The transient stability of traditional power systems is concerned with the ability of generators to stay synchronized with the positive-sequence voltage of the network, whether for symmetrical or asymmetrical faults. In contrast, both positive- and negative-sequence synchronizations should be of concern for inverter-based generation (IBG) under asymmetrical faults. This is because the latest grid codes stipulate that IBG should inject dual-sequence current when riding through asymmetrical faults. Currently, much less is known about the synchronization stability during asymmetrical faults. This significantly differs from the positive-sequence synchronization alone because the coupled dual-sequence synchronization is involved. This paper aims to fill this gap. Considering the sequence coupling under asymmetrical faults, the dual-sequence synchronization model of IBG is developed. Based on the model, the conditions that steady-state equilibrium points should follow are identified. The conditions throw light on the possible types of synchronization instability, including the positive-sequence dominated instability and the negative-sequence dominated one. For different types of instability, the dominant factors are analyzed quantitatively, which are reflected by the limit on the current injection amplitude. Exceeding the limit will lead to the loss of both positive- and negative-sequence synchronizations. The model and the analysis are verified by simulations and hardware-in-the-loop experiments.
The synchronized operation of power generators is the foundation of electric power network stability and a key to the prevention of undesired power outages and blackouts. Here, we derive the conditions that guarantee synchronization in power networks with inherent generator heterogeneity when subjected to small perturbations, and perform a parametric sensitivity analysis to understand synchronization with varied types of generators. As inverter-based resources, which are the primary interfacing technology for many renewable sources of energy, have supplanted synchronous generators in ever grow…
Power electronic converters for integrating renewable energy resources into power systems can be divided into grid-forming and grid-following inverters. They possess certain similarities, but several important differences, which means that the relationship between them is quite subtle and sometimes obscure. In this article, a new perspective based on duality is proposed to create new insights. It successfully unifies the grid interfacing and synchronization characteristics of the two inverter types in a symmetric, elegant, and technology-neutral form. Analysis shows that the grid-forming and g…
Virtual oscillator control is the latest control technique for grid-forming inverters. Virtual Oscillator based Controllers (VOCs) provide all the steady-state droop functionalities of conventional droop controllers and, in addition, the time-domain synchronization with a connected electrical network. However, existing literature does not consider the aspect of dynamic control over the voltage stiffness of a VOC. Voltage stiffness is a vital parameter for a grid-forming inverter. If the voltage stiffness is too high, the inverter picks up all the reactive power demand of the PCC. In contrast, …
This study examines the transient stability of low-inertia power systems with inverter-based generation (IBG) and proposes a sufficient stability criterion. In low-inertia grids, transient interactions are induced between the electromagnetic dynamics of the IBG and the electromechanical dynamics of the synchronous generator (SG) under a fault. For this, a hybrid IBG-SG system is established and a delta-power-frequency model is developed. Based on this model, new mechanisms of transient instability different from those of conventional power systems from the energy perspective are discovered. Fi…
A graph theoretic view on small signal stability of inverter-based power grids
Forschungszentrum Jülich（于利希研究中心）；Institute of Climate and Energy Systems: Energy Systems Engineering (ICE-1)（气候与能源系统研究所：能源系统工程）；Institute of Physics Belgrade, University of Belgrade（贝尔格莱德大学物理研究所）；Potsdam Institute for Climate Impact Research (PIK)（波茨坦气候影响研究所）；Technische Universität Berlin（柏林工业大学）；Institute for Theoretical Physics, University of Cologne（科隆大学理论物理研究所）；Department of Environmental Sciences, Open University of the Netherlands（荷兰开放大学环境科学系）；Institute of Physics, Norwegian University of Life Sciences（挪威生命大学物理研究所）；RWTH Aachen（亚琛工业大学）
Virtual Oscillator (VO) control is the latest and promising control technique for grid-forming and grid-supporting inverters. VO Controllers (VOCs) provide time-domain synchronization with a connected electrical network. At the same time, a VOC can incorporate additional nested control loops to meet the system-level requirements such as fault ride-through capability. However, existing VOCs have two limitations. Firstly, the existing VOCs do not have decoupled control over individual phases. As a result, the performance of the existing VOCs is not satisfactory in presence of unbalanced grid vol…
