Abstract
Shunt active power filter (APF), with higher switching frequency, has a potential market for suppressing wideband harmonics in the more electric aircraft (MEA) or high-speed train (HST) power grid. In this case, the selective extraction of harmonic current reference is essential. Recursive discrete Fourier transform (RDFT) is widely adopted for its excellent selectivity and low computational burden. However, RDFT is tough to implement in the higher frequency APF with the shorter switching cycle, and suffers from the demerit of slow dynamics. To alleviate those, three improved RDFTs are proposed in this paper. Firstly, it is revealed that RDFT can be regarded as a series of DFT and inverse-DFT (IFT), and its settling time depends on the comb filter of DFT. Then, by rearranging the sampling frequency of DFT according to the extracted harmonic, the computational burden of twice-sampling RDFT (TS-RDFT) is reduced while satisfying the same extraction accuracy. Furtherly, to improve the dynamics of TS-RDFT while retaining the merits of simplicity and selectivity, the comb filter is reconfigured based on the load current spectrum. For balanced loads, the settling time of TS-RDFT can be shortened from 1 to 1/6 fundamental cycle in the synchronous reference frame. For unbalanced loads, it is shortened to 1/2 cycle in the $\alpha \beta $ frame. Finally, the simulations with Matlab for harmonic extraction in 400Hz grid and the experiments with the APF prototype for harmonic suppression in 50Hz grid are carried out to verify the effectiveness of the proposed methods.
Highlights
Shunt active power filter (APF), with the representative 10k-20kHz switching frequency, is an effective apparatus to suppress 5th-31st dominant harmonic currents in the 50/60Hz utility grid [1]–[3]
With the increase of high-speed train (HST), more electric aircraft (MEA), distributed power generation system (DPGS), and other grid-tied converters based on the pulse width modulation (PWM), more and more attention has been paid to the suppression of high frequency
Selective extraction is essential but tough to implement for higher switching frequency APF, due to the shorter switching cycle and digital signal processors (DSP) limited computational resources
Summary
Shunt active power filter (APF), with the representative 10k-20kHz switching frequency, is an effective apparatus to suppress 5th-31st dominant harmonic currents in the 50/60Hz utility grid [1]–[3]. To realize selective and fast-dynamic harmonic extraction for higher frequency APF, three improved RDFTs with less computational burden are proposed in this paper. To lower computational burden, the DFT sampling frequency of specific extracted harmonic can be rearranged according to the given maximum error Aerr_m and (13) This method is defined as the twice sampling in this paper. The same as RDFT, the common adjustment factor and comb filter are set for TS-RDFT It takes 2M2 additions and 2M1 multiplications, instead of 2M2 additions and multiplications, to obtain Ak and Bk. But, M2 additions and 2M2 multiplications are still required for IFTk. Because the execution time of addition in DSP is much shorter than that of multiplication, the computational burden for TS-RDFT to extract kth harmonic can be reduced by almost (M2 − M1)/2M2. The settling time of TS6-RDFT can be shortened to T1/6 and computational burden can be reduced by half, while maintaining the selectivity
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