Distribution networks may exhibit series/shunt resonance at specific frequencies, which can significantly amplify background harmonics injected by nonlinear loads, thereby threatening system power quality and equipment security. Active power filters represent a mainstream solution for harmonic mitigation in low-voltage distribution networks. However, their configuration difficulty and cost increase substantially when applied to medium-voltage distribution networks. Recently a growing number of renewable energy generation systems and energy storage units have been integrated into medium-voltage distribution networks via modular multilevel converters (MMCs). A novel approach to harmonic mitigation is to adjust MMC control to suppress harmonics without incurring additional primary equipment investments. So the paper proposes a harmonic voltage mitigation strategy for MMCs based on multiplexed quasi-proportional resonant (QPR) controls. First, the fundamental voltage component of the point of common coupling (PCC) is transformed into a DC component through d-q transformation, enabling real-time harmonic voltage monitoring via low-pass filters that separate fundamental and harmonic components. Second, a quasi-proportional resonant controller is developed to provide high gain. and approximately zero phase deviation at targeted harmonic frequencies, ensuring precise harmonic mitigation. By implementing multiple parallel control channels, coordinated mitigation of various targeted harmonics is realized. Finally, a series of simulations carried out with Matlab/Simulink confirm the validity of the presented control strategy.

Extreme disasters frequently trigger system-level cascading failures that result in large-scale blackouts, subsequently inducing severe socio-economic cascading consequences. Existing emergency control models often fail to conduct a refined assessment of the Value of Lost Load (VoLL) at different load nodes, rendering load shedding strategies insufficiently targeted and selective, which in turn leads to an unnecessary amplification of outage losses. To address these issues, this paper constructs quantitative calculation formulas for both System-level Economic Losses and Socio-Economic VoLL, and further derives a practical refined formulation of the total outage losses suitable for engineering applications. On this basis, by coordinating generation re-dispatch and load shedding measures, a mixed-integer optimization model for emergency control based on the refined assessment of outage losses is established. Simulation results demonstrate that the proposed model can effectively mitigate the evolution of system cascading failures and significantly avoid the amplification of outage losses induced by traditional uniform VoLL assessments.

The systems considering current limiting in transient stability analysis manifest as switching systems, yet their switching mechanism is not clear. To address this issue, this paper establishes a dual DC voltage synchronous control (DVSC) system employing Current Saturation Algorithm (CSA) for current limiting. This paper derives the four operating states and the switching boundaries. The stable equilibrium points of the system under different switching states are calculated, and the conditions for the system to stabilize at its original stable equilibrium point after being disturbed are explained. Finally, simulations verify the correctness and effectiveness of the proposed theory.

Grid-tied virtual synchronous generators (VSGs) face significant uncertainties arising from encapsulated control parameters, volatile renewable injections, and shifting operating conditions. Conventional deterministic stability analysis fails to ensure conservatism under such perturbations. This paper proposes a practical stability framework that focuses on bounded performance within a prescribed error region rather than strict asymptotic convergence. First, a perturbed system model incorporating parameter errors, power fluctuations, and condition variations is established, which is approximated by a deterministic comparison system. Using the unstable equilibrium point of this comparison system, a closed-form critical energy that provides a provably conservative estimate of the region of attraction is derived. Furthermore, the limit cycle of the comparison system is characterized via the Poincaré-Bendixson theorem. Finally, an analytical method is developed to determine the ultimate bound of the perturbed system, thereby delineating the motion range of VSGs under multiple uncertainties.

This paper proposes a general power flow modeling method for power systems with control mode switching. Due to the diverse control strategies of power electronic device, a single converter-interfaced generator (CIG) may operate under multiple modes. Traditional Newton-based solvers typically rely on hard switching logic, which can lead to oscillations and non-convergence. To address this, This paper first reviews and analyzes two mainstream Newton-based iteration frameworks and reveal the mechanisms behind their convergence failures. Then a unified multi-mode modeling approach is presented. A set of binary variables are used to represent multiply mode. Based on the voltage and reactive power limits, the constraints are reformulated as a set of linear constraints exactly. Building on this, second-order cone (SOC) relaxation is applied to recast the power flow problem as a convex optimization model. This optimization-based framework not only facilitates computation but also allows tuning the objective function to reflect different operational preferences. An adaptive tightening strategy are developed to ensure the relaxed solutions can be recovered to physically feasible power flow solutions. Case studies on 33-bus and 118-bus systems demonstrate that the proposed method matches the Newton method when the latter converges, and still obtains feasible solutions when traditional algorithms oscillate, highlighting its robustness and applicability.

