Coordinating Capacitor Fuses With Unbalance Relays and Upstream Protection

The protection layers in a capacitor bank

A relay maker's unbalance paper sums up the arrangement: banks are protected by a combination of fuses, which remove the failed unit or element, and protective relays, which alarm and trip the bank offline. Around those two sit phase overvoltage elements, bank overcurrent or group fuses, and the upstream feeder or line devices. Coordination is the job of making each one operate in its own territory and wait for the device below it.

One relay paper adds a reason the layers cannot be skipped: unbalance protection does not substitute for bank overvoltage protection, so phase overvoltage elements are needed as well. A software vendor's guide adds another: the relay and breaker combination is generally not fast enough to protect the capacitor from case rupture due to internal arcing faults, which is the purpose of the fuse.

Rule one: the fuse operates before the relay trips

A relay maker's handbook, citing IEEE C37.99-2000, states that the unbalance protection should coordinate with the individual capacitor unit fuses so that the fuses operate to isolate the faulty unit before the protection trips the whole bank. For an externally fused bank, the same source says the alarm level is selected according to the first blown fuse, with a delayed trip when additional losses cause group overvoltage above 110 percent of unit rated voltage or the capacitor maker's recommendation, and a high-set stage for cascading or arcing faults. The PJM shunt capacitor guideline sets the same pattern for its members: one blown fuse alarms, and more than one in the same series group trips when overvoltage on the remaining units exceeds 10 percent.

Lockout relays that act on unit loss build in time for the fuse. One relay maker's instruction sheet describes a lockout delay factory-set at 10 seconds to allow individual unit fuses to respond to evolving faults inside the units, so that failed units can be identified visually. It states that coordination is achieved when the level and delay are set per its procedure and a fusing ratio of 1.25 or less is used for individual unit fuses. Check that the unit fuse basis matches the relay maker's assumption before settling either one.

Fuseless banks are the exception: a relay paper notes that in fuseless banks the unbalance protection does not have to be delayed to coordinate with fuses.

Worked example: when to alarm and when to trip

A relay maker's instruction sheet for ungrounded single wye banks gives the voltage on the surviving units in the affected group as Vo = 3P x VL-n / (2F + 3S (P - F)), where P is units per group, S is series groups, F is failed units and VL-n is line-to-neutral voltage. The critical step is the value of F at which Vo reaches the capacitor maker's maximum working voltage, generally 1.1 per unit.

Its example: VL-n = 20.2 kV, S = 1, P = 5, units rated 19.92 kV. With one failed unit, Vo = 3 x 5 x 20.2 / (2 + 3 x 1 x 4) = 303 / 14 = 21.64 kV, which is 21.64 / 19.92 = 1.086 per unit. With two, Vo = 303 / (4 + 9) = 23.31 kV, or 1.170 per unit. One failure stays under 1.1 and should alarm; two exceed it and should trip.

The same sheet gives the neutral voltage the relay sees: Vn = F x VL-n / (2F + 3S (P - F)). For S = 2, P = 10 and VL-n = 20 kV, one failed unit gives 20,000 / (2 + 6 x 9) = 20,000 / 56 = 357 V and two give 40,000 / (4 + 48) = 769 V. The example sets lockout at the midpoint, (357 + 769) / 2 = 563 V, so the relay trips on the second loss but not the first.

Unbalance trip delays and false trips

Once the relay decides to trip, the delay is short. A relay maker's paper says the trip delay should be minimized to reduce damage from an arcing fault, yet be long enough to avoid false operations due to inrush, system ground faults, switching of nearby equipment and non-simultaneous pole operation of the energizing switch; for most applications 0.1 s should be adequate, and longer delays increase the probability of catastrophic bank failures. A 2025 conference paper quotes the same reference time from IEEE C37.99-2012, along with a delay of 15 to 25 cycles that may provide adequate coordination for faults external to the bank. After spurious trips on external faults, that paper's authors recommend negative-sequence delays of not less than 0.6 s for high-voltage banks. Alarm delays of 5 to 10 seconds are normal in one relay maker's simulations.

False trips without a blown fuse usually come from standing, or inherent, unbalance. A relay maker's paper lists system voltage unbalance and capacitor manufacturing tolerance as the main sources, plus sensing device tolerance and differences in unit temperature, and says compensation should be provided if inherent error approaches 50 percent of the alarm setting. Another paper reports a 10 °C variation from the bottom of an outdoor cabinet to the top on a sunny day, and relay operations when the rising sun hits one side of a bank. Bank builders generally place units to limit unbalance to less than 0.5 percent, and compensation should be re-nulled whenever a unit is changed. The relay also needs power-frequency filtering so harmonics do not move the signal.

