Fusing Substation Capacitor Banks: Externally Fused, Internally Fused and Fuseless

How a substation capacitor bank is put together

A substation shunt capacitor bank is built from single-phase capacitor units. Units are connected in parallel to make up the kvar of a group, and parallel groups are connected in series to reach the nameplate voltage of the phase. A phase can also hold more than one string of series groups. Published unit ranges run from 216 V to 24,940 V and from 2.5 kvar to around 800 kvar, so a 138 kV or 230 kV bank always has several series groups per phase.

Most distribution and transmission banks are wye connected, grounded or ungrounded, and one relay maker's protection paper states that virtually all substation banks are wye connected. Grounded wye banks give a low impedance path to ground for surge currents and triplen harmonics. Ungrounded wye banks give no path for zero-sequence currents or for capacitor discharge currents during system faults, but the neutral must be insulated to full line voltage. Larger banks are often split into a double wye, or wired as an H-bridge, so that unbalance can be measured between two equal halves.

A published shunt capacitor protection paper notes that choosing units with the highest available voltage rating gives the fewest series groups and the greatest sensitivity for unbalance protection. That choice also fixes the unit voltage, and the unit voltage sets the voltage class of any external fuse.

Four ways to fuse a substation bank

IEEE C37.99, the IEEE guide for the protection of shunt capacitor banks (now inactive-reserved), describes four common configurations: externally fused, internally fused, fuseless and unfused. A protection paper citing that guide notes that as dielectric materials improved, practice moved from externally fused toward internally fused, fuseless and finally unfused designs. The choice decides where the fuse sits, what a single failure looks like and how sensitive the unbalance relay must be.

Externally fused banks: two limits on group size

External fusing puts most of the design work into the size of each parallel group, because the group has to pass two checks that pull in opposite directions.

The first is overvoltage. When a fuse removes a unit, the remaining units in that group take a larger share of the phase voltage. A published shunt capacitor protection paper gives the rule: select the minimum number of units in parallel greater than 10, so that one open fuse does not leave more than 10 percent overvoltage on the remaining units. The arithmetic behind it: as the number of series groups grows, the overvoltage on the group after F failures approaches P / (P - F), where P is units per group. With P = 10, one open fuse gives 10 / 9 = 1.111.

The second is energy. Every unit in the group is charged, and when one fails the others discharge into it through its fuse. The same paper limits the total energy a parallel group stores to less than 15 kJ for all-film dielectric or 10 kJ for all-paper or paper-film dielectric, at peak maximum voltage. The PJM design guideline for shunt capacitors says that, generally, less than 4,650 kvar of effective capacitance at nominal voltage (15,000 watt-seconds) should be connected in parallel. It adds that 6,300 kvar in parallel has been used successfully since 1993 with standard expulsion fuses, and that adjacent banks on a common bus add energy.

When a group would be too large, published remedies include more series groups, splitting the bank into a double wye, or moving to current-limiting fuses.

Worked example: overvoltage after blown fuses

A relay maker's unbalance paper works through a 138 kV ungrounded single wye bank, externally fused, with 4 series groups, 14 units per group, one string per phase and 21.6 kV units. Normal unit voltage is 138 / (1.732 x 4) = 19.92 kV. The unit rating is 21.6 / 19.92 = 1.084 times that, so the permissible overvoltage factor at 110 percent of unit rating is 1.1 x 1.084 = 1.192.

Using the paper's formula for an ungrounded single wye fused bank, the overvoltage factor on the most stressed healthy units is 1.063 with one failed unit, 1.135 with two and 1.217 with three. In the paper's words, the bank can tolerate two failed units in the same group of 14, but not three. The fuses do their part by removing each failed unit. The unbalance relay has to alarm on the first loss and trip before the third.

Worked example: parallel energy into a failed unit

One manufacturer's capacitor fuse selection guide calculates the available energy by assuming the parallel capacitance is charged to 1.1 times the crest of rated voltage, which gives 3.19 joules per kvar. The figure can be checked from first principles. Energy is one half C V squared, and C = kvar x 1000 / (2 pi f V squared) with V in volts, so energy per kvar at 1.1 per unit crest is 1.21 x 1000 / (2 pi x 60) = 1.21 x 1000 / 377 = 3.21 J at 60 Hz. The small difference from 3.19 is rounding in the guide's constant. At 50 Hz the same arithmetic gives 1.21 x 1000 / 314.2 = 3.85 J per kvar.

The guide's own example has 5 series groups of 10 x 200 kvar units at 13,280 V. When one unit fails, the other 9 discharge into it: 3.19 x 9 x 200 = 5,742 J, which the guide finds within the capability of its expulsion fuse. For the 138 kV bank above, with 14 units per group, take 200 kvar units for illustration: 13 x 200 = 2,600 kvar of healthy parallel units, and 2,600 x 3.19 = 8,294 J. That is under both the 10 kJ and 15 kJ figures from the protection paper.

Fuses carry their own energy ratings. One maker lists 20 kJ for 50 A expulsion fuse tubes and 30 kJ for 80 A tubes when used with the unit spacing in IEEE Std 18-2012. One manufacturer's current-limiting capacitor fuse data lists maximum parallel discharge energy of 85, 77, 75, 88 and 50 kJ for its 2.8, 5.5, 8.3, 15.5 and 23 kV classes. For DENCO fuses, ask DENCO engineering for the figure that applies to the rating you are considering.

