Classification and Principles of APF (Active Power Filter)

In the field of power quality management, the APF (Active Power Filter) is the core equipment for dealing with harmonic pollution. However, some users have just started to use this equipment and are not familiar with its workings. Therefore, today the APF filter manufacturers are here to explain the classification and principles of APF filters to everyone.

I. Classification System of Topological Structures

1. APF is classified by topology into parallel type, series type and hybrid type. The parallel type can directly compensate for the harmonic current of the load, and it accounts for over 75% in the current-source type harmonic mitigation.

2. Series type eliminates voltage harmonics by injecting compensation voltage, and is suitable for voltage-sensitive applications.

3. The hybrid type combines the advantages of both, enabling simultaneous management of voltage and current quality issues. Its advanced version, the Unified Power Quality Controller (UPQC), has a high degree of integration but the cost increases by 30 - 40%.

II. Analysis of Circuit Architecture Types

From the perspective of circuit architecture, APFs can be mainly classified into two-level and multi-level types.

The two-level APF has a simple structure and low cost, but it has high switching losses and an efficiency of 92 - 95%.

2. Multi-level structures (three-level and above) have a cost increase of approximately 25%, but they have the advantages of low harmonic content (THD < 3%) and low switching loss (efficiency ≥ 97%). Modular multi-level APF (MMC - APF) is suitable for medium and high voltage scenarios. Each power module has a voltage of 1 - 2 kV and can be cascaded for higher voltages.

III. Core Work Principle Analysis

APF operates based on the instantaneous reactive power theory. It uses a high-speed DSP to detect the harmonic components of the load current (sampling rate ≥ 128 points per cycle), and controls the IGBT inverter to generate compensatory currents with equal amplitudes and opposite phases. The key technologies include harmonic detection algorithms (such as ip-iq, d-q transformation), current tracking control (predictive current or hysteresis control), and PWM modulation strategies. The high-performance APF has a response time of ≤ 5ms, and the harmonic filtering rate is 95 - 98%.