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.

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

Cascaded H-bridge (CHB) inverter has the advantages of easy modularization, the low harmonic content of output current, and that all power units are capable of achieving independent maximum power point tracking (MPPT). However, aging or partial shielding of photovoltaic (PV) panels will lead to the unbalanced output power of each PV module, which will result in the overmodulation of H-bridges and increased harmonic components of the grid current. In this article, a novel harmonic compensation strategy for single-phase CHB inverter is proposed, it can extend the linear modulation range of single-phase CHB inverters to about 1.27. Moreover, this strategy will not reduce energy harvesting, and also can maintain the system to operate in the unit power factor. Simulation and experimental results all verify the effectiveness of the proposed strategy.

Harmonic Compensation Strategy for Single-Phase Cascaded H-Bridge PV Inverter Under Unbalanced Power Conditions

Cascaded H-bridge (CHB) -based solid-state transformer (SST) consists of CHBs and dual active bridge (DAB) converters. Series-resonant DAB converters (SRCs) are widely used for its advantages of soft switching characteristic and simple control. However, the high-frequency-link (HFL) currents of SRCs contain double-line-frequency harmonic fluctuation in three-phase CHB-based SST, resulting in high current stress. In this article, the SRC is substituted with dc terminal dynamic equivalent circuit, and the analytical expression of the HFL current fluctuation is deduced. With consideration of third-order harmonic voltage injection (THVI), the HFL current fluctuation characteristics with this method are analyzed and the optimal amplitude and phase of the injected third-order harmonic voltage are derived, with which the HFL current stress can be minimized. Both the simulation and experiment results indicate that the HFL current stress of SRCs can achieve a reduction of about 20% with the optimized THVI method.

High-Frequency-Link Current Stress Optimization of Cascaded H-Bridge-Based Solid-State Transformer With Third-Order Harmonic Voltage Injection

Cascaded multilevel inverters, such as the cascaded H-bridge (CHB) converter, are an attractive solution for multistring photovoltaic (PV) systems, because they enable direct connection to the medium voltage grid and maximum power point tracking (MPPT) of multiple strings. As a challenge of the topology, the operation with high power imbalance in the strings is constrained by the overmodulation. This limitation is analyzed for sinusoidal modulation and the impact on the maximum power imbalance is demonstrated. For increasing the operating range with maximum power tracking in the strings, a discontinuous modulation with extended maximum power imbalance and reduced losses is proposed. The method is analyzed in terms of maximum power imbalance, efficiency and power quality. In addition, the method is validated on an experimental test bench.

Modulation for Cascaded Multilevel Converters in PV Applications With High Input Power Imbalance

 Abstract—Among all the renewable energy sources, the installed capacity of solar power generation is the fastest growing in recent years, so photovoltaic (PV) power generation still has great market potential. Compared with low-power systems, large-scale PV systems are more commercially attractive, because they can reduce the cost of the system per watt. The PV inverters with centralized and string structure have been applied in large-scale PV plant, but it is difficult to further increase the voltage and power levels for a single converter. In addition, the line-frequency isolation transformer requires a large amount of materials and has a large volume and weight. Therefore, it is a current trend for large-scale PV system to increase the voltage and power levels to directly connect to the medium-voltage power grid. Based on this, this paper investigates and compares several topologies of PV inverters without line-frequency transformer, including the MMC structure and the three-phase cascaded H-bridge (CHB) structure, which are able to directly connect to the 35kV medium-voltage power grid, and can not only make the voltage and power levels higher, but also further reduce the cost and volume of the whole system.

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Topological comparison and analysis of medium-voltage and high-power direct-linked PV inverter

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.

