Fusing Metal-Enclosed Capacitor Banks

What a metal-enclosed capacitor bank is

A metal-enclosed bank puts capacitor units, fuses, switching devices, reactors and controls inside grounded metal compartments, usually fed from switchgear. One maker's medium-voltage line illustrates the scale: voltages from 2.4 kV through 38 kV, reactive power ratings to 40,000 kvar in a single enclosure, short circuit ratings to 61 kA, and up to 15 capacitor steps, labeled to the C37.20.3-2001 standard for metal-enclosed switchgear. A published example is a 60 Mvar, 38 kV, five-stage, two-section bank at a solar plant.

The same maker states that each capacitor is individually fused with current-limiting fuses to improve bank reliability and protect against case rupture, that its harmonic filter banks use 40 kA current-limiting fuses with blown fuse indicators, and that every stage receives its own blown fuse detection system. Inside a closed enclosure, the unit fuse is the device that keeps a single failed can from becoming an enclosure fault.

Why current-limiting unit fuses suit enclosures

Published manufacturer data describes current-limiting fuses as noiseless and expelling no hot gases or burning particles while interrupting, and recommends them where burning debris cannot be tolerated and where let-through current must be restricted. An expulsion fuse, by contrast, breaks current by exhausting gases out of the fuse area, which is a poor fit inside a compartment full of other equipment. One capacitor fuse maker's indoor current-limiting type is described as designed for indoor use only, for individual unit fusing in metal-enclosed equipment.

DENCO's catalog describes its C-rated capacitor fuses as current-limiting with silent operation and no discharge, available with or without a blown-fuse indicator, for use indoors or outdoors within an enclosure. The DNX series is clip style, 2.8 kV to 38 kV, 1C to 140C, 50 kA asymmetrical. The DNXC series is full-range current-limiting for bus bar to lead mounting, 8.3 kV to 23 kV, 10 A to 65 A, 50 kA. Below 8.3 kV, DENCO also catalogs Types 7, 8 and 9 at 4.3 kV and 5.5 kV, 65 kA symmetrical.

Connection sets the fault current the fuse must clear

One maker's metal-enclosed owner guide says 15 kV class banks are normally connected ungrounded wye and 5 kV class banks normally delta; another maker's banks are connected ungrounded wye as standard. That choice matters to the fuse because it decides how much current flows into a failed unit.

One capacitor fuse maker's selection guide explains: ungrounded or multi-series-group banks are capacitively limited, with available fault current typically less than two or three times the bank's load current, while single-series-group grounded wye or delta banks are inductively limited, limited only by the available system fault current. In a 5 kV class delta bank fed from switchgear, a shorted unit can see the full bus fault current, so the fuse's maximum interrupting rating has to cover the bus, and its let-through has to stay inside the unit's case rupture curve. The same guide says its current-chopping fuse type should not be used on single-series-group grounded wye or delta banks.

Fuse voltage class

For individual unit fusing, the fuse voltage class follows the unit rating. One maker's indoor current-limiting table maps 2,400, 2,770, 4,160 and 4,800 V units to a 5.5 kV fuse; 6,640 to 7,960 V units to 8.3 kV; 8,320 to 14,400 V units to 15.5 kV; and 19,920 and 21,600 V units to 23 kV. It lists rated maximum voltage at 110% of class, referencing IEEE C37.40: 6.05, 9.13, 17.05 and 25.3 kV. Tables from different makers do not always agree, particularly for 8,320 V units, so confirm the class with the fuse maker.

The unit rating comes from the connection. One maker's installation instructions say delta units must be rated at line-to-line voltage and wye units at line-to-neutral voltage. A 13.2 kV ungrounded wye bank uses 7,620 V units (13.2 / 1.732), an 8.3 kV class fuse in the table above. A 4,160 V delta bank uses 4,160 V units, a 5.5 kV class fuse. For the lowest voltages, the same maker offers a 4.3/2.5 kV rating for 2,500 V single-phase or 4,300 V three-phase applications, and says to protect a 4,800 V single-phase capacitor with two 4.3/2.5 kV fuses in series.

Ampere rating and enclosure temperature: worked example

Start from unit current, I = kvar / kV, and apply a margin. One manufacturer's application method uses fuse current = capacitor current x 1.35, with the margin covering normal overvoltages, harmonics, capacitor tolerance and a 25 °C ambient. Enclosures run hotter than that, and DENCO's DNXC catalog requires derating above 25 °C with AT = A25 [1 - 0.0065 (T - 25)], where T is the maximum ambient the fuse will see.

