Best Protection Strategies for Capacitor Banks

About this list

This list describes protection approaches, not products or companies. The technical descriptions come from two published references we read in full: Schweitzer Engineering Laboratories' paper "Principles of Shunt Capacitor Bank Application and Protection" and ABB's capacitor fuse selection guide (Technical Data 38-852). Standards status was checked on the IEEE SA site on 22 September 2026.

Nothing here replaces the capacitor manufacturer's recommendations or a protection study. We name no numeric sizing rules; where a limit is quoted, it is attributed to the document that states it.

1. 1. Current-limiting unit fuses where energy or fault current is high

In short: Indoor and enclosed banks, and banks with high parallel stored energy or high available fault current

When one capacitor unit fails, the healthy units in parallel with it discharge into the fault, and the power system adds fault current on top. The fuse and the failed can must absorb or hold off that energy without rupturing the case. ABB's selection guide describes parallel energy and available fault current as the main criteria for choosing between expulsion and current-limiting fuses, and applies current-limiting fuses above its expulsion fuse's limits.

A current-limiting fuse develops a back voltage as its sand-surrounded element melts, which limits the peak current and absorbs part of the energy itself instead of leaving it all to the failed can. Operation is contained within the fuse body, which matters inside metal-enclosed equipment.

DENCO's C-rated capacitor fuses are current-limiting and silent, available with or without a blown-fuse indicator. Send the bank data and DENCO engineering will confirm a suitable fuse.

See the DENCO range

2. 2. Expulsion unit fuses on outdoor, externally fused racks

In short: Outdoor externally fused banks where parallel energy and fault current are within the fuse's rating

An expulsion fuse melts a fusible link; the arc generates gas from the tube lining, and the link is cooled and stretched as it is forced out of the tube. It conducts until a natural current zero. ABB's guide notes that in this case most of the parallel energy is absorbed in the failed capacitor, which is why expulsion fuses are limited to banks where that energy stays below the fuse and can ratings.

SEL's paper notes that identifying the failed unit is easy in an externally fused bank, because the operated fuse shows the crew which can to change. Allow for the expulsion discharge in rack clearances.

3. 3. Distinguish capacitively limited from inductively limited faults

In short: Deciding which fuse type can actually clear the fault it will see

ABB's guide separates two cases. In ungrounded banks or banks with several series groups, a unit fault is limited by the other capacitors in series, so the available fault current is low. In single-series-group grounded-wye or delta banks, the fault is limited only by the power system, so the fuse may see full system fault current.

The guide also notes that some current-limiting capacitor fuse designs are intended for capacitively limited faults only, while others can interrupt inductively limited faults. Check which case applies before you choose a fuse design, and confirm it against the fuse maker's data.

4. 4. Internally fused units: many elements, each with its own fuse

In short: Larger substation banks that want to lose one element, not a whole can, per failure

In an internally fused unit, each element inside the can has its own fuse. SEL's paper explains that when an element fails, its fuse removes only that element; with many elements in parallel, the capacitance lost is small and the voltage rise on the healthy elements is small, so many elements can fail before unbalance tripping is needed.

The trade-off is visibility. There is no external fuse to show which can is degrading, so detection depends on unbalance protection. SEL notes that identifying the faulty unit is easy in an externally fused bank but more complex with the other fusing methods, which makes maintenance and fault investigation harder.

5. 5. Fuseless banks: series strings sized so a shorted element is tolerable

In short: Higher-voltage banks built from all-film units

SEL's paper describes how modern polypropylene film dielectrics fail by welding the foils together rather than arcing. A fuseless bank uses that behavior: units are connected in long series strings, and a failed element simply shorts out its row. There is no fuse to blow, so the effect is permanent, and the bank is built with enough elements in series that the remaining ones stay within their overvoltage limit.

Because there is no fuse, protection rests entirely on unbalance relaying that is sensitive enough to count shorted elements and alarm or trip in time.

