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With the expansion in recent power systems, the boost in renewable energy resources with multiple interconnections, overloading of existing power networks, increased load growth, and equipment failures, power system transient instability issues have exponentially increased. Transient stability in power systems is of particular importance even while performing the steady state analysis of a power system. Power system transient stability assessment is mandatory regularly for power system operation and has a major impact on power system planning. Potential risks of blackouts and failures in power systems can be avoided or minimized with the prompt prediction of transient instability. This research work presents a predictive approach for power system transient instability. The case study is a 735kV, 29-bus, 7-powerplant network involving precise modeling of generators. In particular, the proposed method is represented by different machine learning models through extraction and regression, in which the variables of the power-generating units are used as primary sources/features. Data cleaning and sorting out techniques are being used for refining data. Feature extraction has also been implemented for further cleansing of the data. The result is in the form of concrete and robust classifier models that can overcome power systems’ instability concerns through prompt prediction.

Cloud energy storage is considered a promising application in future power systems. It focuses on optimally leveraging the capacity of centralized large‐scale energy storage compared with the requirements of small‐scale localized users. In this paper, to satisfy the small‐ and medium‐scale timely energy storage requirement from localized users, the concept of the cloud‐based location sharing energy storage is proposed. The modular mobile energy storage system is flexibly configured and deployed at different sites to fulfil the long‐term seasonally dynamic transformer capacity increment and short‐term daily energy arbitrage based on economic values. To optimize the overall incomes of the energy storage investment, a two‐step user potential identification algorithm is proposed to discover the most valuable users at different time scales from the regional power usage profile. Then, the number/capacity optimal planning algorithm is proposed to optimally share the mobile energy storage system among users seasonally and total profits are further increased through daily energy arbitrage at less valuable seasons. Finally, the structure and dispatching strategy of the cloud‐based location sharing energy storage management system is illustrated. Case study results prove that the proposed identification algorithm can excavate the most valuable users at different time scales and the optimal planning and operation strategy is able to guarantee the overall income benefits. [ABSTRACT FROM AUTHOR]

New techniques and approaches are constantly being introduced to analyze and enhance the transient stability of renewable energy-source-dominated power systems. This review article extensively discusses recent papers that have proposed novel and innovative techniques for analyzing and enhancing the renewable source-dominated power system's transient stability. The inherent low-inertia characteristics of renewable energy sources combined with fast-acting power electronic devices pose new challenges in power systems. Different stability concerns exist for grid-following and subsequent grid-forming converter/inverter connections to power grids; hence, distinct solutions for enhancing the transient stability have been devised for each. Moreover, the fundamental concepts and characteristics of converter/inverter topologies are briefly discussed in this study. Recent discussions and reviews of analysis and enhancement techniques in transient stability could lead to new ways to solve problems in power systems that rely primarily on renewable energy sources. [ABSTRACT FROM AUTHOR]

Under fault conditions, virtual synchronous generators (VSGs) are prone to lose transient stability, similar to synchronous generators. However, the existing studies on VSG transient stability are not comprehensive. The transient characteristics of VSGs are highly affected by control parameters, but the transient models of VSGs in existing studies are overly simplified. In addition, the critical clearing angle (CCA) and critical clearing time (CCT) are two indices used to measure the transient stability of a system, but there are few studies that consider both CCA and CCT. In this article, we analyze the CCA and CCT quantitatively to describe the transient stability boundary of a VSG under different control parameters and fault conditions. Then, an equal proportional area criterion is proposed to provide guidance for the control of virtual inertia in the VSG. Based on a theoretical analysis, a transient control method for a VSG is proposed, which can improve the transient stability of the VSG in terms of both CCA and CCT. Finally, simulation and experimental tests are performed to validate the correctness of the theoretical analysis and the proposed method. [ABSTRACT FROM AUTHOR]

This article reveals that the saturation block (SB) used with the power-angle control loop can enhance the transient stability of grid-forming (GFM) inverters. Three methods of using the SB in the power-angle loop are examined based on power-angle curves. It is found that all can enhance the transient stability of GFM inverters by either decreasing the acceleration area or increasing the deceleration area. At the same time, the threshold value selection of SB in the three schemes is also explained, considering the tradeoff between the transient stability and the response speed. A holistic comparison of the three schemes recommends the scheme with the SB used for power error adopted by the GFM inverters. Finally, experimental tests confirm the effectiveness and correctness of the analysis. [ABSTRACT FROM AUTHOR]

This article investigates the impacts of different current limiting strategies on the transient stability of the virtual synchronous generator (VSG). The power–angle curve of the VSG under different operating conditions is theoretically characterized in detail and experimentally verified through tests conducted on hardware-implemented lab-scale VSGs. It is shown that the reference current saturation approaches prioritizing the current vector angle, the d-axis current, and the q-axis current reshape the VSG power–angle curve in different ways. The resulting impacts on the VSG transient stability are comprehensively investigated through time-domain simulation of a battery energy storage system that is operated as a VSG and connected to a medium-voltage Canadian distribution feeder. The transient stability margin of the VSG is evaluated by determining the critical clearing time (CCT) of various fault scenarios. The studies conducted in the PSCAD/EMTDC software environment utilizing a detailed switching model of the VSG indicate that 1) the current limit of the VSG significantly impacts its power output during and after faults; and 2) the q-axis priority current limiting strategy provides a larger transient stability margin (CCT) as compared with the current vector angle and d-axis current priority approaches. [ABSTRACT FROM AUTHOR]

