Fusing Harmonic Filter Banks

What a harmonic filter bank is

IEEE 1531-2020, IEEE Guide for the Application and Specification of Harmonic Filters, provides guidelines for the specification of components, protection and control of harmonic filters. It applies to 50 Hz and 60 Hz passive shunt power harmonic filters on low-, medium- and high-voltage systems, including industrial low-voltage facilities, utility medium-voltage systems and arc furnace installations. On the capacitor side, IEC 60871-1 covers capacitors intended for use in power filter circuits, and IEEE C37.99 (now inactive-reserved) covered protection of filter capacitor banks as well as shunt banks.

One manufacturer's power factor guide for plant engineers describes a shunt harmonic filter as a power factor correction capacitor combined with a series iron core reactor, and says its filters are almost always tuned below the 5th harmonic. The capacitor in a filter is still a capacitor unit with a case rupture limit, so it still needs a fuse that clears a failed unit and stays in during normal duty.

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

Why capacitors and harmonics interact

Capacitive reactance is Xc = 1 / (2 pi f C), so a capacitor's impedance falls as frequency rises. That is why harmonic currents flow into capacitors. USDA Rural Utilities Service Bulletin 1724D-112 notes that the 9th harmonic, a triplen harmonic, can have the greatest effect on capacitors because it is a common transformer-generated harmonic and capacitors offer low impedance at 540 Hz.

A plain capacitor bank also forms a parallel resonance with the system inductance. One maker's guide notes that capacitors can aggravate existing harmonic conditions and that resonance can amplify harmonics manyfold. The same guide gives a rule of thumb: when nonlinear load is more than 25 percent of the main transformer capacity, filters will almost always be required; when it is less than 15 percent, capacitors can usually be applied without problems. Between the two, a study decides.

Worked example: where a plain capacitor bank would resonate

The same plant guide estimates the resonant harmonic order as h = sqrt(kVAsys / kvar), where kVAsys is the short-circuit kVA at the capacitor and kvar is the bank size. Its example is a 1,500 kVA transformer at 5.5 percent impedance fed from a utility source of 48,000 kVA short circuit. The transformer alone allows 1,500 / 0.055 = 27,273 kVA. Combining the two in series, 1 / (1 / 27,273 + 1 / 48,000) = 17,391 kVA, which is the guide's figure. With a 350 kvar bank, h = sqrt(17,391 / 350) = sqrt(49.7) = 7.05, which the guide describes as right on the 7th harmonic.

The table repeats the same arithmetic for other bank sizes on the same 17,391 kVA system. As kvar rises, the resonance moves down through the 7th and 5th harmonics. A bank that lands on or near a harmonic the plant's loads produce is the case where a tuned filter, rather than a plain capacitor bank, is considered. This is a screening step, not a harmonic study.

The current a filter capacitor fuse must carry

A filter capacitor carries its fundamental current plus the harmonic current it was tuned to absorb. The unit's own limits still apply. According to the IEEE interpretation of IEEE 18-2002, capacitors must be capable of continuous operation provided none of these is exceeded: 110 percent of rated rms voltage, 120 percent of rated peak voltage including harmonics, 135 percent of nominal rms current and 135 percent of rated kvar. Earlier editions allowed 180 percent current; IEEE 18-2002 changed it to 135 percent.

On the fuse side, one manufacturer's selection guide says the 1.3 to 1.65 fusing factor exists partly because capacitors provide a low impedance path for harmonics, and adds that if the application is known to have a large harmonic content, the increase in current should be included in the fuse selection process. Current-limiting capacitor fuse catalogs use a 1.35 protective margin that covers normal overvoltages, harmonics, capacitor tolerances and a 25 °C ambient. A filter bank can use up much of that margin on harmonics alone.

The same plant guide offers a field check: if the true-rms capacitor current exceeds the rated current at system voltage by more than about 5 percent, harmonic voltage distortion is likely. For feeders and disconnects, one bank maker's manual recommends at least 150 percent of full load amperes for harmonic filter capacitor banks, against a 135 percent minimum for other banks.

Worked example: a filter capacitor fuse with derating

Take a single-phase 200 kvar unit rated 7,200 V in a metal-enclosed filter bank. Nameplate current is 200 / 7.2 = 27.8 A. Published fuse class tables put 7,200 V units in the 8.3 kV class, where DENCO's DNXC8 ratings are 10, 18, 25, 30, 40, 45 and 65 A.

