When the relative size of the measured grid-connected inverter of renewable energy power generation equipment's impedance and the grid's impedance changes in different frequencies, only using the current-injected impedance measuring device or the voltage-injected impedance measuring device can not accurately measure the wide-bandwidth frequency impedance of the measured equipment. In this paper, the basic theory of impedance measurement under single current-disturbed injection method and single voltage-disturbed injection method is introduced. Then, aiming at the problem of low measurement precision under single perturbation injection method, the impedance measurement of grid-connected three-phase LC-type inverter is taken as the research object, and a UPQC (Unified Power Quality Conditioner) -based impedance measurement device and its switching control method is proposed. The proposed method can realize the switch of the perturbation injection mode: when the impedance of the grid is higher than that of the measured equipment, the impedance measurement device works on the current injection mode; when the impedance of the grid is lower than that of the measured equipment, the device works on the voltage injection mode. So that the high proportion of the perturbation is distributed on the side of the measured equipment to improve the precision of impedance measurement. After measuring the impedance of resistance and grid-connected inverter of renewable energy power generation equipment, the effectiveness of the proposed device is verified by comparison and analysis.

In the maritime VSC-HVDC system supplying power to passive network on the island, the dc side of the system is prone to oscillation and even instability, which is resulted from the negative incremental input resistance characteristic of the inverter station. First, a virtual phase-lead impedance stability control strategy aiming at rectifier station is proposed. This control strategy can effectively mitigate the dc-side oscillation of the VSC-HVDC system without affecting the load performance of the inverter station. Then, the dc impedance model of the VSC-HVDC system is built, including the output impedance of the rectifier station, dc cable impedance, and the input impedance of the inverter station. In addition, the oscillation mechanism of the VSC-HVDC system is analyzed by impedance-based Nyquist stability criterion. The reason why rectifier station fails to mitigate the dc-side oscillation using traditional virtual resistance stability control is that the output impedance of the rectifier station exhibits negative damping characteristic outside the control bandwidth of voltage outer loop. For this issue, the proposed control strategy can correct the output impedance of the rectifier station to exhibit positive damping characteristic outside the control bandwidth of voltage outer loop, thus mitigating the dc-side oscillation of the VSC-HVDC system. Finally, simulation and experiment results validate the proposed control strategy and analysis.

As the fluctuation of sub-module capacitance voltage when MMC operates at low frequency, the application of MMC in the field of motor drive has been limited. Therefore, this paper proposes a low frequency control method for the MMC with tuned filter circuit. Based on the theoretical analysis for this tuned filter, the circulating current of 2ndfrequency could be suppressed effectively at rated frequency. The tuned filter also enables the minimum voltage that is adopted to control high frequency circulating current at low frequency by feasible parameters. In addition, design of circulating current controller is simplified, because the high frequency circulating current with mixed and variable frequency is no longer needed in this strategy. Zero-error tracking to the hybrid reference current is achieved by proportional integral resonant (PIR) control. Finally, the accuracy of analysis and effectiveness of proposed strategy are proved by a prototype experiment.

In the VSC-HVDC system supplying power to passive network, the dc-side of the system is prone to oscillation and even instability, which is resulted from the constant power characteristic of the inverter station and the interactions of the dc impedance between converter stations. In view of this, firstly, the output impedance model of the rectifier station, dc cable impedance model and the input impedance model of the inverter station are built in this paper. Then, the effect on system stability caused by the length of the dc cable, and the output impedance of the rectifier station is examined by the impedance-based Nyquist stability criterion. And the reason why the rectifier station fails to mitigate oscillation on the dc-side of the VSC-HVDC system using traditional virtual resistance stability control method is revealed. Based on the virtual resistance stability control method, a resistive-inductive impedance active stabilization method aiming at rectifier station is proposed. This method can effectively mitigate the oscillation on the dc-side of the VSC-HVDC system without affecting the load performance of the inverter station. Finally, simulation results validate the proposed method and analysis in this paper.

