Capacitor Fuse Interrupting Rating and Parallel Stored Energy

Two sources of fault energy

When a capacitor unit fails short, current flows into it from two places. The power system supplies fault current at power frequency, and the healthy units in the same parallel group discharge their stored energy into the failed unit at high frequency. A fuse has to deal with both, and the two checks are different: the first is the interrupting rating, the second is energy.

One manufacturer's capacitor fuse selection guide puts the energy problem directly: when a capacitor fails, the energy stored in its series group of capacitors is available to dump into the combination of the failed capacitor and fuse, and the failed capacitor and fuse must be able to absorb or hold off this energy with a low probability of case rupture.

How the bank connection sets the fault current

The same guide explains that in ungrounded banks, or banks with more than one series group, faults are capacitively limited and the available fault current is typically less than two or three times the capacitor bank load current. In single series group grounded wye or delta banks, faults are inductively limited and the current is limited only by the available system fault current. The second case is where the interrupting rating decides.

Worked example. A 600 kvar pole bank of three 200 kvar, 7,200 V units draws 200 / 7.2 = 27.8 A per phase; USDA RUS Bulletin 1724D-112 gives about 28 A for a 600 kvar bank at 7,200 V. One manufacturer's pole bank fusing guide says operating ungrounded wye limits fault current to about three times line current, here about 3 x 27.8 = 83 A. Connect the same bank grounded wye at a point with several thousand amperes available and the fuse must interrupt the full system fault current.

High available fault current also brings case rupture into play. One capacitor maker's pole bank guide lists the largest expulsion links that coordinate with its tank rupture curves, 100K and 65T, and the fault currents up to which they do. Above those values it points to current-limiting fuses, ungrounded wye operation or moving the rack. A municipal utility standard states the same limit in its own words: fusing greater than 100K is not allowed because of the capacitor tank rupture curve.

Interrupting rating: compare like with like

The interrupting rating is the highest fault current the fuse is rated to clear at its rated voltage. Check it against the maximum available fault current at the bank, including future system growth if your utility publishes it.

Ratings are printed on different bases. The tutorial presented with IEEE C37.48.1-2002 notes that the instantaneous crest of a symmetrical fault is 1.414 times the rms symmetrical value, and that the crest of a fully asymmetrical fault may be double that, about 2.8 times, depending on circuit X/R. DENCO's catalog prints its DNX rating as 50kA asymmetrical and its 1 kV to 5.5 kV Types in kA symmetrical. Do not convert one to the other without the X/R data and the test basis; ask DENCO which basis applies.

Fuse type changes the numbers sharply. One manufacturer publishes its expulsion capacitor fuses at 7,400 A symmetrical (10,000 A asymmetrical) in its 9.7 kV class, 3,600 A (5,000 A) at 16.6 kV and 1,800 A (2,500 A) at 26.2 kV, while its current-limiting capacitor fuse classes are rated 40 to 90 kA symmetrical for inductive faults. That gap is one reason current-limiting fuses are used where fault current is high.

Calculating parallel stored energy

A capacitor stores E = 0.5 C V^2. Writing the capacitance in terms of kvar, C = kvar x 1000 / (2 pi f V^2), and charging it to 1.1 times the crest of rated voltage, V = 1.1 x 1.414 x Vrated, the voltage terms cancel and the energy per kvar is fixed by frequency alone: E = 1.21 x kvar x 1000 / (2 pi f).

At 60 Hz that is 1.21 x 1000 / 377 = 3.21 J per kvar. At 1.2 times crest it is 1.44 x 1000 / 377 = 3.82 J per kvar, and at 50 Hz, 1.1 times crest, 1.21 x 1000 / 314.2 = 3.85 J per kvar. One manufacturer's selection guide uses 3.19 J per kvar at 10% overvoltage and 3.80 J per kvar at 20%; its base constant of 2.64 is slightly below the exact 2.653, and the difference is about 0.6%.

The energy that matters is the energy of the units that can discharge into the failed one, normally the other units in its parallel group. A PJM guideline adds that adjacent banks on a common bus add energy. In fuseless banks, a relay maker's paper notes, the discharge energy is small because no capacitor units are connected directly in parallel.