Grid forming (GFM) control strategy has been increasingly used in grid-connected converters to regulate system frequency and mimic the inertia of synchronous generator. However, the description of its power angle dynamics under large disturbances is still lacking up to date, which may pose great challenges to stable operation and economic configuration of grid-connected converters in the future power system. To overcome the aforementioned obstacle, a Krylov–Bogoliubov–Mitropolsky (KBM) asymptotic method with the perturbation theory is presented first in this article to derive an analytical solution for power angle trajectory under large disturbances. Considering that the antiderivative problem is usually unsolvable due to the complicated perturbation function, a tailored integral method is further proposed to address this problem and simplify the final result, which facilitates the KBM asymptotic method to the analysis and calculation of power angle trajectory. As a result, an explicit and accurate time-domain expression is obtained, and it reveals the quantitative relationships between control parameters and dynamic characteristics, which may provide a good potential for system transient stability analysis and GFM converter design. In the end, the effectiveness and accuracy of the proposed method are verified by both simulations and control-hardware-in-loop experiments. [ABSTRACT FROM AUTHOR]

Grid-forming converters (GFMCs) prevail for their capability of providing voltage and frequency support in modern power grids with large integration of intermittent distributed power sources. As GFMCs behave as voltage sources, the current limiting of GFMCs under large disturbances requires special efforts. Furthermore, a reliable GFMC should also be capable of remaining synchronized with the grid during and after large voltage disturbances. To avoid the instability caused by phase-locked loops during low-voltage faults, GFMCs usually retain the active power synchronization under low-voltage faults when the current saturation is triggered. When voltage sags are severe, the transient stability highly depends on the fault duration. This article proposed a robust fault ride through strategy for GFMCs based on reactive power synchronization (Q-Syn) under voltage sags. The Q-Syn method ensures transient stability of GFMCs regardless of the fault duration or level. Furthermore, fast postfault recovery can be achieved with the Q-Syn method. The effectiveness of the proposed method is verified by experiments. [ABSTRACT FROM AUTHOR]

The inverter-driven renewable energy sources (RESs), such as wind energy conversion systems (WECS), pose major threats toward system stability due to lack of inertia. Hence, virtual inertia concepts have gained popularity, for control and improvisation of the dynamic behavior of RESs, by simulating the kinetic inertia of the synchronous generator. This article focuses on developing an improved self-regulating virtual synchronous generator (VSG) control for grid-tied doubly fed induction generator (DFIG)-wind farms (WFs). The proposed scheme provides frequency support to the system while ensuring the low-voltage ride through (LVRT) capability at transient conditions, as per grid code requirements (GCRs). This has been achieved by introducing an additional control at grid side converter (GSC). This auxiliary control consists of a combined approach of VSG control and a current limiting approach. The VSG loop that alters the inertia of the system improves the frequency of the system and the current limiting loop provides the required inductance to limit fault current. This overall loop uses a self-regulating approach, and the developed concept helps to suppress the transients in stator current. The study obtained on a multimachine system and also for a weak grid system confirms the effectiveness and viability of the modified converter control structure. [ABSTRACT FROM AUTHOR]

The decarbonization plans are leading to the decommissioning of conventional power plants, resulting in a deep transformation of power systems. Balearic Islands are not an exception and, in order to compensate this generation reduction and increase the security of supply, a second HVDC link is planned to interconnect the Balearic power system to the Spanish mainland. In this application paper, Modular Multilevel Converters (MMCs) have been assumed as the preferred technology for this HVDC link. A transient analysis has been presented to evaluate the system performance after the introduction of this new HVDC link. Grid-following and grid-forming control strategies have been implemented, comparing their impact on the system performance and stability. The transient events considered are the disconnections of the main synchronous generators of the islands and AC faults in several locations of the Balearic power system. A final case based on a penetration of 100% of converter-based resources has been also simulated to study the islanded operation of the MMC-HVDC link. [ABSTRACT FROM AUTHOR]

This work proposes the Hybrid Control Converter (HCC), a new kind of VSC control. This method aims to have one single converter mimic the behavior of two individual, virtual converters working in parallel (one with grid-following and the other with grid-forming control). This way, the hybrid converter can show behavioral characteristics of both kinds of converter control at the same time. Two possible applications for the HCC are presented. The first one is related to synthetic inertia capability of wind turbines. Results show that the HCC presents superior performance when compared to conventional techniques based on grid following converters, resulting in a smaller grid frequency drop during transients. The second application shows that the HCC allows for autonomous connection of multiple distinct AC systems using a meshed DC transmission system. This enables automatic frequency support and generation and load sharing between the AC grids without requiring any specific action by the system operator. [ABSTRACT FROM AUTHOR]

The increasing complexity of modern AC/DC power systems poses a significant challenge to a fast solution of large-scale transient stability simulation problems. This paper proposes the hybrid parallel-in-time-and-space (PiT+PiS) transient simulation on the CPU-GPU platform to thoroughly exploit the parallelism from time and spatial perspectives, thereby fully utilizing parallel processing hardware. The respective electromechanical and electromagnetic aspects of the AC and DC grids demand a combination of transient stability (TS) simulation and electromagnetic transient (EMT) simulation to reflect both system-level and equipment-level transients. The TS simulation is performed on GPUs in the co-simulation, while the Parareal parallel-in-time (PiT) scheduling and EMT simulation are conducted on CPUs. Therefore, the heterogeneous CPU-GPU scheme can utilize asynchronous computing features to offset the data transfer latency between different processors. Higher scalability and extensibility than GPU-only dynamic parallelism design is achieved by utilizing concurrent GPU streams for coarse-grid and fine-grid computation. A synthetic AC/DC grid based on IEEE-118 Bus and CIGRÉ DCS2 systems showed a good accuracy compared to commercial TSAT software, and a speedup of 165 is achieved with 48 IEEE-118 Bus systems and 192 201-Level detail-modeled MMCs. Furthermore, the proposed method is also applicable to multi-GPU implementation where it demonstrates decent efficacy. [ABSTRACT FROM AUTHOR]

