Search Results (349)

Modern power grids with high renewable energy penetration are vulnerable to fast voltage disturbances caused by grid faults. Among these, voltage sags are critical because they develop within milliseconds and require rapid reactive current support to maintain grid stability and power reliability. Reinforcement learning has previously shown potential for reactive current injection control during voltage sag events due to its fast response and adaptability to changing system conditions. However, existing approaches rely on separate policies for specific subsets of the operating space, which limits their ability to provide optimal actions when the system operates across broader or combined state regions. To address this limitation, this paper proposes a unified Soft Actor–Critic (SAC) target policy trained over the full state and action space by integrating multi-source transfer learning with potential-based reward shaping approach. Results show that the proposed multi-source transfer approach enables the target agent to converge faster and reach a higher reward solution than the baseline SAC and single-source transfer approach. The trained policy also improved prediction accuracy, achieving reactive-current errors below 0.2 A with respect to the ground-truth reference generated through extensive simulations over the full observation and action space. The reference follows the grid-code requirement for minimum reactive current injection during faults and provides a benchmark for evaluating prediction accuracy. This can help distributed generation sources respond more effectively during severe perturbations such as voltage sags, support voltage recovery, and reduce the risk of cascaded disconnections that could lead to unwanted blackouts. Additionally, the inference execution time is also sufficiently fast to satisfy the response-time requirement of voltage sag events, confirming the real-time feasibility of the proposed controller. Full article

Modern power systems are becoming increasingly complex due to the rapid integration of renewable energy sources, the widespread use of nonlinear power-electronic devices, and the deployment of microgrids operating in parallel with conventional power grids. These evolving conditions intensify the occurrence of diverse and highly complex power quality disturbances (PQDs), demanding accurate and computationally efficient monitoring strategies. This paper presents a novel multi-stage hierarchical framework for PQD detection and classification, comprising an initial training stage with a dedicated 1D Convolutional Neural Network (1D-CNN), a transfer learning stage, and a subsequent fine-tuning stage. The proposed approach operates directly on raw voltage waveforms, eliminating the need for any signal preprocessing, as the CNN performs internal feature extraction. The framework is evaluated using a comprehensive dataset that includes synthetic signals, Matlab/Simulink (version R2022a) time-domain simulations, and real voltage sag events. Additionally, up to 29 types of disturbances, including complex multi-event combinations defined by the IEEE-1159 Standard, are generated using the PQ-SyDa toolbox. The proposed model achieves an F1-score of 97.8% using a three-cycle analysis window and further improves to 98.86% when five cycles are used. These results highlight the robustness and generalization capability of the proposed approach for the real-time PQD monitoring task in modern electrical networks. Full article

Due to the influence of the control strategy of the battery energy storage station (BESS), the degree of voltage sag, and the battery state of charge (SOC), the fault current characteristics of the BESS outlet line differ significantly. Traditional overcurrent protection faces the risk of failure to operate. To evaluate the operational performance of overcurrent protection of outlet line connecting BESS, this work first analyzes the topological structure and control strategy of BESS and further investigates the fault current characteristics of its outlet line. Based on this, the operational performance of overcurrent protection relay is studied. In addition, an overcurrent protection method of outlet line connecting BESS considering the battery SOC is proposed. By calculating and setting the SOC boundary, reliable protection of outlet line within different SOC intervals is achieved. Finally, a grid-connected model of BESS is built based on an electromagnetic transient simulation software to verify the operational characteristics of the proposed method. Full article

This paper presents detailed small-signal modeling and modal analysis of a 1.5 MW grid-connected doubly fed induction generator (DFIG) wind turbine. A full nonlinear model capturing stator, rotor, and grid-side converter dynamics, DC-link voltage behavior, and the wind-turbine electromechanical subsystem is developed in the synchronously rotating d-q frame and linearized around a realistic steady-state operating point. The resulting state-space representation is utilized to investigate the intrinsic dynamic characteristics of the DFIG through eigenvalue analysis, modal classification, and participation factor evaluation. The results show that the open-loop DFIG contains a weakly damped electrical mode, a slowly growing unstable mode, and a near-integrator mode linked to the DC-link voltage, all of which strongly influence system behavior under disturbances. Parameter-sensitivity studies reveal how rotor speed, stator voltage, and rotor resistance affect the dominant modes, highlighting significant deterioration under low-voltage and low-speed operating conditions. Time-domain small-signal responses to temporary voltage sags further expose the vulnerability of DC-link voltage and power outputs when no coordinated control is applied. Overall, the study establishes a rigorous dynamic baseline for DFIG systems and provides the foundational insight needed for a follow-up paper focused on advanced damping and robustness-enhancing controllers. Full article

