Di Wu’s research while affiliated with Hangzhou University and other places

Storage energies (SEs) have the flexibility of bi-directional power regulation, so they have been integrated into the power system via power electronic converters to reduce the power fluctuations introduced by converter-based renewables (CBRs), such as solar and wind. However, converter-based SEs may interact with CBRs through the power network, which increases the complexity and difficulty of the phase-lock-loop (PLL)-induced small-signal stability analysis in multi-CBR systems, especially in weak grids. In this context, it remains unclear how the placement of SEs influences the PLL-induced small-signal stability, particularly when the SEs absorb active power from grids. To fill this gap, this paper analyzes the impact of SEs on the PLL-induced small-signal stability in multi-converter systems from the perspective of grid strength and proposes a method for optimally placing SEs to enhance the small-signal stability. First, the analytic results reveal that when the SEs absorb active power from the power grid, they can enhance grid strength and thus the PLL-induced small-signal stability; moreover, the degree of system stability improvement depends on the location of SE placement. On this foundation, a grid-strength-based method is proposed for the PLL-induced small-signal stability improvement via SE placements. The proposed method is validated based on three test systems. This paper provides an effective way of better understanding the interaction among SEs and CBRs through power network, and coordinating the placements of SEs in future converter-dominated power systems.

The increasing integration of renewable resources via power electronic inverters is shifting a modern power system toward a 100% inverter-based power system (IBPS). To maintain the stable operation of a 100% IBPS, it is important to identify the small-signal stability issues resulting from the interaction between the power network and inverter-based apparatuses. While grid strength assessment is a useful tool for quickly identifying the small-signal stability issues, the existing methods are not applicable to the 100% IBPS dominated by grid-following (GFL) and grid-forming (GFM) inverters. To fill this gap, the paper proposes a method for assessing small-signal grid strength of the 100% IBPS in order to quickly identify the small-signal stability issues from the perspective of grid strength. First, we formulate a multi-inverter system modeling for the small-signal stability analysis of the 100% IBPS. Then, based on the analysis results, an index is proposed for quantifying grid strength, and its threshold is also analytically defined to characterize the system stability boundary. Also, an analytical expression is derived to determine the threshold and analyze the impacts of GFL and GFM inverters on the stability boundary. With the defined index and its threshold, our method is proposed and then validated on a modified IEEE 39-bus system.

The increasing integration of renewable resources via power electronic inverters is shifting a modern power system toward a 100% inverter-based power system (IBPS). To maintain the stable operation of a 100% IBPS, it is important to identify the small-signal stability issues resulting from the interaction between the power network and inverter-based apparatuses. While grid strength assessment is a useful tool for quickly identifying the small-signal stability issues, the existing methods are not applicable to the 100% IBPS dominated by grid-following (GFL) and grid-forming (GFM) inverters. To fill this gap, the paper proposes a method for assessing small-signal grid strength of the 100% IBPS in order to quickly identify the small-signal stability issues from the perspective of grid strength. First, we formulate a multi-inverter system modeling for the small-signal stability analysis of the 100% IBPS. Then, based on the analysis results, an index is proposed for quantifying grid strength, and its threshold is also analytically defined to characterize the system stability boundary. Also, an analytical expression is derived to determine the threshold and analyze the impacts of GFL and GFM inverters on the stability boundary. With the defined index and its threshold, our method is proposed and then validated on a modified IEEE 39-bus system.

The increasing level of harmonics in the power grid, driven by a substantial presence of coupled inverter-based energy resources (IBRs), poses a new challenge to power grid transient stability. This paper presents the findings from experiments and analytical studies on the impact of the topological configuration of coupled IBRs on the level of power flow harmonics in a distribution grid: (i) our findings report that the impact of grid topology on harmonics is nonlinear, which is in contrast to the common perception that the power grid operates as a large linear low-pass filter for harmonics; (ii) importantly, this study highlights that the influence of the topological configuration of inverters on the reduction of system-level harmonics is more substantial than the effect of line impedance, emphasizing the significance of grid topological configuration; (iii) furthermore, the observed reduction in harmonics is attributed to a harmonic cancellation effect achieved through self-compensation by all the coupled inverters without affecting the active power flow in the power grid. These findings propose a new approach to limit the penetration of complex IBR harmonics in the power grid from a system-wide perspective. This approach significantly differs from the component-level or localized solutions used today, such as inverter control, power filtering, and transformer tap changes.

