The fuse of a capacitor must quickly isolate a dielectric failure and interrupt the energy available at the fault point. That energy is what the healthy units of the parallel group discharge at their peak voltage. If it exceeds what the unit or the expulsion fuses can withstand, IEEE C37.99-2000 recommends current-limiting fuses with adequate energy rating, and requires them in ungrounded-wye banks inside an enclosure.
Short answer
The fuse of a capacitor unit has two jobs: to isolate quickly a dielectric failure and to interrupt the energy available at the fault point. IEEE C37.99-2000 (cl. 7.1.1 and 7.1.2) explains that this energy is what the healthy units of its group discharge, charged to peak voltage, into the failed unit and its fuse. With little energy, an expulsion fuse will do. If the energy exceeds what the units or the expulsion fuses can withstand, the guide recommends current-limiting fuses with an adequate energy rating.
Two different fuses
The guide defines two fuse uses in a bank:
- Individual capacitor fuse (cl. 3.14): disconnects a failed unit from its bank.
- Line or group fuse (cl. 3.6): disconnects a failed phase from the whole bank.
This guide deals with the first. For the fuse as medium-voltage equipment, see medium-voltage fuse.
They are used by the externally fused bank; the internally fused bank carries the fuse inside the unit.
What energy it must interrupt
When a unit fails, the other units of its parallel group are charged. Their charge is discharged as a high-frequency transient current through the failed unit and its fuse. The fuse must be able to interrupt the energy delivered by the parallel group charged to its peak voltage (cl. 7.1.2).
Energy reference: NEMA CP1-1988 suggests a limit of 15 kJ, equivalent to 4,650 kvar in a parallel group, for film capacitors. Expulsion fuses are frequently applied with higher energy, up to 30 kJ. This is acceptable if the total available discharge energy of the bank does not exceed what the fuse can interrupt or what the failed unit withstands (cl. 7.1.2).
The unit power limit is in capacitor unit limits: 110% voltage and 135% power.
Expulsion or current-limiting fuse
| Situation | Fuse | Clause |
|---|---|---|
| Discharge energy within what units and expulsion fuses withstand | Expulsion | 7.1.2 |
| Discharge energy higher than what units or expulsion fuses withstand | Current-limiting with adequate energy rating | 7.1.2 |
| Ungrounded wye in an enclosure | Current-limiting, mandatory: expulsion fuses generate arc products | 7.1.2 |
| Ungrounded wye in an enclosure, one group, two-bushing units with grounded case | May require current-limiting fuses: the fuse must interrupt the system fault current | 7.1.2 |
| Grounded wye, one unit per phase | The fuse must be able to interrupt the system fault current | 5.1.1 |
| Grounded wye with several series groups | Normally no current-limiting fuses, unless the parallel power exceeds 4,650 kvar or the manufacturer’s level | 5.1.2 |
| Delta with one group per leg | Must interrupt the phase-to-phase fault current; may require current-limiting fuses | 5.3 |
There is a useful exception. In an ungrounded wye with a single group per phase, the current through a failed unit is limited to three times the normal phase current, so current-limiting fuses are normally not needed (cl. 5.2.1).
How an external fuse is selected
The guide refers to IEEE C37.48-1997 for application guidelines for external capacitor fuses, and sets these conditions:
- That they are designed and rated for externally fused capacitor banks.
- That they quickly isolate the failed unit.
- That their voltage interrupting capability is coordinated with the voltage the unit withstands.
- That they withstand the transient inrush and outrush currents.
- That they are sized for the continuous current, including harmonics and adequate margins.
- That they interrupt inductive and capacitive currents.
- That their characteristics are coordinated among the different protection schemes of the bank and among the fuse types (expulsion, current-limiting or combined).
Case rupture curve. The manufacturer must supply the rupture curve of each unit, because it changes with the construction. The protection is adequate if the total clearing curve of the fuse lies to the left of and below that unit curve (cl. 7.1.2).
Internal fuses
In banks with internal fuses, the link of each element has its own rules (cl. 7.1.3): it must isolate the element properly, be able to interrupt transient overvoltages, not operate on external faults and withstand the inrush and outrush current throughout the life of the unit. The fastest fuse that meets those requirements is chosen. The healthy fuses of a unit with a fault must be able to safely interrupt other internal faults.
Relation to unbalance
The fuse must be fast to coordinate with unbalance: the standard asks that the fuse operate before unbalance takes the bank out, and that it not operate during switching or external faults (Table 1). If the fuse is slow, unbalance needs a longer delay to coordinate with it (cl. 4.3.1), whereas for an external arc the shortest possible delay is preferable (cl. 7.1.4). For the effect on the unbalance signal, see capacitor bank unbalance protection.
The signals produced when fuses blow are calculated in unbalance calculation with external fuses.
What not to do
- Replace fuses carelessly in an ungrounded-wye bank. If there are failed units in different phases, replacing the fuses can form a phase-to-phase fault (cl. 5.2.1).
- Change the fuse type without checking the parallel energy when adding units to the group.
- Assume an expulsion fuse is safe in any enclosure. In a closed enclosure, arc products can make the fault worse.
What we do
The choice between expulsion and current-limiting fuses depends on the parallel energy, the connection and the enclosure of your bank. At Tiza Group we resolve it within the bank’s engineering design and studies and quote the study according to the scope.
Related guides
- IEEE C37.99: shunt capacitor bank protection guide
- Externally fused capacitor bank: design and limits
- Internally fused capacitor bank: how it is protected
- Capacitor unit limits: 110% voltage and 135% kvar
- Capacitor bank protection
References
- IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks: cl. 3.6, 3.14, 4.3.1, 5.1.1, 5.1.2, 5.2.1, 5.3, 7.1.1, 7.1.2, 7.1.3 and Table 1.
- IEEE Std C37.48-1997, cited by the guide for the application of external capacitor fuses.
- NEMA CP1-1988, cited by the guide for the parallel energy limit.
Frequently asked questions
How do you know whether a fuse protects the unit against case rupture?
The total clearing curve of the fuse is compared with the case rupture curve supplied by the capacitor manufacturer. The protection is adequate if the clearing curve lies to the left of and below the rupture curve.
Why does an ungrounded-wye bank inside a cabinet use current-limiting fuses?
Because an expulsion fuse releases gases and arc products, and inside a closed enclosure they can make the fault evolve. The guide requires current-limiting fuses in that case.
Up to what parallel energy are expulsion fuses applied?
NEMA CP1-1988 suggests 15 kJ (4,650 kvar) for film capacitors. The guide adds that they are frequently applied with more energy, up to 30 kJ, if the bank's available discharge energy does not exceed what the fuse and the failed unit can withstand.