Direct Answer
A capacitor stores electrical energy in an electric field created by separated charge. Its basic relationship is \(Q=CV\): more capacitance or voltage means more stored charge. In AC circuits, capacitor current leads voltage and capacitive reactance decreases as frequency rises.
In power systems, capacitor banks are commonly connected in shunt to supply leading reactive power, support voltage, reduce upstream reactive current, and improve system capacity utilization. Their application must also account for voltage rise, harmonic resonance, switching transients, failed units, protection, discharge time, and changing load conditions.
How a Capacitor Stores Charge and Energy
A capacitor consists of conductive electrodes separated by a dielectric. Applying voltage moves charge onto the electrodes and establishes an electric field in the dielectric. Energy is stored in that field rather than as a continuous flow of current through the dielectric.
What Determines Capacitance?
For an ideal parallel-plate capacitor, capacitance increases with plate area and dielectric permittivity and decreases as plate spacing increases:
- \(C\)Capacitance in farads.
- \(\varepsilon\)Permittivity of the dielectric.
- \(A\)Effective electrode area.
- \(d\)Effective dielectric spacing.
A capacitor stores separated charge and electric-field energy. It does not “store current,” and in AC power systems it does not create free real power.
Key Capacitor Equations
Charge
Stored Energy
Current-Voltage Relationship
Capacitive Reactance
Capacitive reactance falls as frequency rises. This is why capacitors can behave very differently at harmonic frequencies than they do at the 50 or 60 Hz fundamental.
Series and Parallel Capacitors
Ideal series equations are not enough for high-voltage assemblies. Unit tolerances, internal discharge elements, voltage distribution, failed units, fusing, and bank connection all affect actual stress.
Capacitor Behavior in DC and AC Circuits
With steady DC, ideal capacitor current falls to zero after charging. With AC, changing voltage continuously moves charge, so alternating current flows.
DC Charging
An initially uncharged capacitor draws charging current when connected to a DC source. As capacitor voltage approaches source voltage, charging current decays. The charging rate is determined by the surrounding circuit resistance and capacitance.
AC Phase Relationship
For an ideal sinusoidal capacitor, current leads voltage by 90°. Real capacitors introduce losses and parasitic inductance, so the phase relationship is not perfectly ideal.
Real Capacitor Model
Practical capacitors have equivalent series resistance (ESR), equivalent series inductance (ESL), dielectric loss, leakage, tolerance, and temperature-dependent behavior. At sufficiently high frequency, ESL can dominate and the device can stop behaving capacitively.
Capacitor Types and Ratings
Capacitance value alone is never enough to select a capacitor. Voltage, current, dielectric technology, temperature, losses, duty cycle, frequency, and expected life must match the application.
| Type | Common use | Important limitation |
|---|---|---|
| Ceramic | Decoupling, filtering, RF, electronics | Some dielectric classes vary strongly with DC bias and temperature |
| Aluminum electrolytic | Bulk DC filtering and energy buffering | Polarity, ripple current, temperature, and lifetime are critical |
| Film | AC duty, pulse duty, power electronics, motor/run applications | Larger physical size for a given capacitance than some alternatives |
| Supercapacitor | Short-duration energy buffering | Low cell voltage, balancing, leakage, and energy-density tradeoffs |
| Shunt power capacitor | Power factor / reactive support / voltage support | Must be applied for system voltage, frequency, harmonics, switching, protection, and discharge |
Swipe horizontally to view all table columns.
Ratings Engineers Actually Check
| Rating / property | Why it matters |
|---|---|
| Capacitance and tolerance | Sets reactance, stored charge, filter response, and reactive output |
| Voltage rating | Controls dielectric stress and failure margin |
| RMS / ripple current | Controls internal heating from AC current |
| ESR / loss tangent | Determines losses and temperature rise |
| Temperature range | Affects capacitance, losses, and aging |
| Frequency | Changes reactance and harmonic-current stress |
| Duty / switching frequency | Important for pulse, power-electronic, and switched-bank service |
| Life / reliability rating | Important where capacitor aging is a maintenance driver |
Swipe horizontally to view all table columns.
Power-System Capacitor Banks
Power-system capacitors are commonly assembled into shunt banks connected to distribution feeders, substations, industrial buses, or individual loads. Their main function is to supply leading reactive power locally.
