Active Harmonic Filters (AHFs), also known as Active Power Filters (APFs), are modern devices used to eliminate harmonic currents and improve the power quality of electrical systems. They actively monitor and dynamically compensate for harmonics and other power quality issues in real-time.
Construction and Working:
- Sensing Mechanism: AHFs continuously monitor the load current to detect the harmonic distortion present.
- Processing: Once the harmonics are detected, the AHF’s control system calculates the exact compensating current needed to cancel out the harmonics.
- Injection of Compensating Current: The AHF then injects a current into the system that is the exact opposite (or inverse) of the detected harmonics. This phase opposition cancels out the harmonics, resulting in a cleaner waveform.
- Power Electronics: AHFs use IGBTs (Insulated Gate Bipolar Transistors) or similar power electronics to produce the compensating current.
Advantages:
- Dynamic Response: AHFs can adapt to varying harmonic conditions in real-time.
- Broadband Compensation: Unlike passive filters, which are usually tuned to specific frequencies, AHFs can address a broad spectrum of harmonics.
- Additional Functions: Many AHFs can also correct power factor, balance phase currents, and mitigate voltage sags or swells.
- Avoids Resonance Issues: AHFs can be designed to avoid creating resonance conditions, which can sometimes occur with passive filters.
- Compact Size: They often require less space compared to the equivalent capacity of passive filters.
Disadvantages:
- Cost: AHFs are generally more expensive than passive harmonic filters.
- Complexity: They rely on advanced control systems and power electronics, which can make them more complex to install, maintain, and repair.
- Reliability: The reliance on power electronics and control systems means there are more components that could potentially fail, although this is mitigated with high-quality design and manufacturing.
Implementation Tips:
- It’s beneficial to conduct a thorough harmonic study before installing an AHF to determine the type and magnitude of harmonics present.
- While AHFs can handle a wide range of harmonic conditions, it’s still important to size and configure them correctly based on the specific requirements of the electrical system.
- AHFs often complement passive filters in applications where both fixed and dynamic harmonic compensation is needed.
In summary, Active Harmonic Filters are advanced devices that offer dynamic, real-time harmonic compensation. They are particularly useful in situations where the harmonic profile is complex or varies over time, such as in facilities with many variable-frequency drives or rapidly changing loads.