Transient Angle Research Articles

GCN-LSTM Based Transient Angle Stability Assessment Method for Future Power Systems Considering Spatial-Temporal Disturbance Response Characteristics

GCN-LSTM Based Transient Angle Stability Assessment Method for Future Power Systems Considering Spatial-Temporal Disturbance Response Characteristics

Two-Stage Transient Control for VSG Considering Fault Current Limitation and Transient Angle Stability

Two-Stage Transient Control for VSG Considering Fault Current Limitation and Transient Angle Stability

Transient angle stability analysis of maximum Lyapunov exponent based on the key branch response information

Based on real-time access to response information on wide-area tributaries, in order to assess the system stability more quickly, this paper combines the simplified branch transient transmission capability (sBTTC) index and the largest Lyapunov exponent (MLE). An online identification method for transient power angle stability is proposed. Firstly, the method is based on a two-machine system to study the consistency of the power angle difference between the two ends of the critical branch and the change of the phase difference of the fleet. Secondly, the transient power angle stability problem is transformed into the MLE analysis problem according to the change rule of sBTTC index of the key branch. Then, by combining the characteristics of MLE curve and sBTTC index curve, the transient power angle stability criterion is given. Finally, the proposed method is verified and analysed by simulation examples. The method has strong industrial applicability because it does not need system model, the information source is easy to measure, it has strong anti-noise ability, and it overcomes the defect of the traditional method that needs to set a fixed symbolic observation window.

Transient Angle and Voltage Stability of Grid-Forming Converters with Typical Reactive Power Control Schemes

Transient Angle and Voltage Stability of Grid-Forming Converters with Typical Reactive Power Control Schemes

Transient Stability of Paralleled Virtual Synchronous Generator and Grid-Following Inverter

The majority of inverter-based resources (IBRs) currently operate as grid-following inverters (GFLIs). However, these inverters exhibit certain stability issues when integrated in low-strength areas of the grid. To enhance the grid strength in a GFLI-dominant area, virtual synchronous generators (VSGs) can be installed. If the VSG and the GFLI are investigated as a paralleled system, despite the benefits brought by the VSG, in some cases, the transient angle instability process, caused by a voltage sag, is accelerated, making the whole system more prone to instability. Therefore, this paper aims to study the transient angle stability of a paralleled VSG-GFLI system via investigating the voltage at a common bus between the two IBRs. Subsequently, a stable region of the voltage angles of the two IBRs is determined. Based on this stable region, the stability margin of the paralleled system can be determined and used to quantitatively evaluate the system stability. Moreover, an additional control loop is proposed to improve the transient stability of the paralleled system in this paper. The proposed controller can adaptively reduce the power set-point of the VSG to avoid a complete failure of the VSG even when no stable equilibrium point exists during a voltage sag. The stability investigation and performance evaluation of the proposed method are conducted in PSCAD/EMTDC and experimentally validated.

Investigating the transient angle in a unidirectional composite in tension and compression

In unidirectional composite laminates, when there is a slight deviation between fiber and load-applying direction, the failure mode in tensile and compression will be changed from fiber breakage and buckling, respectively, and will be turned to matrix cracking. The transient angle is the angle of misalignment between fiber and load-applying direction, in which the strength properties of the composite will sharply and considerably decrease due to the change of failure mode. In this paper, the transient angle has been investigated experimentally in fiber-reinforced glass epoxy unidirectional composites. To determine the transient angle, certain unidirectional composite specimens with specific digression angles from the load-applying direction were manufactured due to standards, and tensile and compression tests were performed on them. Results demonstrate that the tensile transient angle is between 4° and 8°, and the compressive transient angle is between 8° and 12°. Moreover, experimental results were compared with analytical calculations using Hashin’s failure criteria. Investigations showed that Hashin failure criteria can adequately predict the amount of load causing failure in unidirectional composite laminates before the transient angle. Still, by approaching this angle, Hashin’s failure criteria can’t correctly predict the load causing failure, and error will increase gradually as it passes the transient angle. This error will occur after the transient angle because Hashin’s Failure Criteria can’t predict changes in failure modes in unidirectional composite laminates.