Online transient analysis plays an increasingly important role in dynamic power grids as the renewable generation continues growing. Traditional numerical methods for transient analysis not only are computationally intensive but also require precise contingency information as input, and therefore, are not suitable for online applications. Existing online transient assessment studies focus on the determination of post-contingency system stability or stability margin. This paper develops a novel graph-learning framework, Deep-learning Neural Representation or DNR, for online prediction, of the time-series trajectories of the system states using initial system responses that can be measured by phasor measurement units (PMUs). The proposed DNR framework consists of two sequential modules: a Network Constructor that captures network dependencies among generators, and a Dynamics Predictor that predicts the system trajectories. The key to improved prediction performance is the introduction of the spatio-temporal message-passing operations into graph neural networks with structural knowledge. Its effectiveness and scalability are validated through comparative studies, demonstrating the prediction performance under different contingency scenarios for systems of different sizes. This framework provides a solution to online predicting post-fault system dynamics based on real-time PMU measurements. Additionally, it can also be applied to facilitate the offline transient simulation without simulating the entire trajectories. [ABSTRACT FROM AUTHOR]

This article proposes a novel and easily implementable overcurrent suppression control for voltage-controlled virtual synchronous generators (VSGs). The VSG is vulnerable to grid voltage disturbances, such as voltage dip/recovery, because it consists of voltage source inverters (VSIs). In the worst case, the overcurrent caused by the voltage disturbance damages the VSI. The proposed VSG control can suppress the overcurrent caused by not only symmetrical but also asymmetrical voltage dip/recovery. The key idea is to use novel virtual grid voltages for the VSG reference output voltage. The proposed VSG control requires no complicated analysis/tuning for the implementation and no additional equipment, such as a fault-current limiter. Because of these advantages, the overcurrent in the VSG can be suppressed without increasing the cost and the tuning effort. The effectiveness of the proposed method is verified through simulations and experiments. [ABSTRACT FROM AUTHOR]

Due to the requirement of synchronism and power sharing, droop control and its variations have become one essential component for distributed generator (DG)-powered microgrids. However, the power sharing accuracy and system stability margin may be threatened by the randomness from the load demand and renewable generation. In this paper, a dynamic stability analysis is first performed on a DG-powered microgrid through a produced system frequency response model (SFR). The results point out that (i) the critical system eigenvalues directly vary with the system operating condition; (ii) a fixed-gain power sharing control is prone to be less damped and loses stability easily under some operating conditions. Then, the heuristic adaptive dynamic programming (HDP) strategy is used for power sharing control with the benefit of adapting to real-time disturbances and uncertainties. Through Lyapunov theorem, stability analysis is provided to demonstrate the reliability of the HDP-based power sharing control in islanded microgrids. Finally, simulation tests verify the analysis results and demonstrate the favorable performance of the HDP-based power sharing control under uncertain load disturbances. [ABSTRACT FROM AUTHOR]

The installation of phasor measurement units (PMUs) in contemporary electrical networks provides enhanced monitoring and control capabilities of the entire system. However, the placement of additional PMU devices is constrained by the relatively high cost and complicated communication infrastructure. Consequently, optimizing the allocation of PMU units is required to achieve complete visibility of the power system operation while minimizing the associated cost. This paper presents a coherent approach for solving the optimal PMU placement (OPP) in order to reduce the total number of PMUs that is required to completely observe the network. Furthermore, the proposed formulation of the OPP allocation problem considers several objectives such as cost minimization, redundancy and efficiency maximization in addition to various constraints like incorporation of zero-injection buses, single PMU failure, single line outage and consideration of flow measurements. Moreover, a two-stage approach is proposed to ensure the numerical observability of the obtained PMU placement. The proposed framework relies on solving a mixed integer linear programming problem for the OPP placement problem based on conventional measurements. The rank of the gain matrix is thereafter computed to check if the solution is numerically observable. In contrast to reported studies in the literature, transient stability enhancement is augmented in the formulation of the optimization problem such that the redundancy of predetermined buses is guaranteed without increasing the total number of installed PMU units. A novel spectral cluster-based online coherency grouping is employed to identify buses which possess the ideal characteristics in terms of transient stability support. [ABSTRACT FROM AUTHOR]

This letter analyzes the impact of hybrid voltage- and power-based synchronization control on the transient stability of grid-forming (GFM) inverters. It is found that the added voltage-based synchronization loop exploits the phase angle difference to resist frequency deviation, whose function is similar to the damper winding of synchronous generators. By resembling the large-signal model of such hybrid synchronization control (HSC) to the power-angle swing equation, it is proved that the HSC brings higher damping to the system and thus, enhances the transient stability of GFM inverters. Further, a comparative study of two active power controllers, i.e., the droop controller with a low-pass filter and the proportional-integral (PI) controller, is given. It is revealed that the HSC with the droop controller equivalently decreases the active power reference, which leads to better transient stability behavior than that of using the PI controller. The theoretical findings are corroborated by experimental tests. [ABSTRACT FROM AUTHOR]

