In the actual operation of the grid-connected inverter, there will be a large number of low-order harmonics in the grid-connected current, resulting in a decrease of the grid current quality. Therefore, this paper adopts a harmonic resonant controller to suppress harmonics of grid current, and the parameters specific design constraints are presented. At the same time, in view of its weakening for system phase margin and the complex design of many parameters, this paper proposes multi-objective particle swarm algorithm to further optimize the parameters of the system, so that the system has good control performance. Finally, the simulation results verify that after the system adopts the algorithm optimized harmonic resonant controller, low-order harmonics can be significantly inhibited, and the dynamic performance can also be good at the same time.

The multi-module input-parallel output-series (IPOS) connected inverter system, which is composed of standard modules parallel in the input side and series in the output side, have many advantages in high voltage applications. For example, in the renewable energy power generation fed to the grid, the IPOS inverter system can reduce the transformation ratio of the transformer between the inverter and grid, thus the transformer can be smaller and lighter. In order to get the high quality of grid current, the LCL filter is adopted in the output side of standard modules. Multiple objectives such as unity power factor, damping the resonant spike of LCL filter and output voltage sharing within standard modules, need to be achieved for this LCL-type IPOS grid-connected inverter system. This paper proposes a single-current feedback scheme which only samples the inverter-side current of one module to reach these goals simultaneously. It greatly simplifies the control structure and cuts down the cost sharply. In the end, the experimental results of the prototype consisting of two modules are shown to verify the effectiveness of the proposed scheme.

The input-parallel output-series inverter system(IPOS) is very suitable for low input-voltage and high output-voltage applications, which is composed of multiple inverter modules connected in parallel on the input side and in series on the output side. The output voltage sharing(OVS) is the key to ensure the stable operation of the IPOS inverter system, and the existing control strategies have shortcomings such as low modularity and reliability. Therefore, this paper proposes a droop control strategy based on the droop relationship between the inverter-side inductor current and filter capacitor voltage. It realizes disturbed control with no interconnection bus between modules. In the case of inconsistent filter parameters, it can significantly improve the system output voltage sharing and ensure the stable operation of the system by increasing the droop coefficient. This paper introduces the working principle and characteristic of the droop control strategy firstly. The influence of the droop coefficient on the system stability is also analyzed, and simulation results are presented at last.

In the practical application of grid-connected inverters, the parameter uncertainty will affect the stable operation of the system, and the μ-synthesis theory can be used to improve the robustness of the system by designing a robust controller. However, the controller designed using this method is of high order and thus difficult to implement in engineering. Therefore, this paper proposes adopting genetic algorithm to optimize the parameters of a fixed-order controller based on μ-synthesis, so that it can meet the robust performance requirement that the structured singular value is less than 1. The simulation results show that the proposed controller has good robustness and robust performance.

In this paper, the stability problem of multi-paralleled grid-connected inverter system in the weak grid is investigated. Due to the existence of grid impedance, the inverters in the system have coupling problems with each other. This may cause resonance and lead the system to be unstable. This paper focuses on the general system of inverters with different parameters, and then establishes the Norton equivalent model of the system. Based on the equivalent transformation of the Norton model, the grid impedance is equivalently allocated to each inverter, and a stability criterion for a multi-paralleled grid-connected inverter system is proposed. In addition, considering the influence of the digital-control-delay on system stability, a phase-lead compensator is added to the capacitor-current-feedback path to improve the system robustness against grid impedance variation. The effectiveness of the theoretical analysis is verified by experimental results.

Input-series-output-series (ISOS) inverter combined system is generally used in the occasion of high input and high output voltage. If the combined system topology is employed in the grid-connected application, multiple small-capacity standardized inverter modules could constitute a bulky-capacity grid-connected inverter system, which will effectively improve system reliability and facilitate system design. Nevertheless, for the sake of enhancing the system reliability authentically, distributed control and redundancy are urgently needed for the grid-tied ISOS inverter system. This paper firstly proposes a novel distributed control strategy to achieve multiple control objectives of grid-connected combined system, which includes power balance, suppression of resonance spikes, and high power factor of grid current. Then a hot plugging scheme dedicated to ISOS grid-connected inverter system is put forward to actualize redundancy. Meanwhile, the timing sequence is introduced. Finally, the prototypes are fabricated in the lab and the experimental results are exploited to verify the effectiveness of the proposed methods.

Input-series-output-parallel (ISOP) inverter system is very suitable for high input voltage and high output current power conversion applications. If the inverter system topology is applied to the grid-connected application, a large-capacity grid-connected inverter system can be built with multiple small-capacity standardized inverter modules, which will improve system reliability and facilitate system design. This paper investigates on ISOP LCL-type grid-connected inverter system, and it proposes a topological optimization method to cut the number of inductor and reduce system volume. Then a new solution is presented, which can meet multiple control targets, i.e., power balance among modules, damping resonant peak, and high power factor of the grid current. Simulation and experimental results validate the effectiveness of the proposed topology and solution.

Input-series-output-parallel (ISOP) inverter system is very suitable for high input voltage and high output current power conversion applications. The objective of the system is input voltage sharing (IVS) and output current sharing (OCS). This paper put forward two specific implementations of compound control strategy which can realize IVS and OCS. One of two approaches is IVS control combined with the same inductor current phase angle and the other is IVS control combined with the same inductor current amplitude. In the meanwhile, a new solution dedicated to distributed configuration is proposed to achieve fully modularity and communications among the modules are realized through three buses. The experimental results validate the effectiveness of the above two methods and the distributed configuration.

This paper explores a configuration for modular inverter system, namely, input-series-output-series (ISOS) connected inverter system, which is suitable for high-input-voltage and high-output-voltage applications. The control objective of this configuration is to realize input voltage sharing and output voltage sharing. On the basis of existing centralized control method, a novel distributed voltage sharing control strategy is proposed. Compared with the centralized control strategy, the distributed control strategy makes each module in the system work independently and fully modularity is achieved. The principle of the control method is elaborated and the experimental results are presented to verify the theoretical analysis.