A single capacitor unit is not enough for a substation: large capacitances at medium voltages are needed, so a "bank" is built — an array of units connected in parallel for capacitance and in series for voltage, following its own engineering rules for connection and protection.
Why Banks and Not Single Units?
- Parallel adds capacitance: each unit added in parallel increases the group's compensating power (kVAr).
- Series divides voltage: a string of units shares the medium voltage that a single unit cannot withstand.
- So the bank is built as an array: series groups to withstand voltage, each group containing parallel units to reach the required capacitance.
Three-Phase Bank Connections
| Connection | Characteristics |
|---|---|
| Grounded star | Common at higher voltages; reduces voltage stress and simplifies unbalance protection |
| Ungrounded star | Prevents the flow of zero-sequence harmonics; common in distribution |
| Delta | Mostly for lower voltages (power factor correction panels in facilities) |
| Double star | Enables sensitive unbalance protection by comparing the currents of the two star points |
Protecting Banks
- Fuses for each unit (external or internal) that isolate the faulty unit while keeping the bank operational.
- A circuit breaker for the entire bank with overcurrent relays — protected exactly like transformers.
- Unbalance protection: the most distinctive protection for banks — the failure of units or blowing of fuses disturbs the internal voltage/current balance, detecting it before stress escalates on the remaining units.
- Inrush-limiting reactors: small series coils that suppress the inrush charging current at the moment of connection, especially when energizing a bank adjacent to an already-charged bank.
A blown fuse on one unit raises the voltage on its neighbors in the same series group — don't delay replacing failed units, as the excess stress cascades.
Sample answer: Parallel connection adds the units' capacitances to achieve the required compensating power in kVAr, while series connection divides the medium voltage across a string of units because a single unit cannot withstand the full busbar voltage. So the bank is built as an array: series groups to divide the voltage, with each group containing parallel units for capacitance — and the configuration is set so that the voltage on each unit remains within its rating while achieving the total required kVAr.
Re-energizing a bank immediately after disconnecting it without waiting for its units to discharge. Connecting a charged capacitor to the network with opposite polarity doubles the inrush current and stress — a discharge delay (per the internal discharge equipment) is required before any re-energizing.
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