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.

Distributed power generators in islanded microgrid usually adopt droop control strategy or virtual synchronous generator control strategy to simulate Q-U droop characteristic of synchronous generators for voltage support ability enhancement and reliability improvement. However, because the distributed power generators' capacities and locations are random, they cannot share reactive load proportional to their rated capacity, resulting in overload protection, and even system instability. In order to improve reactive power sharing precision, voltage control precision and system stability simultaneously, an improved reactive power sharing algorithm combining virtual impedance and virtual capacitor is proposed in this paper. Firstly, the three existing open-loop reactive power sharing strategies are summarized and the effect of their parameters on reactive power sharing error, steady-state voltage deviation and system stability are analyzed. Afterwards, based on the analysis results, a reasonable configuration of virtual impedance and virtual capacitor values are put forward to realize the control targets of reducing steady-state reactive power sharing error, improving voltage control precision and system stability meanwhile. Finally, simulation and experimental platforms are built to verify the effectiveness of the proposed strategy.

During the past decades, the small-signal stability of the interconnected converters has been extensively researched, and several impedance/admittance-based criteria were reported and effectively verified. However, the equivalence and correlation among these criteria have not been clearly expounded. In this article, the internal relationship among the existing five stability criteria is firstly studied by modeling and stability analysis for different interconnected converters, and proving the equivalence of these stability criteria. Then, from the perspective of the application complexity, the convenience for system design and the reliability against zero-pole cancellation risk, the optimal stability criteria for different types of interconnected converter systems are recommended. Finally, based on a cascaded system composed of a boost and a buck converter and a paralleled system consists of two single-phase LCL inverters, the correctness of the proposed viewpoints is demonstrated by case studies and experiments.

Finite control set model predictive control (FCS-MPC) is widely used in rectifiers due to its fast response. When there are multiple control objectives, the optimal switching sequence is selected by minimizing the cost function with weighting factors. However, the reduction of current ripple caused by output error and switching loss is in conflict to a certain extent for FCS-MPC. In addition, when multiple objectives are controlled, the selection of weighting factor is a serious challenge. Therefore, a modulation method based on continuous control set model predictive control is proposed in this article. Two phases are clamped and one phase is modulated, which not only can significantly reduce the switching loss without increasing the computational burden, but also can control the neutral point voltage without using weighting factors. In addition, when the proposed method is adopted, current zero-crossing distortion can be suppressed over the full range of modulation index. The effectiveness of the proposed method is verified by simulation and experimental results.

The study of stability criterion and interaction analysis for grid-connected inverter under weak grid is of great value. Different from traditional impedance stability criterion, this paper proposes control loop stability criterion for grid-connected inverter from the perspective of controller bandwidth overlap. Firstly, the system d-q overall small-signal model G0 considering phase-locked loop (PLL) of grid-connected inverter under weak grid is given and split into three multiplied independent parts: grid impedance, phase-locked loop and current controller. Then using the equivalent loop ratio expression obtained by combining PLL and grid impedance together and then divided by the current controller, the control loop stability criterion is proposed. The proposed stability criterion can not only maintain the independence of each single loop, but also can analyze the overall system stability. So, further analysis of the interaction law among the three parts under this control loop stability criterion is carried out by deducing the expression of the bandwidth ratio n of the PLL and current controller. And it is found that under the weak grid, the interaction between the links will be generated when the bandwidth ratio is greater than the threshold of n. This phenomenon can be represented by the overlapping area of amplitude-frequency curves in the bode diagram for G0. And the weaker the grid or the closer the bandwidth of PLL is to current controller, the larger the overlapping area of amplitude-frequency curves, and the more likely it is to lead the closed-loop gain to infinity. Furthermore, when the bandwidth of the current controller is fixed, the threshold of n varies in the regions of less than 1 as well as more than 1 with the change of the grid strength. Therefore, the system can remain stable no matter what the bandwidth ratio of the phase-locked loop to the current controller is greater or less than 1. Accuracy of the proposed control loop stability criterion and interaction analysis is verified through simulation and experimental results.