Renewable energy sources (RESs) are highly penetrated in power systems through phase-locked loop (PLL) synchronized converters. Under severe grid voltage sags, PLL-synchronized RES system is susceptive to transient instability and tripping from the grid due to inappropriate current injection. The existing literatures mainly focus on synchronization instability issues of a single grid-connected RES system, and the effect of the interaction between them is rarely studied. To fill this gap, this paper aims to clarify the interaction mechanism and characterize stability boundary of paralleled PLL-synchronized RES system with different converter parameters. More specifically, the effect of current injection angle and magnitude on transient synchronization stability is investigated for guiding current injection reference design during low voltage ride-through (LVRT) and rated capacity option of RESs. First, the transient interaction model of the paralleled converter system is developed, and interaction mechanism under different current injection angles is revealed. Next, stability region analysis is performed to evaluate the impact of current injection magnitude, wherein current injection boundary is obtained. Then, a current injection design method is proposed for improving both transient stability and voltage support. The theoretical analysis and the proposed method are validated by simulation results. [ABSTRACT FROM AUTHOR]

Deep learning (DL) techniques have shown promising performance for designing data-driven power system transient stability assessment (TSA) models. However, due to the deep structure of the DL, the resulting model is always a black-box and hard to explain, which hinders its practical adoption by the industry. This paper proposes an interpretable DL-based TSA model to balance the TSA accuracy and transparency. The proposed method combines the strong nonlinear modelling capability of a deep neural network and the interpretability of a decision tree (DT). Through regularizing DL-based model with the average DT path length in the training process, the proposed interpretable DL-based TSA method can visually explain the TSA decision-making process. Simulation results have shown that the proposed method can deliver highly accurate TSA results and interpretable TSA decision-making rules, which can be used for designing preventive control actions. [ABSTRACT FROM AUTHOR]

Attributed to the ability of inertia provision and good regulation performance, the virtual synchronous generator (VSG) has been proven as a promising solution to the problems introduced by using converter interfaced generation. Although the small-signal stability control of the VSG has been widely analyzed, the transient characteristics still needs more study, which belongs to a large-signal problem. In this article, the transient angle stability control of the VSG is investigated. Two possible VSG emulation methods, i.e., the power-emulation and the torque-emulation, are compared from the perspective of transient characteristics. Then, the transient of the VSG internal voltage and its impact on the transient angle stability are quantitatively studied in details. Thereafter, an enhanced VSG controller is proposed, where a large-signal analysis is also presented to evaluate its influence on the acceleration and deceleration areas. Finally, the analysis and effectiveness of the enhanced VSG method are validated by the experimental results. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this article, for fast transient stability status (TSS) prediction, a new scheme is proposed, which does not require any postfault data and predicts TSS using data measured until the end of the fault-on period. In this method, phasor measurement units (PMUs) are located at synchronous generators' (SGs) buses to gather time-synchronized data. Then, these measurements are applied to the proposed procedure to calculate the feature set utilizing fault-on and prefault information and predict TSS by a decision tree classifier. Since the feature set does not include any postfault (i.e., after short-circuit clearance) data, the proposed method has the capability of fast prediction of postdisturbance TSS, and hence, it provides more opportunity for system operators and/or special protection systems to implement timely corrective actions to prevent instability. Also, the results of the time-domain analysis carried out in IEEE14-bus, IEEE39-bus, 16-Machine (68-bus), and IEEE118-bus test systems and comparison of them with previous techniques reveal that although the proposed method requires fewer PMUs, it can forecast TSS accurately under N-1 and N-2 conditions. Besides, these results confirm that, considering the measurement error, when the rotor angle of SGs is estimated using the data measured by WAMS, the proposed method accurately predicts TSS. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This letter develops an equivalent circuit model of a grid-forming (GFM) converter with a circular current limiter and analyzes its transient stability. It is revealed that the inner control loop can be simplified as a voltage source behind an equivalent resistor. Based on the developed model, theoretical analysis and experimental tests demonstrate that the transient stability of a $P-f$ droop-controlled GFM converter with the circular current limiter can be assured whenever there exist stable equilibrium points. [ABSTRACT FROM AUTHOR]

This letter proposes a novel transient stability criterion for power systems with multi-wind farms, which is deduced from power flow and inertia distribution. The variations of acceleration and deceleration power with and without wind integration, instead of the acceleration and deceleration areas themselves, are calculated, in which the power angle swing direction, the transient stability control of wind turbines, the position of wind farms access, and the unbalanced output of synchronous clusters are also considered. The simulation results show the effectiveness of the proposed criterion to distinguish transient stability. [ABSTRACT FROM AUTHOR]

This paper proposes a Principal Component Analysis (PCA)-based method to construct composite indices to rapidly assess power system transient stability. By assigning appropriate weights to a few selected sub-indicators, a composite index can be constructed to guarantee a better coverage of worst-case stability scenarios of the power system. This paper provides an automatic method to construct a composite index with four sub-indicators calculated by the integral of acceleration, speed deviation and rotor angle of generators over a certain integration period using PCA. The effectiveness of the sub-indicators is validated first, followed by a comprehensive example of process of composite index construction, and the assessment of the influence of the effect of integration period on the performance of the composite index. High accuracy of classification has been demonstrated using composite index with appropriate settings of the integration periods of the sub-indicators. [ABSTRACT FROM AUTHOR]

