Active power filters (APFs), as power electronic devices that dynamically suppress harmonics and compensate for reactive power, exhibit unique advantages in managing interharmonics. Interharmonics are frequency components between integer multiples of the fundamental power frequency. Their frequency fluctuations and aperiodicity make them difficult for traditional passive filters to effectively suppress. APFs, however, can accurately track the instantaneous changes of interharmonics through real-time detection and dynamic compensation mechanisms, achieving high-precision management. Their core principle involves collecting load current through a current transformer, extracting interharmonic components through a high-speed digital signal processor (DSP), and controlling the inverter to generate a compensation current with equal amplitude but opposite phase, which is then injected into the grid to counteract the impact of interharmonics on the system.
The harmonic detection stage of the active power filter (APF) is a crucial foundation for interharmonic compensation. Traditional FFT algorithms suffer from spectral leakage when handling non-integer harmonics, leading to decreased detection accuracy. Therefore, APFs often employ the IP-IQ method based on instantaneous reactive power theory or an improved synchronous rotating coordinate transformation method. These methods decompose three-phase current into positive-sequence, negative-sequence, and zero-sequence components through coordinate transformation, then extract the fundamental component using a low-pass filter, and finally obtain the interharmonic current through difference calculation. For example, the ip-iq method constructs an α-β stationary coordinate system to map the interharmonic components to a specific frequency band, and combined with adaptive filtering technology, can effectively separate the target frequency band signal, providing accurate instructions for subsequent compensation.
In the compensation current generation stage, the active power filter (APF) needs to address the control challenges posed by interharmonic frequency fluctuations. The frequency of interharmonics may drift with load changes, requiring simultaneous improvement in the tracking speed and accuracy of the compensation current. APF typically employs PWM hysteresis control or predictive current control strategies: the former achieves fast response by setting a hysteresis bandwidth, but excessive bandwidth can lead to increased compensation current ripple; the latter predicts the interharmonic current at the next moment based on the system model, adjusting the inverter output in advance to achieve a balance between dynamic performance and steady-state accuracy. Furthermore, the application of multi-level inverter technology can further improve the waveform quality of the compensation current, reduce switching losses, and adapt to the needs of high-frequency interharmonic mitigation.
To address the non-periodic characteristics of interharmonics, Active Power Filters (APFs) require optimized control algorithms to enhance their anti-interference capabilities. For example, sliding mode variable structure control can improve the system's robustness to parameter perturbations and external disturbances, ensuring stable output compensation current even during sudden changes in interharmonic amplitude. Fuzzy control, on the other hand, achieves adaptive parameter adjustment by simulating human experience, avoiding overshoot or undershoot problems caused by fixed parameters in traditional PID control. Some advanced APFs also incorporate artificial intelligence technologies, such as neural network algorithms, to build an interharmonic feature library through offline training and quickly match optimal control parameters during online identification, significantly improving compensation efficiency.
The hardware design of the Active Power Filter (APF) also significantly impacts interharmonic compensation performance. To adapt to high-frequency interharmonic mitigation, the inverter must use IGBT devices with low switching losses and high withstand voltage, and optimize the heat dissipation structure to prevent overheating and derating. Simultaneously, the selection of the DC-side capacitor's capacity must balance dynamic response speed and voltage fluctuation range to avoid discontinuous compensation current due to insufficient capacitance. Furthermore, the design of the output filter requires a trade-off between the cutoff frequency and impedance characteristics. It must effectively filter out harmonics near the switching frequency while avoiding attenuation of interharmonic compensation.
When multiple APFs operate in parallel, interharmonic compensation needs to address circulating current suppression and coordinated control. Circulating current causes ineffective flow of compensation current between devices, reducing efficiency and even causing overload. To address this, APFs typically employ master-slave control or distributed coordinated control strategies: the former uses a master unit to uniformly allocate compensation tasks, with slave units strictly following master commands; the latter uses a communication network to achieve information exchange between devices, dynamically adjusting the output current to eliminate circulating current. Some solutions also introduce virtual impedance technology, simulating resistance characteristics to change the device's output impedance, suppressing circulating current at its source.
In practical applications, the interharmonic compensation effect of APFs needs further improvement through system-level optimization. For example, in renewable energy integration scenarios, interharmonics generated by photovoltaic inverters or wind power converters may overlap with grid background harmonics, increasing the difficulty of compensation. At this point, the APF needs to work in conjunction with devices such as the SVG (Static Var Generator) to achieve joint management of harmonics and reactive power through hierarchical control. Furthermore, combined with a power quality monitoring system, the APF can analyze the sources and propagation paths of interharmonics in real time, providing data support for adjusting management strategies and ultimately achieving global optimization of the power grid's harmonic levels.
