Power Factor Correction (PFC) & SVC Guide: Capacitor & Shunt Reactor Sizing
Calculating kVAr capacitor banks with Qc = P × (tan φ1 − tan φ2), stopping leading capacitive over-compensation from LEDs and UPS systems, and sizing thyristor SVC shunt reactors.
Power factor correction (PFC) and Static Var Compensator (SVC) systems balance lagging inductive reactive power from motors and transformers as well as leading capacitive reactive power from LED drivers, SMPS/UPS filters, and long underground cables. Maintaining `cos φ` near `1.00` eliminates utility reactive penalties and reduces transformer and feeder `I²R` losses.
> Related Category: Source complete PFC switchboard components in SVC Solutions, Reactive Power Control Relays, Power Capacitors, and Shunt Reactors.
1. Reactive energy ratio limits (`tan φ` / `cos φ` targets)
In commercial and industrial tariffs (such as Turkey's EPDK grid code for `≥ 50 kVA` connections), monthly reactive-to-active energy ratios must stay strictly inside a narrow deadband:
| Connection tier | Max Lagging (Inductive) / Active | Max Leading (Capacitive) / Active | Equivalent target `cos φ` |
|---|---|---|---|
| 9 kW ≤ Demand < 50 kVA | ≤ 33% (`tan φ ≤ 0.33`) | ≤ 20% (`tan φ ≤ 0.20`) | Inductive `cos φ ≥ 0.95` |
| Demand ≥ 50 kVA | ≤ 20% (`tan φ ≤ 0.20`) | ≤ 15% (`tan φ ≤ 0.15`) | Inductive `cos φ ≥ 0.98` |
2. Capacitor bank (`kVAr`) sizing formula and voltage ratings
To raise an inductive plant from existing `cos φ₁` to target `cos φ₂`:
- Formula: `Qc (kVAr) = P (kW) × (tan φ₁ − tan φ₂)`
- Worked example: Raising a `100 kW` motor plant from `cos φ₁ = 0.75` (`tan φ₁ = 0.882`) to `cos φ₂ = 0.98` (`tan φ₂ = 0.203`) requires `Qc = 100 × (0.882 − 0.203) ≈ 68 kVAr`.
- Capacitor voltage class: Specify `400 V` or heavy-duty `450 V` power capacitors on low-THD grids, and `525 V` capacitors whenever detuned harmonic filter reactors (`7%` / `189 Hz` or `14%` / `134 Hz`) are connected in series.
- AC-6b capacitor switching contactors: Energizing a capacitor step draws `100–150 × In` peak inrush current. Always switch steps via dedicated power factor correction contactors equipped with early-make pre-charge damping resistors (`IEC 60947-4-1 AC-6b`) to prevent contact welding.
3. Why modern facilities need stepless thyristor SVC + shunt reactors
Offices, shopping malls, hotels, hospitals, and LED-lit warehouses turn strongly capacitive overnight and on weekends when motors stop while LED drivers, UPS EMC filters, IT supplies, and lightly loaded XLPE feeders keep injecting leading `kVAr`—often asymmetrically across `L1, L2, L3`:
- SVC-ready reactive power control relays: Sample all three phase CTs independently, switch coarse capacitor/reactor steps, and send `0–100%` PWM/trigger commands for fine per-phase trim.
- SVC inductive load drivers + SVC shunt reactors: Phase-angle thyristors modulate inductive reactive draw steplessly (`0%` to `100%` per phase in milliseconds), neutralizing asymmetric leading `kVAr` right down to unity (`cos φ ≈ 1.00`).
For correct 3-phase CT placement upstream of both load and PFC busbars, review Current Transformer K-L / S1-S2 Wiring and Ratio Selection.
Frequently asked questions
Why do facilities incur capacitive reactive penalties at night when production is off?
At night active `kWh` consumption drops near zero while emergency/perimeter LED drivers, server UPS filters, and long underground cables generate constant leading `kVArh`. Because the tariff limit (`15%`) is a ratio against active `kWh`, even a few leading `kVAr` quickly exceed 15% of low night-time `kWh`.
What happens if standard AC-3 motor contactors are used on capacitor banks?
Without pre-charge damping resistors, capacitor inrush spikes weld main silver contacts shut within weeks, locking the capacitor bank permanently online and causing severe overvoltage and leading-PF penalties.
How do I size an SVC driver and shunt reactor?
Measure peak night/weekend leading `kVAr` across all three phases and size the SVC shunt reactor ~20% above that leading peak (e.g., `10 kVAr`, `25 kVAr`, or `50 kVAr`), adding fixed contactor-switched shunt reactors when base capacitive load exceeds `50 kVAr`.
Why is my PFC relay constantly hunting (switching C1 on and off)?
Common causes are reversed CT `S1–S2` polarity, phase-to-CT cross-wiring (`L1` voltage paired with `L2` CT), mounting CTs downstream of the PFC feeder instead of on the main incoming incomer, or an oversized smallest step (`C1`) relative to light load.