Worked examples: energy in a parallel group

One manufacturer's guide works this example: a bank of 5 series groups, each of 10 x 200 kvar units at 13,280 V. A failed unit's fuse sees the energy of the other nine units, 3.19 x 9 x 200 = 5,742 J, which that guide finds suitable for its 15 kV expulsion fuse.

The table extends the same arithmetic to larger groups. The last row is above every published expulsion figure quoted in the next section and is a case for current-limiting fuses or a different bank arrangement.

Published parallel-energy limits

Several limits appear in the literature. They come from different sources and apply to different things, so cite each on its own. Converted at 3.19 J per kvar, 10 kJ is about 3,135 kvar, 15 kJ about 4,700 kvar and 50 kJ about 15,670 kvar.

DENCO's catalog data used for this guide does not list a parallel-energy capability for DNX or DNXC fuses. If your group energy is high, send the kvar per unit and the number of units in parallel, and DENCO engineering will confirm the application.

• Bank design rule: a published protection paper recommends keeping a parallel group's stored energy below 15 kJ for all-film dielectric or 10 kJ for all-paper or paper-film dielectric, at peak maximum voltage, in externally fused banks.
• Utility guideline: PJM says generally less than 4,650 kvar of effective capacitance at nominal voltage (15,000 watt-seconds) should be connected in parallel, and notes that 6,300 kvar in parallel has been used successfully since 1993 with standard expulsion fuses.
• Expulsion fuse tubes: one maker rates its capacitor expulsion fuse tubes at 20 kJ (50 A tubes) or 30 kJ (80 A tubes) of parallel energy when used with the unit spacing recommended per IEEE Std 18-2012.
• Expulsion application: another maker's guide says its expulsion fuse is not normally applied above 20 kJ, equivalent to 6,000 kvar, because of case rupture, and applies current-limiting fuses above that.
• Current-limiting fuses: one maker publishes its full-range current-limiting capacitor fuse as safely fusing at least 50 kJ; another lists maximum parallel discharge energy for its current-limiting classes of 85, 77, 75, 88 and 50 kJ at 2.8, 5.5, 8.3, 15.5 and 23 kV.

How the fuse type changes the energy split

In an expulsion fuse the link melts, the arc generates gas from the tube lining, and the fuse conducts until a natural current zero. One selection guide notes that most of the parallel energy is then absorbed in the failed capacitor. A back-voltage current-limiting fuse absorbs a large portion of the energy itself, and the same guide says a current-chopping design leaves trapped voltage on the other units.

A current-limiting fuse also caps the power-system current. One manufacturer's catalog gives an example: on an 8.3 kV circuit with 20,000 A rms symmetrical available and an unlimited peak of 48,000 A, a 40 A current-limiting fuse lets through 7,800 A peak. What still reaches the failed unit is the fuse's total clearing I2t, which is also its let-through I2t.

Compare that with the capacitor's withstand. One capacitor maker prints a minimum of 2,000,000 A2s on its standard and heavy-duty tank rupture curves for fault currents of 10,000 A or less, rising to 5,600,000 A2s up to 15,000 A for extreme-duty units. At 10,000 A, 2,000,000 A2s corresponds to 2,000,000 / 10,000^2 = 0.02 s, about 1.2 cycles at 60 Hz.

When energy is too high, the remedies in a relay maker's paper are more series groups, splitting the bank into a double wye, or current-limiting fuses. That paper suggests the split when a group's parallel kvar rises above about 4,650 kvar.

Common mistakes

These errors show up in interrupting and energy checks.

• Comparing an asymmetrical rating with a symmetrical fault study figure, or the reverse.
• Checking interrupting rating against the capacitively limited fault current when the bank is a single series group grounded wye that sees full system fault current.
• Counting only the units in one bank when an adjacent bank on the same bus adds energy.
• Merging limits from different sources, for example applying a fuse maker's 20 kJ figure as if it were a bank design rule.
• Assuming a larger expulsion link adds safety margin when available fault current is already above the level at which it coordinates with the tank rupture curve.
• Forgetting future growth in system fault current at the bank location.

Frequently asked questions