In an externally fused capacitor bank, each phase has one or more series groups of parallel units, and each unit has its own fuse. The number of parallel units per group has a minimum (so that, when a unit is isolated, the others do not exceed 110% of rated voltage and the fuse operates quickly) and a maximum (the energy the healthy units discharge into the failed one). IEEE C37.99-2000 gives these criteria in 4.3.1 and 4.4.1.
Short answer
An externally fused capacitor bank has, in each phase, one or more series groups of parallel units. Each unit has its own fuse, mounted between the unit and the bank’s fuse bus. If a unit short-circuits, its fuse isolates it and the bank stays in service with the others. IEEE C37.99-2000 (clauses 4.2.1, 4.3.1 and 4.4.1) sets how many parallel units each group must have: enough so that the healthy ones are not overstressed, and not so many that their energy damages the fuse or the failed unit.
How it is built
- A series group is the set of units connected in parallel between two points.
- A phase is formed by one or more series groups, from the line terminal to the neutral.
- Each unit has a fuse. With units of high rated voltage, the bank needs fewer series groups and its structure is simpler.
The unit can have a relatively high voltage because the external fuse can interrupt a high-voltage fault. In exchange, the reactive power of each unit must be smaller, because each group needs a minimum number of parallel units.
The fuse that isolates each unit is described separately in capacitor fuses: expulsion or current-limiting; the opposite arrangement, with the fuse inside the unit, is in internally fused bank.
What happens when a unit fails
- A unit short-circuits. The healthy units of the group discharge into it
- The fuse of that unit operates. The unit is isolated and the fuse gives a visual indication of which one it was
- The remaining units of the group take more voltage. The group capacitance dropped, so it carries more voltage
- Unbalance measures the change. Alarm if one unit was lost; trip if the remaining ones exceed 110%
The standard clarifies that in an externally fused bank several element breakdowns can occur before the fuse removes the whole unit (cl. 7.1). The fuse operates when the unit is essentially short-circuited.
Minimum parallel units per group
The minimum is determined by two conditions (cl. 4.3.1):
- Overvoltage when a unit is isolated. The general rule: isolating one unit of the group must not put more than 110% of rated voltage on the other units of the group. That 110% is the maximum continuous voltage of the units under IEEE 18-1992.
- Enough current to blow the fuse. The failed unit must receive enough current for its fuse to operate in a reasonably short time. That way unbalance does not need a long delay.
Our worked example. For a grounded-wye bank with S series groups and P parallel units per group, the voltage left on the remaining units when a fuse blows, relative to normal, is S·P / (S·P − S + 1). It comes from the equations of Table 2 of the guide (group capacitance, phase capacitance and voltage division). With S = 4:
| Parallel units (P) | Voltage on the remaining units |
|---|---|
| 6 | 114.3% |
| 8 | 110.3% |
| 9 | 109.1% |
| 10 | 108.1% |
With 8 parallel units the 110% is exceeded by a small margin. With 9 the rule is met. Choosing units of a somewhat higher rated voltage also lowers that percentage (cl. 8.3.2).
Maximum parallel units per group
If a unit fails, the other units of the group discharge their stored energy as a high-frequency transient current that flows through the failed unit and its fuse. The fuseholder and the unit must withstand that discharge (cl. 4.3.1).
To limit the risk of fuseholder failure or case rupture, NEMA CP1-1988 recommends that the total energy stored in a parallel group not exceed 15,000 J for film capacitors (10,000 J for the older paper or paper-film types), at maximum peak voltage (rated voltage × 1.1 × √2). At 60 Hz that is equivalent to 4,650 kvar per parallel group. The guide mentions film banks applied up to 9,600 kvar in parallel with expulsion fuses. Before going above 4,650 kvar, the capacitor and fuse manufacturers must be consulted.
If the bank exceeds that energy with the minimum number of series groups, the guide proposes three ways out: lower-voltage units with more series groups and fewer parallel units (with less unbalance sensitivity), splitting the bank into two sections (double wye), or using current-limiting fuses in a single wye.
Overvoltage when units are isolated
The higher the power of each unit, the fewer parallel units there are per group and the larger the voltage change when one is isolated (cl. 4.4.1). A second failure is more likely in the same group, because its remaining units carry the highest stress. If two units fail in different groups, the overvoltage is lower than if they fail in the same one.
That is why higher-voltage units and fewer series groups give the greatest unbalance sensitivity (cl. 4.3.1), but the signal drops as series groups or parallel units are added. Bank design and protection are decided together.
The 110% reference for the units that stay healthy comes from the capacitor unit limits: 110% voltage and 135% power.
What depends on this arrangement
Fuse selection (its curve and its energy) and the unbalance calculation for each connection start from the arrangement defined here. For the protection overview, see capacitor bank protection and unbalance protection. For the complete equipment, capacitor bank; for the fuse as equipment, medium-voltage fuse.
What we do
The number of units per group, the unit voltage and the phase arrangement are decided with your system data: maximum voltage, bank size and parallel energy. At Tiza Group we carry out that engineering design and studies work and quote the study according to the scope.
Related guides
- IEEE C37.99: shunt capacitor bank protection guide
- Internally fused capacitor bank: how it is protected
- Bank connection: grounded wye, ungrounded wye or delta
- How to size a capacitor bank
- Capacitor bank trips
References
- IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks: cl. 4.2.1 (externally fused banks), 4.3.1 (design), 4.4.1 (overvoltage on the remaining units), 5.1.1 and 5.1.2, 7.1 and 8.3.1; Table 2 (equations of figure 16).
- NEMA CP1-1988, cited by IEEE C37.99-2000 for the parallel energy limit.
- IEEE Std 18-1992, IEEE Standard for Shunt Power Capacitors.
Frequently asked questions
Why does an externally fused bank need a minimum number of parallel units per group?
Because when a unit's fuse blows, the remaining units of the group take on its share of the voltage. The general rule of IEEE C37.99 is that they must not end up above 110% of rated voltage. With few parallel units, that rule is not met.
What limits the maximum number of parallel units?
The energy that the healthy units of a group discharge into the failed unit and its fuse. The guide cites 15,000 J for film capacitors (4,650 kvar at 60 Hz) as the value recommended by NEMA CP1-1988. Exceeding it requires consulting the capacitor and fuse manufacturers.
Why do more series groups reduce the unbalance signal?
Because the phase voltage is shared among more groups. When a unit drops out of one, the voltage change at the neutral or at the measuring point is smaller, and the unbalance relay sees a weaker signal.