Where Banks Are Installed
| Location | Primary objective | Tradeoff |
|---|---|---|
| Individual motor / load | Local VAR support and feeder-current reduction | Must switch appropriately with the load |
| MCC / facility bus | Group-load power-factor correction | Automatic steps may be needed for variable load |
| Distribution feeder | Voltage support and loss reduction along feeder | Placement and switching influence feeder voltage profile |
| Substation bus | Bulk reactive support and voltage control | Less local current reduction downstream than distributed placement |
Swipe horizontally to view all table columns.
Fixed vs. Switched Banks
A fixed bank remains connected continuously. A switched bank is divided into steps that can be connected or disconnected based on voltage, VAR demand, power factor, time schedule, or other control logic. Switched banks reduce the risk of overcompensation when load changes significantly.
For detailed kVAR sizing and economic application, see Power Factor Correction. This page focuses on capacitor behavior and power-capacitor application.
Capacitor Reactive Power, kVAR/MVAR, and Voltage Dependence
Power capacitor banks are usually specified by reactive-power output rather than farads because the operational question is how many VARs the bank supplies at the actual system voltage and frequency.
Single-Phase Capacitor Reactive Power
Why Capacitor kVAR Changes With Voltage
At constant capacitance and frequency, reactive output varies approximately with the square of applied voltage:
Example: Bank Output at Lower Voltage
A 1,000 kVAR bank is rated at 13.8 kV. If the bus operates at 13.2 kV and frequency is unchanged:
A shunt capacitor produces less VAR support when voltage is already depressed. That characteristic is important in voltage-control and stability studies.
Capacitor Banks, Harmonics, and Resonance
This is one of the most important power-system capacitor topics. Because capacitive reactance falls with frequency, harmonic currents can be much larger than a simple 60 Hz calculation suggests.
Parallel Resonance
System inductance and shunt capacitance can form a parallel resonant circuit near a harmonic frequency. If that resonant frequency is close to a strong harmonic source, bus voltage distortion and capacitor current can increase substantially.
Series Resonance
Capacitors and reactors may also form series-resonant paths with low impedance at selected frequencies. This principle is used intentionally in passive filters but can be damaging when it occurs unintentionally.
Detuned Capacitor Banks
Detuned banks add series reactors so the capacitor-reactor combination is tuned below a dominant harmonic, reducing the chance that the bank will resonate directly at that harmonic. The exact tuning, reactor rating, capacitor voltage stress, and thermal duty must be engineered for the application.
Facilities with significant VFDs, rectifiers, UPS systems, EV chargers, inverters, or other nonlinear loads should evaluate harmonic spectrum and system resonance before adding conventional power-factor correction banks.
IEEE 519-2022 provides the current IEEE harmonic-control framework at the point of common coupling. See Power Quality for harmonic measurement and troubleshooting context.
Capacitor Switching, Protection, Failed Units, and Discharge
Capacitor banks can experience high transient current and voltage during energization, de-energization, restrike, back-to-back switching, and nearby fault events. Switching duty is therefore part of bank design—not an afterthought.
Switching Transients and Inrush
Energizing an uncharged bank can produce high-frequency inrush current. Switching one bank when another energized bank is nearby can create particularly severe back-to-back inrush. Breaker or contactor selection, pre-insertion components where used, reactor application, and switching sequence may all matter.
Common Capacitor-Bank Protection
| Function | What it detects / limits | Why it matters |
|---|---|---|
| Unit / group fuses | Failed capacitor units or elements depending on bank design | Isolates failed portions and limits damage |
| Unbalance protection | Capacitance loss or failed elements/units | Detects internal bank deterioration before excessive stress develops |
| Overcurrent | Major faults and abnormal bank current | Backup / fault protection |
| Overvoltage | Excessive bus or unit voltage | Limits dielectric stress |
| Neutral / residual measurement | Bank imbalance depending on connection | Sensitive indication of failed units/elements |
| Temperature / enclosure monitoring | Abnormal heating where provided | Supports condition assessment |
Swipe horizontally to view all table columns.
Why a Failed Unit Can Overstress the Remaining Units
In some series-group bank arrangements, loss of capacitance in one unit or group changes voltage distribution. Remaining healthy units can experience increased voltage stress even though the overall bank remains connected. Unbalance protection is designed around the actual bank connection and failure mode.
Stored Charge and Discharge Safety
Capacitors can remain charged after disconnection. Power-capacitor installations therefore use discharge devices and operating procedures to reduce residual voltage. Personnel should never assume a disconnected bank is discharged without following the applicable procedure and verifying the condition.