The review presents a comprehensive review of the influence of power quality indicators and operating conditions at industrial enterprises on the technical condition and reliability of electrical equipment. Harmonic distortion, voltage fluctuations and sags, load surges, overvoltages, and voltage unbalance are considered factors that increase thermal, electrical, and mechanical stresses in transformers, induction motors, cable lines, and overhead power lines. It is shown that these disturbances can increase RMS currents, additional losses, hot-spot temperature, vibration, and insulation aging rate, reducing equipment service life and increasing failure probability. The review links power quality disturbances with thermal aging models, remaining useful life assessment, and probabilistic reliability models, including the Weibull distribution. It is established that a correct remaining service life assessment requires considering not only individual disturbances but also the combined influence of voltage and current quality, load conditions, ambient temperature, and humidity. Particular attention is paid to modern monitoring and forecasting technologies, including IoT systems, multi-agent models, machine learning, and predictive diagnostics. These technologies enable the transition from scheduled maintenance to continuous multiparameter monitoring. A structure for quantitative risk assessment and practical recommendations for predictive maintenance of industrial electrical equipment are proposed. Full article

This study investigates the coordinated impact of a synchronous condenser (SC), battery energy storage system (BESS), and static synchronous compensator (STATCOM) on enhancing voltage and frequency stability in a modified IEEE 9-bus power system under severe disturbances. The aim is to quantify the individual and combined contributions of these technologies during both fault ride-through (FRT) and load-increment events. The methodology includes dynamic modelling of all three devices in DIgSILENT PowerFactory. The SC is represented as a synchronous machine with inertia and AVR-based voltage control; the BESS employs converter-based active power and frequency-droop control; and the STATCOM provides fast reactive power injection through a dual-loop voltage regulator. Key indicators include nadir (minimum frequency), Rate of Change of Frequency (RoCoF), steady-state deviation, voltage sag depth, and recovery characteristics. Results indicate distinct roles for each device. The SC increases inertia and improves damping, but it also introduces small, well-damped oscillations. The BESS significantly enhances frequency stability by mitigating nadir, reducing RoCoF, and accelerating recovery, with negligible effect on voltage regulation. The STATCOM substantially reduces voltage sag and speeds up voltage recovery, but it does not influence frequency behaviour. When combined, the hybrid SC–BESS–STATCOM system demonstrates strong complementarity: the SC supports inertia, the BESS stabilizes active-power imbalance, and the STATCOM ensures fast reactive-power compensation. Full article

Grid-forming (GFM) converters enhance power system stability by emulating synchronous generators, but their limited overcurrent capability under grid faults poses a critical challenge to transient stability. Existing current-limiting methods often force a trade-off between fault current suppression and voltage support. To address this, a direct internal voltage control (DIVC)-based fault current-limiting strategy is proposed. The DIVC framework eliminates inner control loops and directly regulates the internal voltage amplitude and phase by leveraging measurements at the point of common coupling (PCC) and the converter output, enabling fast, accurate current control within a virtual synchronous generator (VSG) architecture. Under mild faults, the strategy prioritizes maintaining the terminal voltage to preserve voltage source behavior; under severe faults, it smoothly transitions to a current-limiting mode that preserves the terminal voltage phase angle to support transient synchronization. The scheme incorporates compensation-enabling criteria, dual-mode amplitude/phase compensation, and power reference modification. Experimental results under an 80% voltage sag demonstrate that the proposed method limits the transient current peak to 1.1 p.u. and ensures oscillation-free recovery within 0.1 s, significantly outperforming conventional current saturation and virtual impedance techniques. The proposed approach also exhibits strong current-limiting capability under unbalanced faults. Full article

