Power electronic-interfaced renewable energy sources (RES) exhibit lower inertia compared to traditional synchronous generators. The large-scale integration of RES has led to a significant reduction in system inertia, posing significant challenges for maintaining frequency stability in future power systems. This issue has garnered considerable attention in recent years. However, the existing research has not yet achieved a comprehensive understanding of system inertia and frequency stability in the context of low-inertia systems. To this end, this paper provides a comprehensive review of the definition, modeling, analysis, evaluation, and control for frequency stability. It commences with an exploration of inertia and frequency characteristics in low-inertia systems, followed by a novel definition of frequency stability. A summary of frequency stability modeling, analysis, and evaluation methods is then provided, along with their respective applicability in various scenarios. Additionally, the two critical factors of frequency control—energy sources at the system level and control strategies at the device level—are examined. Finally, an outlook on future research in low-inertia power systems is discussed.

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. First, two loss-of-synchronization (LOS) types are identified based on the relative power imbalance owing to the mismatch between the inertia of the IBG and SG under a fault. Second, the relative angle and frequency will jump at the moment of a fault, thus affecting the system energy. Third, the cosine damping coefficient induces a positive energy dissipation, thereby contributing to the system stability. A unified criterion for identifying the two LOS types is proposed using the energy function method. This criterion is proved to be a sufficient stability condition for addressing the effects of the jumps and cosine damping coefficient on the system stability. The new mechanisms and effectiveness of the criterion are verified based on simulation results.

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.

In a multi-voltage-source-converter (multi-VSC) system, there may be a scenario where some VSCs lose synchronization stability and others keep stable under grid faults. To describe and analyze the transient behavior of the system under such a scenario, accurate equivalent models are indispensable. In this article, a coherency-based equivalent aggregated modeling method of multi-VSC systems is developed for transient stability analysis. By investigating the factors that reflect the dynamic characteristics of VSCs, the pre-fault and post-fault q -axis voltage drops on collector impedance of each VSC are selected as the coherency identification indicators, and a coherency criterion based on these indicators is proposed. Then, by assessing the transient synchronization stability of multiple VSCs, a criterion based on electrical distances is provided to ensure that VSCs with different stability will not be divided into a coherent group. Combining these two criteria, VSCs can be divided into different coherent groups and aggregated as several equivalent VSCs. Thus, the equivalent model of the multi-VSC system can be obtained. The proposed model is verified by simulations of a multi-VSC system connected to the IEEE 39-bus power grid. Compared with the existing equivalent model, the proposed model shows advantages in precisely reflecting the power dynamics of the multi-VSC system where VSCs present different transient stability under faults. It provides an effective tool for behavior prediction and transient synchronization stability analysis of multiple VSCs.

Lyapunov's direct method plays an important role as a very commonly used analytical tool in studies on transient synchronization stability (TSS) of grid-following converters. To bridge the gap that energy-type Lyapunov function (LF) is only effective in the positive damping area and thus too conservative, a novel LF whose domain of definition includes part of negative damping area is proposed for TSS analysis of grid-following converters. Its corresponding TSS criterion and estimated region of attraction (ROA) exhibit significantly reduced yet still guaranteed conservativeness, endowing them with more practical value and higher efficiency in analytical TSS assessment works.

The operation mode of grid-forming voltage source converters (GFM-VSCs) may switch between voltage source mode (VSM) and current source mode (CSM) under some situations such as grid faults, owing to the current limitation control. During the mode-switching process, there is state transfer from the final state of the last mode to the initial state of the next mode, which impacts the transient synchronization stability (TSS) of GFM-VSCs. This paper primarily focuses on analyzing and improving the TSS of GFM-VSCs by considering the effect of state transfer. A novel transient instability mechanism is revealed through the existence analysis of equilibrium points. It clarifies that the state transfer may cause the operating trajectory during faults to bypass the stable equilibrium point in CSM before diverging to the next cycle, thereby resulting in transient synchronization instability. Besides, to further analyze the TSS of mode-switched VSCs considering the dynamics during faults, multiple Lyapunov functions are adopted to derive the TSS criteria and boundaries. It has been identified that lowering the minimum critical current and adjusting the saturated current phase in accordance with virtual power angle (VPA) dynamics can enhance the TSS. Therefore, a VPA feedback-based current limiting strategy is proposed to safeguard GFM-VSCs against overcurrent and ensure the TSS. The validity of the new transient instability mechanism and the efficacy of the proposed strategy are confirmed through simulations of a GFM-VSC connected to an IEEE 39-bus power grid and hardware-in-the-loop experiments.