Sizing an external unit fuse

Unit current is unit kvar divided by unit kV. A 200 kvar unit rated 19.92 kV draws 200 / 19.92 = 10.04 A. The multiplier applied on top of that depends on the source, and the published figures do not agree:

• The PJM guideline builds a multiplier from overvoltage 1.1, tolerance 1.15 and harmonics 1.1, and recommends 1.25 rather than the 1.35 sum. Its unit fuse table lists 10T or 12K links for a 200 kvar, 19.92 kV unit on a 230 kV bank. On that basis, 10.04 x 1.25 = 12.6 A.
• Current-limiting capacitor fuse catalogs use capacitor current x 1.35, a protective margin that covers normal overvoltages, harmonics, capacitor tolerances and a 25 °C ambient: 10.04 x 1.35 = 13.6 A.
• One manufacturer's selection guide gives a fusing factor typically in the range of 1.3 to 1.65 per unit.
• A relay maker's lockout relay instructions state that coordination with its unbalance relay assumes a fusing ratio of 1.25 or less for individual capacitor-unit fuses.

Matching the margin to the bank

These multipliers are not interchangeable. A larger multiplier reduces nuisance operations from inrush and outrush but slows clearing of a failed unit and can upset relay coordination. A smaller one does the opposite. Pick one basis, use the fuse maker's curves for that basis, and confirm it in the protection study.

Voltage class follows the unit voltage. In one manufacturer's full-range current-limiting table, 19,920 V and 21,600 V units use a 23 kV fuse. DENCO's DNXC23 ratings are 12, 18 and 25 A, and DNX23 covers 3C to 40C and 50C to 140C. On the 1.35 arithmetic, 13.6 A is above the 12 A size, so the next DNXC23 rating, 18 A, is a calculated starting point only. DENCO's catalog lists its C-rated fuses for indoor or outdoor use within an enclosure, so for an open-rack substation bank send the layout to DENCO engineering before going further.

Interrupting duty depends on the configuration. One manufacturer's guide explains that in ungrounded banks or banks with several series groups, faults are capacitively limited, typically less than two or three times bank load current. In single series group grounded wye or delta banks, faults are inductively limited and limited only by the available system fault current. The same maker says its current-chopping fuse should not be used on single series group grounded wye or delta banks.

Internally fused and fuseless banks: what changes

In an internally fused unit, a relay maker's paper explains, the element fuse isolates a faulty element within a few cycles. Little capacitance is lost, so a more sensitive unbalance protection scheme is required. The paper adds that internally fused banks use fewer units in parallel and more series groups.

In a fuseless string, a failed element shorts and stays in service, and the rest of the string takes a slightly higher voltage, about E / (E - 1) where E is the number of elements in series. One relay paper gives 48 in series as 48 / 47, about a 2 percent increase; another gives 100 in series as 100 / 99, about 1 percent. A utility and relay maker paper on a 230 kV fuseless bank tabulates 4.35 percent overvoltage with 3 failed elements and 9.09 percent with 6, and set the alarm at 3 and the trip at 6. Because no units sit directly in parallel, discharge energy is small, and unbalance protection does not have to be delayed to coordinate with fuses.

Fuses can still appear at bank level. IEC 60549 separates unit fuses, which clear a fault inside one unit so the rest of the bank stays in service, from line fuses, which isolate the whole bank. One manufacturer's bulletin describes power fuses for group fusing of station capacitor banks where available fault currents are high, without nuisance operations from inrush or outrush. A software vendor's application guide notes that the relay and breaker combination is generally not fast enough to protect a capacitor from case rupture caused by internal arcing, which is why externally fused banks rely on the unit fuse for that job.

Pairing unit fuses with unbalance protection

Banks are protected by a combination of fuses, which remove the failed unit or element, and relays, which alarm and trip. In an externally fused bank the relay sees the fail-open state left after the fuse has removed the unit. A relay maker citing IEEE C37.99-2000 says the alarm level is selected according to the first blown fuse, a delayed trip follows when group overvoltage exceeds 110 percent of unit rated voltage or the capacitor maker's recommendation, and a high-set stage covers cascading or arcing faults. The PJM guideline matches: one blown fuse should alarm, and more than one in the same series group should trip the bank when overvoltage on the remaining units exceeds 10 percent.

The relay also has to wait for the fuse. One relay maker's lockout relay is factory-set for a 10-second delay so that individual unit fuses can respond to evolving faults and the failed units can be identified by eye. The coordination guide covers the delays, pickups and backup devices in detail.

Common mistakes

These come up repeatedly in published guidance and field reports on substation banks.

• Sizing parallel groups for overvoltage only. A group large enough for the 10 percent rule can still exceed the 10 kJ or 15 kJ energy figures; check both.
• Forgetting adjacent banks. The PJM guideline notes that adjacent banks on a common bus add energy to what a failed unit sees.
• Mixing multiplier bases. A 1.65 fusing factor used with a relay scheme that assumes 1.25 or less can let the relay trip the bank before the unit fuse clears.
• Reading a tank rupture curve without asymmetry. One capacitor maker notes the curve should be compensated for asymmetry at high fault currents.
• Treating intact fuses as proof of healthy units. A utility and relay maker paper says intact fuses do not necessarily mean the units are in perfect condition; capacitance measurement finds partially failed units.
• Applying a fuse below the available fault current in a single series group grounded wye or delta bank, where faults are limited only by the system.

Frequently asked questions