To address DC-link voltage fluctuation, active-power imbalance between the machine side and the grid side, and double-frequency distortion in the grid current of a flywheel energy storage system (FESS) under symmetrical and asymmetrical voltage sag faults, this paper proposes a coordinated control strategy for the machine-side and grid-side converters to enhance low-voltage ride-through (LVRT) capability. Taking the DC-side energy imbalance as the coordination criterion, the machine-side converter adopts an online active-current-command reconstruction method based on cascaded limiting of DC-link voltage deviation. Under reactive-power-priority support and constrained active-power output on the grid side, the FESS can actively adjust its active-current command according to the DC-side energy state, thereby suppressing DC-link overvoltage/undervoltage and restoring the power balance between the machine side and the grid side. On the grid side, an improved linear active disturbance rejection control (LADRC) is introduced into the current inner loop. By optimizing the structure of the extended state observer, the observation and compensation capability for double-frequency disturbances is enhanced, thus improving grid-current quality under asymmetrical faults. In this way, power rebalancing between the machine side and the grid side, DC-link voltage stabilization, and grid-current disturbance suppression are incorporated into a unified coordinated control framework. Hardware-in-the-loop experimental results show that the proposed strategy can maintain DC-link voltage stability during both symmetrical and asymmetrical voltage sags, while keeping the maximum grid-current total harmonic distortion (THD) below 0.13%. Under asymmetrical voltage sag, the improved LADRC reduces the maximum interphase peak-current deviation from approximately 52 A under conventional PI control to 4.57 A, corresponding to a reduction of about 91.2%. These results indicate that the proposed strategy can effectively enhance DC-link voltage stabilization and improve grid-current quality during faults. Full article

Photovoltaic (PV)-storage systems operating in weak grids are affected by high grid impedance, transient voltage disturbances, and measurement noise, which can degrade frequency regulation, increase converter current stress, and impose high-frequency current fluctuations on the battery. To address these issues, this paper proposes a multi-timescale transient-state sensing and signal-processing framework for grid-forming PV-hybrid storage systems. The proposed framework combines three coordinated functions. First, a frequency-domain HESS power-decoupling mechanism separates high-frequency transient power components and assigns them to the supercapacitor, while the battery mainly handles low-frequency energy variations. Second, a voltage-deviation-driven adaptive virtual inductance is introduced to increase the equivalent output impedance during voltage-sag events and reduce transient inrush current. Third, a noise-resilient frequency sensing strategy based on a filtered frequency derivative and a dead-band for false-trigger suppression is developed to reduce noise-induced false triggering in adaptive inertia and damping control. Comparative simulations indicate that under the tested weak-grid conditions, the proposed method reduces the transient inrush-current peak by 53.2%, decreases the maximum dynamic frequency deviation by approximately 75%, and improves the active-power regulation speed by more than 50%. These results indicate that the proposed sensing-oriented framework can improve transient response while reducing converter and battery current stress in PV-storage systems connected to high-impedance grids. Full article

In response to issues such as the strong output power fluctuations of photovoltaic units caused by external environmental factors like solar irradiance, and the susceptibility of traditional control methods to grid disconnection incidents under significant system disturbances, a control strategy based on cascaded linear-nonlinear Extended State Observer (ESO) Active Disturbance Rejection Control (ADRC) is proposed for photovoltaic-storage hybrid systems. A model of the photovoltaic-storage hybrid system is constructed to achieve high- and low-frequency power distribution among energy storage units. The cascaded linear-nonlinear ESO-based ADRC is employed to estimate and compensate for system disturbances in real-time. Simulation results verify the effectiveness of the proposed control strategy, demonstrating a reduction in frequency deviation by 0.08 Hz under normal frequency dips. More critically, during severe AC bus voltage sags, the proposed strategy prevents the system frequency from falling below 49.0 Hz, thereby avoiding the under-frequency load shedding (UFLS) that occurs with conventional PI control. Additionally, the DC bus voltage fluctuation is limited to within 8 V under grid disturbances, and unlike the severe oscillations exhibited by PI control during system recovery, the proposed cascade strategy ensures an immediate and smooth transient response. Full article

Virtual synchronous generators (VSGs) are prone to transient power angle instability and short-circuit current overshoot under symmetrical short-circuit grid faults. To address the limitation that existing transient control strategies fail to simultaneously guarantee power angle stability and fault current limiting, a coordinated control strategy combining dynamic active power reference regulation and adaptive virtual impedance is designed. Specifically, the active power reference is dynamically adjusted in accordance with the voltage sag magnitude at the point of common coupling (PCC), which effectively narrows the acceleration area of the virtual rotor and maintains the transient power angle near its rated value to prevent the risk of system loss of synchronism. On this basis, an adaptive virtual impedance control scheme is designed to accurately calculate and implement the optimal current-limiting impedance on demand, confining the steady-state fault current within the allowable threshold. Finally, the effectiveness of the designed strategy is verified on the Matlab/Simulink simulation platform. Simulation results demonstrate that the designed strategy achieves the coordination between transient power angle stability and fault current limiting, thus improving the operational stability of the VSG grid-connected system under symmetrical short-circuit grid faults. Full article

