The high penetration of distributed energy resources, based on power electronics, is giving rise to stability issues in the electrical network, as initially those plants were not required to provide either voltage or frequency support. Due to this, TSOs and DSOs have set new requirements for them to provide grid support functionalities. This paper presents a grid forming control strategy for power converters which is based on a virtual admittance loop. By means of this strategy it is possible to use a conventional current control based inverter, just adding this outer loop. By doing so, the converter is not only able to perform current control, but also frequency and voltage support functionalities. In this work, the proposed control is described and analyzed, checking as well its stability boundaries. After this analysis, the results obtained in a hardware-in-the-loop platform, where faulty scenarios, islanding or black start conditions, will be shown. Finally, the results obtained in an experimental true scale workbench will show its effectiveness for providing such services.

The increasing participation of renewables in power systems has forced system operators to strengthen grid codes requiring renewable power plants (RPPs) to provide fast frequency support such as inertia response and power oscillation damping. To cope with such new requirements, this article proposes the implementation of a virtual synchronous power plant controller (VSPPC) for RPPs. The VSPPC takes the most from emulating the behavior of a synchronous generator, especially in the case of grid events, as it permits emulating inertia and provides power oscillation damping capabilities. If compared with conventional control schemes, the main advantage of the VSPPC lays in the fact that it does not require to implement any modification in the existing controllers of the plant's converters, which are often controlled as grid-following generation units. This feature makes VSPPC attractive for incorporating advanced grid supporting functionalities in utility-scale RPPs. In this work, the main principles behind the VSPPC and comparative analysis based on simulation and experimental results are provided to validate its performance.

The advent of renewable energy has posed difficulties in the operation of power systems whose net inertia is becoming critically low. To face such challenges, grid-forming power has been one of the potential solutions pursued by the industry and research community. Although promising, grid-forming power converters are still immature for mass deployment in power systems. In the meanwhile, an enormous amount of grid-following power converters has been underexploited when it comes to grid-supporting functionalities. Therefore, this article proposes an external inertia emulation controller (eIEC) for grid-following power converter to provide frequency support to the grid. For the purpose of minimizing installation efforts and resources, the controller is designed in such a way that it can be implemented in an external controller communicating with the grid-following power converter via an industrial communication link. This article also investigates the effect of communication delay on the stability performance of the proposed controller. In addition to the detailed analysis, hardware-in-the-loop experiments are also carried out to validate the proposed eIEC.

The integration of renewable energies based on photovoltaic power plants is increasing exponentially fast. In consequence, stability problems associated to the lack of inertia is becoming a serious concern. Transmission and distribution system operators are demanding more requirements in photovoltaic plants to provide grid functionalities in terms of frequency and voltage support. Therefore, power converters based on virtual synchronous controllers have been proposed to emulate the dynamic behavior of synchronous generators. Control strategies such as the Synchronous Power Controller has been proposed for photovoltaic applications to fulfill this requirement. However, most of these control strategies are integrated locally as an add-on firmware embedded in each converter, providing the demanded support in their connection point. For photovoltaic power plants, the inherent synchronous characteristic is expected at the connection point with the electrical grid, which is not always the connection point of each power converter, thus, unexpected power support levels can be obtained. The aggregation of several power converters operating in a coordinated synchronous action would solve this issue, due to the fact that they can provide voltage and frequency support in a specific point. In this paper a centralized control strategy based on a synchronous central architecture is proposed. The control strategy emulates an aggregated synchronous control to regulate a photovoltaic power plant as a coordinated system. Simulation and experimental results validate the effectiveness of the control proposed.

Inertia emulation is claimed to play a decisive role in the regulation and management of frequency in modern electrical systems. The support offered by renewable energy power plants and distributed generators is key to diminish the rate of change of frequency (RoCoF), as many synchronous generators are being replaced all around the globe. It is a reality that the implementation of the swing equation in the power converter control has been the core of several proposals on grid-forming controllers to emulate inertia. This kind of controller has been heavily studied and integrated in some demonstrators around the world during the last years, providing dynamic inertia support functionalities. However, the need to modify the synchronization strategy in already deployed power units has been one of the key opposition factors on industry, leading to a severe shortcoming on the integration. In contrast to the traditional swing equation implementation this paper presents a lightweight inertial phase-locked loop (IPLL) able to take the most of inertial features introducing minor changes on classical power converter control and synchronization structures. As shown in this work, the straightforward implementation significantly reduces the technological and computational effort compared to other synchronous emulation proposals. Moreover, it integrates not only dynamic inertial response to the converter, but also all grid-forming capacities to the power conversion unit. This modification on the synchronization structure enables the converter to work in grid-following mode in grid-tied applications, and grid-forming in islanded ones. The integration of the proposed IPLL, the stability analysis and a sample of its performance in HIL and experimental environments will be presented in this paper.

The expansion of renewable energy sources highly increased the number of grid-connected converters associated to PV and wind in power plants as well as in residential areas. The appearance of voltage harmonics, which are typically generated by a large number of devices, is a common issue in weak grid environments and it is transitioning to the main utility grid. This harmonic distortion is significantly increasing due to the interaction between electronic power converters and the network, which not only contribute to damage other equipment connected to the same point of common connection (PCC), but also increase the losses and saturation of the transformers in the line. While previous studies have extensively addressed this issue for grid-following power converters, the harmonic control in parallel grid-forming converters, particularly those based on virtual synchronous emulation, remains underexplored. This paper intends to provide a solution focused on providing a selective harmonic control for synchronous power controller (SPC) based power converters. The proposed solution enables the parallelization of power converters to naturally share the harmonics attenuation effect at the PCC. The methodology is presented and validated through real-time simulations as well as experimental tests conducted in a 200 kVA microgrid testbed.