Low-Pass Filter Design for Harmonic Detection Based on the Instantaneous Reactive Power Theory in AC Electric Railway Systems
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Abstract
This paper presents the design of a Low-Pass Filter (LPF) for extracting the harmonic component of active power in the harmonic detection process based on instantaneous reactive power theory (PQ theory). The objective is to accurately compute the harmonic current to be used as a reference current for Active Power Filters (APFs), which inject compensating currents to eliminate harmonics in AC electric railway systems. The performance of the proposed LPF is evaluated through MATLAB/Simulink simulations, with test cases classified according to the selected cutoff frequency. The simulation results indicate that the appropriate cutoff frequency should be selected within the mid-range between 0 Hz and the frequency of the first harmonic order of active power. Furthermore, to validate the harmonic detection performance of the PQ theory, this study conducts Hardware-in-the-Loop (HIL) simulations for harmonic mitigation in AC electric railway systems. The results confirm that harmonic detection based on PQ theory, combined with the LPF designed using the proposed method, can accurately compute the reference current. Consequently, the APF can effectively inject compensating currents to mitigate harmonics. The Total Harmonic Distortion (THD) values of the three-phase source currents (iSA, iSB, iSC) are 3.51%, 3.53%, and 3.54%, respectively, which are all below the 5% limit specified in IEEE Std. 519-2022.
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