Fusing Power Factor Correction and Harmonic Filter Capacitors

Where power factor and filter capacitors sit

Shunt capacitor banks are used for power factor correction and voltage support, and from a system operator's view a bank can also provide harmonic filtering. IEC 60871-1 covers shunt capacitors above 1 000 V, particularly for power-factor correction, and capacitors intended for power filter circuits. IEEE 18-2025 covers shunt power capacitor units from 216 V upward, and IEEE 1036-2020 is the application guide for shunt power capacitors rated 2400 Vac and above.

Harmonic filters have their own guide, IEEE 1531-2020, the IEEE Guide for the Application and Specification of Harmonic Filters, which covers the specification of components, protection and control of passive shunt harmonic filters on low-, medium- and high-voltage systems. IEEE C37.99 covers shunt and filter bank protection but excludes pole-mounted distribution banks and capacitors connected to rotating machines.

DENCO's catalog lists power factor correction equipment and harmonic filtering equipment among the applications for its C-rated capacitor fuses, along with capacitor bank protection, induction heating systems, and high power drives and welders.

How much kvar a correction takes

USDA Rural Utilities Service Bulletin 1724D-112 defines power factor as the ratio of real power (kW) to total power (kVA), equal to the cosine of the phase angle between voltage and current. The kvar needed to move from one power factor to another is Q = P x (tan phi1 - tan phi2), which is the form behind the multiplier tables in one maker's plant engineer guide.

Worked example from that guide: raising a 400 kW load from 0.87 to 0.97 power factor uses a multiplier of 0.316. The check is tan(acos 0.87) - tan(acos 0.97) = 0.5667 - 0.2506 = 0.316, and 400 x 0.316 = 126 kvar; the guide rounds up to a 140 kvar standard size. The same guide notes that raising power factor from 70% to 90% releases 0.32 kVA per kW, or 128 kVA on a 400 kW load.

The reasons to do it are mostly economic. The RUS bulletin lists voltage regulation problems, increased losses, power factor penalties and reduced system capacity, and gives a billing example: metered demand times target PF over measured PF, so 0.95 / 0.83 = 1.145, a 14.5% higher billed demand.

Why fuse current runs above nameplate

One manufacturer's capacitor fuse selection guide gives three reasons fuses are applied with continuous current margin. Rated kvar of a shunt capacitor unit is a minimum, so actual current can be higher. Harmonics find a low impedance path through capacitors, so more current flows through the fuse. And units must be able to operate continuously at 1.1 times rated voltage or 1.35 times rated kvar. If an application is known to have large harmonic content, the guide says, the increase in current should be included in fuse selection.

The IEEE limits behind that third point come from IEEE's 2005 interpretation of IEEE 18-2002: continuous contingency operation is allowed provided none of these is exceeded: 110% of rated rms voltage, 120% of rated peak voltage including harmonics, 135% of nominal rms current and 135% of rated kvar. The 110% voltage limit was meant for contingencies, not as a nominal design basis, and fuses should be rated to withstand that overvoltage too. Earlier editions allowed 180% rms current; IEEE 18-2002 changed it to 135%, and whether later editions changed these limits was not verified for this guide.

Voltage also moves kvar the other way. One manufacturer's guide gives actual kvar = rated kvar x (actual V / rated V) squared, with a worked example: a 900 kvar bank rated 13.2 kV on a 12.47 kV system delivers 900 x (12.47 / 13.2) squared = 802 kvar and draws 37.2 A, and equipment and cable sizing must be at least 37.2 x 1.35 = 50.2 A.

Worked example: a unit fuse in a hot enclosure

Take a 200 kvar, 7,200 V single-phase unit in a metal-enclosed bank. Published fuse class tables place 7,200 V units in the 8.3kV class. Unit current is 200 / 7.2 = 27.8 A. One manufacturer's selection guide sets minimum fuse current at capacitor current times a 1.35 protective margin that assumes a 25°C ambient: 27.8 x 1.35 = 37.5 A.

DENCO's DNXC catalog (p.74) requires derating above 25°C with the formula AT = A25 [1 - 0.0065 (T - 25)], where A25 is the rating at 25°C and T is the maximum ambient the fuse will see. At 55°C the factor is 1 - 0.0065 x 30 = 0.805, so the 25°C rating has to be at least 37.5 / 0.805 = 46.6 A. Among the DNXC8 ratings of 10, 18, 25, 30, 40, 45 and 65 A, a 40 A fuse derates to 32.2 A and a 45 A fuse to 36.2 A, both short of 37.5 A; 65 A derates to 52.3 A. The table repeats the arithmetic at three ambients.

This is a calculated starting point, not a recommendation. Outrush and inrush I2t withstand and the capacitor's case rupture curve still have to be checked with DENCO and the capacitor maker before a rating is chosen.