Bank overcurrent and the feeder breaker

Bank overcurrent protection backs up the unit fuses and clears faults on the bus and racks. A relay maker's handbook citing IEEE C37.99-2000 gives a desirable minimum starting level of 135 percent of nominal phase current for grounded wye banks or 125 percent for ungrounded banks, and a high-set stage at three to four times rated current to override back-to-back switching. A software vendor's guide for a switchgear-fed capacitor gives wider ranges: time overcurrent pickup 135 to 165 percent of capacitor current and instantaneous pickup 200 to 450 percent, with the fuse below the case rupture curve.

Worked numbers for a 1,200 kvar, 12.47 kV grounded wye bank: phase current is 1,200 / (1.732 x 12.47) = 55.6 A. At 135 percent the minimum pickup is 75.0 A (69.5 A at 125 percent if the bank were ungrounded). The high-set stage at 3 to 4 times rated current is 167 to 222 A. Those settings must sit above the unit fuses' curves in time so the fuse clears a failed unit first, and above inrush so energizing the bank does not trip it.

In metal-enclosed equipment, one bank maker's manual says circuit breaker trip and fuse characteristics should coordinate with the capacitor protection fuses installed in the unit, and another maker's banks use main incoming fuses that provide main bus protection and backup for the capacitor fuses.

Group fuses on pole and line banks

On distribution lines, banks are usually fused by phase rather than per unit; RUS Bulletin 1724D-112 says capacitor units are not normally fused individually. Group fuse guidance from a pole-bank fusing guide: continuous capability at least 135 percent of rated current for grounded wye banks and 125 percent for ungrounded wye; tin T and K links carry 150 percent of rating continuously, silver links 100 percent. The RUS bulletin gives a nominal rating of 135 to 140 percent of capacitor line current, says a link should melt in 5 minutes at 150 to 300 percent of rated current, and tabulates, for example, a 600 kvar bank on 12,470/7,200 V grounded wye at 27.8 A with a 25T link.

Group fuses must also coordinate with the tank rupture curve. The same pole-bank guide gives the largest links that coordinate with its standard and heavy duty units, 100K and 65T, only up to 10,000, 7,800, 6,900 and 6,500 A rms symmetrical at X/R of 0, 5, 10 and 15; above that it points to current-limiting fuses, ungrounded wye operation or moving the rack. The City of Columbus protection standard does not allow fusing greater than 100K because of the capacitor tank rupture curve.

Upstream, the RUS bulletin says capacitor fuses should be coordinated with any line sectionalizing devices. The Columbus standard gives a fuse-to-fuse rule: the protecting fuse should clear before the protected fuse has reached 75 percent of its melting time. If an upstream fuse would melt in 0.20 s at a given fault current, the capacitor group fuse must clear that current by 0.75 x 0.20 = 0.15 s.

Coordinating with the case rupture curve

Every layer above the unit fuse is too slow to protect the case, so the unit or group fuse carries that job alone. One capacitor maker's pole-bank fusing guide states it directly: the fuse maximum clear TCC must fall below the tank rupture TCC at and below the level of available fault current, and at high fault currents the tank rupture curve should be compensated for asymmetry. The NEC 2023-cycle draft text of Article 460 says protective devices over 1000 V should be rated or adjusted to operate within the limits of the safe zone for individual capacitors, and defines the safe zone as low probability of damage other than slight swelling of the case, as identified by the case rupture curve.

In practice this sets the upper bound on fuse size, and inrush and outrush set the lower bound. Relay coordination works inside the window those two leave. If the window closes, the fixes are on the bank side: more series groups, a double wye, inrush reactors, or a current-limiting fuse with lower let-through.

Common mistakes

These are the coordination errors that published guidance warns about most often.

• Setting the unbalance trip so sensitive that it trips on the first blown fuse in a bank designed to tolerate it, leaving no alarm stage.
• Using unit fuses larger than the relay maker's assumed fusing ratio, so the relay trips before the fuse clears a failing unit.
• Long unbalance trip delays. One relay paper warns longer delays increase the probability of catastrophic bank failure.
• Leaving standing unbalance uncompensated, or forgetting to re-null after replacing a unit.
• Setting bank overcurrent high-set below back-to-back inrush.
• Picking a group link larger than the tank rupture curve allows at the available fault current.

What a coordination study needs

To coordinate DENCO fuses with the rest of the scheme, collect the following and send it with the request. DENCO's catalog includes DNXC time-current curves on p.73; ask DENCO engineering for the curves that match the rating you are considering.

• Bank configuration: grounded or ungrounded wye, single or double wye, delta or H-bridge; S, P and strings per phase.
• Unit kvar, rated voltage and the capacitor maker's tank rupture curve.
• Unbalance relay type, alarm and trip levels and delays.
• Bank overcurrent or group fuse settings and upstream device curves.
• Available fault current and X/R at the bank.
• Inrush and outrush figures from the switching study.

Frequently asked questions