This paper proposes a Transient Stability Constrained Optimal Power Flow (TSCOPF) formulation that models non-synchronous renewable generation equipped with synthetic inertia. The proposed optimization problem calculates the optimal operating point of the system, accommodating high shares of non-synchronous renewable generation while ensuring transient stability in the event of critical incidents. Synthetic inertia controllers are used to improve the dynamic stability of the system in cases of very high share of renewable generation. The proposed tool is tested in the North-West Spanish system, a network with a high penetration of wind energy that causes a reduction in the total system inertia. The results of the study show that 1) synthetic inertia in renewable power plants can diminish electromechanical oscillations after a severe contingency, reducing the cost of ensuring transient stability; 2) using synthetic inertia the system becomes more stable when conventional generation is decommissioned following de-carbonization and renewable promotion policies; and 3) the proposed model can be used to calculate the parameters of the synthetic inertia control. [ABSTRACT FROM AUTHOR]

A data-driven approach for improving wide-area situational awareness (SA) in modern power systems for building a more resilient grid is proposed in this article. SA is of paramount importance when it comes to accurate sensing of events for better visualization of the power system dynamics. Various events uniquely excite particular states of the power systems resulting in a maximum change in the entropy for certain power system components. A new conceptual framework utilizing structure preserving energy function (SPEF) for identifying the specific component of the energy function and correlating it with the appropriate event has been suggested. Cubature Kalman filter-unknown input based dynamic state estimation technique has been employed for the extraction of states and unknown inputs from the phasor measurement unit (PMU) measurements for the derivation of the SPEF-based event indicators. In order to demonstrate the efficacy of the proposed technique, it has been validated on the IEEE 39-bus system and also on actual PMU data obtained from the Indian power grid. The results indicate the suggested technique to be accurate and robust to noise and thus can be a promising tool for real-time power system monitoring. [ABSTRACT FROM AUTHOR]

In this paper, we use a nonlinear model of a grid-connected Archimedes wave swing (AWS) to explore the actual generation potential of this wave energy conversion system. Our model has never been used before for a grid-connected AWS. The control system employs six proportional-integral (PI) controllers to maximize the energy harvest from waves, to minimize the generator power losses, and to maintain both the grid and DC link voltages at their reference values. The PI controller parameters were selected using a hybrid augmented grey wolf optimizer and cuckoo search (AGWO-CS) algorithm. The results obtained from MATLAB Simulink for different types of grid fault, with successful and unsuccessful reclosing of the circuit breakers, were compared with results from the particle swarm optimization (PSO) and COOT algorithms. To verify the accuracy of our control system, a grid-connected AWS was tested experimentally using a real-time RT-LAB simulator combined with OP4510. The results validated the efficiency and superiority of the control system using AGWO-CS and the results from simulation and experiment were similar. [ABSTRACT FROM AUTHOR]

This paper demonstrates the potential of inverter-based loads to support grid reliability during power system transients thereby enabling reliable integration of renewable energy in power systems. Such loads are referred to in this paper as grid-supportive loads (GSLs). A new GSL model is developed that simulates the transient response capabilities that can be programmed in electronic loads. The model’s design enables it to be easily integrated in widely used commercial power system transient analysis software. Theoretical expressions are derived that explain the workings of the GSL model. The performance, numerical stability, and impact of the GSL model is validated on 9-bus and 2000-bus synthetic power system models using generator tripping and bus fault disturbances. Results on the 2000 bus system show that in the absence of frequency support from wind/solar generation resources, just 20% of loads with grid-supportive capabilities can improve frequency response by up to 2000 MW/0.1 Hz and reduce deviation in frequency at nadir by up to 60% compared to the situation when GSLs are absent. Power system reliability also improves under fault events. It is further shown that GSLs can aid in integrating more renewable generation without degrading the overall transient response of the power system. [ABSTRACT FROM AUTHOR]

With widespread application of voltage source converter (VSC) as a key energy-conversion power electronic device by using the phase-locked loop (PLL) technique for synchronization, the system dynamics has become much complicated. In this paper, the nonlinear dynamics and transient stability of the PLL-based VSC system are investigated, within a unified framework of the (normalized) generalized swing equation. It is found that there are three different types of bifurcation, including the generalized saddle-node, Hopf, and homoclinic bifurcations. Within the coexistence parameter region, the basin boundary of the stable equilibrium point shows either a closed-loop or a fish-like pattern. With the help of the equal area criterion (EAC), the transient stabilities under different transient disturbances including short circuit, voltage dip, and power rise are analyzed. Because the equivalent damping of the VSC is state-dependent, the theoretical results based on the EAC are examined. Furthermore, based on the analytical results from the generalized swing equation, the principle for all major transient stability enhancement methods is uncovered. All these findings are well verified by extensive electromagnetic transient simulations. Therefore, the generalized swing equation provides a deeper physical insight and plays a crucial role in transient stability problems in power-electronic-dominated power systems, similar to the swing equation in traditional power systems. [ABSTRACT FROM AUTHOR]

