The increasing penetration of renewable generation in power systems is causing growing concern about system stability. Grid-forming (GFM) control has been appointed as the technology required for achieving a high penetration of renewables in the grid, as it successfully contributes to the power system stability. This paper proposes a novel grid-forming control scheme for voltage source converters (VSC) achieving the operational characteristics of a virtual synchronous machine (VSM). The proposed scheme is based on the orientation of a defined virtual flux to a reference axis obtained from the emulation of the synchronous generator swing equation. The virtual flux is obtained by the integration of the VSC internal voltage and it is oriented to the reference axis by means of a flux controller that also controls the flux magnitude. In this way, the VSC synchronism is maintained, operating as a voltage source connected to the grid. Moreover, the flux orientation in turn allows to control the VSC active power, while the flux magnitude control allows to regulate the VSC reactive power or terminal voltage in isolated mode. The paper also demonstrates that the stability of GFM converters is negatively affected when emulating high inertia values. So, a stabilizer is also proposed for damping these modes. Finally, due to fact that GFM converters behave as voltage sources, their low voltage ride through capability becomes an issue, because they naturally respond to faults with high short-circuit currents, that cannot be withstand by power electronics converters. Therefore, a method for limiting fault currents is proposed as well in the paper. The proposed grid-forming control scheme has been implemented in a real-time control platform and validated using comprehensive simulation models in a real-time simulator for assessing its grid-forming capability.

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.

In recent years, the use of synchronous generators (SGs) has been displaced due to the increased use of renewable energy sources. These types of plants mostly use power electronic converters to connect to power grids, which, due to their mode of operation, cannot provide the same services. This paper analyzes the synchronization of Grid-Forming converters (GFM) without phase-locked loop (PLL) through the active power control loop. Stability analysis shows that when increasing the emulated moment of inertia in a voltage source converter (VSC) using grid-forming control, the system becomes oscillatory. The paper proposes a novel compensation mechanism in order to damp the system oscillation, allowing the implementation of inertia emulation. Finally, the real-time implementation is executed using a Hardware in the Loop experimental set-up. The response of VSC under grid disturbances is simulated in a real time simulator, while the proposed control system is implemented in a real-time controller platform.

Renewable power generation has increased in recent years, which has led to a decrease in the use of synchronous generators (SGs). These power plants are mainly connected to the power system through electronic converters. One of the main differences between electronic converters connected to power systems and SGs connected to the grid is the current contribution during faults, which can have an impact on protection systems. New grid codes set requirements for fast current injection, but the converters’ maximum current limitation during faults make it challenging to develop control strategies for such current contribution. This paper presents a positive and negative sequence current injection strategy according to the new Spanish grid code requirements for the novel grid-forming converter control algorithm based on virtual-flux orientation. The behavior of the proposed strategy is tested in a hardware in the loop (HiL) experimental set-up under balanced faults, meaning that the fault is symmetrically distributed among the three phases, and unbalanced faults, where the fault current is distributed asymmetrically between the phases.

The electric power system is undergoing a significant transition. Renewable energy generation capacity has increased, leading to the displacement of synchronous generators (SG) and a growing concern about grid stability due to the decrease of rotating inertial energy. Grid-forming (GFM) converters have become a technological solution to this challenge, as they are capable of operating in low strengh system conditions and helping to stabilize the voltage and frequency of the grid. In this paper, a novel control scheme for GFM converters is validated in a commercial converter according to the National Grid ESO, called National Energy System Operator (NESO) since October 2024. The control scheme uses the flux vector as a simple and effective method for limiting the active and reactive currents without internal current loops. For this purpose, a hardware test bed has been implemented consisting of two DC sources emulating a photovoltaic (PV) plant which feed the DC bus a voltage source converter (VSC), a grid emulator which allows to generate the desired disturbances in the grid to evaluate the converter’s response and a three-phase load. The results obtained have validated that these GFM converters based on the flux vector are capable of synchronizing with the grid, responding against frequency disturbances like phase jumps or faults in the grid and operating in islanded mode. In addition, the innovative method used to limit the active and reactive current has also been tested, comparing its performance with that of a conventional GFM scheme, showing an improvement in the response stability.

The increasing penetration of renewable energy generation in recent years has introduced significant changes and challenges to modern power systems. One of the most critical challenges is the reduction in system inertia, which decreases grid stability and subsequently weakens the electrical network. To address this issue, grid-forming (GFM) converters have emerged as a promising solution to maintain stability in weak grids. This paper proposes three novel control schemes for GFM converters and compares them with the performance of another topology recently published by the same authors. The four evaluated control schemes are based on the virtual flux variable which allows current limiting without using internal current loops, improving the stability of the control system. The assessment includes methods based on PI regulators, using the mathematical flatness property of differential algebra, direct control (DC), and model predictive control (MPC). The results demonstrate the robustness and correct operation of all four control strategies as GFM converters. Furthermore, through tests involving disturbances such as frequency variations, voltage sags, phase jumps, and transitions to islanded mode, their differences in terms of dynamic response, switching frequency, and current quality are clearly evidenced.