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

Thanks to the ability of the inertia provision and the voltage support, the virtual synchronous generator with reactive power control (RPC) is a promising solution among various grid-forming control schemes. This paper analyzes the transient angle stability of the grid-connected type-4 wind turbine with virtual synchronous generator and reactive power control strategy during the fault. By deriving the analytical criterion with the transient energy function, various key factors on the transient stability of the system can be revealed comprehensively and jointly. Additionally, the impacts of the integral coefficient of the RPC and the power reference switching delay on the transient stability are also analyzed by plotting the phase portraits of the derived third-order large-signal model of the grid-connected wind turbine system. The analytical result can be helpful for the determination of the parameters of the grid-connected wind turbine in order to keep transient stable and keep connected with the grid during the fault. Finally, the robustness and the effectiveness of the theoretical analysis results are validated by the electromagnetic transient simulation in MATLAB/Simulink under various conditions.

Integrated advance assessment of power system transient voltage and transient angle stability based on two-stage ensemble spatio-temporal graph neural network

Transient angle stability (TAS) and transient voltage stability (TVS) are important bases for safe operation of power system. With the replacement of a high percentage of new energy, the voltage and power angle problems are closely coupled, so an integrated assessment method that takes into account the rapidity, accuracy and stability is urgently needed to deal with the system risks. In order to solve this problem, firstly, we propose a multi-task spatio-temporal graph neural network model based on two-stage ensemble learning (TSEL-MT-STGNN) for integrated assessment of TAS&TVS. In the first stage of this model, the topological spatial features of the system are extracted through multi-type graph neural networks, and the obtained knowledge is input to the gated recurrent unit (GRU) for further temporal feature extraction, so as to achieve spatio-temporal information fusion. In this stage, TAS and TVS were combined through the multi-task framework to achieve integrated assessment under the shared features. In the second stage, meta-learner multi-layer perceptron (MLP) is used to fuse the output results of multi-type spatio-temporal graph neural network. Secondly, an integrated assessment method is proposed based on the model, which enables end-to-end assessment, and gives the stability result and safety margin of the system in the current state without knowing the fault clearing time. Thirdly, in order to ensure the timeliness of the assessment method, a multi-graph parallel training scheme is proposed to accelerate the training process. By introducing time constrained loss function and dynamic weight training, the response time is greatly reduced on the basis of maintaining the existing accuracy, thus achieving advance assessment. Finally, the improved New England 39-bus system is taken as an example for verification and analysis. The results show that the proposed method can achieve high-precision advance assessment of TAS&TVS, and has strong robustness and anti-noise capability.© 2017 Elsevier Inc. All rights reserved.

Multi-task transient stability assessment of power system based on graph neural network with interpretable attribution analysis

Transient power angle stability analysis and voltage stability analysis are the key basis of power system security and stability control. The diversified operation modes of power system put forward higher requirements for real-time and topology generalization of transient stability assessment. Based on Convolution Neural Network (CNN) and Graph Attention Networks (GAT), this paper uses the wide-area transient response features to evaluate the transient angle stability and voltage stability of power system. Firstly, CNN is used to encode the temporal variation characteristics of transient features of electrical components in hidden layer space. Secondly, the encoded hidden layer features are mapped into graph data samples according to the spatial topological correlation of electrical components, so as to train and build a multi-task transient stability assessment model based on GAT. The model can adaptively capture the interaction relationship between encoded hidden layer features of topologically correlated components, and realizes real-time evaluation of transient angle stability and transient voltage stability by perceiving the temporal–spatial correlation of transient features. Finally, Shapley additive explanation (SHAP) method is used to explain the decision-making mechanism of the model and extract the key features that affect the transient stability, so as to enhance the ability of power grid operators to perceive the transient stability situation. The results of IEEE-39 system show that the transient stability assessment model has good accuracy and topology generalization, and the results of interpretable analysis are in line with the conclusions of traditional mechanism cognition, which helps to improve the credibility of the model results.