In this paper, the transient stability for dc-link voltage of doubly-fed induction generator (DFIG)-based wind turbine (WT) is studied in detail during low voltage ride-through (LVRT). Firstly, referring to the rotor swing equation of synchronous generator (SG), the nonlinear large-signal model of dc-link voltage is established. In addition, the damping power, static slip power, and dynamic slip power are derived. Consequently, the transient performance of dc-link voltage under the different operation conditions of DFIG is revealed during LVRT. Furthermore, the instability form of dc-link voltage with the moving process of equivalent power angle is analyzed by the energy function of dc-link voltage. In addition, the impacts of dc voltage control loop's parameter, the active current of the stator, and the slip on the stabilization process of dc-link voltage are studied. Analysis result indicates that the insufficient damping and large unbalanced power would deteriorate the transient behavior and steady-state level of dc-link voltage. Therefore, an additional damping and slip power feedforward control strategy is proposed, which can make the transition process of dc-link voltage smoother and significantly improve the voltage steady-state level. Finally, simulation and experimental results validate the effectiveness of theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper investigates the transient stability of power systems co-dominated by different types of grid-forming (GFM) devices. Synchronous generators (SGs and VSGs) and droop-controlled inverters are typical GFM devices in modern power systems. SGs/VSGs are able to provide inertia while droop-controlled inverters are generally inertialess. The transient stability of power systems dominated by homogeneous GFM devices has been extensively studied. Regarding the hybrid system jointly dominated by heterogeneous GFM devices, the transient stability is rarely reported. This paper aims to fill this gap. It is found that the synchronization behavior of the hybrid system can be described by a second-order motion equation, resembling the swing equation of SGs. Moreover, two significant differences from conventional power systems are discovered. The first is that the droop control dramatically enhances the damping effect, greatly affecting the transient stability region. The second is that the frequency state variable exhibits a jump at the moment of fault disturbances, thus impacting the post-fault initial-state location and stability assessment. The underlying mechanism behind the two new characteristics is clarified and the impact on the transient stability performance is analyzed and verified. The findings provide new insights into transient stability of power systems hosting heterogeneous devices. [ABSTRACT FROM AUTHOR]

The safety of grid-connected voltage-controlled inverter is threatened by transient angle instability due to fault. The impact of different active power control (APC) and reactive power control (RPC) on the inverter's transient angle stability hasn't been fully investigated. To fill this gap, firstly, a unified model of the voltage-controlled inverter considering different APC and RPC is put forward. Based on the proposed unified model, the effect of different APC and RPC loops on the inverter's transient angle stability are qualitatively analyzed by using equal area criterion (EAC). Then, the TS multimodeling method which is based on the Lyapunov function is applied to evaluate the effect of control parameters on the inverter's transient angle stability quantificationally. Finally, the above analysis is verified based on an experiment. It is found that lowering inertia improves the inverter's global stability but decreases the inverter's first swing stability. Transient stability of the inverter can be improved by increasing damping component of the inverter. Transient angle stability of the inverter is influenced by the RPC loop. With the increase of the output voltage of the RPC loop, reference reactive power, and output voltage of the inverter, transient angle stability of the inverter can be improved. [ABSTRACT FROM AUTHOR]

The impact of dc-bus voltage control (DVC) on transient synchronization stability of grid-tied inverters operating in grid-feeding mode has been assessed in this brief. A nonlinear quasi-static model magnifying the dynamics of phase-locked loop (PLL) and DVC of grid-tied inverter has been developed first. Following this, the steady-state and transient stability boundaries of the model are analytically characterized and numerically evaluated. The proposed model and the stability assessment method eliminates the inaccuracies associated with the previously developed model which considers the dynamics of PLL alone. [ABSTRACT FROM AUTHOR]

This article presents a distributed approach to predict transient and short-term voltage instability impending during post-fault interval. The proposed approach employs multiagent system and considers each generation bus as an intelligent agent, in order to predict stability status locally. Since transient and short-term voltage stability may exist solely or simultaneously, a universal model-free prediction method has been employed to distinguish between stable, critical stable, and unstable cases. The proposed prediction algorithm gives an early prediction of transient and short-term voltage instability by investigating four patterns on the trajectory of relative frequency deviation and inertia frequency response and simple negotiation between generation bus agents. In order to evaluate the performance of the proposed scheme, it has been tested on the IEEE 39-bus system, IEEE 118-bus system, and IEEE Nordic system, where the highest performances have been achieved. [ABSTRACT FROM AUTHOR]

By increasing the penetration of wind power in the modern power systems, to investigate transient stability is of special importance. This article presents an innovative index based on the concept of the potential energy boundary surface along with kinetic energy, which aims to detect the transient instability of a power system comprising doubly fed induction generator (DFIG)-based wind farms. Accordingly, transient instability detection (TID) is performed without directly calculating the unstable equilibrium point and thus the computational load is decreased. Since the proposed approach requires only the postfault data that can be easily measured by the phasor measurement units, it is suitable for real-time applications. Therefore, the proposed approach can be applied as a general tool to any power system with any change in topology and operating conditions. Moreover, considering that the current-balance form is the preferred industry model for the implementation of transient stability simulation, the network equations in the current-balance form are extracted for grid-connected DFIG and synchronous generators. Different scenarios are simulated in the Western System Coordinating Council 3-machine, 9-bus system, and the 10-generator New England system. To validate the new index for TID, simulation results are compared with the transient stability index and out-of-step distance relay. The obtained results validate the correctness and effectiveness of the presented new index. [ABSTRACT FROM AUTHOR]