The new power system serves as a key carrier for achieving the national goals of carbon peaking and carbon neutrality.This paper focuses on the structural formation of the new power system and analyzes the critical challenges encountered during its development from five perspectives:the physical grid architecture and transmission forms,the mechanisms for power-energy balance and steady-state construction,the voltage-synchronization-frequency-oscillation stability mechanisms,the system fault defense and emergency recovery mechanism,and the electricity-carbon coordinated market mechanism.Furthermore,based on a comprehensive assessment of domestic and international research progress and development trends,this study explores the underlying scientific issues and future technological pathways.Finally,the paper summarizes the key and disruptive technologies essential for shaping the new power system and proposes relevant policy recommendations,aiming to advance the construction of a clean,low-carbon,secure,adequate,economically efficient,supply-demand coordinated,and flexible-intelligent power system,thereby contributing to national energy security.

This paper addresses issues of large power fluctuations and low voltage levels in 100% renewable energy power systems without synchronous power support. The control mechanism of energy storage converters is analyzed, and a reactive power optimization model is established, considering energy storage control and DC transmission characteristics. A reactive power optimization method based on energy storage converter control is proposed. This method utilizes energy storage converter control to optimize the system's reactive power, enhancing the voltage regulation capability while participating in active power balance and ensuring economical operation. Additionally, a mathematical model based on DC channel operational characteristics is established to meet the demands for DC transmission in renewable energy systems. The optimization results can be applied to DC transmission curve planning. Finally, a case study is conducted on a county-level power grid in Northwest China without synchronous power support. The active and reactive power decision variables at each node in the grid are solved using the Yalmip platform in MATLAB, verifying the effectiveness of the proposed method.

This paper studies the synchronization stability of a bundled power export system where a large-scale solar plant and a hydropower station are interconnected and delivered to the receiving grid through a VSC-HVDC link. Under several simplifying assumptions, the system is modeled for both normal operation and a severe three-phase fault, with the converter current-limiting behavior explicitly included because it reshapes the fault-time power transfer and directly impacts the rotor-angle dynamics of the hydropower synchronous generator. Using Thevenin’s theorem, a fault-time equivalent model is reconstructed to capture the interaction among the sending-end converter, AC network, and generator under current saturation, and the saturated-current control is parameterized by a current saturation angle that determines the active/reactive current allocation during faults. Based on the resulting simplified model, the optimal saturation angle is obtained by formulating a constrained optimization problem and solving it via the Lagrange multiplier method in combination with the equal-area criterion, aiming to maximize the transient synchronizing margin and avoid loss of synchronism. Numerical simulations validate that the derived saturation angle provides superior synchronization-stability performance compared with conventional current-limiting settings, thereby enhancing fault ride-through capability for hydro–PV bundled VSC-HVDC transmission systems.

The rising load density and increasingly uneven spatial distribution in urban AC grids are undermining power flow controllability. Congestion in AC corridors and short-circuit limits have become dominant constraints. To address these challenges, an optimized planning approach for embedded voltage source converter based high voltage direct current (VSC-HVDC) is developed, by adding converters and DC lines to form a hybrid AC/DC architecture, which enables rapid inter-area power flow redistribution and mitigates AC bottlenecks. A mixed-integer linear programming (MILP) model minimizes total planning-period cost by unifying DC terminal siting, converter sizing, generator dispatch, load-shedding decisions, and line power flow control within a single decision framework. Nonconvex couplings between DC terminal siting and DC line connectivity are handled via linear relaxation. The total-cost objective integrates capital expenditures of converters and DC lines with operating costs from generation and load shedding. Case studies on the IEEE 30-bus system compute planning solutions at several discrete load levels and demonstrate that optimized DC configurations effectively relieve AC congestion, ensure secure operation, and minimize planning-period cost. These findings highlight the potential of embedded VSC-HVDC for precise power flow steering and improved techno-economic performance, offering both theoretical support and a practical pathway for flexible interconnection upgrades in high-load-density urban grids.

As modern power systems expand, short-circuit (SC) currents increasingly approach equipment ratings, making SC constraints a vital security prerequisite for grid planning. However, integrating these constraints into hybrid AC/DC transmission expansion planning (TEP) poses significant mathematical challenges due to inherent nonlinearities. To address these issues, this paper proposes a multi-period robust SC-constrained TEP (SC-TEP) framework. Specifically, a linear approximation method based on the Woodbury matrix identity and the incremental method is developed to convert the nonlinear SC constraints, which account for both candidate AC lines and HVDC systems, into tractable mixed-integer linear programming (MILP) expressions. These linearized SC constraints are then embedded into the robust co-expansion planning model, which is solved using the Column-and-Constraint Generation (C&CG) algorithm. Numerical experiments on the modified IEEE 24bus and 118-bus systems validate the model's effectiveness. Notably, these SC constraints shrink the feasible investment space, accelerating computation to even outperform conventional robust models without SC limits.