Sizing margins in published guidance

None of these margins is a standard's fuse rule. Each is a source's own practice, and they differ by voltage class, application and region. They are listed so you know what range to expect when a vendor quotes a fuse, not as a substitute for a sizing calculation.

NEC Article 460 and motor capacitors

The 2023-cycle draft text of NEC Article 460 requires an overcurrent device in each ungrounded conductor for each capacitor bank, with a rating or setting as low as practicable (460.8(B)), with an exception for a capacitor connected on the load side of a motor overload device. In that case 460.9 says the overload device rating is based on the improved power factor of the motor circuit. Older references put the Part II and Part III boundary at 600 V; the 2023-cycle text uses 1000 V. Confirm wording against the edition you work under.

Motor capacitors bring their own limits. One plant engineer's guide says not to exceed the maximum kvar specified by the motor manufacturer, to avoid self-excitation, and to put the capacitor on the line side of the starter for motors that are jogged, plugged, reversed, multi-speed, frequently started or high inertia. The RUS bulletin adds that with capacitors on the motor side of the overload device, either the breaker relay needs adjusting or a lower-rated fuse is needed.

Harmonics and resonance

Capacitive reactance is 1/(2 pi f C), so impedance falls as frequency rises and harmonic currents flow into capacitors. The RUS bulletin singles out the 9th harmonic, a common transformer-generated triplen harmonic at which capacitors offer low impedance (540 Hz on a 60 Hz system).

Capacitors also form a parallel resonance with the supply. One maker's guide estimates the resonant harmonic as h = sqrt(kVAsys / kvar). Its example: a 1,500 kVA, 5.5% impedance transformer on a 48,000 kVA utility short circuit gives a system of 17,391 kVA; adding 350 kvar gives h = sqrt(17,391 / 350) = 7.0, right on the 7th harmonic. Its rule of thumb: if nonlinear load is more than 25% of main transformer capacity, filters will almost always be required; below 15%, capacitors can usually be applied without problems.

Fuses are often the first sign. A metal-enclosed bank manual lists frequent unexplained fuse blowing as a possible sign of parallel resonance and calls for harmonic analysis. The plant guide suggests that true rms capacitor current more than about 5% above rated current at system voltage points to harmonic voltage distortion.

Harmonic filter banks

A shunt harmonic filter is, in one maker's words, a power factor correction capacitor combined with a series iron core reactor, and that maker's filters are almost always tuned below the 5th harmonic. In a tuned filter the capacitor is meant to carry harmonic current, so the fuse sees the fundamental plus the harmonic current the filter absorbs. One selection guide defines fuse rating as the maximum current the fuse can carry continuously without deterioration, including harmonics, which is the figure to compare against the filter's rms current.

Published practice shows the extra allowance. One metal-enclosed bank maker recommends sizing feeders for harmonic filter banks at least 150% of full load amperes. Another uses 40kA current-limiting fuses with blown-fuse indicators in its filter banks, and notes that filter reactors also reduce the frequency and magnitude of back-to-back inrush. One maker's bank warranty excludes capacitor failure due to harmonic distortion. Get the rms current through each unit from the filter design and give the harmonic spectrum to both the capacitor maker and the fuse maker.

Enclosures, temperature and DENCO options

Air inside an enclosure can run well above outdoor ambient. One metal-enclosed bank manual calls for shading where daytime temperatures exceed 40°C (104°F) and has fans turn on at about 85°F; the same manual warns not to switch capacitors on, off and on again in less than 200 seconds. DENCO's catalog lists three capacitor fuse ranges for this work.

• Types 1 to 9, 1kV to 5.5kV: 25A to 200A at 1kV to 2.5kV, 50A to 175A at 3kV, 6A to 100A at 4.3kV and 6A to 75A at 5.5kV, current-limiting, designed to ANSI C37-41 specs in the catalog's words, and designable to customer specifications.
• DNX, 2.8kV to 38kV: clip-style current-limiting capacitor fuses, 1C to 140C, 50kA asymmetrical.
• DNXC, 8.3kV to 23kV: full-range current-limiting capacitor fuses for bus bar to lead mounting, 10A to 65A, 50kA, with the derating formula above.

Common mistakes

These are the errors most often behind fuse trouble in PF correction and filter equipment.

• Sizing the fuse on nameplate current with no allowance for harmonics, tolerance and contingency overvoltage.
• Using the 25°C rating inside a hot enclosure. At 55°C, DENCO's DNXC formula leaves 80.5% of the 25°C rating.
• Replacing blown fuses one size larger without checking for parallel resonance first.
• Treating a filter capacitor like a plain PF capacitor. Its rms current includes the harmonic current it was designed to absorb.
• Exceeding the motor maker's maximum kvar on a motor-switched capacitor, which risks self-excitation.

Frequently asked questions