In photovoltaic grid-connected inverter based on the deadbeat current control, the filter inductance variation and one-step-delay control affect the distortion of the grid current, stability and dynamic of the system. In this paper, a fast robust PWM method for photovoltaic grid-connected inverter is proposed, which reduces the distortion of the grid current caused by the filter inductance variation, effectively solves the delay of the one-step-delay control, and improves the stability and dynamic response speed of the system. Considering the parasitic resistance of the filter inductance and the line equivalent resistance, the influence of the filter inductance deviation coefficient on the system performance is analyzed, and the appropriate control parameter range is given. Simulation and experimental results verify the effectiveness of the proposed method.

Microgrid cluster as an interconnected system of multiple AC subgrids and DC subgrids, its complex structure increases the difficulty of power coordination control for it. To address this issue, a microgrid cluster structure and its autonomous coordination control strategy are proposed in this paper. The proposed microgrid cluster structure is composed of AC subgrids, DC subgrids, power exchange unit (PEU) and energy pool (EP). The aim of PEU is mainly to coordinate power exchange between the subgrids and EP. Meanwhile, the power coordinated control method is proposed to design the controller of PEU, which can realize mutual power support among each sub-grid and reduce the bus voltage deviation in each subgrid. Besides, EP is mainly used to balance the system power, and the dc-link voltage stability control method of EP is proposed to stabilize the dc-link voltage and to reasonably allocate the net exchange power of PEU. The proposed autonomous coordination control strategy has been verified by the simulation results.

For LCL-type distributed generation (DG) systems connected to weak grid, a novel two frequency-bandwidths real-time sensitive inverter-current-feedback (TFBRTS-ICF) control method is proposed to enhance the system reliability and robustness, which mainly includes the synchronous reference frame quasi-proportional-integral (SRFQPI) control, un-attenuated inverter-current-feedback active damping (UICFAD) control, and high-sensitivity real-time PWM algorithm. The SRFQPI control can compensate reactive power, and regulate the instantaneous current without steady-state error regardless of the fundamental frequency fluctuation. The UICFAD control can suppress the LCL-resonance peak well without requiring additional sensors. The proposed PWM algorithm can solve the robust problem of the UICFAD controller against grid-impedance variation through removing the computation delay. Meanwhile, the proposed PWM algorithm can achieve a full range of duty-ratio between 0~1 and offer half of switching period for the inverter-current sampling and the duty-ratio calculating, which is easy to implement using a digital microcontroller. Experimental results confirm the theoretical expectations and the effectiveness of the proposed method.

Abstract Summary: To annotate newly sequenced organisms, cross-species sequence comparison algorithms can be applied to align gene sequences to the genome of a related species. To improve the accuracy of alignment, spaced seeds must be optimized for each comparison. As the number and diversity of genomes increase, an efficient alternative is to cluster pairwise comparisons into groups and identify seeds for groups instead of individual comparisons. Here we investigate a measure of comparison closeness and identify classes of comparisons that show similar seed behavior and therefore can employ the same seed. Availability: Source code is freely available at http://dna.cs.gwu.edu and from Bioinformatics online. Contact: [email protected], [email protected] Supplementary information: Supplementary data are available at http://dna.cs.gwu.edu and Bioinformatics online.

BACKGROUND: To meet the needs of gene annotation for newly sequenced organisms, optimized spaced seeds can be implemented into cross-species sequence alignment programs to accurately align gene sequences to the genome of a related species. So far, seed performance has been tested for comparisons between closely related species, such as human and mouse, or on simulated data. As the number and variety of genomes increases, it becomes desirable to identify a small set of universal seeds that perform optimally or near-optimally on a large range of comparisons. RESULTS: Using statistical regression methods, we investigate the sensitivity of seeds, in particular good seeds, between four cDNA-to-genome comparisons at different evolutionary distances (human-dog, human-mouse, human-chicken and human-zebrafish), and identify classes of comparisons that show similar seed behavior and therefore can employ the same seed. In addition, we find that with high confidence good seeds for more distant comparisons perform well on closer comparisons, within 98-99% of the optimal seeds, and thus represent universal good seeds. CONCLUSION: We show for the first time that optimal and near-optimal seeds for distant species-to-species comparisons are more generally applicable to a wide range of comparisons. This finding will be instrumental in developing practical and user-friendly cDNA-to-genome alignment applications, to aid in the annotation of new model organisms.