In order to address the synchronization stability of grid-forming converters considering current limiting constraints，a switching dynamic model of the system of a single converter connected to the grid based on direct current voltage synchronization control （DVSC） strategy considering current limiting is established in this paper. Grid-forming converters without current limiting constraints will only operate in the constant voltage control mode after fault clearance，while those with current limiting constraints may stabilize in different modes like constant voltage control mode or constant current control mode. Therefore，the transient stability of studied model is analyzed using the segmental equal area criteria （SEAC） in this paper and the analytical expression of the critical clearing angle （CCA） is derived，revealing the mechanism by which the saturation current angle affects the stability of the system. In addition，a setting method of current saturation angle that can enhance the transient stability margin is proposed in this paper，which not only prevents the system from being locked into the current limiting mode after fault clearance but also achieves the maximum critical clearing angle and deceleration area. Finally，simulations based on MATLAB/Simulink are carried out to verify the correctness of the expression of critical clearing angle and setting method of current saturation angle.

As the proportion of renewable energy in the power system continues to increase, the inertia level of the system gradually decreases. Utilizing energy storage to provide inertia and primary frequency support for photovoltaic generation has become an effective means to address frequency stability issues. Parameters for frequency support that are too small cannot fully utilize the frequency modulation capability of optical energy storage systems, while parameters that are too large will lead to active power output limits of inverters during transient processes. In response to this, the paper proposes a method for optimizing the inertia and primary frequency modulation parameters of optical energy storage systems, taking capacity limitations into account. First, based on the control equations of the grid-type optical energy storage system, the analytical relationship between frequency deviation, frequency deadband, primary frequency modulation coefficient, and inverter output is established to derive the feasible boundaries of the primary frequency modulation coefficient to ensure that the inverter and energy storage output do not exceed limitations. Secondly, with the constraints of optical energy storage system capacity limitations and the dynamic equations participating in transient frequency support, an optimization model for maximizing the frequency nadir of the optical energy storage system is established to determine the optimal virtual inertia coefficient. Finally, simulation analysis verifies the effectiveness of the proposed frequency support parameter optimization method.

This paper investigates the transient stability of Grid-Forming (GFM) Permanent Magnet Synchronous Generator (PMSG) systems during grid faults. An analysis demonstrates how a fixed saturated current angle can trap the system in undesirable operating points, while reactive power coupling can degrade performance. Both factors pose a risk of turbine overspeed and instability. To overcome these vulnerabilities, a dual-mechanism control strategy is proposed, featuring an adaptive saturated current angle control that, unlike conventional fixed-angle methods, which risk creating Current Limiting Control (CLC) equilibrium points, dynamically aligns the current vector with the grid voltage to guarantee a stable post-fault trajectory. The effectiveness of the proposed strategy is validated through time-domain simulations in MATLAB/Simulink. The results show that the proposed control not only prevents overspeed trip failures seen in conventional methods but also reduces post-fault recovery time by over 60% and significantly improves system damping, ensuring robust fault ride-through and enhancing overall system stability.

This paper investigates a grid-forming converter-based power system considering current-limiting switching characteristics and establishes a phase-angle switching model for the system based on state variables. Using the segmented equal-area criterion, the traditional stabilization strategy for saturated current angle parameters in current-limiting control is analyzed. It is found that under this strategy, regardless of the configuration of the saturated current angle, the system always has unstable equilibrium points (UEPs) in the current-limiting mode. Once the system trajectory crosses the UEPs in this mode, the system is at risk of instability. To address this issue, a rotating saturated current angle control strategy is proposed, enabling the system to maintain constant power output in the current-limiting mode. Under this strategy, no UEPs exist in the system trajectory, thus ensuring stability.

This paper investigates the synchronization stability of one multiple virtual synchronous generators (VSGs) embedded power system with the current saturation and limits of frequency. The dynamic model of the multiple VSGs integrated system with controller limits is built. Accordingly, the transient energy function (TEF) for the autonomous system without touching controller limits is established, and the stability region is determined via the method of the closet unstable equilibrium point (UEP). Further, it is found that the system leaves away from the bounds of frequency at several leaving bound points . Among these, there exists two special ones with the minimal transient energy, and they are fixed irrespective of the switching times. Finally, one stabilizing condition is proposed that only if the energy of two special leaving bound points is less than the system critical energy, the global stability is ensured. The system critical energy is determined by the minimum of two values, namely, the method by closet UEP, and the maximum potential energy to ensure the system does not enter the mode of current limiting control (CLC). The numerical simulations of 9-buses and 39-buses power systems have verified the correctness of the proposed analysis.

