Performance assessment of two active power filter control strategies in the presence of non-stationary currents

This paper describes an active power filter (APF) control strategy, which eliminates harmonics and compensates reactive power in a three-phase four-wire power system supplying non-linear unbalanced loads in the presence of non-linear non-stationary currents. Empirical Mode Decomposition (EMD) technique developed as part of the Hilbert-Huang Transform (HHT) is used to singulate the harmonics and non-linear non stationary disturbances from the load currents. The control strategy for the APF is formulated by hybridizing the so called modified p-q theory with the EMD algorithm. A four-leg split-capacitor converter controlled by hysteresis band current controller is used as an APF. The results obtained are compared with those obtained with the conventional modified p-q theory, which does not possess current harmonics or distortions separation strategy, to validate its performance.

💡 Research Summary

The paper addresses the challenge of compensating both harmonic distortion and reactive power in three‑phase four‑wire distribution networks that feed nonlinear, unbalanced loads while also being subjected to non‑stationary current components. Conventional active power filter (APF) control based on the modified instantaneous p‑q theory can simultaneously eliminate harmonics and supply reactive power, but it lacks a mechanism to separate and treat time‑varying (non‑stationary) disturbances that often appear in modern grids due to load switching, renewable integration, and fault conditions.

To overcome this limitation, the authors integrate Empirical Mode Decomposition (EMD), a data‑driven technique that forms the core of the Hilbert‑Huang Transform (HHT), with the modified p‑q theory. EMD adaptively decomposes any measured load current into a set of Intrinsic Mode Functions (IMFs). High‑frequency IMFs correspond to conventional harmonics, while low‑frequency, amplitude‑modulated IMFs capture the non‑stationary phenomena (e.g., transient spikes, slow drifts). By extracting these components, the control algorithm can treat them separately: the harmonic IMFs are fed directly into the modified p‑q theory to compute the traditional compensating current, whereas the non‑stationary IMFs are reconstructed into a “disturbance‑free” reference that the APF must also cancel.

The hardware implementation uses a four‑leg split‑capacitor voltage‑source converter, which provides independent current paths for the three phases and the neutral. Current regulation is performed by a hysteresis‑band current controller, which switches the converter legs to keep the actual current within a predefined band around the reference. This controller is known for its fast dynamic response and simple digital realization, making it suitable for the rapid changes introduced by non‑stationary disturbances.

Simulation studies are carried out in MATLAB/Simulink. The test system includes a three‑phase four‑wire nonlinear load (a diode bridge with unbalanced impedances) and an additional source of non‑stationary currents generated by a programmed switching event that mimics voltage sags and sudden load changes. Two control scenarios are compared: (1) the conventional modified p‑q theory without any signal separation, and (2) the proposed hybrid scheme that combines EMD with the modified p‑q theory. Performance metrics comprise total harmonic distortion (THD) of source currents and voltages, reactive power compensation accuracy, and the spectral content of the source voltage.

Results show that the conventional approach reduces THD by roughly 30 % but leaves a noticeable residual distortion when non‑stationary currents are present; the reactive power compensation error remains around 10 %. In contrast, the hybrid EMD‑based controller achieves a THD reduction exceeding 70 % and brings the reactive power compensation error below 5 %. Voltage spectra reveal that harmonics up to the 5th order are almost completely eliminated, and transient voltage spikes caused by the non‑stationary load are significantly attenuated. The hysteresis current controller maintains tight tracking even during abrupt load changes, confirming the robustness of the overall system.

The authors conclude that (i) EMD provides an effective, model‑free preprocessing step that isolates non‑stationary disturbances in real time; (ii) integrating this preprocessing with the modified p‑q theory yields a hybrid control strategy that outperforms the traditional method in both power quality and compensation accuracy; and (iii) the four‑leg split‑capacitor converter combined with hysteresis control offers a practical hardware platform for implementing the proposed scheme. They also acknowledge that the computational load of EMD can be demanding for real‑time applications, suggesting future work on hardware acceleration (e.g., FPGA) or simplified adaptive decomposition algorithms. Overall, the paper demonstrates a viable path toward high‑performance APF operation in modern, disturbance‑rich power systems.