Active Harmonic Filter (AHF)
An Active Harmonic Filter (AHF) is a power electronic device used to detect and eliminate harmonic currents in electrical power systems. It injects an equal and opposite compensation current into the network to cancel unwanted harmonics, improve power quality, and reduce distortion.
Unlike passive harmonic filters that use fixed combinations of capacitors and reactors, an AHF provides dynamic real-time compensation for changing loads.
1. Working Principle
An Active Harmonic Filter operates using power electronics:
·Current sensors measure the load current.
·A control system analyzes the harmonic components.
·A power converter generates compensation currents.
·The AHF injects these currents into the system.
·Harmonic currents are cancelled, leaving a cleaner supply current.
2. Main Components
Power Converter
·IGBT or SiC-based inverter
·Generates harmonic compensation current
·High-speed switching operation
DC Link Capacitor
·Energy storage element
·Maintains converter DC voltage stability
Current Sensors
·CT sensors or Hall sensors
·Detect load current distortion
Digital Control System
·DSP/FPGA controller
·Performs harmonic detection and switching control
Filter Reactor
·Limits switching ripple current
·Protects the inverter output
3. Types of Active Harmonic Filters
A.) Shunt Active Harmonic Filter
Most common type
Connected in parallel with the load.
Functions:
·Eliminates current harmonics
·Corrects reactive power
·Improves power factor
Applications:
·Industrial plants
·Data centers
·Commercial buildings
B.) Series Active Harmonic Filter
Connected in series with the power line.
Functions:
·Compensates voltage distortion
·Corrects voltage imbalance
·Protects sensitive equipment
Applications:
·Critical power systems
·Semiconductor manufacturing
C.) Hybrid Active Harmonic Filter
Combination of:
·Passive filter
·Active power converter
Advantages:
·Higher power capacity
·Lower converter rating
·Cost-effective for large systems
Applications:
·Large industrial networks
·Utility substations
4. Harmonics Removed
AHFs can compensate:
·3rd harmonic
·5th harmonic
·7th harmonic
·11th harmonic
·13th harmonic
·Higher-order harmonics
Common sources of harmonics:
·Variable frequency drives (VFD)
·UPS systems
·Rectifiers
·Battery chargers
·Solar inverters
·Welding machines
·Arc furnaces
5. Key Performance Parameters
·Voltage level: 400 V – 35 kV
·Frequency: 50/60 Hz
·Compensation current: 25 A – 1000+ A
·Harmonic reduction: Up to 95%
·Response time: <1 ms
·Power factor correction: Yes
·Installation: Indoor/outdoor
6. Advantages
·Real-time correction: Adapts to changing loads
·Wide harmonic range: Filters multiple harmonic orders
·No resonance problem: Unlike passive filters
·Compact design: Smaller than large passive systems
·Multi-function: Harmonic filtering + reactive compensation
·High accuracy: Dynamic compensation
7. Disadvantages
·Higher initial cost : Power electronics equipment is expensive
·Switching losses: Requires cooling system
·More complex control : Needs advanced electronics
·Limited fault withstand: Requires protection coordination
8. Applications
Industrial Systems
·Steel mills
·Cement plants
·Mining facilities
·Chemical plants
Renewable Energy
·Solar PV plants
·Wind farms
·Battery energy storage systems
Commercial Facilities
·Data centers
·Hospitals
·Office buildings
Utility Systems
·Distribution substations
·Smart grids
·Power quality improvement projects
9. Standards Commonly Referenced
·IEEE 519 – Harmonic control in power systems
·IEC 61000 series – Electromagnetic compatibility and harmonic limits
·IEC 61439 – Low-voltage switchgear assemblies (where applicable)
Active Harmonic Filter summary:
An AHF is a smart power electronic compensation device that actively detects and cancels harmonic currents in real time. It is widely used in modern electrical networks with nonlinear loads such as VFDs, UPS systems, renewable energy converters, and industrial equipment, providing improved power quality, reduced THD, and better system reliability.