Insufficient inertia is the paramount challenge in power system transition toward $ 100\%$ converter-based generation. A promising solution to this issue is utilizing distributed virtual inertia (DVI) concept, i.e., releasing energy stored in the dc-link capacitors of power converters employed in the grid following a frequency disturbance. Nevertheless, this method has two drawbacks: $ 1$) the dc-link voltage is not reverted to its reference value after the power mismatch between generation and demand, and $ 2$) it yields instability of a local mode associated with the control system when the converter operates in weak grids. To overcome the aforesaid problems, the conventional DVI loop is modified so as not to affect the outer loop voltage regulator after transient. Moreover, an efficient compensator is presented in this article which eliminates the adverse impact of DVI technique on the converter stability in weak grid connections. The efficacy of proposed control scheme is depicted by simulations in MATLAB. The results illustrate that an improvement of $ 23\%$ is evident in the grid frequency rate of change following a frequency contingency collated with the case in which the DVI loop staying nullified. [ABSTRACT FROM AUTHOR]

It is well known that grid-feeding converters that synchronize to the grid through a Phase-Locked Loop (PLL) can become unstable after a fault. An often-neglected element that plays an important role in the converter synchronization stability is the PLL frequency limiter. While it slows down the phase change during the fault, the frequency limiter also constrains the error of the PLL input, thus leading to a longer settling time. This letter investigates the mechanism of the converter synchronization stability caused by the frequency limiter and provides a taxonomy to evaluate its impact on the overall system dynamic response. [ABSTRACT FROM AUTHOR]

Since the wide application of virtual synchronous generators (VSGs), the power grid faces great challenges in the safe and stable operation due to their limited thermal capacity and weak anti-disturbance ability. During transient period, for example, a fault occurs in the transmission line, the VSG may lose the transient angle stability and provoke the current hard limit. Even if the fault is cleared by tripping of line, it still faces the problem of instability and voltage dips. To address this problem, in this paper, the post-fault large-signal model of VSG is derived first via the travelling waves based fault information acquisition. Subsequently, with the effect of both active and reactive power loops taken into account, a two-stage simultaneous control scheme is proposed for improving the transient stability of VSG, while considering the current limitation during fault state and voltage support after fault clearance. This method is fulfilled by mode switching and an additional feedback control based on the fault signal. Finally, the effectiveness of the proposed method under both symmetrical and asymmetrical faults is verified. Moreover, the application of the proposed method in a multiple VSGs system is also verified. Besides, the robustness to parameter mismatch and the feasible operating region for the method are discussed. [ABSTRACT FROM AUTHOR]

Machine learning approaches have exhibited high potential in power system transient stability assessment (TSA), yet their initial preparation stages of stability knowledge base generation (SKBG) based on time-domain simulations often undergo high computational costs. In fact, how to ease the heavy computational burden of SKBG without sacrificing the reliability is still a significant challenge. To address this problem, this paper develops a semi-supervised ensemble learning (SSEL) framework for reliable SKBG acceleration, without additional need for computing hardware upgrading. In particular, it performs detailed simulations for a minority of cases and fast simulations for the majority ones to reduce the total computation time. Considering the absence of stability status information of those fast simulated cases, an SSEL scheme is systematically designed to reliably label their stability status. Before implementing SSEL, two concise feature descriptors are first introduced to efficiently extract transient features from multiplex system trajectories. Then, all the cases are characterized in a unified feature space, whereby a series of semi-supervised support vector machines are trained in randomly formed subspaces. Afterward, these single machines are systematically combined to derive an enhanced SSEL model, which is able to make reliable and robust labeling decisions. Further, a backtrace strategy is carefully devised for SSEL, so as to maintain the high reliability of SKBG. Test results on the IEEE 39-bus system and the realistic GD Power Grid in South China illustrate the excellent performances of the proposed framework on SKBG acceleration. [ABSTRACT FROM AUTHOR]

In this paper, a distributed predefined-time fractional-order sliding mode controller (DPTFOSMC) is proposed for performance improvement of power system. The proposed DPTFOSMC is designed based on the multi-agent framework and distributed control theory, which can reduce the pressure of central controller in communication and calculation, thus obtaining fast and coordinated response. The DPTFOSMC can achieve the advantages of sliding mode control (SMC) with fast response and strong robustness for the power system. In addition, the power system under the proposed DPTFOSMC can be stabilized in stable range within a predefined time which is directly equal to the tunable parameter, thus meeting the high requirement for convergence time of the power system. The prescribed performance function (PPF) is carried out to design the DPTFOSMC, which can ensure that the power system can achieve the expected transient and steady responses. Meanwhile, fractional calculus is implemented to devise the DPTFOSMC to eliminate the chattering phenomenon of conventional SMC. The Lyapunov function is adopted to analysis the predefined-time stability of the power system under the proposed DPTFOSMC. Simulations are taken to validate the effectiveness and superior performance of the proposed DPTFOSMC for the power system. The results show that the DPTFOSMC can ensure the power system to achieve superior performances than the existing control methods. [ABSTRACT FROM AUTHOR]

Few studies have focused on assessing the transient and steady-state voltage stability status of dynamic systems simultaneously. This motivated us to propose a new concept referred to as joint voltage stability assessment (JVSA). Towards this end, this paper proposes a novel data-driven JVSA method considering load uncertainty. It combines multiple convolutional neural networks (multi-CNNs) and a novel variational Bayes (VB) inference for better JVSA accuracy. First, the multi-CNN model is utilized to fast estimate the maximum voltage deviations during the transient and steady-state process. Uncertain load scenarios and system topology under $N$ -1 contingency with are chosen as inputs of each CNN model. Second, estimated voltage deviations are put into the VB inference to automatically infer the transient and steady-state voltage stability status. To validate its effectiveness, numerical simulations are performed on the modified WECC 179-bus system by comparing with benchmark algorithms. It is demonstrated that the proposed data-driven JVSA method is more accurate and faster than the conventional VSA method. [ABSTRACT FROM AUTHOR]

