Safe Reinforcement Learning-Based Transient Stability Control for Islanded Microgrids With Topology Reconfiguration

The prevalence of Line‐Commutated Converter (LCC) HVDC makes the coupling and interaction phenomenon between AC system and DC system of particular complicated. There are significant challenges to determine control strategy using state‐of‐the‐art approaches especially when discrete events (e.g. switching and state resetting) happen frequently due to successive commutation failures (CFs). An enhanced transient stability emergency control (TSEC) for AC/DC grids based on the optimal control is presented to maintain the transient stability and mitigate the successive CFs caused by AC faults. In the proposed TSEC method, an extinction advance angle constraint is added to the TSEC model to mitigate the successive CFs. The emergency power control of LCC‐HVDC is employed as an additional control measure to reduce generator‐tripping and load‐shedding amount. The direct sequential approach is adopted to obtain the optimal solution of TSEC to improve the calculation efficiency. In the direct sequential approach, the modified adjoint sensitivity analysis (MASA) with the consideration of adjoint jump conditions is utilized to increase the gradient calculation accuracy. The effectiveness of the control strategies and the accuracy of the sensitivity calculation are verified on two test systems.

An on-line transient stability emergency control approach is proposed, which couples an open-loop and a closed-loop emergency control technique. The open-loop technique uses on-line transient stability assessment in order to adapt the settings of automatic system protection schemes to the current operating conditions. On the other hand, the closed-loop technique uses measurements in order to design and trigger countermeasures, after the contingency has actually happened, then to continue monitoring in a closed-loop fashion. The approach aims at combining advantages of event-based and measurement-based system protection schemes, namely, speed of action and robustness with respect to uncertainties in system modeling. It can also comply with economic criteria.

The power transient stability analysis is one of the basis for determining the control strategy of power system security and stability. Considering the influence of power grid topology on the transient stability of power system, the transient stability evaluation model is constructed based on the graph attention neural network. The electrical components and their transient operation data are mapped to the graph data with the spatial topology characteristics of power system for model training, so as to improve the topological generalization performance of the model. The marginal contribution of input characteristics to the output of transient power angle stability evaluation model is quantitatively calculated based on Shapley additive explanation (SHAP), so as to improve the interpretability of data-driven method for transient power angle stability evaluation. The effectiveness of the proposed method is verified by the IEEE 39-bus system.

peer reviewed

The deep adjustment of power grid structure puts forward new requirements for power system emergency control technology. Aiming at the problem that traditional transient stability control methods only consider power angle stability constraints, resulting in low control accuracy and unable to guarantee the transient voltage stability of power grid, a hybrid model based on deep learning is proposed for power system emergency control. Firstly, the power system emergency control margin model of random forest is established to accurately judge the stability of the system and avoid improper decision-making due to the failure of the model. Secondly, the long short-term memory network is used for real-time power curve prediction to provide accurate implementation scheme of machine and load shedding, so as to realize the stable operation of power system. Finally, simulation analysis is carried out in IEEE 16 machine 68 bus interconnected system. The results show that our method has better system stability judgment accuracy than the comparison method, and the emergency control method can effectively ensure the stable operation of the system after failure.

The integration of large-scale new energy (NE) into the grid has brought great problems to the transient stability of the power grid. Therefore, the transient angle stability improvement strategy of the power grid based on a grid-forming energy storage system (GFM-ESS) is proposed in this paper. First, the mechanism of GFM-ESS to improve the power grid’s transient stability is examined from the standpoint of regional power balancing in the power grid. Secondly, a hierarchical control strategy for GFM-ESS is proposed, including the control modules at the power station and converter level. The inertial control link is used to create the power station level control model, and the converter level control model takes into account the SOC constraints of the GFM-ESS. Finally, the 36-node (CEPRI-36) system of the China Electric Power Research Institute in PSASP is used for simulation verification. The simulation results show that the control strategy proposed in this paper is effective.