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

Due to the nonuniform solar irradiance, unequal ambient temperatures, or inconsistent degradation of photovoltaic (PV) modules in three-phase cascaded H-bridge (CHB) PV inverter, the unbalanced output power among PV modules will lead to the imbalanced power between phases and bridges, resulting in unbalanced or even distorted grid current between three phases, which cannot meet the requirements of grid codes. Concerning this issue, this article proposes a novel power equalization method based on the combination of hybrid modulation strategy and zero-sequence injection methods, which deals with interbridge power imbalance by hybrid modulation and interphase power imbalance by zero-sequence injection methods. By utilizing the proposed method, three-phase-balanced grid currents with low total harmonic distortion are able to be achieved even when the interbridge and the interphase power are seriously unbalanced. In addition, each H-bridge cell in three-phase CHB inverter uses square-wave modulation, which maximizes the dc-side voltage utilization of each H-bridge cell, thus extending its power balance region. Experimental results achieved by a laboratory prototype of a three-phase seven-level CHB inverter demonstrate both feasibility and validity of the proposed method.

Research on Power Equalization of Three-Phase Cascaded H-Bridge Photovoltaic Inverter Based on the Combination of Hybrid Modulation Strategy and Zero-Sequence Injection Methods

The invention discloses a power equilibrium control method for reducing DC voltage fluctuation of a cascaded H-bridge inverter, and aims to solve the problem that power generation quantity is reduced caused by relatively large DC-side voltage fluctuation. The power equilibrium control method comprises the following steps of controlling a total DC-side voltage to achieve tracking of a maximum power point voltage of the DC-side voltage of each H-bridge unit and obtain an active current instruction value of a system; controlling decoupling of a network-side current, where independent control on an active current and reactive current can be achieved, and meanwhile, a total modulation wave voltage of the inverter is generated; and performing power equilibrium control between units, an accurate on-off state of each H-bridge unit is determined and power equilibrium control of each H-bridge unit and maximum power tracking of each H-bridge unit are achieved by sequencing an error of the DC-side voltage of each H-bridge unit. By the method, various working conditions can be adapted, stable running of the cascaded H-bridge photovoltaic inverter within a relatively large range can be achieved, moreover, the DC-side voltage fluctuation can be reduced, and the power generation quantity of the system is improved.

Power equilibrium control method for reducing DC voltage fluctuation of cascaded H-bridge inverter

The invention discloses a three-phase cascaded H-bridge photovoltaic inverter DC-side voltage fluctuation suppression method whih can solve the problem that the the DC side voltage of the cascaded H-bridge photovoltaic inverter has a large fluctuation of the second frequency The method comprises the following steps: (1) total DC side voltage control, which is used for realizing that the DC sidevoltage of each H-bridge unit tracks its maximum power point voltage and obtains the system active current command value; (2) current decoupling control at the grid side, which can realize the independent control of active and reactive currents and generate the original modulated signal of the inverter; (3) DC voltage ripple suppressing control and correcting three-phase original modulation signalby injecting third harmonic to reduce DC voltage ripple of H-bridge This control method can reduce the DC voltage fluctuation and the capacitor cost under the stable operation of cascaded H-bridge photovoltaic inverter, and has the advantages of simple algorithm and easy engineering implementation

Three-phase cascaded H-bridge photovoltaic inverter DC-side voltage fluctuation suppression method

The invention discloses a single-phase cascade H-bridge grid-connected inverter control method under the weak grid and aims to realize stable operation of a cascaded photovoltaic inverter under the extremely weak grid. The method comprises following steps of (1) of DC side voltage control, the DC side voltage of each H-bridge unit is realized to track the maximum power point voltage; (2) of droopcontrol, a grid side current control command is obtained through a droop control equation and an AC voltage loop; and (3) of network side current control, independent control of the active current andthe reactive current can be realized, and an inverter modulation signal is generated. The method is advantaged in that advantages of both the droop control strategy and maximum power tracking are integrated, not only can the stable operation range of the cascaded H-bridge PV grid-connected inverter be expanded, but also maximum power tracking of each H-bridge unit can be realized.

Dai Zhiqiang

Papers

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

Research on Power Equalization of Three-Phase Cascaded H-Bridge Photovoltaic Inverter Based on the Combination of Hybrid Modulation Strategy and Zero-Sequence Injection Methods

Power equilibrium control method for reducing DC voltage fluctuation of cascaded H-bridge inverter

Three-phase cascaded H-bridge photovoltaic inverter DC-side voltage fluctuation suppression method

Single-phase cascade H-bridge grid-connected inverter control method under weak grid