Cosimulation techniques are gaining popularity amongst the power system research community to analyze future scalable smart grid solutions. However, complications such as multiple communication protocols, uncertainty in latencies are holding up the widespread usage of these techniques for the power system analysis. These issues are even further exacerbated when applied to digital real-time simulators (DRTSs) with strict real-time constraints for power hardware-in-the-loop (PHIL) tests. In this article, we present an innovative digital real-time cosimulation infrastructure that allows interconnecting different DRTS through the Aurora 8B/10B protocol to reduce the effects of communication latency and respect real-time constraints. The proposed solution synchronizes the DRTS interconnection by means of the IEEE 1588 precision time protocol (PTP) standard to align executions and results obtained by the cosimulated scenario. The ideal transformer method interface algorithm, commonly used in PHIL applications, is used to interface the DRTS. Finally, we present time-domain and frequency-domain accuracy analyzes on the obtained experimental results to demonstrate the potential of the proposed infrastructure. With the presented setup, a time step duration down to 50 $\mu$ s is shown to be stable and accurate in running an electromagnetic transients cosimulated power grid scenario by interconnecting two commercial DRTS (i.e., RTDS NovaCor), extending the scalability of future smart grid real-time simulations. [ABSTRACT FROM AUTHOR]

The virtual synchronous generator (VSG) control of a grid-connected converter is an attractive interfacing solution for high-penetration renewable generation systems. Unfortunately, the synchronous resonance can appear due to the power control loops, which are usually damped by adopting a virtual resistance (VR). However, the effects of VR on the transient stability of the VSG are rarely studied. In this article, a virtual point of common coupling and a virtual power angle concept are proposed to represent the mathematical model of the VSG with VR damping. Based on the model, the transient stability is further analyzed using the phase portrait and the attraction regions of the nonlinear system. It reveals that the VR has negative and different impacts on the transient stability compared with the real grid resistor. In order to keep the VSG with VR to work normally during the grid voltage sag, an enhanced transient stability method, by reducing the active power commands when the grid fault is detected, is introduced. A design-oriented analysis and the parameter design with different VRs are also presented. Finally, the theoretical analysis is verified by the experimental results. [ABSTRACT FROM AUTHOR]

Most dc power distribution architectures are organized as a typical cascaded converter system that is composed of source and load converters, and may experience a system oscillation due to the negative impedance effect of constant power loads. The load-side virtual impedance method is a possible solution to such stability problem, which however makes the dynamic response of the load converter slow, and thus increases the requirement of the filter capacitance. In this article, an auxiliary circuit that is connected to the output port of the load converter is proposed to enhance the transient performance of the power converter system, while the system can be stabilized by the virtual impedance method. The control and parameter design of the proposed auxiliary circuit is introduced. The capacitance requirement of the system for an expected transient deviation of the output voltage is significantly reduced by using the proposed auxiliary circuit. An example design is given to verify the feasibility of the proposed method with simulation analysis and experimental results. [ABSTRACT FROM AUTHOR]

This article develops a novel decentralized controller for multi-area secondary load frequency control (LFC) of power systems. The proposed robust-adaptive approach estimates the external disturbance input and uncertainties, which are assumed to be matched with the control input. In this regard, a disturbance-observer state-feedback controller is designed. To offer a systematic approach, controller design conditions are derived in terms of linear matrix inequality constraints. Thereby, for any given representation of the LFC area, the controller gains can be straightforwardly obtained by using numerical solvers. Moreover, in order to enhance the steady-state performance, the controller is modified through the heuristic genetic algorithm. The controller design procedure is wholly offline and can be simply implemented. The developed decentralized approach does not need the information of the other areas, which reduces the cost and the number of measuring units. It is also robust against the power fluctuations of the load and renewable energy sources. To show the superiorities of the developed controller, four scenarios are considered. These scenarios comprise the aggregation of the photovoltaic, wind turbines, and electric vehicle to the power system. OPAL-RT experiments are given to verify the transient and steady-state performance and robustness of the proposed controller. [ABSTRACT FROM AUTHOR]

Under grid fault, the grid-connected voltage source converters (VSCs) have the risk of transient synchronization instability during low voltage ride through. Based on the equivalent rotor swing equation of VSC, this letter proposes an improved transient synchronization stability control strategy, which improves the existence of equilibrium points and transient synchronization behaviors of the VSC system. Thus, the transient synchronization stability of grid-connected VSC can be significantly enhanced. The simulation and experimental results demonstrate the validity of the proposed strategy. [ABSTRACT FROM AUTHOR]

Grid-connected converters are exposed to the loss of synchronisation with the grid during severe voltage sags particularly when operating under weak grid condition which introduces voltage and frequency volatility. This paper presents employing machine learning methods besides modifying the converter control scheme to enhance the transient stability of power synchronization control (PSC). For early detection of synchronization instability of PSC to provide adequate time for taking correcting control actions, an encoder stacked classifier is proposed which is trained to be robust against data corruption and added noise. Then, by integrating the proposed instability detection scheme to the synchronization loop of PSC, a phase freezing mode is introduced to avoid losing synchronism during grid faults. It is disclosed that the frozen synchronization loop, which is activated by the proposed instability detection scheme, can ensure synchronization stability of PSC. Time-domain simulations are conducted to confirm the presented findings. [ABSTRACT FROM AUTHOR]