The increasing integration of renewable resources via power electronic inverters is shifting a modern power system toward a 100% inverter-based power system (IBPS). To maintain the stable operation of a 100% IBPS, it is important to identify the small-signal stability issues resulting from the interaction between the power network and inverter-based apparatuses. While grid strength assessment is a useful tool for quickly identifying the stability issues, the existing methods are not applicable to the 100% IBPS dominated by grid-following (GFL) and grid-forming (GFM) inverters. To fill this gap, the paper proposes a method for fast assessing grid strength in terms of small-signal stability in the 100% IBPS. First, we formulate a multi-inverter system modeling for the small-signal stability analysis of the 100% IBPS. Then, based on the analysis results, an index is proposed for quantifying grid strength, and its threshold is also defined to characterize the system stability boundary. Also, an analytical expression is derived to determine the threshold and analyze the impacts of GFL and GFM inverters on the stability boundary. With the defined index and its threshold, our method is proposed and then validated on a modified IEEE 39-bus system.

The complexity of modern power grids, caused by integrating renewable energy sources, especially inverter-based resources, presents a significant challenge to grid operation and planning, since linear models are unable to capture the complex nonlinear dynamics of power systems with coupled muti-scale dynamics, and it necessitate an alternative approach utilizing more advanced and data-driven algorithms to improve modeling accuracy and system optimization. This study employs the sparse identification of nonlinear dynamics method by leveraging compressed sensing and sparse modeling principles, offering robustness and the potential for generalization, allowing for identifying key dynamical features with relatively few measurements, and providing deeper theoretical understanding in the field of power system analysis. Taking advantage of the this method in recognizing the active terms (first and high order) in the system’s governing equation, this paper also introduces the novel Volterra-based nonlinearity index to characterize system-level nonlinearity. The distinction of dynamics into first-order linearizable terms, second-order nonlinear dynamics, and third-order noise is adopted to clearly show the intricacy of power systems. The findings demonstrate a fundamental shift in system dynamics as power sources transit to inverter-based resources, revealing system-level (second-order) nonlinearity compared to module-level (first order) nonlinearity in conventional synchronous generators. The proposed index quantifies nonlinear-to-linear relationships, enriching our comprehension of power system behavior and offering a tool for distinguishing between different nonlinearities and visualizing their distinct patterns through the profile of the proposed index.

Integrating static synchronous compensators (STATCOMs) in a multiple inverter-based-resource (IBR) system for voltage support can deteriorate sub/sup-synchronous oscillation issues caused by the interaction among IBRs and power networks, especially in low short-circuit-level grids. However, it is still challenging to fully understand the impact mechanism of STATCOMs on IBR-induced oscillation issues and to effectively design STATCOMs' control for dampening these oscillation issues in a multi-IBR system due to complex system dynamics and varying operating conditions. To tackle these challenges, this paper proposes a novel method to reveal how STATCOMs influence IBR-induced oscillation issues in a multi-IBR system from the viewpoint of grid strength, which can consider varying operating conditions. Furthermore, we investigate the robust small-signal stability of the multi-IBR system with STATCOMs by designing STATCOMs' control parameters to ensure the robust small-signal stability of multiple subsystems under critical operating conditions. This avoids exhaustive studies on many operating conditions with detailed system models. The proposed methods are validated on a modified IEEE 39-node test system. This paper provides an effective way of better understanding the interaction among diversified devices through power network, and coordinating their controls to ensure the system's robust small-signal stability in modern power systems integrated with large-scale power converters.