curves so your relay settings can be checked against them.

Fused, fuseless or unfused near 34.5 kV

34.5 kV is roughly where bank designs change. One relay maker's paper says fuseless designs are not usually applied below about 34.5 kV, because they need more than 10 elements in series, and that unfused designs are normally used on banks below 34.5 kV where series strings are not practical, or on higher voltage banks with modest parallel energy. That is one paper's view, not a standard, but it explains why 23 kV class unit fuses show up both on 34.5 kV distribution banks and on externally fused transmission banks.

Fault current at a failed unit depends on the arrangement. One manufacturer's guide says faults in ungrounded or multi-series-group banks are capacitively limited, typically to less than two or three times bank load current. DENCO prints 50 kA for the DNXC and 50 kA asymmetrical for the DNX.

Quote checklist for 23 kV capacitor fuses

• Unit voltage and kvar, and the unit data sheet if the voltage is 15125, 20800, 22130 or 22800 V.
• Bank voltage, connection, number of series groups and units per group.
• The fusing margin your standard uses (1.35, 1.25 or other).
• Available fault current and maximum ambient at the fuse.
• DNX23 clip mounting or DNXC23 bus bar to lead mounting, indicator option, quantity.
• Part number of any fuse being replaced, so DENCO engineering can confirm a replacement.

Common mistakes at 23 kV

• Choosing the class from the 34.5 kV system voltage. Each unit in a 34.5 kV wye bank sees 19.92 kV.
• Assuming every unit above 19 kV fits 23 kV. 21600 V and 22800 V units exceed 23 kV at 110%, and three catalogued voltages are in no table.
• Expecting a DNXC23 above 25 A. The list is 12, 18 and 25 A.
• Ignoring derating because the currents look small. With three ratings, one size step is the whole margin.
• Applying 1.35 when the owner's standard uses 1.25, or the reverse, without saying so in the request.

Frequently asked questions