Externally Fused vs Internally Fused vs Fuseless Capacitor Banks

Four ways to build a bank

A widely cited technical paper on shunt capacitor bank protection, drawing on IEEE C37.99, names four common configurations: externally fused, internally fused, fuseless and unfused. It also traces how practice moved as dielectrics changed: as new dielectric materials came to market, fusing practice shifted from externally fused toward internally fused, fuseless and finally unfused banks.

The same paper notes that capacitor units are available from 216 V to 24,940 V and from 2.5 kvar to around 800 kvar. In every configuration, units connected in parallel form a parallel group sized for the kvar requirement, and groups connected in series make up the voltage rating. Which configuration a bank uses shapes how its units are built, how a failure shows up and what the external fuse, if there is one, has to do.

Externally fused banks

In an externally fused bank, a relay maker's paper notes, an individual fuse mounted between each capacitor unit and the bank's fuse bus typically protects the unit. The units have a few elements in parallel and many in series. When an element fails, it shorts its row and the unit draws more current, and successive failures within the same unit make the external fuse operate and remove the whole unit. Because the external fuse can interrupt a high-voltage fault, the units can be designed for a relatively high voltage.

The trade-off is lost capacity. Each blown fuse takes out a whole unit, so in a bank built from a few large units a significant amount of kvar can be lost per operation. The upside is maintenance: the protection paper observes that identifying faulty units is easy in an externally fused bank and more complex with the other types. A utility co-authored paper calls that a perceived visual advantage and adds a caution: pollution, corrosion and fluctuating climatic conditions reduce the reliability of external fuses, so they must be checked and replaced periodically.

The protection paper gives two design rules for externally fused banks, citing IEEE references. Use more than 10 units in parallel so that one open fuse does not leave more than 10 percent overvoltage on the remaining units. And limit the units in parallel so the total energy a parallel group stores stays below 15 kJ for all-film dielectric, or 10 kJ for all-paper or paper-film dielectric, at peak maximum voltage. The first rule pushes the parallel count up; the second caps it.

Worked example: voltage on the surviving units

A relay maker's unbalance paper gives exact formulas for the overvoltage factor kOV, the voltage on the most stressed healthy unit divided by its normal voltage. With P units in parallel per group, S series groups, R strings per phase and F units lost to blown fuses, an ungrounded single wye bank gives kOV = 3SPR / (3SPR - F (3SR - 3R + 1)) and a grounded wye bank gives kOV = SP / (SP - F (S - 1)).

The paper's own example is a 138 kV ungrounded single wye bank, externally fused, with S = 4, P = 14, R = 1 and 21.6 kV units. Normal unit voltage is 138 / (1.732 x 4) = 19.92 kV, so the 110% limit allows kOV = 1.1 x 21.6 / 19.92 = 1.192. Here 3SPR = 168 and 3SR - 3R + 1 = 10. One failed unit gives 168 / 158 = 1.063, two give 168 / 148 = 1.135 and three give 168 / 138 = 1.217. As the paper puts it, the bank can tolerate two failed units in the same group of 14, but not three.

The grounded wye column applies the same paper's grounded formula to the same S and P, as arithmetic for comparison: SP = 56, and each failure subtracts S - 1 = 3. The paper also notes that system overvoltage stacks on the fractional rise; two failed units at kOV 1.135 plus 10% system overvoltage give 1 + 1.1 x 0.135 = 1.149.

Internally fused banks

Internally fused units have many elements in parallel, each with its own fuse inside the can. When an element breaks down it is disconnected, failing open, and a relay maker's paper notes that the internal fuse isolates it within a few cycles. The capacitance lost is small, and many elements can fail before unbalance tripping is needed. Because each failure changes so little, the same paper notes that a more sensitive unbalance protection scheme is required.

Internally fused banks use fewer capacitor units in parallel and more series groups. The protection paper calls for a minimum of two units in parallel, with the maximum set by the discharge current from the parallel units according to the capacitor manufacturer's recommendation. A utility co-authored paper adds a rupture benefit: instant disconnection of a failed element prevents the unit from being exposed to sustained arcing, minimizing the risk of can rupture. There is no external unit fuse to replace, but finding a degraded unit takes capacitance measurement rather than a visual check.

Fuseless and unfused banks

Fuseless units are built much like externally fused units, but nothing is fused. A failed element shorts its section, the effect is permanent, and the unit remains in service. Fuseless banks are built with at least 10 elements in series so that, after an element fails, the remaining series elements stay within the 110 percent overvoltage requirement. Voltage on the remaining elements rises by about E / (E - 1), where E is elements in series: published examples give 48 / 47, about 2%, and 100 / 99, about 1%.

Because no units are connected directly in parallel, a relay maker's paper notes, the discharge energy into a failure is small, and the unbalance protection does not have to be delayed to coordinate with fuses. The same paper says fuseless design is not usually applied for system voltages below about 34.5 kV, and that unfused banks would normally be used below 34.5 kV, where series strings of units are not practical, or on higher voltage banks with modest parallel energy. Those are one paper's observations, not a standard rule.

The table shows a published 230 kV fuseless example from a utility co-authored paper, with alarm at three failed elements and trip at six.

The four configurations side by side

The table summarizes the papers' descriptions and the unit ranges two capacitor makers catalog for each type. It compares behavior; it is not a recommendation for any voltage or kvar range.

Grounding and the external fuse

How the bank is connected changes the duty on an external fuse. The protection paper notes that grounded wye banks offer a low impedance path to ground for lightning surge currents and for triplen harmonic currents, and reduce recovery voltages for switching equipment to approximately twice normal peak voltage. A relay maker's paper adds the downside: circulation of inrush currents and harmonics in grounded wye banks may cause misoperation or over-operation of relays and fuses. Ungrounded wye banks give no path for zero-sequence or triplen harmonic currents, and their neutral must be insulated for full line voltage.

Fault current through an external fuse depends on the connection too. One manufacturer's selection guide explains that in single series group grounded wye or delta banks, faults are limited only by the available system fault current, while in ungrounded or multi-series-group banks they are capacitively limited, typically to less than two or three times the bank load current. Delta banks have a single series group, so a relay maker's paper notes that isolating a faulted unit leaves no overvoltage on the rest and unbalance detection is not required. When a group's parallel energy grows too large, the same paper says the bank may be split into two wye sections.

Where DENCO external fuses fit

External unit fuses are the job DENCO's capacitor fuses are made for. DNX fuses are clip-style current-limiting capacitor fuses rated 2.8kV to 38kV and 1C to 140C at 50kA asymmetrical. DNXC fuses are full-range current-limiting fuses for bus bar to lead mounting, rated 8.3kV to 23kV and 10A to 65A at 50kA. For 1kV to 5.5kV units, DENCO catalogs Types 1 to 9. To match a fuse to an existing externally fused bank, send the current part number and the unit data and DENCO engineering will confirm a replacement.

Common mistakes

These errors come from treating the four configurations as interchangeable.

• Assuming one blown fuse is harmless. The survivors in that group now run above normal voltage, and the next failure is most likely in the same group, as a relay maker's paper notes.
• Forgetting system overvoltage. It stacks on the internal rise, so a bank that tolerates two failures at nominal voltage has less margin on a high-voltage day.
• Adding units in parallel to reduce the per-failure rise without rechecking parallel energy against the fuse's limit.
• Treating an internally fused or fuseless bank as fault-free because no fuse is visible. Failures there show up only in capacitance or unbalance readings.
• Leaving external fuses in service for years without inspection. Pollution, corrosion and climate reduce their reliability.

Frequently asked questions