Enhanced transient angle stability control of isolated power grid based on phase sequence exchange

An isolated power grid (IPG) lacks the support of the external power grid, and a single generator or load accounts for a large proportion of the total capacity. Once transient angle instability, the generator and load shedding is not effectively. Phase sequence exchange (PSE) uses power electronic devices to switch the three-phase sequence, thereby reducing the power angle of the generator by 120° and preventing the system from losing stability. This paper proposed a PSE scheme for transient angle instability applied to IPG. The local measurements of IPG are used to identify the critical generator, and the system trajectory is obtained through large-step integration. The stability is assessed by the concavity of the trajectory. Once the trajectory swings from the concave area to the convex area, the IPG loses transient stability, at which point the generators in the critical cluster simultaneously PSE prevent transient instability. Based on Simulink and RTDS platform, the scheme is verified in a 39-bus 10-machine system and an existing IPG system. The results show that the proposed scheme can effectively restore the stability of the IPG, outperforming generators and load shedding.

New Energy Power System Dynamic Security and Stability Region Calculation Based on AVURPSO-RLS Hybrid Algorithm

Using a high proportion of new energy is becoming the development trend of the modern power industry, with broad application prospects and potential threats to power system operation safety. This paper proposes a hybrid adaptive velocity update relaxation particle swarm optimization algorithm (AVURPSO) and recursive least square (RLS) method to quickly estimate the DSSR boundary using hyper-plane expression. Firstly, the operating point data in the high-dimension nodal injection space are analyzed using the AVURPSO algorithm to identify the key generators, equivalent search space, and critical points, which have relatively great effects on transient angle stability. The hyper-plane expression of the DSSR boundary, which matches the critical points best, is finally fitted by the RLS approach. Hence, the adopted algorithm is applied to rapidly approximate the DSSR boundary by hyper-plane expression in power injection spaces. Finally, the proposed algorithm is validated using a simulation case study on three wind farm regions of the actual Hami Power Grid of China using the DIgSILENT/Power Factory software. Consequently, the mentioned method effectively captures the security stability boundary of the new energy power system and realizes the three-dimensional visualization space of DSSR. By leveraging the DSSR, the state analysis can be conducted rapidly on several parameters, including security and stability assessments in relation to various energy supply capabilities. Meanwhile, these indices are calculated offline and applied online. The findings of this investigation confirm the efficacy and accuracy of the suggested modeling used in the analyzed system, offering technical assistance ensuring the stability of the new energy power system. The DSSR allows the rapid analysis of several parameters, including security and stability assessments with various energy supply capabilities.

Angle Instability and Oscillations Control using SVC: A Deep Reinforcement Learning Enhanced Local Controller

An advanced control scheme to deal with transient angle instability and low-frequency angle oscillations in power systems is proposed in this paper. The control combines an existing controller (derived from the concept of Lyapunov energy functions) and deep Q-network (a deep reinforcement learning algorithm) to control a static VAr compensator. This control is modified in this paper in a way to directly control thyristor controller reactor part of the compensator, offering an easier implementation. The aim is to improve performances of existing control through the use of deep Q-network while retaining its basic system stabilization characteristics. Advantages of the proposed control scheme are illustrated by implementation of a model of four-machine test power system (this system is considered as the benchmark when studying the phenomena dealt with in this paper) in MATLAB/Simulink environment and using TensorFlow toolkit for deep Q-network usage.

Transient stability analysis and enhancement control strategies for interconnected AC systems with VSC-Based generations

This paper analyzes the transient angle stability mechanism of interconnected AC power systems with voltage source converter (VSC) based generations and presents a stability improvement control strategy for the VSCs. Firstly, the mathematical model of the prototype AC system with both synchronous generator (SG) and VSC is developed, considering the dynamics of the pre-fault, fault-on, and post-fault systems. For the heterogenous three-machine system, the analytical method for the critical clearing time (CCT) is proposed for transient stability assessment. Then, the evolution of the transient stability margin of the AC system is studied, considering the impact of the VSCs. Under the conditions that VSCs can keep synchronism with the grid during the fault, reducing the active current of the VSCs benefits the transient angle stability of SGs. Based on the mechanism analysis, an enhancement control strategy is developed. It achieves active and reactive current modulation of the VSC using the frequency measurements of the critical SGs (CSGs) in the AC system. The proposed method devotes to boosting the electromagnetic power of CSGs during the entire transient process. Finally, time-domain simulations in PSCAD/EMTDC demonstrate the theoretical analysis. The effectiveness and robustness of the enhancement controller are further validated by the time-domain simulations in a 2-area interconnected system with 4 SGs and a modified New England 39-bus test system.

