Grid-forming inverter technology is a novel and evolving concept for systems with high penetration of renewables. The integration of these grid-independent units with the existing conventional generators and gradually replacing them should contribute to stabilizing the power grid. Several control strategies are proposed in order to design grid-forming inverters to emulate the kinetic energy and the self-synchronization features of synchronous machines with robust control on voltage and frequency to deal with the dynamic issues of the grid. In this paper, a PV-based grid-forming inverter with a modified virtual synchronous machine control in parallel with a battery supported inverter with an enhanced droop control has been considered to operate under non-ideal grid voltage conditions and in the isolated mode of operation. The control methodology encloses a photovoltaic synchronous generator, along with the nonlinear feedback linearization current-limiting control with voltage ridethrough capabilities. They enable the grid-forming PV-inverter and the grid-following battery inverter to provide active and reactive power to the load during unbalanced grid conditions seamlessly. A hardware co-simulation environment in Simulink using the software tool of Piecewise Linear Electrical Circuit Simulation (PLECS) and System Generator from Xilinx has been applied to evaluate the operation of the controller and verify its flexibility and effectiveness in different case studies.

Recent technical advances in control, protection and interconnection of distributed power generation units implies that it is practically viable and economically profitable to keep them as backup generators in isolated operating modes. However, more sophisticated control strategies are needed to recognize the existing working condition and adjust the performance to meet the strict standards of grid interconnection. This paper presents a new adaptive control structure, based on internal model control (IMC), which uses multiple models and an inherent islanding detection method through an optimized switching mechanism to tune the operation of a three phase inverter under transitions between islanded and gridtied conditions. By applying a power synchronization method, the system emulates the operation of a synchronous machine which is needless to rely on a phase-locked loop to synchronize during the transitions. Hardware co-simulation environment in Simulink/PLECS and Xilinx System Generator have been utilized to evaluate the transient behavior of the controller in discretized domain and verify its robustness during parameter variations, unbalanced grid conditions and load switching conditions. Various switching rules have been applied and a comparison of their effect in transient response is demonstrated.

The power synchronization control strategy for grid-connected voltage-source converters (VSCs) provides an operation similar to synchronous machines. It is able to avoid the instability caused by a standard phase-locked loop in integration into weak grids. However, the non-minimum phase phenomenon in the developed dynamics places a fundamental limitation on the ac system's stability. This paper proposes a one-degree-of-freedom internal-model-based control methodology. It introduces a control approach to incorporate the dynamics of the system's nominal model in the control structure. It also rectifies the unwanted effects of the right-half plane zeros. The explicit incorporation of the model enhances the tracking capabilities of the controller in a PV-based VSC. Besides, this article shows that a single-loop of control will suffice to regulate active and reactive power. Validating results are generated via a hardware-in-the-loop system based on a Xilinx Zynq ®-7000 SoC field-programmable gate array (FPGA). Furthermore, experimental results are conducted for low-power prototyping to examine the satisfactory performance of the proposed control architecture.

Multilevel converters are becoming favorable topologies to maximize the power transfer from solar panels to utility grids. As the harmonic emission of multilevel inverters are generally lower than their conventional two-level counterparts, not a bulky passive filter is required to eliminate the harmonic currents. However, the zero impedance of LCL filters at certain frequencies may expose the network to instability. In this paper, an embedded active damping technique based on virtual resistance method, within an internal model control scheme, has been implemented on a PV-flying capacitor multilevel inverter. An internal model of the LCL filter and the feeder line resistances are encapsulated in the controller and the difference between the control current output and the actual one passing through a compensator leads to the accurate control objective. Hardware-in-the-Loop case studies in Matlab/Simulink and Xilinx System Generator environment proves the robustness of the proposed control method.