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A useful time-domain algorithm to characterise harmonic currents produced by a cluster of three-phase AC/DC converters operated under either balanced or imbalanced supply voltages and asymmetrical source impedances with the consideration of load variations is proposed. The proposed method is based on the Poincaré map concept for increasing the solution speed. An actual industrial rolling mill system with several DC motor drives operating in parallel is under test to show the usefulness of the proposed method. Simulations results are compared with those obtained by actual measurements. It shows that the harmonic currents determined by the proposed method agree well with those obtained by actual measurements. The proposed algorithm is relatively accurate, computationally efficient and robust. [ABSTRACT FROM AUTHOR]

The fast Fourier transform (FFT) has been widely used for the signal processing because of its computational efficiency. Because of the spectral leakage and picket-fence effects associated with the system fundamental frequency variation and improperly selected sampling time window, a direct application of the FFT algorithm with a constant sampling rate may lead to inaccurate results for continuously measuring power system harmonics and interharmonics. An improved FFT-based algorithm to measure harmonics and interharmonics accurately is proposed. In the proposed algorithm, a frequency-domain interpolation approach is adopted to determine the system fundamental frequency, and the interpolatory polynomial method is applied to reconstruct the sampled time-domain signal; it is followed by using the FFT to calculate the actual harmonic components. Then, the frequency-domain interpolation is again applied to find the interharmonic components. The performance of the proposed algorithm is validated by testing the actual measured waveforms. Results are compared with those obtained by directly applying a typical FFT algorithm and by the IEC grouping method. It shows that the solutions determined by the proposed algorithm are more accurate, and a reasonable computational efficiency is maintained. [ABSTRACT FROM AUTHOR]

An efficient approach to characterising harmonic currents generated by a cluster of parallel-operated AC/DC converters in the steady state is presented. The approach is carried out in the time domain and the interactions between converters are considered. In calculating harmonic currents generated by a single converter, the Poincaré map-based approach is applied to increase the computational efficiency. A solution procedure that is sequentially solving a single-converter problem with detailed model in the time domain, while the others are fixed as equivalent AC current sources, is proposed to find the overall source-side harmonic currents. The computed AC-side harmonic components of each converter current are then extracted via FFT. Solutions obtained by the proposed method are compared with those obtained by using a brute-force time-domain simulation tool, Simulink. It is shown that the AC-side harmonic currents determined by the proposed approach agree well with those obtained by the time-domain simulation tool, but the solution time is significantly reduced. Therefore, the proposed approach is useful for quantifying harmonic currents generated by a cluster of AC/DC converters and to facilitate the harmonic mitigation implementations, such as passive and active harmonic filter designs, for complying with harmonic standards such as IEC 61000-3-4 and IEEE-519. In addition, the proposed approach is suitable for characterising harmonic currents generated by other types of power-electronic devices, such as AC and/or DC drives, operated in parallel simultaneously. The solution algorithm can also be integrated into a harmonic power flow program for penetration studies to provide a fast quantification of the impact of multiple harmonic sources on the power system. [ABSTRACT FROM AUTHOR]

A new theory and a conceptual design for the three-phase shunt active power filter (AFF) control strategy under imbalanced and distorted supply voltages are presented. Conventionally, the compensation currents injected by the APF are determined according to reference frame transformation-based approaches, where the load instantaneous real and reactive powers are calculated in the new reference frame. The APF injection currents are then computed to maintain the balanced and sinusoidal source currents, as well as the least active power consumption by the APF. However, if current harmonics set by harmonic standards or other constraints are considered, the load harmonic currents after APF compensation can be controlled to meet the requirements. Therefore, the APF may have an adjustable performance. An optimal algorithm for the APF control in steady state is proposed, in which the individual and total harmonic current distortion limits, the minimum load power factor, levels of current imbalance and the size of the APF can be taken into account. The planned APF does not consume or produce active power and only the passive energy-storage elements are required in the new APF design. Results obtained by simulations with Matlab and Simulink show that the proposed approach is more flexible than conventional approaches for compensating reactive power and harmonic/neutral currents of the load, even if the source voltages are severely distorted and imbalanced. The new control strategy is very suitable for planning the APF when several alternative objective functions and constraints are under considerations. [ABSTRACT FROM AUTHOR]