Short answer
Capacitor unbalance internally fused protection works by detecting small capacitance changes. An internally fused capacitor unit contains many elements arranged in parallel groups, with groups connected in series. Each element has its own fuse. When an element fails, its fuse blows, removing only that element; the unit stays in service with slightly lower capacitance. The remaining elements in that group then carry a slightly higher share of the voltage. In an illustrative unit with four series groups of 12 elements, one blown fuse lowers that group’s capacitance to 11/12 and raises its voltage by about 6.7 %. Unbalance protection, for example by measuring current between the neutrals of a double-wye bank, sees the capacitance change. Natural unbalance from manufacturing tolerances must be compensated so that small changes can be detected. Settings depend on bank structure; no generic values are given.
Element versus unit failure
| Fuses blown in group | Group voltage share | Rise |
|---|---|---|
| 0 | 0.250 | 0 |
| 1 | ≈ 0.267 | ≈ +6.7 % |
| 2 | ≈ 0.286 | ≈ +14.3 % |
- Internally fused unit: element failure blows an internal fuse; unit continues with reduced capacitance.
- Externally fused unit: unit failure blows an external fuse, removing the whole unit; see external-fuse capacitor banks.
- Fuseless: elements fail short, shorting a series group within a string.
Voltage rise in the affected group
Illustrative example. Unit with 4 series groups, each 12 elements in parallel; one element fuse operates. Values chosen for demonstration.
Group capacitances: three at 12 C_e, one at 11 C_e. Voltage divides inversely to capacitance:
shareaffected = (1/11) / (3/12 + 1/11) ≈ 0.2667
Normal share is 0.25, so voltage on the remaining 11 elements rises by about 6.7 %. A second blown fuse in the same group raises it further. Protection must alarm or trip before element voltage exceeds the manufacturer’s limit.
Detecting the change
| Scheme | Measures |
|---|---|
| Double-wye neutral current | Current between two wye neutrals |
| Neutral voltage (ungrounded wye) | Neutral shift |
| H-bridge | Current across bridge between two strings |
| Phase voltage differential | Tap point voltage vs bus |
Each scheme’s sensitivity depends on bank layout: number of units per group, groups in series and strings in parallel.
Natural unbalance
Manufacturing tolerances mean the bank is not perfectly balanced when new. The protection measures this inherent signal at commissioning and subtracts it, so that changes from blown fuses are visible. Temperature differences between parts of the bank can also shift readings. Records of commissioning values matter.
Settings logic
- Calculate unbalance signal per blown fuse for the bank structure.
- Alarm at a number of fuses that leaves margin to the element voltage limit.
- Trip before the limit is reached.
- Compensate natural unbalance and check periodically.
Tuning of filter banks shifts as capacitance falls; see tolerance drift in tuned harmonic filters.
What this article does not cover
It provides no generic unbalance alarm or trip settings; these follow manufacturer data and bank-specific calculation.
Where this reasoning stops. The calculation is per unit; whole-bank signals need the bank’s full arrangement.
Practical takeaway
Calculate how element losses raise group voltage and change the unbalance signal for the actual bank layout, compensate natural unbalance at commissioning, and set alarm and trip before element voltage limits.
Reference framework
- IEEE C37.99, Guide for the protection of shunt capacitor banks.
- IEC 60871-4, Shunt capacitors for a.c. power systems above 1 000 V: internal fuses.