Short answer
External fuse capacitor bank unit loss changes voltage across the remaining units. In an externally fused bank, each unit has its own fuse, and a failed unit is removed completely. Units are arranged in parallel groups, with groups connected in series in each phase leg. When a group loses a unit, its capacitance falls and it takes a larger share of the phase voltage, so the remaining units in that group are overstressed. In an illustrative leg of two series groups of six units each, losing one unit raises the affected group’s voltage by about 9.1 % and lowers the leg’s reactive output by about 9.1 %. A second loss in the same group raises its voltage by 20 %. The remaining units’ overvoltage capability decides how many losses are tolerable before the bank must trip.
Series-parallel structure
| Units lost in A | Group A share | Rise | Leg kvar |
|---|---|---|---|
| 0 | 0.500 | 0 | 100 % |
| 1 | ≈ 0.545 | ≈ +9.1 % | ≈ 90.9 % |
| 2 | 0.600 | +20 % | 80 % |
A phase leg may have S series groups, each with P units in parallel. Units in a group share the same voltage; groups share the leg voltage inversely to their capacitance.
Illustrative redistribution
Illustrative example. Phase leg with 2 series groups, 6 units each; one unit in group A lost. Values chosen for demonstration.
Capacitance: group A 5 C_u, group B 6 C_u. Voltage shares:
share_A = (1/5) / (1/5 + 1/6) = 0.2 / 0.3667 ≈ 0.545
Normal share is 0.5, so group A voltage rises by about 9.1 %. Leg capacitance:
Cleg = (5 × 6) / (5 + 6) C_u ≈ 2.727 C_u (vs 3.0 C_u)
Reactive output of the leg falls about 9.1 %.
With a second unit lost in group A (4 C_u):
share_A = (1/4) / (1/4 + 1/6) = 0.6
Group A voltage rises 20 % above normal.
Voltage rise versus reactive change
| Effect | Concern |
|---|---|
| Voltage rise on survivors | Dielectric stress, cascade failure |
| Reactive output loss | Power factor, filter tuning |
| Phase unbalance | Neutral current or voltage, used for detection |
The two effects are related but separate: the reactive change may look acceptable while survivors are overstressed.
Limits and protection
- Overvoltage capability of remaining units, from manufacturer data and standards.
- Units lost per group that keeps survivors within limits.
- Unbalance protection set to alarm at the first loss and trip before the limit; see capacitor unbalance protection with internally fused units for detection schemes.
- Larger groups (more units in parallel) reduce voltage rise per loss.
Filter tuning shifts with lost units; see tolerance drift in tuned harmonic filters. Harmonic duty on survivors is covered in capacitor overload from background harmonics.
What this article does not cover
Element-level failure criteria for internally fused units are not repeated here.
Where this reasoning stops. Simple series-parallel arithmetic; bank grounding and phase interaction change details.
Practical takeaway
Calculate voltage on survivors for each unit loss in the actual series-parallel arrangement, treat voltage rise and kvar loss separately, and trip before surviving units exceed their overvoltage capability.
Reference framework
- IEEE C37.99, Guide for the protection of shunt capacitor banks.
- IEC 60871-1, Shunt capacitors for a.c. power systems above 1 000 V.