Worked example, a 150 kvar, 7,620 V unit in a 13.2 kV ungrounded wye bank. Unit current = 150 / 7.62 = 19.7 A. Required = 19.7 x 1.35 = 26.6 A. At 40 °C the derating factor is 1 - 0.0065 x 15 = 0.9025: a 25 A DNXC8 gives 22.6 A (short), a 30 A gives 27.1 A (meets 26.6 A). At 55 °C the factor is 1 - 0.0065 x 30 = 0.805: the 30 A falls to 24.2 A (short), and the 40 A gives 32.2 A (meets it).

The 15 °C difference in enclosure temperature moves the arithmetic one rating. One maker's metal-enclosed guide recommends shading where daytime temperatures exceed 40 °C and sets fans to turn on at about 85 °F, which shows how much the enclosure temperature is a design variable. This is a calculated starting point only; outrush I2t, inrush between steps and the unit's case rupture curve still have to be checked with DENCO and the capacitor maker.

Inrush between switched steps

In a multi-step bank, each step closes next to steps that are already energized, so the inrush is back-to-back and limited only by the inductance between them. One manufacturer's capacitor fuse guide gives a peak estimate: Ir (kA peak) = 1.333 x sqrt(kvar_eff / L), with L in microhenries and kvar_eff = kvar1 x kvar2 / (kvar1 + kvar2). For two 3,000 kvar steps, kvar_eff = 1,500. With the guide's assumed 10 uH, Ir = 1.333 x sqrt(1,500 / 10) = 16.3 kA peak. With 44 uH, the per-stage inrush reactor value shown in one maker's metal-enclosed drawings, Ir = 1.333 x sqrt(1,500 / 44) = 7.8 kA peak.

The unit fuses carry their share of that current every time a step closes, and repeated heating near the melting point slowly fatigues an element. A university facilities design standard requires inrush reactors that limit high-frequency inrush below the I2t rating of the fuses, and asks the bank supplier to submit peak inrush, rms inrush, inrush frequency and duration, and the unit tank rupture curve. Those are exactly the numbers a fuse maker needs to check melting I2t. One owner guide also warns not to switch capacitors on-off-on in less than 200 seconds, which keeps steps from reclosing onto trapped charge.

Coordination and code items around the fuses

The unit fuses are one layer. The bank also has to fit the system and the code.

• Feeder protection: one maker's owner guide says circuit breaker trip and fuse characteristics should coordinate with the capacitor protection fuses in the bank; another uses main incoming fuses for bus protection and backup protection of the capacitors.
• Bus and disconnect sizing: one maker sizes phase bus at a minimum of 135% of bank nominal current; another says disconnects at a minimum of 135% of full load amperes, recommends 150%, and 150% for harmonic filter banks.
• NEC Article 460 (2023-cycle text): conductors and disconnecting means not less than 135% of capacitor rated current; above 1000 V, switches able to carry 135% and withstand maximum inrush including adjacent installations, an isolating means with a visible gap, and protective devices rated or adjusted to operate within the safe zone for individual capacitors.
• Discharge: the same text requires residual voltage reduced to 50 V or less within 5 minutes for capacitors above 1000 V; one maker's banks use discharge resistors to that limit, and its manual says to wait 10 minutes after disconnecting before entering.

Commissioning and maintenance

One maker's commissioning guide checks that stage phase currents are balanced within 3%, and its maintenance schedule records power factor, control voltage, stage operations and currents every three months, tightens power connections yearly, and lists a routine check for open or damaged fuses. Its torque table gives 17 lb-ft for capacitor bushing terminals and 20 lb-ft for power fuse terminals. A test-equipment maker recommends infrared scans twice a year with the bank energized at least an hour, since a blown fuse shows as a temperature difference, and capacitance checks per stage against the maker's values.

Unbalanced stage current or a cold capacitor case points to a blown fuse. Find out why it blew before replacing it.

Common mistakes

• Taking the 25 °C fuse rating into a 55 °C enclosure. Derate with the fuse maker's formula for the maximum enclosure ambient.
• Assuming every metal-enclosed bank is ungrounded wye. A 5 kV class delta bank exposes the unit fuse to full bus fault current.
• Using the system voltage instead of the unit voltage for fuse class on wye banks, or the reverse on delta banks.
• Ignoring back-to-back inrush between steps when choosing the smallest fuse rating that meets continuous current.
• Re-energizing a step faster than the bank maker allows.
• Mixing fuse types or ratings within a stage after replacements.

Frequently asked questions