Abstract
The massive penetration of renewable energy sources in electrical systems has been displacing synchronous generators (SGs) from conventional power plants in the last few years. Renewable generation plants are usually connected to power grids through electronic power converters, which cannot provide the same power generation services as SGs due to their mode of operation. Recently, different concepts have been proposed for electronic converter control in an attempt to emulate the performance of SGs, resulting in the so-called grid-forming converters (GFCs). This paper proposes a new GFC control strategy based on the reactive power synchronization (RPS) method, which decouples the synchronizing power and the active power control of renewable generation source to which a converter is connected. For this purpose, this study assesses three power sources: batteries, photovoltaic (PV) plants, and full-converter wind turbines. Moreover, the study proposes models and controls for each of these sources, whose dynamics exert a decisive influence on the grid services provided by renewable energy plants. Thereafter, the study proposes a GFC–RPS control scheme and verifies its effectiveness in different applications; for example, inertial response, which provides power immediately through a fast frequency response after a grid has experienced a load variation. Unlike storage systems and wind turbines, PV plants can only render these services if they are not operating at maximum power. Further, the study validates the GFC–RPS control strategy for regulating AC voltage at the output terminals of a converter. Finally, the paper assesses GFC hot swapping during the transition from a grid-connected to an isolated-operation mode while feeding a dynamic load. Results revealed that both the voltage and frequency remain stable, thereby demonstrating that the proposed GFC–RPS control indeed acts as a true voltage source and emulates the behavior of a conventional SG.
Highlights
The concept of grid-forming converters (GFCs) was first introduced within the context of microgrids [1], [2] as a novel control method for voltage source converters (VSCs) acting as uninterruptible power supplies aimed at maintaining voltage and frequency at the load when it becomes disconnected from microgrids
The last block assesses the dynamic response from the GFC during the AC voltage control hot-swapping process when the GFC becomes disconnected from the grid, and the converter remains in an islanded mode supplying a dynamic load at its terminals
The results presented validate the robustness of the GFC–reactive power synchronization (RPS) control based on the hotswapping strategy during the transition from a gridconnected mode to an islanded mode
Summary
The concept of grid-forming converters (GFCs) was first introduced within the context of microgrids [1], [2] as a novel control method for voltage source converters (VSCs) acting as uninterruptible power supplies aimed at maintaining voltage and frequency at the load when it becomes disconnected from microgrids. In these instances, the control system’s orientation angle (θ) that is determined by measuring voltage directly at converter terminals through a PLL, it is replaced by the integral action of the frequency deviation, Δω, which it is calculated through an active power synchronization loop, or through reactive power as the current study proposes. REACTIVE POWER SYNCHRONIZATION To assess the existing relationship between the instantaneous active and reactive powers generated by the GFC against an increase in voltage and angle, the model illustrated in Fig. 3 is used In this figure, the GFC is represented as a voltage source behind an impedance connected to a grid with frequency, ω0.
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