Due to the severe global warming, the frequency and intensity of typhoon extreme weather events have increased around the world. These events would cause serious <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$N-k$</tex-math></inline-formula> contingency of coastal AC/multi-terminal high voltage DC (MTDC) power grid, resulting in a high possibility of transient instability. To address this problem, this article develops a novel two-stage resilient control strategy. In the first stage, an optimization model is established to obtain a control scheme that involves generator rescheduling, load shedding and MMC operation point adjustment after <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$N-k$</tex-math></inline-formula> contingency. In the second stage, an active power modulation method of modular multi-level converters (MMC) based on the model predictive control algorithm is proposed to improve the transient stability of the coastal AC/MTDC grid. The proposed control strategy makes full use of the flexible control resources of MMC-MTDC, thereby increasing the transient stability margin of the coastal AC/MTDC grid. The effectiveness of the proposed strategy has been validated on an AC/MTDC power grid built on MATMTDC that is an open source software based on MATLAB. Simulation results demonstrate that the proposed two-stage resilience control strategy exhibits promising performance in enhancing the transient stability of the coastal AC/MTDC power system when suffering a typhoon event.

The increasingly prominent pattern of ac and hvdc hybrid connection and the continuous growth of new energy grid connected capacity have led to profound changes in the disturbed behavior and stability characteristics of the power grid. It is increasingly urgent to implement dynamic monitoring and stability control of large power grid based on wide area measurement system (WAMS). In this paper, firstly, the trajectory characteristics of the ac branch under the impact of large disturbances and the relationship between them and transient stability are studied. Secondly, the continuous tracking method of the branch voltage center based on the node voltage amplitude and phase is proposed, and the relationship between the branch voltage center and the system oscillation center is analyzed. Thirdly, for the problem that a single characteristic is necessary but not sufficient to identification the key branch, a method based on wide area response information and combined multiple characteristic parameters is proposed, furthermore, a transient stability emergency control strategy is put forward. Finally, the effectiveness of the key branch discrimination and control strategy is verified by simulation for ac and HVDC hybrid power grid.

As a flexible and reliable way for distributed energy consumption and integration, the converters-dominated microgrid has attracted more and more attention recently. Owing to the low inertia and high nonlinearity of power converters, the islanded microgrid under large-signal disturbances is easily suffer to transient stability problems. To support the stable operation of microgrid, grid-forming converters are extensively investigated and applied. In this paper, an improved equal area criterion (IEAC) is proposed for the transient stability analysis of a simple microgrid equipped with both grid-following and grid-forming converters. Firstly, a simplified second-order model of the microgrid is established, which contains a nonlinear damping term relying on the power angle. Then, the limitation of conventional equal area criterion (EAC) for transient stability analysis is revealed, which completely ignores the unfavorable influence of negative damping and may lead to erroneous judgement. To fill this gap, an improved equal area criterion is proposed to derive the transient stability conditions for islanded microgrid, which could improve the estimation accuracy of stability boundaries. Moreover, impacts of system parameters and controller parameters on transient stability are studied, which provides useful guidelines for the parameter optimization of grid-forming and grid-following converters. Eventually, simulation and hardware-in-loop experiments are conducted to verify the effectiveness and superiority of the proposed method.

Power systems are subjected to a wide range of disturbances during daily operations. Severe disturbances, such as a loss of a large generator, a three-phase bolted fault on a generator bus, or a loss of a transmission line, can lead to the loss of synchronism of a generator or group of generators. The ability of a power system to maintain synchronism during the few seconds after being subjected to a severe disturbance is known as transient stability. Most of the modern methods of controlling transient stability involve special protection schemes or remedial action schemes. These special protection schemes sense predetermined system conditions and take corrective actions, such as generator tripping or generation re-dispatch, in real time to maintain transient stability. Another method is the use of a real-time feedback control system to modulate the output of an actuator in response to a signal. This paper provides a fundamental evaluation of the use of feedback control strategies to improve transient stability in a power system. An optimal feedback control strategy that modulates the real power injected and absorbed by distributed energy-storage devices is proposed. Its performance is evaluated on a four-machine power system and on a 34-machine reduced-order model of the Western North American Power System. The result shows that the feedback control strategy can increase the critical fault clearing time by 60%, thereby improving the transient stability of the power system.