This work provides a theoretical/methodological contribution to the transient analysis of asymmetrical faults in three-phase systems. Transient analysis of three-phase systems is usually performed by resorting either to the instantaneous Symmetrical Component Transformation (SCT) or to numerical methods. In this paper, an analytical methodology based on the time-domain Clarke transformation is presented for the transient analysis of the most common asymmetrical faults. For each kind of asymmetrical fault, a specific circuit coupling between the Clarke αβ0 circuits is derived. Two main advantages are obtained over the SCT approach. First, the Clarke circuits involve real-valued voltages/currents, instead of complex variables as with the SCT. Second, the Clarke circuits αβ0 are not all coupled to each other. Therefore, the dynamic order of the Clarke equivalent circuits is lower than that of the SCT circuits. This property can be of interest in both the derivation of analytical and numerical solutions. A simple radial system is used to exemplify the proposed methodology. Full article

The grid-forming (GFM) wind turbine with energy storage is regarded as a promising solution for the integration of renewable energy sources (RESs) into power systems. However, the system faces the risk of instability during large grid disturbances, such as grid voltage sags and frequency variations. To address this issue, this paper proposes a coordinated control method to enhance the transient stability of GFM wind turbines with energy storage. First, a permanent magnet synchronous generator (PMSG)-based wind turbine employing grid-forming control and integrated with an energy storage system is introduced. Then, transient stability cases are identified based on the equal area criterion (EAC) within the virtual synchronous generator (VSG) control framework. On this basis, a low-voltage ride-through (LVRT) method is developed by coordinately adjusting inertia, damping, and active power reference according to fault severity, thereby ensuring system stability under low-voltage grid fault. Furthermore, a frequency fluctuation mitigation (FFM) is proposed to suppress power oscillations under frequency disturbances. The coordinated LVRT and FFM methods enable effective stabilization of the system under grid voltage and frequency faults. Finally, simulation results validate the theoretical analysis and demonstrate the effectiveness of the proposed control strategy. Full article

With the high penetration of distributed photovoltaic and storage systems, active distribution grids are prone to experiencing “active power surplus and reactive power shortage” during the evening peak, leading to voltage sags at the network end. Although electric vehicle (EV) grid-connected inverters possess four-quadrant reactive power regulation capabilities without causing the additional chemical cyclic aging of the battery cells, existing dispatch systems often treat them as unconditional response resources, overlooking users’ actual willingness to cede control and the associated strategic interactions. To address this, this paper proposes a “grid-load” coordinated reactive power optimization strategy that accounts for EV users’ willingness to respond: a Logit model incorporating price incentives, initial energy consumption, and parking duration is constructed based on discrete choice theory. By combining a truncated normal distribution with the Monte Carlo method to eliminate micro-sampling errors, a model of the expected reactive power capacity of charging stations under dynamic incentives is established; considering the physical constraints of SVCs and EVs, a scalarized single-objective optimization model is constructed with grid loss-equivalent costs, ancillary service costs, and voltage deviation as objectives, and solved using an improved particle swarm optimization algorithm with linearly decreasing weights. Simulations on a modified 33-node IEEE system incorporating storage indicate that this strategy can assign optimal compensation prices to each node based on the spatial value of reactive power. Compared to traditional single-voltage regulation and fixed subsidies, it not only stabilizes the grid-wide voltage within a safe range but also avoids overcompensation, achieving global optimization of both power quality and economic efficiency. Full article

When a fault occurs in the power grid to which the Virtual Synchronous Generator (VSG) is connected, it leads to overcurrent phenomena, which threatens the safety of the inverter and easily results in device damage. Although existing direct current limiting unit (CLU) control strategies can restrict the fault current, the input active power command far exceeds the power output, causing the virtual rotor to continuously accelerate. This leads to power angle divergence and a subsequent loss of synchronization. To address the conflict between direct current-limiting control and system transient stability, this paper proposes a control strategy based on the Particle Swarm Optimization (PSO) algorithm to modify the active power command, building upon existing direct current-limiting VSG control. During grid faults, the output current is constrained to its maximum value, leading to a reduction in the system’s output power. By leveraging the PSO algorithm, the proposed strategy decreases the active power command to minimize the power mismatch between the command and the output. This maximizes the system’s transient stability by minimizing the rotor acceleration torque and effectively suppressing excessive power angle deviation. Meanwhile, the active power command reduction is introduced as a penalty term to maximize the active power output capability during the fault period. Simulation results demonstrate that, compared to VSG with only direct current-limiting control, the proposed strategy significantly enhances the transient stability and transmission efficiency of the VSG under long-term fault conditions across various grid voltage sag scenarios. Furthermore, it ensures a seamless transition from the fault state to normal operation during short-term faults. Full article