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 arisen for the grid stability due to the deficiency of converter inertia and damping. To improve the system inertia besides the energy efficiency, this paper 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 non-reserved 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 energy storage device and the full use of PV power is realized, which are verified by the simulation and experimental results.

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.

Intelligent photovoltaic (PV) module monitoring can automatically locate the position of panels and monitor the status of PV modules, which is of great significance for PV plants operation and maintenance. As a new communication method in the field of PV monitoring, the monitoring of PV modules based on power line communication is the future trend of PV power plants monitoring due to its low maintenance cost and strong anti-interference ability. This paper presents an intelligent PV module monitoring scheme based on the parallel resonant coupling unit, which uses the dc bus as the communication channel and modula-tes the monitoring data into a high-frequency form to carry out the carrier communication. By using the parallel resonant coupling, the high-frequency carrier wave is injected into the dc bus, which improves the system efficiency and anti-interference ability. In addition, an adaptive level decision circuit is proposed to improve the flexibility and scalability of the communication system by adaptively changing the reference level of the decision circuit. Experimental results achieved by a developed power line transceiver demonstrate both feasibility and validity of the proposed scheme.

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 paper proposes a novel power equalization method based on the combination of hybrid modulation strategy and zero-sequence injection methods, which deals with inter-bridge power imbalance by hybrid modulation and inter-phase 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 inter-bridge and the inter-phase 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.

Featured with the advantages of expandable modular structure, simple layout, and low dependence on copper and steel, three-phase solid state transformer based cascaded H-bridge topology has been replacing traditional inverters in high-voltage and high-power photovoltaic power generation applications. However, due to the double line-frequency power fluctuation on the dc-side of H-bridge converters, if the LLC topology is used as the front-stage isolated DC/DC converter, its resonant current will produce an envelope ripple with the same frequency. The envelope ripple will not only increase the current stress of switching devices and diodes in LLC converter, but also increase the losses of switching devices and magnetic components. Aiming at this problem, this paper analyzes the mechanism of enveloping ripple in detail by mean of the concept of calculus, expounds the influence of envelope ripple on the whole system, and then proposes a control method to suppress envelope ripple of resonant current. The validity of the proposed method is verified by simulation and experimental results.