In this paper, a super-twisting algorithm (STA) second-order sliding mode control (SOSMC) is proposed for single-phase photovoltaic grid-connected voltage source inverters. The SOSMC approach is aimed to inject sinusoidal current to grid with low total harmonic distortion (THD), strong robustness to parameter variations, and fast dynamic response to solar irradiance changes. As the discontinuous sign function in the traditional sliding mode control (SMC) is replaced by the continuous super-twisting function, the chattering problem is eliminated. Furthermore, a robust sliding mode differentiator (SMD) is proposed to acquire the derivative of current reference which is difficult to obtain in practice. The stability of the system under the proposed method is proved through the Lyapunov theory. The experimental results validate that the proposed method has satisfactory performance in both dynamic and steady-state conditions, as well as disturbance rejection.

Multilevel inverters (MLIs) featuring reactive power ability and common ground (CG) have become much popular in transformerless grid-tied photovoltaic applications. A switched-capacitor (SC) based five-level transformerless inverter configuration is thus proposed in this article, which requires only six switches, one diode, three capacitors, and one input dc voltage supply. Notably, only four power switches are operated in high frequency, thus, the switching losses are reduced. Moreover, the presented topology can effectively tackle the leakage current problem using a CG architecture. A multicarrier phase disposition pulsewidth modulation strategy is employed to achieve reactive power regulation and SC voltage self-balancing. To achieve the neutral point voltage balance of the presented five-level inverter, a simple closed-loop voltage-balancing control method is proposed. The operation principles with modulation strategy and voltage self-balancing of SC are discussed in depth. Comparisons between the proposed and the state-of-the-art MLIs are presented in detail. Finally, experimental tests on a single-phase 1.1-kW prototype validate the appropriate performance of the proposed inverter.

This letter presents a transformerless single-phase PV inverter, which combines with common-ground and active power decoupling techniques. The leakage current is eliminated and bulky electrolytic capacitors are removed, thus improving the system safety and reliability. In addition, it needs only four switches and can be implemented conveniently by a commercial H-bridge module. The validity of the inverter is verified with a 2-kW prototype.

Owing to the filter resonance and background harmonics, the current control for the LCL-filtered grid-connected inverter should be carefully designed to ensure stable operation. Prior-art current control methods normally require extra sensors to achieve the damping of the resonance, or sophisticated active damping should be employed. Both increase the overall system cost and complexity. In this context, an improved control strategy is proposed for LCL-filtered inverters. The proposed control method utilizes a novel reduced-order observer in a way that only one current sensor is required for stable operation (i.e., resonance and harmonics are effectively attenuated). More specifically, the reduced-order observer embeds the dynamics of the grid voltage, where the estimated grid information is used for synchronization in a phase-locked loop. The estimated state variables of the observer are then used for the controller design. Furthermore, to achieve active damping and suppress the influence of grid voltage distortions on the current quality, a multi-resonant state-space controller is proposed, where a linear quadratic regulator method is employed to obtain the optimal gain. Simulations and experimental tests are performed on a 3-kW grid-connected inverter system with an LCL filter. The results demonstrate the effectiveness of the proposed method in terms of robust active damping and strong harmonic attenuation, and thus the inverter achieves a good power quality with only one current sensor.