Robust Stability Analysis of Synchronverters Operating in Parallel

Recent studies have shown how synchronization units of converters operating nearby may interact with each other, affecting the stability of the system. Synchronverters are able to self-synchronize to the grid without the need of a dedicated unit because they can reproduce the power synchronization mechanism of synchronous machines. Recently, the robust stability of a synchronverter has been investigated by means of structured singular values (commonly called μ-analysis). In this paper, μanalysis is performed to investigate how the robust stability of a synchronverter is affected by the presence of another converter of the same type operating in parallel. It is demonstrated that the parallel operation of synchronverters reduces their robust stability and a possible solution is proposed, based on the implementation of virtual impedances in the control algorithm. An accurate state-space model of the system under study is developed by adopting the component connection method and the robust stability analysis is validated against time-domain simulations in MATLAB/Simulink/PLECS and experimental results with a power-hardware-in-the-loop test bench

Robust Stability Investigation of the Interactions Among Grid-Forming and Grid-Following Converters

State-of-the-art grid-connected converters can be classified as “grid-following,” meaning that they require a dedicated synchronization unit in order to inject active and reactive currents into the grid. Recently, other converter control concepts have been proposed in the literature, such as the synchronverter, which can instead achieve synchronization without a dedicated unit and, within its physical limitations, make the converter behave as an ideal voltage source. Since it should be expected that the grid-connected converters having different control philosophies will coexist for many years, in this article, the interaction among the converters operating nearby are addressed. First, the component connection method (CCM) technique is introduced, as a means for obtaining the state-space representation of a complex system with several units operating nearby. Due to the complexity of the grid and the difficulty in obtaining its exact representation, μ-analysis is adopted in this article for assessing the robust stability of the converter under different operating conditions, according to a defined set of plant uncertainties. Simulation results and experimental tests in a laboratory environment by means of a power hardware-in-the-loop (PHIL) test bench are performed to demonstrate the validity of the presented analysis

Robust stability analysis of LCL filter based synchronverter under different grid conditions

Synchronverters have gained interest due to their capability of emulating synchronous machines (SMs), offering self-synchronization to the grid. Despite the simplicity of the control structure, the adoption of an LCL-filter makes the overall model complex again, posing questions regarding the tuning of the synchronverter and its robustness. The multi-inputs multi-outputs (MIMO) formulation of the problem requires multivariable analysis. In this paper, the effects of control parameter and grid conditions on the stability of the system are investigated by means of structured singular values (SSV or μ-analysis). A step-by-step design procedure for the control is introduced based on a linearized smallsignal model of the system. Then the design repercussions on the stability performance are evaluated through the performed robustness analysis. The developed linearized model is validated against timedomain simulations and laboratory experiments. These have been carried out using a power hardwarein- the-loop (PHIL) test bench, which allows to test the synchronverter under different grid conditions. As a conclusion the paper offers a simple guide to tune synchronverters but also a theoretical solid framework to assess the robustness of the adopted design

Analysis and design of LCL filter based synchronverter

Synchronverters have gained interest due to their capability of emulating synchronous machines (SMs), offering self-synchronization to the grid. Despite the simplicity of the control structure, the adoption of an LCL-filter makes the overall model complex again, posing questions regarding the tuning of the synchronverter and its robustness. The multi-inputs multi-outputs (MIMO) formulation of the problem requires multivariable analysis. In this paper, the effects of control parameter and grid conditions on the stability of the system are investigated by means of structured singular values (SSV or μ-analysis). A step-by-step design procedure for the control is introduced based on a linearized smallsignal model of the system. Then the design repercussions on the stability performance are evaluated through the performed robustness analysis. The developed linearized model is validated against timedomain simulations and laboratory experiments. These have been carried out using a power hardwarein- the-loop (PHIL) test bench, which allows to test the synchronverter under different grid conditions. As a conclusion the paper offers a simple guide to tune synchronverters but also a theoretical solid framework to assess the robustness of the adopted design

Current Limitation Strategy For Grid-Forming Converters Under Symmetrical And Asymmetrical Grid Faults

The potentials of grid-forming (GFM) converters in stabilizing a power system with high penetration of power electronics-based generation have been recently investigated. Due to their intrinsic behaviour of voltage source behind impedance, a crucial aspect of any GFM converter control strategy will be the handling of fault-ride through (FRT) scenarios. This paper proposes a FRT strategy for GFM converters, which respects the converter hardware limitations (i.e. current limitations) even under sever fault conditions, while maintaining GFM behavior before, during, and after the fault. The presented strategy addresses both symmetrical and asymmetrical faults, and is compliant with recently proposed draft grid codes requirements published by the British system operator NGESO. The issues related to FRT of GFM converters are first discussed in detail, and a comprehensive overview on the solutions proposed in the literature is reported. Then a proper strategy is presented, and its effectiveness is demonstrated by means of simulations and Power-Hardware-in-the-Loop (PHIL) measurements in a laboratory environment

Grid-forming converters:An overview of control approaches and future trends

In the last decade, the concept of grid-forming (GFM) converters has been introduced for microgrids and islanded power systems. Recently, the concept has been proposed for applications in wider and interconnected transmission networks, and several control structures have thus been developed, giving rise to discussions about the characteristics and the functionalities of such converters. In this paper, an overview of control schemes for GFM converters is provided. By identifying the main subsystems in respect to their functionalities, a generalized control structure is derived and different solutions for each of the main subsystems composing the controller are analyzed and compared. Subsequently, several selected open issues and challenges regarding GFM converters, i. e., angle stability, fault ride-through (FRT) capabilities, and transition between islanded to grid-connected modes are discussed. Perspectives on challenges and future trends are lastly shared