Power-Capacitor Design and Field Review Checklist
A capacitor-bank application should begin with the electrical objective and end with verification under multiple operating conditions.
| Review item | Verify | Risk if missed |
|---|---|---|
| Objective | PF correction, feeder voltage, substation VAR support, filter duty | Bank solves the wrong problem |
| Voltage / frequency | Actual normal and abnormal range | Wrong kVAR output or dielectric overstress |
| Load profile | Peak, normal, light load, seasonal operation | Overcorrection and voltage rise |
| Harmonics | Spectrum, nonlinear load, resonance scan where needed | Excess current / voltage distortion / failure |
| Switching duty | Single-bank and back-to-back inrush, switching frequency | Breaker/contact damage or transient overvoltage |
| Bank connection | Grounded/ungrounded wye, delta, series groups, fuse scheme | Protection and failure behavior misunderstood |
| Protection | Fuse, unbalance, overcurrent, overvoltage, alarms | Failed units remain undetected |
| Discharge | Discharge devices, wait time, grounding procedure | Residual-voltage hazard |
| Field condition | Bulging, leaks, blown fuses, hot joints, corrosion, contamination | Progressive failure or outage |
Swipe horizontally to view all table columns.
Field Warning Signs
- Repeated fuse operation or unbalance alarms
- Bulging, leaking, cracked, or discolored capacitor units
- Abnormal noise or vibration in reactors/contactors
- Hot terminals or bus connections
- Higher-than-expected harmonic current
- Voltage rise or leading PF at light load
- Frequent switching or controller hunting
Capacitor Engineering References
- IEEE 18-2025 — Standard for Shunt Power Capacitors Active IEEE standard covering shunt power capacitors rated 216 V or higher and 2.5 kVAR or more for 50/60 Hz AC transmission and distribution systems.
- IEEE 1036-2020 — Guide for the Application of Shunt Power Capacitors Active application guide for shunt power capacitors rated 2400 Vac and above; IEEE also has an active P1036 revision project.
- IEEE 519-2022 — Harmonic Control in Electric Power Systems Current IEEE framework for steady-state harmonic voltage and current distortion at the point of common coupling.
- Eaton — Power Factor Correction: A Guide for the Plant Engineer Practical industry guidance covering capacitor selection, kVAR sizing, installation location, harmonics, filters, and maintenance.
Frequently Asked Questions
What does a capacitor do?
A capacitor stores energy in an electric field and exchanges charge when its voltage changes. In power systems, capacitor banks are used primarily to supply leading reactive power and support voltage.
Why do capacitors block DC but pass AC?
After an ideal capacitor charges to a steady DC voltage, its current falls to zero. With AC, voltage keeps changing, so charge continually moves on and off the electrodes and alternating current flows.
Why are power capacitor banks rated in kVAR instead of microfarads?
For power-system operation, the useful quantity is reactive-power output at a specified voltage and frequency. Capacitance still determines that output, but kVAR or MVAR directly describes the VAR support the bank provides to the system.
Does a capacitor bank always improve voltage?
A shunt capacitor often raises local voltage by supplying reactive power, but the amount depends on system impedance, load, bank size, and operating condition. Too much capacitance can cause excessive voltage at light load.
Can capacitor banks cause harmonics or resonance?
Yes. Capacitors can resonate with system inductance and amplify harmonic current or voltage. Harmonic-rich systems may require detuned banks, filters, or a dedicated harmonic/resonance study.
Why is capacitor-bank switching difficult?
Capacitor energization can create high-frequency inrush current and switching transients. Back-to-back switching can be especially severe when another energized bank is already connected nearby.
Can a capacitor remain dangerous after it is disconnected?
Yes. Capacitors can retain stored charge after disconnection. Power capacitor installations use discharge devices and procedures, but maintenance still requires the applicable wait, grounding, and verification steps.
What happens when one capacitor unit fails in a bank?
The effect depends on the bank connection and fuse arrangement. In some series-group designs, failed capacitance changes voltage distribution and can overstress remaining units, which is why unbalance protection is important.
Summary and Next Step
Capacitors store electric-field energy, oppose rapid voltage change, and draw leading current in AC systems. Those fundamentals explain their role in electronics, power electronics, motors, filters, and utility capacitor banks.
For power-system work, the most important practical concepts are reactive-power output, voltage-squared behavior, bank location, harmonics and resonance, switching duty, protection, failed-unit voltage stress, discharge safety, and load variation. A correct kVAR number is only the start of a complete capacitor-bank application.