Capacitor Bank Symbol
Definition: The Capacitor Bank symbol represents a group of capacitors connected in parallel (or series-parallel) as a single power system element — used in power engineering diagrams per IEC 60617 and IEEE standards — to provide reactive power compensation (VAr support), harmonic filtering, power factor correction, or energy storage in AC and DC power systems.
Also known as: capacitor bank, capacitor bank for power factor correction, shunt capacitor bank, VAr bank, power factor correction capacitor, filter bank, switched capacitor bank.
What the Capacitor Bank symbol means
The Capacitor Bank symbol represents multiple capacitors grouped into a single assembly that collectively provides a large capacitance or reactive power rating beyond what a single capacitor can achieve practically. The positive terminal ('+') is on the left and the negative terminal ('−') is on the right. In AC power systems, capacitor banks supply reactive power (capacitive VArs) to offset the lagging reactive power consumed by inductive loads such as motors and transformers, improving the power factor of the supply network.
In power factor correction applications, a capacitor bank is typically connected in shunt (line-to-neutral or line-to-line) at the point of common coupling. In harmonic filter applications, capacitors are tuned in series with inductors to form LC filters at specific harmonic frequencies. In DC systems (e.g. welding machines, battery chargers, and UPS systems), capacitor banks provide energy storage and voltage ripple reduction at the DC bus.
How to identify the Capacitor Bank symbol
The Capacitor Bank symbol is drawn as a rectangular block (wider than tall) with a '+' label on the left connection pin and a '−' label on the right connection pin, representing the aggregate of parallel capacitors. Some representations show multiple parallel capacitor symbols stacked side by side within the block. The designator is typically C or CB in power engineering drawings. The block may be annotated with the total rated power in kVAr and the rated voltage.
Function in a circuit
A capacitor bank improves the reactive power balance of an electrical network by supplying leading reactive current (capacitive VArs) to offset the lagging reactive current drawn by inductive equipment. This raises the power factor (towards unity), reduces the reactive current component in supply cables and transformers, lowers I²R losses, improves voltage regulation, and can reduce electricity tariff demand charges. In harmonic filter applications, the capacitor bank tuned with a reactor provides a low-impedance shunt path for specific harmonic frequencies, reducing harmonic distortion in the network.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60831-1/-2 covers shunt power capacitors for AC power systems above 1 kV. IEC 60252-1/-2 covers AC motor capacitors. IEC 61921 covers power capacitors for low-voltage power factor correction banks. IEC 60617 provides the schematic symbol. IEC 61642 covers industrial AC networks affected by harmonics and covers filter capacitor bank design. |
|---|---|
| ANSI/IEEE 315 | IEEE Std 18 (Shunt Power Capacitors) and IEEE Std 1036 (Application Guide for Shunt Power Capacitors) govern capacitor bank applications in North America. ANSI C37 series covers capacitor bank switching and protection. The symbol follows ANSI Y32.2 / IEEE 315 conventions for a labelled rectangular block. |
| Key difference | IEC drawings show the capacitor bank as a rectangle with terminal labels; ANSI/IEEE drawings similarly use a rectangular block often annotated with kVAr rating and voltage. Both standards use the same general rectangular block representation, with minor differences in label position and annotation style. |
Terminals / pins
| Pin | Name |
|---|---|
| pos | + |
| neg | - |
Typical values
Low-voltage (LV) banks: 5 kVAr to 500 kVAr at 400 V / 440 V / 480 V (50 or 60 Hz). Medium-voltage (MV) banks: 100 kVAr to 10 MVAr at 6.6 kV, 11 kV, 33 kV. Capacitance: depends on rating and voltage — C = Q / (ω × V²) where Q is in VAr and V is in volts. Voltage rating: typically 1.1× to 1.3× the nominal system voltage for MV banks. DC bus capacitor banks: microfarads (µF) to farads (supercapacitors) at 24 V to 800 V DC.
