Capacitor energy storage — PFC, harmonic filters, DC-link, supercaps

Reference notes

Capacitors are ubiquitous in power systems — power-factor correction, harmonic filtering, DC-link buffering in inverters, snubbers, and supercapacitor-based high-power short-duration storage. Use Next → to walk through capacitor fundamentals, the major types used in power applications, the three flagship applications (PF correction, harmonic filtering, DC-link), and the Ragone-plot trade-off between conventional capacitors, supercapacitors, and batteries.

Fundamentals

• Capacitors deliver POWER very quickly (microseconds to milliseconds), store relatively little ENERGY per unit volume — opposite to most battery chemistries.
• Round-trip efficiency: 95–99 % (losses only in series resistance and dielectric leakage).
• Cycle life: essentially infinite for most types — millions of cycles, no chemistry to degrade.

Major capacitor types in power systems

• Power-factor correction capacitors — supply local leading reactive power to cancel inductive load's lagging Q. µF (residential) to MVAR (substation banks).
• Harmonic filter capacitors — combined with inductors to form tuned LC filters absorbing specific harmonics (5th, 7th, 11th, 13th).
• DC-link capacitors — stiff DC bus between rectifier and inverter in VFDs, EV traction, solar inverters, battery PCS. mF range.
• Snubber capacitors — small caps across switching devices to limit voltage spikes during turn-off. nF–µF.
• Supercapacitors / ultracapacitors — electrostatic double-layer storage at 2.5–2.7 V/cell. F to kF capacitance; 5–10 Wh/kg; >1M cycle life.
• Series capacitors — long-transmission-line compensation to reduce effective X_L → more power transfer. kvar at line voltage.
• Battery-capacitor hybrids — emerging chemistry combining Li-ion and capacitor electrodes.

Power-factor correction (PFC)

Inductive loads (motors, transformers, fluorescent ballasts) draw lagging Q from the source. S = √(P² + Q²) sets source current. Adding capacitors locally supplies leading Q, cancelling some of the inductive Q → smaller S, smaller source current, lower I²R losses, better voltage profile.

Typical target: improve PF from 0.80 → 0.95 lagging (cancels ~75 % of original Q without crossing into leading PF). Implementation: fixed banks at loads + switched banks at substations + modern STATCOMs for fast continuous control.

Harmonic filters

Tuned LC filter shunts a specific harmonic to ground. Resonant frequency:

• At the harmonic frequency, the filter has near-zero impedance → absorbs harmonic current.
• At fundamental frequency, the filter is capacitive → provides PFC as a bonus.
• Filters are typically DETUNED (slightly below resonance, e.g., 4.9× fundamental for 5th harmonic) to avoid sharp resonance under grid frequency drift.
• Common configurations: 5th + 7th + 11th + 13th tuned, plus a broadband high-pass for above-13th.
• Bare capacitor banks (without inductors) must be sized to AVOID resonance with the upstream source inductance — otherwise they AMPLIFY harmonics.

DC-link capacitors

Essential in any voltage-source inverter. Provides a stiff DC voltage source between rectifier and inverter stages. As inverter switches modulate at kHz rates, DC current pulses rapidly; the DC-link cap supplies these pulses locally → clean PWM output.

• Sizing: typically 1–5 mF per kW of inverter rating, hundreds of V.
• Electrolytic: high C density, low cost, ~10 000-hour life at max temp — often the lifetime-limiting component in commercial VFDs.
• DC-film: longer life (> 100 000 hr), wider temperature range, used in EV traction, premium industrial. Costs more per µF.
• Modern wide-bandgap semiconductor switches (SiC, GaN) reduce required DC-link capacitance — major factor pushing wide-bandgap adoption in EV inverters.

Supercapacitors — the high-power storage

• Energy storage at the electrode-electrolyte double-layer (no chemistry).
• Voltage 2.5–2.7 V/cell; stacked for higher voltage.
• Capacitance: tens of F to thousands of F per cell.
• Energy density: 5–10 Wh/kg (20–40× less than Li-ion).
• Power density: 5–10 kW/kg (10–30× more than Li-ion).
• Cycle life: > 1 million cycles (200× more than Li-ion).
• Round-trip η: 95–98 %.

Supercap application sweet spots

• Regenerative braking — heavy equipment, trams, electric trucks. Capture braking energy in seconds, release during acceleration. Shanghai Maglev, several Chinese tram lines.
• UPS bridging — instant ride-through during a generator's startup (10s of seconds).
• Photo-flash energy buffer — store slowly, release in milliseconds.
• Pulse power — laser cutting, welding, X-ray, radar.
• Hybrid bus / tram propulsion — charged at stations, released during acceleration.

NOT a good supercap fit

• Overnight residential storage (10–30 kWh) — supercap bank would be uneconomically large.
• EV primary traction battery — supercap energy density too low.
• Any energy-dominated application where Li-ion's $/Wh advantage dominates.

Ragone-plot positioning (rough)