Mao Wang

Bio: Mao Wang is an academic researcher from Hefei University of Technology. The author has contributed to research in topics: Inverter & H bridge. The author has an hindex of 7, co-authored 36 publications receiving 226 citations.

Multilevel Inverters for Grid-Connected Photovoltaic Applications: Examining Emerging Trends

Abstract: As the cost of photovoltaic (PV) modules and inverters continues to decline, PV power generation is gaining more and more share in the electricity market. The market and its customers are demanding higher-performance inverters in terms of efficiency, power density, module-level control, and increasingly higher voltage and power levels. Because of their outstanding performance, multilevel inverters have attracted the attention of researchers and technology developers alike. This article presents commonly used multilevel inverter technologies for grid-connected PV applications, including five-level inverters, single-phase nonisolated inverters, and three-phase, isolated cascaded H-bridge inverters. Detailed discussions are presented, along with characteristics of PV applications.

An Optimized Third Harmonic Compensation Strategy for Single-Phase Cascaded H-Bridge Photovoltaic Inverter

Abstract: Due to the unequal solar radiations or dust accumulation of photovoltaic modules in a single-phase cascaded H-bridge photovoltaic inverter, the unbalanced output power among photovoltaic modules will make the H-bridges with higher power overmodulation, resulting in deteriorated grid current. Concerning this issue, this paper proposes an optimized third harmonic compensation strategy that injects moderate amount of third harmonic into the overmodulation H-bridges to keep the amplitudes of their modulation waveforms just being unity, then compensates the same amount of opposite harmonic sequence and properly distributes it to rest of the H-bridges. The proposed method can ensure that all the power units will be free from overmodulation under some heavy power imbalance conditions and will not increase the third harmonic component of the grid current. The validity and effectiveness of the proposed method is verified by simulation and experimental results.

A Modified Hybrid Modulation Strategy for Suppressing DC Voltage Fluctuation of Cascaded H-Bridge Photovoltaic Inverter

Abstract: The hybrid modulation strategy using both low-frequency square-wave modulation and high-frequency pulse-width modulation can extend the stable operating region of a cascaded H-bridge converter. However, it does not control accurately dc voltages of converter but balances them by charging or discharging dc capacitors based on the state of system, which could aggravate fluctuation of dc voltages. If cascaded H-bridge converter is used in photovoltaic field, the aggravated fluctuation on dc capacitor voltages will result in further losses in energy harvesting of solar cells. To address this issue, a modified hybrid modulation strategy is presented in this paper. When dc-bus measured voltages are close to their reference voltages, these H-bridges operate in a zero mode. Adding appropriate zero state can prevent grid current charging or discharging the dc capacitors whose voltages have already reached their control goals. Therefore, the proposed technique is capable of suppressing dc voltage fluctuation, and, thus, improving the output power of solar cells. Simulation and experimental results validate the effectiveness and feasibility of the proposed method.

An Optimized Third Harmonic Injection Method for Reducing DC-Link Voltage Fluctuation and Alleviating Power Imbalance of Three-Phase Cascaded H-Bridge Photovoltaic Inverter

Abstract: Due to different solar radiation, temperature, and other reasons of modules in the three-phase cascaded H-bridge (CHB) photovoltaic (PV) inverter, the output power among PV modules will be unequal and lead to unbalanced grid currents, which cannot meet the requirements of grid codes. On the other hand, the second-order voltage ripple is aroused in the dc-link capacitor since each phase-leg of the CHB inverter is made up of a single-phase inverter. Concerning these two issues, this paper proposes an optimized third harmonic injection method based on fundamental frequency zero sequence injection (FFZSI), which is a conventional method to handle the slight power imbalance problem. In this paper, the FFZSI is adopted to redistribute the power among three phases, and then the third harmonic is injected into the three-phase combined reference waveforms according to the two requirements: all modules are free from overmodulation and the dc-link voltage fluctuation is mostly reduced. Therefore, the proposed method can reduce the dc-link voltage fluctuation and extend the power balance range of FFZSI at the same time. The validity and effectiveness of the proposed method are verified by simulation and experimental results.

A Grid-Supporting Strategy for Cascaded H-Bridge PV Converter Using VSG Algorithm With Modular Active Power Reserve

Abstract: The cascaded H-bridge (CHB) converter is a favorable candidate in the photovoltaic (PV) field due to its modular multilevel construction, which could pursue the maximum energy yield by adopting the maximum power point tracking (MPPT) strategy. However, with the increasing penetration of PV generation, challenges are arising for the grid stability due to the deficiency of converter inertia and damping. To improve the system inertia besides the energy efficiency, this article proposes a control strategy for the CHB converter with the combination of MPPT and virtual synchronous generator (VSG). Taking advantage of the modularity of CHB converter, a proportion of the total PV power is reserved by the selected reserved cells to provide the power buffer between the VSG and PV power for grid frequency supporting, and the nonreserved cells are utilizing the MPPT to ensure the energy efficiency. With the proposed control strategy, the CHB converter could be equipped with the frequency regulation capacity without any energy storage device and the full use of PV power is realized, which is verified by the simulation and experimental results.

