Parallel Operation of Grid-Forming Power Inverters

Abstract: The use of renewable energy sources has been increasing during the last few years, both to obtain a less impacting energy production, and because of the limited fossil resources. Renewable sources are connected to the grid using inverters, which can be controlled in two main modes, grid-following, and grid-forming. Grid-following inverters (GFLIs) operate connected and synchronized to the grid. GFLIs can be considered as current sources, which adjust their output current by varying output voltage to obtain a certain power. Because GFLIs cannot form the grid voltage, they cannot operate in standalone mode. On the other hand, grid-forming inverters (GFMIs) are controlled to form their output voltage by setting the voltage amplitude and frequency. Therefore, GFMIs can be grid-connected or operate in standalone mode. The GFMIs can participate in frequency control. Thanks to this, the integration of renewable sources as GFMIs may increase electrical system reliability. In addition, GFMIs can be disconnected from the grid to work as an isolated microgrid in case of contingencies. This thesis aims to investigate and validate control methods, without communication, for the operation of parallel GFMIs in standalone mode. The thesis is divided into two main parts. The first part is related to single inverters working as GFLI and GFMI. The two operational modes are analyzed and validated in the laboratory. In particular, the disturbance rejection capabilities of the voltage control are observed by using a GFLI as controllable load. Completing the first part, a functioning system is obtained for a single inverter. The second part is dedicated to the study and validation of parallel GFMIs. In this part, two main scenarios are addressed, the case of parallel operation with inductive lines and resistive lines. For each scenario, different types of droop control are discussed. These two cases differ for the relations between the power flows and frequency and voltage variation. For the case of a mostly inductive line, active and reactive power flows are decoupled. Therefore, the proportional droop control is first considered, both for parallel inverters with equal and different droop coefficients. For the case with different coefficients, power oscillations are expected from the theory. For this reason, an improved method, which introduces a derivative term to dampen the system response, is investigated. Another control strategy for renewable energy integration is the virtual synchronous generator approach (VSG), which tries to mimic the behavior of a synchronous generator. This is analyzed, considering the first-order control and the effect of the virtual inertia on frequency variations. The system is then tested in the laboratory and the experimental results are compared for the different methods. The quality of the control is mainly assessed considering the active power-sharing between the inverters and the frequency response for a load step. In addition, the experimental results are confronted with the theory during the validation of the method. Finally, the second scenario is considered where the case of a mostly resistive line is investigated. In particular, two decoupling methods are presented and implemented for the proportional droop control with equal coefficients. The virtual impedance method is addressed, particularly regarding the impact of the virtual impedance on system dynamics. The second method is based on a linear transformation of active and reactive power flows. For this, the line parameters should be known. Therefore, the effect of accuracy on the parameters is considered. The system is then tested with a focus on the coupling behavior and the active power-sharing, and the experimental results are compared for the two decoupling methods.