The difference between passive filter and active power filter
Apr 13, 2026| Passive filters, also known as LC filters, are filtering devices composed of inductors (L), capacitors (C), and resistors (R), which can specifically eliminate single or multiple orders of harmonics. The most commonly used structure in engineering is the LC series resonant branch. When this branch is connected in parallel to the power grid, it presents extremely low impedance at the frequencies of major characteristic harmonics such as the 3rd, 5th, and 7th orders, providing a bypass channel for harmonic currents to achieve harmonic shunting. Single-tuned filters, double-tuned filters, and high-pass filters all belong to passive filters, whose core working mode is passive harmonic suppression, realizing harmonic control only by relying on the resonant characteristics of LC passive components.
Active Power Filter (APF) is a dynamic harmonic suppression and reactive power compensation device based on power electronic technology, which can perform real-time compensation for harmonics with varying amplitudes and frequencies, as well as fluctuating reactive power. Its "active" characteristic is reflected in the fact that the device needs to be equipped with a DC-side power supply and generate compensation current through active control, rather than relying solely on LC passive resonance. APF can effectively overcome the shortcomings of traditional passive filters, such as fixed compensation characteristics and vulnerability to system impedance, realizing full dynamic and real-time tracking compensation, and can complete both harmonic suppression and reactive power compensation.
The instantaneous reactive power theory of three-phase circuits is the core theoretical basis for the development of APF. APF is mainly divided into parallel type and series type, among which the parallel type is the most widely used:
● Parallel APF: Equivalent to a controlled current source, mainly compensating for current harmonics, reactive power, and unbalanced current;
● Series APF: Equivalent to a controlled voltage source, mainly suppressing voltage harmonics, voltage fluctuations, and voltage disturbances.
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Comparison Items |
Passive Filter |
Active Power Filter (APF) |
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Component Composition |
Passive components such as inductors (L), capacitors (C), and resistors (R) |
Power converter, controller, current/voltage sensor, DC-side power supply |
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Working Principle |
Using the series resonance characteristics of LC, it presents low impedance at the target harmonic frequency, providing bypass shunting for harmonic currents (passive type) |
Detecting harmonics through the controller, the power converter actively injects reverse compensation current to offset the original harmonics (active type) |
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Compensation Characteristics |
Fixed frequency compensation; stepped compensation can be achieved through group switching, but continuous dynamic compensation is not possible |
Full-frequency dynamic tracking compensation, adaptable to real-time changes in harmonic amplitude and frequency |
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Harmonic Control Capability |
Only capable of controlling preset low-order harmonics (3rd, 5th, 7th orders), with poor effect on high-order harmonics |
Able to filter both low-order and high-order harmonics simultaneously, and can cope with complex and variable harmonic scenarios |
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System Resonance Risk |
Prone to harmonic resonance amplification with grid system impedance, posing potential safety hazards |
Basically no resonance occurs; only slight interaction with system impedance may occur in extremely weak power grids, with extremely low risk |
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Reactive Power Compensation Capability |
Can simultaneously provide fixed capacitive reactive power compensation, but cannot compensate for inductive reactive power or fluctuating reactive power |
Able to dynamically and smoothly compensate for both capacitive and inductive reactive power, solving both harmonic and reactive power problems |
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Response Speed |
Slow (second/100-millisecond level), affected by component parameters |
Extremely fast (microsecond/millisecond level), capable of real-time tracking of harmonic changes |
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Cost |
Low, with simple components and convenient operation and maintenance |
Relatively high; the core components are power converters and controllers, with slightly higher operation and maintenance requirements |
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Applicable Scenarios |
Scenarios with stable loads and fixed harmonic frequencies (ordinary industrial plants, low-voltage distribution networks)
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Scenarios with large load fluctuations and complex harmonics (frequency converters, electric arc furnaces, power supply systems for precision electronic equipment) |