In the transient stability analysis,the grid-forming converter considering current limiting will appear as a switching system,and its switching mechanism is still unclear.Aiming at a dual power synchronization loop(PSL)system considering current limiting,where the current limiting measure uses a current saturation algorithm.The switching boundaries of the dual-machine system in terms of phase angle are derived.It is found that the dual-machine system considering current limiting has four working states and four switching boundaries within a phase angle period,and the positions of two switching boundaries are related to the saturation current angle settings of the two converters.Based on this,the transient stability of the studied system is further analyzed,obtaining the stable equilibrium points of the four working states.The stable equilibrium points are configured by changing the current saturation angle set values of the two converters,which can avoid the situation that one converter is locked in the current limiting control mode.The simula-tion verifies the correctness and effectiveness of the proposed theory.

This paper provides a new insight for the transient stability of the grid-connected virtual synchronous generator (VSG) with multiple controller limits. The dynamic model of the grid-connected VSG with the current saturation and limits of frequency is firstly established. Accordingly, one conservative stability region of the system without limits of frequency is derived via the Lyapunov method, and the critical energy is determined by the potential energy of the intersection point of the right switching line and x-axis. Furthermore, it is found that the trajectories of the system with limits of frequency leave from the bounds of the frequency at two fixed switching points (FSPs). As a result, the global stabilizing condition is derived that the Lyapunov function values of the two FSPs are less than the defined critical energy of system with no limits of frequency based on the common Lyapunov function theory.

Low-Earth-orbit (LEO) satellite communication systems face challenges due to high satellite mobility, which hinders the reliable acquisition of instantaneous channel state information at the transmitter (CSIT) and subsequently degrades multi-user transmission performance. This paper investigates a downlink multi-user multi-antenna system, and tackles the above challenges by introducing orthogonal time frequency space (OTFS) modulation and rate-splitting multiple access (RSMA) transmission. Specifically, OTFS enables stable characterization of time-varying channels by representing them in the delay-Doppler domain. However, realistic propagation introduces various inter-symbol and inter-user interference due to non-orthogonal yet practical rectangular pulse shaping, fractional delays, Doppler shifts, and imperfect (statistical) CSIT. In this context, RSMA offers promising robustness for interference mitigation and CSIT imperfections, and hence is integrated with OTFS to provide a comprehensive solution. A compact cross-domain input-output relationship for RSMA-OTFS is established, and an ergodic sum-rate maximization problem is formulated and solved using a weighted minimum mean-square-error based alternating optimization algorithm that does not depend on channel sparsity. Simulation results reveal that the considered practical propagation effects significantly degrade performance if unaddressed. Furthermore, the RSMA-OTFS scheme demonstrates improved ergodic sum-rate and robustness against CSIT uncertainty across various user deployments and CSIT qualities.

The grid converter usually needs to take current limiting measures to suppress overcurrent, which will make the power system become a switching system, and the switching mechanism of the switching system is not clear. In this paper, for dual-machine systems considering current limiting, the switching boundary of two converters in constant voltage control mode and current limiting control mode is deduced by circuit principle, and the switching mechanism of the system is defined. It is found that when the current saturation algorithm is used to limit the current, the setting of the saturation current angle will affect the switching boundary and the stable equilibrium point of two converters in the current limiting mode. Finally, by configuring the saturation current angle and changing the stable equilibrium point position of the converter in current limiting mode, the converter is not locked in current limiting mode, and the theory is verified by simulation.

Current limit control can protect converters from overcurrent damage during fault transient but may simultaneously deteriorate the transient synchronization stability. This paper focuses on the transient stability analysis and enhancement of the grid-forming renewable energy system integrated via a voltage source converter based high-voltage direct current (VSC-HVDC) link, considering current limit. First, the switched dynamic model of the studied system under current limit is developed. It is found that switching between voltage control mode and current restricted control mode occurs when the power angle difference between the renewable energy inverter and the VSC-HVDC rectifier reaches a specific value. Accordingly, the transient stability mechanism of the studied system is analyzed using the system's power-angle curve, and the transient stability criterion is derived from the segmented equal-area criterion. On this basis, an optimal tuning strategy for the saturation current angle is further proposed to enhance the transient stability of the system. Simulation results verify the accuracy of the derived stability criterion and the effectiveness of the proposed parameter-tuning strategy.