This paper proposes a local sliding mode control (SMC) strategy for frequency regulation and active power sharing in islanded microgrids (MGs). Unlike advanced strategies, either droop-based or droop-free, that rely on inter-inverter communication, the proposed method operates in a fully decentralized manner, using only measurements available at each inverter. In addition, it adopts a minimalist structure that avoids adaptive laws and consensus mechanisms, which simplifies implementation. A discontinuous control law is derived to enforce sliding dynamics on a frequency-based surface, ensuring robust behavior in the face of disturbances, such as clock drifts, sudden load variations, and topological reconfigurations. A formal Lyapunov-based analysis is conducted to establish the stability of the closed-loop system under the proposed control law. The method guarantees that steady-state frequency deviations remain bounded and predictable as a function of the controller parameters. Simulation results demonstrate that the proposed controller achieves rapid frequency convergence, equitable active power sharing, and sustained stability. Owing to its communication-free design, the proposed strategy is particularly well-suited for MGs operating in rural, isolated, or resource-constrained environments. A comparative evaluation against both conventional droop and communication-based droop-free SMC approaches further highlights the method’s strengths in terms of resilience, implementation simplicity, and practical deployability.

Transient stability analysis of a grid-connected type-4 wind turbine with grid-forming control during the fault

Introduction: MMC-HVDC is an effective means of large-scale new energy power generation network transmission, and when the proportion of MMC-HVDC transmission in the receiving power grid increases, its low inertia and weak support characteristics affect the transient power angle stability of SG in the system. The goal is to achieve a transient power angle stability analysis method of the receiving power grid considering the influence of multiple control links of the MMC-HVDC transmission. Method: Based on the multivariate differential method, a dynamic analysis model of MMC- HVDC transmission considering the influence of outer loop control and limiting link is established. Based on the phase plane method and space vector diagram method, the influence of multiple control links and parameters of MMC-HVDC transmission on the transient power angle stability of receiving end power network are analyzed. Results: A more accurate transient model of hybrid system was established, and the influence law of MMC-HVDC parameters on SG power angle was obtained through analysis. Based on this, a new control strategy was proposed to enable SG to restore stability more quickly. Conclusion: By establishing a mathematical model of SG and MMC, the power interaction characteristics between them can be accurately characterized. Injecting more MMC reactive current during the fault is helpful to the transient power angle stability of SG. The power angle stability of SG can be effectively improved by constructing an adaptive optimal control strategy.

In a high percentage of new energy-islanded microgrids, the overall inertia of the system gradually decreases, and the transient stability requirements of the microgrid frequency and voltage become more and more demanding under low-inertia conditions. To improve the transient stability of low-inertia islanded microgrid frequencies and voltages, this paper proposes a transient stability enhancement strategy for islanded microgrids based on energy storage system (ESS)–virtual synchronous generator (VSG) control. Model predictive control (MPC) is added within the active control loop of the VSG to achieve dynamic correction of the active power reference value of the VSG; PI control link is added within the reactive control loop to achieve a fast dynamic response of the reactive power command value. The ESS achieves fast and accurate regulation of frequency and voltage according to the power reference value of the VSG active control loop and the power command value of the reactive control loop simultaneously. Considering the need to ensure the ability of VSG to operate stably during transients, a comprehensive current-limiting technique combining virtual impedance and phase limiting is used to limit the fault current of VSG and maintain its synchronization and stability. Finally, the simulation results verify the strategy’s effectiveness and the superiority of the transient stability enhancement effect.

The proliferation of next-generation renewable energy systems has driven widespread adoption of electronic devices and nonlinear loads, causing grid distortion that degrades waveform quality in grid-forming (GFM) converters. Additionally, unbalanced grid faults exacerbate overcurrent risks and transient stability challenges when employing conventional virtual impedance strategies. While existing studies have separately examined these challenges, few have comprehensively addressed non-ideal grid conditions. To bridge this gap, a novel control strategy is proposed that reshapes the output current waveforms and enhances transient stability in GFM converters under such conditions. First, a sliding mode controller with an improved composite reaching law to achieve rapid reference tracking while eliminating chattering is designed. Second, a multi-quasi-resonance controller incorporating phase compensation is introduced to suppress harmonic distortion in the converter output current. Third, an individual-phase fuzzy adaptive virtual impedance strategy dynamically reshapes the current amplitude during unbalanced faults and improves the system’s transient stability. Validated through PSCAD/EMTDC simulations and hardware-in-the-loop experiments, the proposed strategy demonstrates superior transient stability and fault ride-through capability compared to state-of-the-art methods, ensuring reliable GFM converter operation under severe harmonic and unbalanced grid conditions.