Reactive power compensation is the use of electrical equipment—most commonly capacitor banks, active power factor correction (APFC) systems, or STATCOM / active filters—to reduce unwanted reactive power (kVAr) in an electrical installation and improve power factor (PF). The goal is to minimize reactive power drawn from the grid, stabilize voltage, and reduce cable and transformer losses.
Why Reactive Power Compensation Matters in EV Charging
EV charging sites can include multiple power-electronic loads (AC chargers, DC chargers, rectifiers, inverters) that affect power factor and grid interaction.
– Utilities may apply penalties or require limits for low power factor or excessive reactive energy
– Improved PF can free up apparent power (kVA) capacity in transformers and switchgear
– Lower reactive current reduces I²R losses, heat, and voltage drop in feeders
– Better voltage stability supports reliable charging, especially at constrained sites
– Helps meet grid connection agreement requirements and avoid forced derating
Reactive Power, Power Factor, and kVA Explained
Reactive power is not “wasted,” but it increases current without delivering useful energy.
– Active power (kW) does useful work and charges the battery
– Reactive power (kVAr) supports magnetic/electric fields in inductive/capacitive loads
– Apparent power (kVA) is the total electrical demand on the supply (kVA ≈ √(kW² + kVAr²))
– Power factor (PF) is kW/kVA; a lower PF means higher current for the same useful power
How Reactive Power Compensation Works
Compensation devices supply reactive power locally so less is drawn from the utility.
– Capacitor banks provide capacitive kVAr to offset inductive loads and raise PF
– Detuned capacitor banks add reactors to avoid resonance with harmonics
– APFC systems automatically switch capacitor steps based on measured PF and load
– STATCOM / active compensators inject or absorb reactive power dynamically, fast and precise
– Active harmonic filters can improve PF and reduce distortion, depending on site conditions
EV Charging Site Scenarios Where Compensation Is Used
– Sites with large transformers and long feeders where voltage drop becomes a constraint
– Mixed loads (HVAC, lifts, lighting, motors) plus EV charging in commercial buildings
– Depot or hub sites scaling to many chargers where kVA capacity is limited
– Installations where the DSO requires PF within a defined range at the point of connection
– Locations with measurable reactive energy charges on the electricity bill
Harmonics and Why “Simple Capacitors” Can Be Risky
Power electronics can introduce harmonics, and capacitors can amplify problems if not designed correctly.
– Capacitors can resonate with network inductance, increasing harmonic currents
– Resonance can cause overheating, nuisance trips, capacitor failure, or voltage distortion
– Sites with DC fast chargers often require detuned solutions or active filtering
– Proper design should include a power quality study and harmonic measurements where needed
Benefits of Reactive Power Compensation
– Reduced utility penalties and improved compliance with grid connection rules
– Increased usable capacity in switchboards and transformers (lower kVA loading)
– Improved voltage stability and reduced voltage drop under high load
– Lower heat and losses in cables, improving reliability and equipment life
– Better overall power quality when combined with harmonic mitigation
Limitations and Considerations
– Incorrect sizing can cause overcompensation (leading power factor) and voltage issues
– Switching capacitor steps can create transients without proper protection
– Compensation equipment adds CAPEX, space requirements, and maintenance needs
– Best results depend on accurate measurement at the point of connection and realistic load profiles
Related Glossary Terms
– Reactive power (kVAr)
– Power factor (PF)
– Apparent power (kVA)
– Power quality study
– Harmonics
– Active harmonic filters
– Grid connection agreement
– Voltage drop