Performance Improvement of Single-Phase Transformerless Grid-Connected PV Inverters Regarding Common-Mode Voltage (CMV) and LVRT

The single-phase transformerless grid-connected photovoltaic (PV) systems, mainly the low-power single-phase systems, require high efficiency, small size, lightweight, and low-cost grid-connected inverters. However, problems such as leakage current, the DC current injection and safety issues are incorporated with transformerless grid-connected PV inverters. Besides, the new standards such as Low-Voltage Ride-Through (LVRT) capability and staying connected to the grid during the fault occurrence should be considered for the next generation of transformerless PV inverters. In this paper, a study is going underway on the LVRT capability and the Common-Mode Voltage (CMV) in a number of most common transformerless grid-connected PV inverters. In fact, by a comprehensive study on all possible switching combinations and the current paths during the freewheeling period of the selected inverters, the proposed control strategy for performance improvement of the PV inverters under the normal and the LVRT conditions is presented. As a matter of fact, a reconfigurable PWM method is proposed, which makes it possible to switch between two PWM methods and hence provide improved performance of the inverters in the LVRT condition. Finally, the results of simulations in the normal and the LVRT operations to verify the theoretical concepts are indicated

Robust nonlinear sliding mode controllers for single-phase inverter interfaced distributed energy resources based on super twisting algorithms

This paper presents a voltage control mode (VCM)-based super-twisting algorithm-sliding mode controller (STA-SMC) and current control mode (CCM)-based STA-SMC for islanded and grid-connected operations of single-phase inverters interfaced with distributed energy resources (DERs), respectively. The external disturbances are modeled by considering the effects of load currents or load voltages depending on operational modes whiles parametric uncertainties are modeled based on their variations from nominal values. Therefore, the proposed controllers are robust against external disturbances, parametric uncertainties, and variations in loading conditions. Simulation studies are conducted in the MATLAB/Simulink platform for both single and multiple DERs which are interfaced through single-phase inverters to the main grid or loads. The results are also analyzed through comparative studies with different controllers and it is found that the proposed controllers perform better than existing controllers

Single-phase Multilevel Current Source Inverter with Reduced Device Count and Current Balancing Capability

Nowadays power converters play an important role in power system and industrial centers. One of the most important and widely used types of conversion is DC to AC conversion that is also called inverters. Generally inverters are divided to voltage source inverter (VSI) and current source inverter (CSI). From another view the inverters are divided to two-level and multilevel types. The multilevel inverters are attractive because of their good output waveform quality. However, there has been less attention to multilevel current source inverter (MLCSI) when compared with multilevel VSI. In this paper, a new topology for MLCSI is proposed. The proposed topology employs reduced number of switches to generate desired multilevel output current. The proposed MLCSI is capable of balancing the currents of the inductors that are used in the MLCSI structure. A multicarrier PWM based switching strategy is also proposed for the MLCSI. The simulation results using PSCAD/EMTDC as well as the experimental results from a single-phase 5-level CSI laboratory prototype demonstrate its validity