Modular Cascaded H-Bridge Multilevel PV Inverter with Distributed MPPT for Grid-Connected Applications

Abstract: Due to the possibility of providing energy with much less dependence on the fossil fuels, renew-competent energy sources, in specified sun photovoltaic (PV) conversion have received elevated acceptance and progress in latest times. Big benefits of PV panels comprise easy and trustworthy power production and suitability for disbursed iteration. In addition the costs for photovoltaic modules is drastically lowering. To comprehend this issue, a control plan with modulation compensation scheme is likewise proposed. An exploratory three-stage seven-level cascaded H-bridge inverter has been manufactured using nine H-bridge modules (three modules for each stage). Each H-bridge module is associated with a 185-W solar panel. Simulation results are introduced to confirm the practicality of the proposed approach.

Voltage Source Multilevel Inverters With Reduced Device Count: Topological Review and Novel Comparative Factors

Abstract: Multilevel inverters (MLIs) have gained increasing interest for advanced energy-conversion systems due to their features of high-quality produced waveforms, modularity, transformerless operation, voltage, and current scalability, and fault-tolerant operation. However, these merits usually come with the cost of a high number of components. Over the past few years, proposing new MLIs with a lower component count has been one of the most active topics in power electronics. The first aim of this article is to update and summarize the recently developed multilevel topologies with a reduced component count, based on their advantages, disadvantages, construction, and specific applications. Within the framework, both single-phase and three-phase topologies with symmetrical and asymmetrical operations are taken into consideration via a detailed comparison in terms of the used component count and type. The second objective is to propose a comparative method with novel factors to take component ratings into account. The effectiveness of the proposed method is verified by a comparative study.

Packed E-Cell (PEC) converter topology operation and experimental validation

Abstract: This paper proposes a novel single-dc-source multilevel inverter called Packed E-Cell (PEC) topology to achieve nine levels with noticeably reduced components count, while dc capacitors are actively balanced. The nine-level PEC (PEC9) is composed of seven active switches and two dc capacitors that are shunted by a four-quadrant switch to from the E-cell, and it makes use of a single dc link. With the proper design of the corresponding PEC9 switching states, the dc capacitors are balanced using the redundant charging/discharging states. Since the shunted capacitors are horizontally extended, both capacitors are simultaneously charged or discharged with the redundant states, so only the auxiliary dc-link voltage needs to be sensed and regulated to half of the input dc source voltage, and consequently, dc capacitors' voltages are inherently balanced to one quarter of the dc bus voltage. To this end, an active capacitor voltage balancing integrated to the level-shifted half-parabola carrier PWM technique has been designed based on the redundant charging/discharging states to regulate the dc capacitors voltages of PEC9. Furthermore, using the E-cell not only reduces components count but also the proposed topology permits multi ac terminal operation. Thus, five-level inverter operation can be achieved during the four-quadrant switch fault, which confers to the structure high reliability. The theoretical analysis as well as the experimental results are presented and discussed, showing the basic operation, multi-functionality, as well as the superior performance of the proposed novel PEC9 inverter topology.

Multilevel Inverters for Grid-Connected Photovoltaic Applications: Examining Emerging Trends

Abstract: As the cost of photovoltaic (PV) modules and inverters continues to decline, PV power generation is gaining more and more share in the electricity market. The market and its customers are demanding higher-performance inverters in terms of efficiency, power density, module-level control, and increasingly higher voltage and power levels. Because of their outstanding performance, multilevel inverters have attracted the attention of researchers and technology developers alike. This article presents commonly used multilevel inverter technologies for grid-connected PV applications, including five-level inverters, single-phase nonisolated inverters, and three-phase, isolated cascaded H-bridge inverters. Detailed discussions are presented, along with characteristics of PV applications.

Design and Implementation of High Efficiency Control Scheme of Dual Active Bridge Based 10 kV/1 MW Solid State Transformer for PV Application

Abstract: One promising topology for solid state transformer (SST) is a modular multilevel cascaded converter, in which submodule is composed of dual active bridge (DAB) and H-bridge. For SST application in PV system, the efficiency could be severely affected especially for DAB due to the wide voltage and power range of PV panels. Thus, the motivation of this paper is to deal with the control strategy to improve DAB efficiency inside SST for PV application. Instead of utilizing time-domain based analysis method, which requires complex modeling process, this paper models DAB under frequency domain by fully considering the effect of both fundamental and harmonic frequency component. To achieve high efficiency operation, a multiorder reactive-current suppression (MRS) strategy is proposed by controlling phase shift angle as well as the duty cycles of primary and secondary side H-bridges. In terms of DAB controller design inside SST, the small signal model under MRS is established and a notch filter is implemented to suppress the second order line frequency fluctuation in the control loop. Finally, a 10 kV/1 MW SST prototype is introduced along with the system control structure and implementation method. The experiment of the submodule and SST confirms the effectiveness of proposed method.