Xia Chen

The increasing integration of power electronic converters in modern power systems has significantly accentuated stability concerns. Among these, the synchronization stability challenges of grid-following (GFL) converters under weak grid conditions are particularly prominent. It has been demonstrated that the introduction of grid-forming (GFM) converters can effectively mitigate these stability issues. However, a theoretical understanding of how various GFM control strategies impact GFL stability is still incomplete, making the small-signal stability assessment of hybrid GFL-GFM systems a significant challenge. Moreover, the reactive power control (RPc) of GFM converters plays a crucial role in maintaining system stability. Research on how RPC affects its support capability remains unexplored. To address these issues, this paper proposes a stability assessment method for GFL-GFM hybrid systems. By employing modal decoupling and matrix perturbation methods, a decoupled low-order system is developed for characterizing the stability of the original system, thereby reducing the analytical complexity and offering physical insights into the supporting capabilities of GFM converters. Furthermore, an enhanced RPC structure for GFM converters is proposed to improve their grid-support capability within different frequency ranges. The effectiveness of the proposed method is validated on the IEEE 39-Node system.

With the rapid development of renewable energy, power systems are witnessing an increasing penetration of power converters. Currently, converters widely employ grid-following (GFL) control, while grid-forming (GFM) control is considered as a grid-friendly strategy to accommodate large-scale power converters. Hence, GFL-GFM hybrid systems (GGHSs) will prevail in the future. However, such systems may encounter small-signal stability issues across a wide frequency range due to the closed-loop interactions among the power network, GFL, and GFM converters. To address this problem, this paper first introduces the concept of grid current strength, a metric that quantitatively characterizes the stability of GFM converters, thereby extending the traditional grid voltage strength. We derive low-dimensional GFL and GFM subsystems to represent the dynamics and stability of the original high-dimensional system. On this basis, a two-dimensional strength index is proposed to quantitatively assess the stability margin of the GGHSs in a tractable manner. Based on this index, we analyze the impact of the proportions of GFL and GFM converters on system stability and demonstrate that inappropriate proportions may lead to instability. Finally, we propose a method to rapidly identify the critical GFL or GFM devices that dominate the system stability using participation factors. On this basis, we introduce stability enhancement strategies for GGHSs in both the system planning and system operational stages. The efficacy of the proposed assessment and enhancement methods is validated through time-domain simulations and hardware-in-the-loop (HIL) tests.

This article investigates the load frequency control (LFC) problem of multiarea power systems subject to network-induced delays. High-control cost is a drawback of conventional LFC algorithms, which is not conducive to energy saving requirements. This article proposes an impulsive control algorithm that acquires area control error information and implements control signals only at discrete instants to save control cost. Considering time delays induced by network communication, a novel system-augmentation-based modeling scheme is provided for the proposed LFC algorithms, such that the closed-loop system is augmented to a delay-free impulse-switching system. Moreover, switching-based discontinuous Lyapunov function is designed to capture the influence of delayed impulses on the stability and $\boldsymbol{H_{\infty}}$ performance of the interconnected power systems. Sufficient conditions for the exponential stability and $\boldsymbol{H_{\infty}}$ performance of the closed-loop system are presented. Further, impulse-delay-dependent design criteria guaranteeing exponential convergence and $\boldsymbol{H_{\infty}}$ performance are derived consequently. Low-energy consumption and effectiveness of the provided scheme are verified by numerical examples and a hardware-in-loop experiment test.

Battery energy storage system (BESS) plays a crucial role in the integration of renewable energy by balancing supply and demand, providing frequency regulation, and supporting voltage stability. However, for multiple grid-connected BESS systems, traditional power control methods, which focus on the dynamics of individual BESS, often neglect the interactions between BESSs. It results in idle power capacity and slow response times during transient events. To address these issues, a distributed cooperative control strategy based on a consensus algorithm has been proposed to improve the overall system’s response speed and ensure steady-state power sharing according to the state of charge (SOC). However, few researchers have conducted the stability analysis of multiple BESSs under the distributed control scheme, and the interaction has not been thoroughly investigated. This paper addresses this gap by developing an impedance model for multiple BESSs by incorporating a secondary control layer. The modeling accuracy is validated through EMT simulations. Using Nyquist stability criterion, the paper compares the stability of BESSs with distributed cooperative control to traditional power control methods, demonstrating the advantages of the proposed approach. Additionally, the impact of cooperative control parameters and communication faults on system stability is analyzed. The time-domain simulations confirm the accuracy of the stability analysis.

A hybrid offshore rectifier system, consisting of a Diode Rectifier and a Modular Multilevel Converter (DR-MMC), is attracting attention in High Voltage Direct Current (HVDC) transmission systems, as it combines the high efficiency and low cost of the DR with the control capabilities of the MMC. It is also applied to offshore wind farms (OWF). The application of offshore DR-MMC rectifiers necessitates a thorough evaluation of system stability. A small-signal DC impedance model for the DR-MMC rectifier is proposed. It includes the impact of the OWF, transformers, AC cable and the DR-MMC rectifier. This model is then used for developing the AC impedance model as seen from the onshore converter station. The OWF is represented using an aggregated model for simplification. Utilizing these small-signal impedance models, the paper explores the effects of the OWF’s control parameter, control parameters of the DR-MMC, and MMC/DR rated capacity ratio on the AC impedance of onshore converter and the onshore stability. Moreover, this paper compares the stability of HVDC systems utilizing the DR-MMC and the conventional MMC, and evaluates the validity of modeling the offshore system using an ideal source in stability analyses of the onshore system. It finds that the DR-MMC offers better stability than the conventional MMC, and modeling the offshore system as a simple DC voltage source leads to inaccurate onshore stability assessments.

Considering the operating mode of power system under weak grid conditions is closely related to the oscillation instability of grid-following (GFL) renewable energy sources (RES), and grid-forming (GFM) energy storage (ES) is one of the effective means to improve GFL stability, this paper further extends the optimal operation objective for ES to ensure small-signal synchronous stability of power system. Firstly, considering the synchronous stability differences of devices at different spatial locations, based on modal decoupling methods, the oscillation stability operating domain dominated by the synchronous loop of multiple GFL RES units (GFLs) is analyzed. Secondly, based on frequency domain analysis and modal decoupling methods, the oscillation stability operating domain of system consisting of multi-point layout GFM ES units(GFMs) and multiple GFLs under different frequency regulation parameters is quantified. Subsequently, considering the frequency regulation reserve and synchronous oscillation stability operating domain constraints, a comprehensive optimal operation model of GFM ES frequency regulation parameters and device output power is proposed. Finally, the effectiveness of the proposed oscillation stability operating domain and optimal operation model is verified in a modified IEEE 39-bus system.

Numerous small-scale energy storage systems (ESSs) are distributed throughout the power system and have the potential to be aggregated for power regulation. In this context, distributed power tracking control with aggregated capacity estimation is proposed for distributed ESSs. The proposed distributed control is free of the measure for total power while achieving accurate total power tracking. In addition, power allocation is performed among ESSs considering their types, SoC levels, and power/energy capacities. To estimate the dispatched power ranges of ESSs, an event-triggered distributed power capacity estimator is proposed with a relatively low communication burden. The power reference for the power tracking control is limited and therefore the reliable operation of the control is ensured. The proposed control and estimator are implemented in a discrete-time fashion and only rely on the sparse communication requirement. A detailed theoretical analysis is conducted to verify the equilibrium and convergence of the proposed approaches. Simulations based on the IEEE 33-bus system demonstrate the effectiveness of the proposed distributed power tracking control and the capacity estimator.