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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]