Where the Capacitor Bank symbol is used
- Industrial power factor correction — shunt capacitor banks at MCC or transformer secondary to raise site power factor to 0.95–0.99 and avoid utility penalty charges
- Utility substation voltage support — large MV/HV switched capacitor banks to support transmission and distribution voltage during peak load
- Harmonic filters — capacitor banks tuned with reactors as 5th, 7th, and 11th harmonic filters at drives and rectifier loads
- Motor start assistance — capacitor banks that temporarily boost terminal voltage during direct-on-line motor starting
- DC link filtering — large electrolytic capacitor banks at the DC bus of VFDs, UPS, and switch-mode power supplies
- Renewable energy inverters — capacitor banks on the DC side of solar and wind inverters for voltage ripple suppression
- Pulsed power systems — large film capacitor banks for energy storage in welding, pulse-forming networks, and electromagnetic research
Example
In a 400 V industrial distribution panel drawing, a capacitor bank symbol annotated '50 kVAr, 440 V, IEC 61921' is shown connected line-to-neutral at the main busbar; the '+' pin connects to the bus via a contactor, and the '−' pin connects to the neutral bar, providing automatic power factor correction that switches in as motor load increases.
Key facts
- A capacitor bank is a group of capacitors connected in parallel to provide high reactive power (kVAr) or capacitance beyond single-unit capability, standardised under IEC 60831 (MV) and IEC 61921 (LV) for power systems.
- The symbol has two pins: '+' (positive / line terminal, left) and '−' (negative / neutral terminal, right).
- Reactive power output Q = V² × ω × C (VAr), where V is the applied voltage and C is the total capacitance; doubling voltage quadruples output kVAr.
- IEC 60831 allows capacitor banks to operate at up to 1.1× rated voltage continuously and up to 1.3× for transients.
- Automatic power factor correction (APFC) panels switch capacitor bank steps in and out via contactors controlled by a power factor relay, maintaining the target power factor at the point of connection.
- Capacitor banks must be protected against overvoltage, overcurrent, and harmonic resonance; individual capacitor units include internal fuses or overpressure disconnectors per IEC 60831.
- Harmonic resonance can occur when the capacitive reactance of the bank equals the inductive reactance of the system at a harmonic frequency; a detuning reactor (typically 5–7% impedance) prevents resonance in LV power factor correction banks.
Frequently asked questions
What does the capacitor bank symbol mean in a power diagram?
The capacitor bank symbol represents a group of capacitors connected together to supply reactive power (capacitive VArs) to an AC power system, or to provide energy storage and filtering in a DC system. It is connected in shunt (parallel with the load) to improve power factor, support voltage, or filter harmonics. The '+' pin connects to the line or DC positive rail, and the '−' pin to neutral or DC negative.
What does the capacitor bank symbol look like?
The capacitor bank symbol is a filled rectangle (wider than tall) with a '+' label on the left pin and a '−' label on the right pin. It represents the aggregate of all capacitor units in the bank. Annotations typically include the total kVAr rating, voltage rating, and relevant standard (e.g. IEC 60831 for MV banks).
What is the difference between a single capacitor symbol and a capacitor bank symbol?
A single capacitor is shown as two parallel plates (IEC: two parallel lines; ANSI: one straight and one curved plate) representing one discrete component. A capacitor bank symbol represents an assembly of multiple capacitors connected in parallel (or series-parallel) to achieve a high total capacitance or reactive power rating, and is drawn as a rectangular block annotated with the total rating.
What standards cover capacitor banks?
IEC 60831-1 and IEC 60831-2 cover shunt power capacitors for AC systems above 1 kV. IEC 61921 covers LV power factor correction banks. In North America, IEEE Std 18 and IEEE Std 1036 govern shunt capacitor application and protection. IEC 61642 and IEEE 519 address harmonic filter capacitor bank design.
Why does a capacitor bank improve power factor?
Inductive loads (motors, transformers) draw lagging reactive current that increases the apparent power (kVA) without doing useful work. A capacitor bank supplies leading reactive current that cancels the lagging component, reducing total reactive current from the utility supply. This lowers the phase angle between voltage and current, raising the power factor towards unity and reducing I²R losses in cables and transformers.
What is harmonic resonance in a capacitor bank and why is it dangerous?
Harmonic resonance occurs when the capacitive reactance of the capacitor bank equals the inductive reactance of the supply system at a harmonic frequency (typically the 5th or 7th). At resonance, harmonic currents from nonlinear loads are amplified many times, causing overheating of capacitors and transformers and potential dielectric failure. A detuning reactor (5–7% impedance) in series with the capacitor bank shifts the resonant frequency below the 5th harmonic, preventing resonance.
Can a capacitor bank be used in a DC circuit?
Yes. In DC circuits, a large electrolytic or film capacitor bank connected across the DC bus stores energy and reduces voltage ripple from rectifiers, inverters, or switching supplies. The '+' pin connects to the positive DC rail and '−' to the negative rail. DC capacitor banks are common in VFD DC links, UPS energy storage, welding machines, and pulsed-power systems.
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