Step 1, the catalog margin: 27.8 x 1.35 = 37.5 A at 25 °C. Step 2, enclosure temperature. DENCO's DNXC catalog (p.74) requires derating above 25 °C with AT = A25 x [1 - 0.0065 x (T - 25)], where T is the maximum ambient the fuse will see. At 40 °C the factor is 1 - 0.0065 x 15 = 0.9025; at 55 °C it is 1 - 0.0065 x 30 = 0.805. The table shows the result: at 40 °C the 40 A size carries 36.1 A, short of 37.5 A, so the 45 A size (40.6 A) is the first that fits; at 55 °C the 45 A size drops to 36.2 A, so the 65 A size (52.3 A) is the first that fits.

Step 3, harmonics. Suppose the measured true-rms current in that unit is 32.0 A, an illustrative value. That is 32.0 / 27.8 = 1.15 times nameplate, well past the 5 percent indicator, and only 37.5 / 32.0 = 1.17 of margin is left inside the 1.35 calculation. If the harmonic increase is carried into the calculation, 32.0 x 1.35 = 43.2 A: at 40 °C the 45 A size (40.6 A) no longer fits and the 65 A size (58.7 A) does. Note that 32.0 A is still under the capacitor's own 135 percent current limit of 37.5 A. Each step up in fuse size also raises let-through, so the case rupture check has to be repeated with the larger fuse. These are calculated starting points only; outrush I2t and case rupture still have to be checked with DENCO.

Inrush, outrush and switching in filter banks

The filter reactor helps the fuse. One maker of metal-enclosed banks notes that filter reactors significantly reduce the frequency and magnitude of inrush currents from back-to-back capacitor bank switching, and shows a 34.5 kV single-step 7 Mvar 5th harmonic filter bank as an example. Its plain metal-enclosed capacitor banks add separate 44 uH transient inrush reactors per stage, with current-limiting fuses one per capacitor. Without reactors, a switchgear maker's guide describes back-to-back inrush on the order of tens of kiloamperes and tens of kilohertz.

The check itself does not change: the fuse's minimum melting I2t must stay above the I2t of energization and outrush transients, with room for repeated events. Get the transient figures from the filter study. A university facilities design standard, for example, requires bank submittals to show peak inrush, rms inrush current, inrush frequency and duration, and the capacitor unit tank rupture curve, and requires inrush reactors to keep high-frequency inrush below the I2t rating of the fuses.

One bank maker's manual also warns not to switch capacitors on-off-on in less than 200 seconds.

Enclosures, indicators and blown-fuse detection

Most medium-voltage filter banks are metal-enclosed. One maker's metal-enclosed harmonic filter banks use 40 kA current-limiting fuses with blown fuse indicators, and every stage receives its own blown fuse detection system.

Heat is the other enclosure issue. One bank maker's manual says shading should be provided where daytime temperatures exceed 40 °C (104 °F), and that enclosure fans turn on at about 85 °F. Use the highest temperature the fuse will actually see, not the room or site average, in the derating step.

DENCO's C-rated fuses are listed with or without a blown fuse indicator. DNXC fuses mount bus bar to lead and are rated 50 kA; DNX fuses are clip style from 2.8 kV to 38 kV. For filter banks at 1 kV to 5.5 kV, DENCO's Types 1 to 9 are catalogued by maximum AC voltage and Type number.

Common mistakes

Most filter fuse problems trace back to one of these.

• Sizing on nameplate current when measured true-rms current is well above it. The harmonic increase belongs in the selection.
• Skipping derating. A fuse that fits at 25 °C can fall short at the 40 °C or 55 °C inside an enclosure.
• Adding a plain capacitor bank where the screening puts resonance on the 5th or 7th harmonic.
• Treating repeated fuse operations as bad fuses. One bank maker's troubleshooting table says frequent unexplained fuse blowing may be a sign of a parallel resonance condition and calls for a harmonic analysis.
• Stepping up the fuse to stop nuisance operations without repeating the case rupture check for the larger size.
• Using generic inrush figures when the filter reactor and study give the real ones.

What to send DENCO for a filter bank

A filter application needs more detail than a plain power factor bank. Send the following and DENCO engineering will review the fuse rating, voltage class and mounting:

• Capacitor unit kvar, rated voltage and frequency, and the number of units per phase.
• Bank connection (grounded wye, ungrounded wye or delta) and system voltage.
• Tuning order and the reactor rating.
• Measured or calculated true-rms current per unit, with the harmonic spectrum if available.
• Maximum ambient temperature inside the enclosure.
• Available fault current at the bank, and the inrush and outrush figures from the study.
• Mounting (clip or bus bar to lead) and whether a blown fuse indicator is needed.

Frequently asked questions