Power-electronics-based microgrids (MGs) consist of a number of voltage source inverters (VSIs) operating in parallel. In this paper, the modeling, control design, and stability analysis of parallel-connected three-phase VSIs are derived. The proposed voltage and current inner control loops and the mathematical models of the VSIs are based on the stationary reference frame. A hierarchical control scheme for the paralleled VSI system is developed comprising two levels. The primary control includes the droop method and the virtual impedance loops, in order to share active and reactive powers. The secondary control restores the frequency and amplitude deviations produced by the primary control. Also, a synchronization algorithm is presented in order to connect the MG to the grid. Experimental results are provided to validate the performance and robustness of the parallel VSI system control architecture.

A new approach of direct power control (DPC) of a voltage source inverter for achieving symmetrical grid currents under unbalanced grid voltage conditions is presented. In these situations, negative sequence components appear on the grid voltage vector, leading to distorted currents if the power is maintained constant. Nevertheless, sinusoidal balanced grid currents can be achieved through control. Several current controllers have already been modified to overcome voltage imbalance with good results. Here, sinusoidal grid currents are obtained by directly controlling power, which becomes a novel approach and keeps DPC philosophy. A phase locked loop is necessary to obtain positive sequence voltage angle. A very simple positive and negative sequence calculator is also used so that power references can be modified in order to obtain balanced grid currents. Simulated and experimental results are presented to confirm the validity of the method.

In this study, a grid simulator based on a back‐to‐back inverter topology with resonant controllers is presented. The simulator is able to generate three‐phase voltages for a range of amplitudes and frequencies with different types of perturbations, such as voltage sags, steady‐state unbalanced voltages, low‐order harmonics and flicker. The aim of this equipment is to test the performance of a given system under such distorted voltages. A prototype of the simulator, consisting of two inverters connected back‐to‐back to a 380 V three‐phase grid and feeding a micro‐grid composed of two‐inverter interfaced distributed generators and a critical load was built and tested. A set of experimental results for linear purely resistive loads, non‐linear loads and current‐controlled inverters is presented to prove the capabilities of the simulator. Finally, a case study is presented by testing a micro‐grid.

In recent years, the increasing integration of renewable energy sources into power systems has led to significant changes in grid operation. Power electronic converters are primarily used to connect these power plants to the electrical system and, as key components in modern power systems, grid-forming (GFM) converters have emerged to provide the ability to autonomously establish and maintain system stability without the need for synchronous generators (SGs). The current contribution during faults is one of the main differences between SGs and power converters, which can affect protective systems. The most recent grid codes require a fast current injection in the event of a fault. This article presents a low-voltage ride-through control strategy for GFM converters based on virtual-flux orientation according to the European Commission Regulation (EU) 2016/631 and the Spanish Technical Supervision Standard requirements. For the validation of this control algorithm, a hardware test bed has been implemented consisting of two dc voltage sources emulating a photovoltaic plant feeding the dc bus of a voltage source converter and, finally, a grid emulator where the different voltage faults have been programmed.