Reduced Model of Droop-controlled Converters for Transient Simulation Analysis

In recent years, the transient stability problems encountered by power grids have increasingly attracted interest due to the connection of renewable energy resources. One of the effective methods for studying the transient characteristics of a system is transient simulation analysis. To reduce the simulation time, especially in a system with numerous renewable energy resources, model reduction has been widely employed. However, when a general reduced model is adopted to analyze different transient characteristics, a tradeoff between accuracy and simulation speed emerges. In this study, reduced model selection is implemented to resolve this problem. Considering the droop-controlled converter as an example, this approach allows the transient simulation analysis of voltage, frequency, and power angle characteristics. First, a full-order model of the droop-controlled inverter is formulated. Thereafter, the singular perturbation method is applied to derive different reduced-order models. Then, considering accuracy and simulation speed simultaneously, the most suitable reduced models for transient angle, voltage, and frequency simulation analyses are determined through time-domain simulation comparison. Finally, the simulation results based on Matlab/Simulink are used to verify the correctness of the reduced model selection.

Enhanced Transient Angle Stability Control of Grid-Forming Converter Based on Virtual Synchronous Generator

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.

A Unified Model of Voltage-Controlled Inverter for Transient Angle Stability Analysis

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.

A Two-Stage Simultaneous Control Scheme for the Transient Angle Stability of VSG Considering Current Limitation and Voltage Support

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.

Coordination of Resistive SFCL and Additional Power Controller for Transient Stability Enhancement of Virtual Synchronous Generator

Virtual synchronous generator (VSG) techniques are friendly applied in inverter interfaced renewable energy sources (IIRESs), and that enables to recover the rotor inertia and provide the damping support. However, the VSG generally owns weak overcurrent ability, and its transient angle stability (TAS) under faults may be destroyed. For solving the issue, This paper puts forward a coordination method of a resistive superconducting fault current limiter (SFCL) and an additional power controller (APC). The relevant mathematical model is built, and the TAS is probed based on the extended equal area criterion (EEAC). In the coordination philosophy, the SFCL represses the fault current and prevents the disconnection of the VSG, and thereupon the designed APC is to actively adjust the VSG's power reference and positively reduce the acceleration of the virtual rotor. It is aimed to fully realize the transient power balance, and the change of the virtual rotor angle can be mitigated. Furthermore, the MATLAB platform is adopted to create a simulation model. Multiple fault scenarios are simulated, and the findings certify the efficacy and superiority of the proposed approach. The VSG successfully rides through the serious and moderate faults, and meanwhile, its transient angle stability is visibly increased.

Power System Transient Stability Assessment Based on Snapshot Ensemble LSTM Network

Data-driven methods using synchrophasor measurements have a broad application prospect in Transient Stability Assessment (TSA). Most previous studies only focused on predicting whether the power system is stable or not after disturbance, which lacked a quantitative analysis of the risk of transient stability. Therefore, this paper proposes a two-stage power system TSA method based on snapshot ensemble long short-term memory (LSTM) network. This method can efficiently build an ensemble model through a single training process, and employ the disturbed trajectory measurements as the inputs, which can realize rapid end-to-end TSA. In the first stage, dynamic hierarchical assessment is carried out through the classifier, so as to screen out credible samples step by step. In the second stage, the regressor is used to predict the transient stability margin of the credible stable samples and the undetermined samples, and combined with the built risk function to realize the risk quantification of transient angle stability. Furthermore, by modifying the loss function of the model, it effectively overcomes sample imbalance and overlapping. The simulation results show that the proposed method can not only accurately predict binary information representing transient stability status of samples, but also reasonably reflect the transient safety risk level of power systems, providing reliable reference for the subsequent control.

Analysis of Instability in Torque Control of Sensorless PMSM Drives in Flux Weakening Region

This article analyzes the effect of angle estimation error on the sensorless torque control of the permanent-magnet synchronous motor (PMSM). The actual torque of PMSM may differ from the torque reference due to the angle error in sensorless control. Furthermore, this torque discrepancy can cause instability to the control system, especially in the flux weakening region. The instability is highly dependent on the gain setting of the controller and observer in the control loop. To mitigate the adverse effect of the angle error, a torque feedforward method considering the transient angle error is proposed. With the proposed torque feedforward method, the gain margin of the control loop can be enhanced without modifying the gain of the control system. The proposed method is verified through the simulation and experimental results.