6. 6. Unfused banks: series and parallel units with no fuses

In short: Designs where the capacitor maker recommends it and relaying can supervise the whole bank

SEL describes unfused banks as similar to fuseless in failure behavior: a failed element shorts its row. The difference is that units are connected in a series and parallel combination. The general requirement still applies: the voltage on the healthy elements must not exceed what the capacitor manufacturer allows.

7. 7. Unbalance relaying: the core protection for every bank type

In short: Detecting failed units or elements that are too small to show in phase current

A single failed unit or element changes bank voltages and currents only slightly, so it is detected by comparing the bank against itself. SEL's paper describes the common methods: neutral voltage for ungrounded single-wye banks (with a compensated version that removes system unbalance), neutral current or neutral voltage between the two halves of an ungrounded double-wye bank, tap-point voltage or phase differential current for grounded single-wye banks, and neutral current differential for grounded double-wye banks.

Which method suits a bank depends on the fusing method, the size of the bank, the grounding and the required CT and PT insulation. In an externally fused bank, the fuse is the first line of protection and unbalance relaying backs it up; in fuseless and unfused banks, unbalance relaying is the primary protection.

• Alarm stage: warns that units or elements have failed
• Trip stage: removes the bank before healthy units exceed their voltage limit
• Settings come from the bank's actual series and parallel configuration

8. 8. Bank-level protection for faults the unit fuses don't see

In short: Faults on the bank bus, connections and switching equipment

Unit fuses and unbalance relaying cover failures inside the bank. Faults on the bank's own bus, leads or switching device, and sustained system overvoltage, need protection at the bank breaker or fuse. SEL's paper puts the goal plainly: protection must cover all faults internal and external to the bank, and it must be immune to transients, fast, sensitive and dependable.

Coordinate the bank's main protection with the unit fuses so a single can failure clears locally rather than tripping the whole bank.

9. 9. Allow for tolerance, harmonics and unit capability when fusing

In short: Avoiding nuisance fuse operation in service

ABB's guide lists why capacitor fuses see more than nameplate current: unit kvar tolerance is positive only, harmonics find a low-impedance path through capacitors, and capacitor units must be able to operate above rated voltage and kvar. SEL's paper, summarizing IEEE C37.99-2000, lists two limits that matter for fusing: abnormal operation limited to 110 percent of rated RMS terminal voltage, and capability to carry 135 percent of nominal RMS current.

If the bank is on a system with significant harmonic content, include the harmonic current in the fuse selection, and ask the capacitor and fuse makers to confirm it.

Which standard covers what

Three IEEE documents frame capacitor bank work. Check the current edition before you rely on any of them.

• IEEE 18, Standard for Shunt Power Capacitors: covers capacitor units rated 216 V or higher and 2.5 kvar or more for shunt connection to 50 Hz or 60 Hz systems. The 2025 edition is active, published 27 March 2026, and supersedes IEEE 18-2012.
• IEEE C37.99, Guide for the Protection of Shunt Capacitor Banks: covers protection of shunt and filter capacitor banks, and excludes pole-mounted banks on distribution circuits and capacitors connected to rotating machines. The latest edition, C37.99-2012, is listed as inactive-reserved since 30 March 2023; it remains the published reference.
• IEEE 1036, Guide for the Application of Shunt Power Capacitors: active 2020 edition, covering application, protection and ratings for capacitors rated 2400 V ac and above.
• IEEE C37.48-2020, Guide and Tutorial for the Application of High-Voltage (> 1000 V) Fuses and Accessories: general background on current-limiting and expulsion fuses.

What to send when you request capacitor fuses

A capacitor fuse is chosen for the bank, not the can alone. Have this ready before you ask DENCO or any other fuse maker for a recommendation.

• Bank configuration: wye or delta, grounded or ungrounded, single or double wye, number of series groups and units per group
• Unit voltage and kvar ratings, and the capacitor manufacturer's fusing recommendation
• Available fault current at the bank
• Indoor or outdoor, enclosed or open rack
• Known harmonic content
• Whether a blown-fuse indicator is required

Frequently asked questions