Autonomous Control of Distributed Storages in Microgrids

Poh Chiang Loh,Frede Blaabjerg 전력전자학회 2011 ICPE(ISPE)논문집 Vol.2011 No.5

Operation of distributed generators in microgrids has widely been discussed, but would not be fully autonomous, if distributed storages are not considered. Storages in general are important, since they provide energy buffering to load changes, energy leveling to source variations and ride-through enhancement to the overall microgrids. Recognizing their importance, this paper presents a scheme for sharing power among multiple distributed storages, in coordination with the distributed sources and loads. The scheme prompts the storages to autonomously sense for system conditions, requesting for maximum charging energy only when excess generation capacity is available. As generation capacity drops or demand increases, energy drawn by the storages decreases spontaneously, until full source capacity is near. At which point, the storages release their stored energy for meeting the extra load demand. The described process takes place in an automated manner with energy sharing always maintained among the storages. To verify the concepts discussed, simulation was performed with its results shown near to the end of the paper.

Cascaded Trans-z-source Inverters

Ding LI,Poh Chiang LOH,Miao ZHU,Feng GAO,Frede Blaabjerg 전력전자학회 2011 ICPE(ISPE)논문집 Vol.2011 No.5

Z-source inverter is a recently proposed single-stage inverter with added voltage-boost capability for complementing the usual voltage-buck operation of a traditional voltage-source inverter. As long as the transformer element added in to the z-source concept, a trans-z-source inverter with one transformer and one capacitor is reported recently. This paper has adapted the cascaded concept into the trans-z-source and trans-quasi-z-source inverters to extend each to the cascaded topologies before combination is made with allowing more sources embedded which reduces the capacitor voltage and enhanced the compatibility for distributed sources. Unlike existing techniques, voltage stresses within the proposed inverters are better distributed among the passive components. Theoretical analysis for explaining these operating features has already been discussed before simulation were performed and an experimental prototype was constructed in laboratory with captured results shown for visual confirmation.

Line-to-line voltage based modulation scheme for single-phase reduced switch ac-dc-ac converters to achieve improved performance

Zian Qin,Poh Chiang Loh,Frede Blaabjerg 전력전자학회 2015 ICPE(ISPE)논문집 Vol.2015 No.6

Modulation methods can significantly influence the performance of the converter in steady state, which is therefore very important. With different objectives like the minimum current ripple, minimum power loss or maximum DC link voltage utilization, SVPWM or DPWM are normally applied. With the SVPWM or DPWM proposed in literatures, the generation of the references is phase voltage based. But the phase voltages actually cannot be controlled directly by the PWM converter, since they can be influenced by both of terminal voltages and the load network while the PWM converter can only control the former. It thus introduces complexity to the generation of the references, especially in unbalanced source or load condition. In this paper, a line-to-line voltage based PWM reference generation method is proposed. The proposed method is easy to be implemented to maximize the DC link voltage utilization, reduce current ripple, or improve the loss distribution. Analysis and experimental test are also given to verify the proposed method.

A Unified Grid Current Control for Grid-Interactive DG Inverters in Microgrids

Xiongfei Wang,Poh Chiang Loh,Frede Blaabjerg 전력전자학회 2015 ICPE(ISPE)논문집 Vol.2015 No.6

This paper proposes a unified grid current control for grid-interactive distributed generation inverters. In the approach, the grid-side current, instead of inverterside current, is controlled as an inner loop, while the filter capacitor voltage is indirectly regulated through a virtual admittance in the outer loop. It, therefore, provides several superior features over traditional control schemes: 1) highquality grid current in the grid-connected mode, 2) inherent derivative-less virtual output impedance control, and 3) the unified active damping for both grid-connected and islanded operations. Root locus analyses in the discrete z-domain are performed for elaborating the controller design. Simulations and experimental results demonstrate the performances of the proposed approach.

Dead-Time Elimination and Zero Common Mode Voltage Operation of Neutral-Point-Clamped Inverter

Feng Gao,Jianhua Yuan,Ding Li,Poh Chiang Loh,Houlei Gao 전력전자학회 2011 ICPE(ISPE)논문집 Vol.2011 No.5

This paper presents the dead-time elimination scheme for modulating neutral-point-clamped inverter. Unlike the dead-time compensation methods reported for mitigating negative effects induced by dead-time insertion, the proposed scheme simply separates each pair of switches by treating every switch independently and triggers the corresponding switches per phase according to the judged current direction. The removal of dead-time interval, together with the introduction of an appropriate modulation scheme, can then help mitigate switching common mode voltage, even without those narrow alternating spikes generated by common dead-time insertion in traditional pulsewidth modulation (PWM) techniques.

Extended Topologies of Tapped-Inductor Z-Source Inverters

Miao ZHU,Ding LI,Feng GAO,Poh Chiang LOH,Frede Blaabjerg 전력전자학회 2011 ICPE(ISPE)논문집 Vol.2011 No.5

In this paper, two distinct types of tapped-inductor Zsource impedance networks are proposed for implementing high-performance voltage-type inverters topologies with strong voltage boost inversion abilities and less components’ stresses. All proposed topologies can in principle be unified into a generic network entity, which is termed hybrid-source tapped-inductor impedance network. The resulting novel Z-source inverters would have a higher output voltage gain and other unique advantages that currently have not yet been investigated. Theoretical analysis for explaining these operating features has already been discussed throughout the paper, before experimental results are presented for verifying them. It is anticipated that these advantages would help the formed inverters find applications in photovoltaic and other renewable systems, where a high voltage gain is usually requested.

Power Decoupling Method for Single Phase Differential Buck Converter

Wenli Yao,Yi Tang,Xiaobin Zhang,Xiongfei Wang,Poh Chiang Loh,Frede Blaabjerg 전력전자학회 2015 ICPE(ISPE)논문집 Vol.2015 No.6

The well-known inherent second-order ripple power in single phase converters imposes harmonic stress on the dc link, resulting in low efficiency and overheating issues. In order to avoid installing bulky electrolytic capacitors or LC filters in the dc-link, this paper presents a differential buck inverter to improve the dc link power quality, and an improved active power decoupling method is proposed to achieve ripple power reduction for both AC-DC and DC-AC conversions. The ripple energy storage is realized by the filter capacitors, which are connected between the output terminal and the negative dc bus. By properly controlling the differential mode voltage of the capacitors, it is possible to transfer desired energy between the DC port and AC port. The common mode voltage is controlled in such a way that the ripple power on the dc side will be reduced. Furthermore, an autonomous reference generation technique is proposed to provide accurate ripple power compensation, and closed-loop controllers are also designed based on small signal models. The effectiveness of this power decoupling method is verified by detailed simulation studies as well as laboratory prototype experimental results.