E-SIME - A method for transient stability closed-loop emergency control: achievements and prospects

Transient voltage stability assessment of renewable energy grid based on residual SDE-Net

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.

With the development of science and technology, the cost of offshore wind power development and transmission has gradually decreased, making offshore wind power technology a research hotspot. The installed capacity has been increasing year by year. China has abundant offshore wind energy resources, which are much richer than those on land and are available for exploitation. Offshore wind speeds are stable and do not produce harmful substances. Therefore, studying the transient voltage stability of offshore wind power grid-connected systems to improve the utilization efficiency of offshore wind power and protect the ecological environment is critical. After large-scale wind power is connected to the grid, ensuring the stability of the system's transient voltage becomes crucial. This paper aims to deeply study the transient voltage stability of offshore wind farm grid-connected systems, providing theoretical support for the subsequent design and operation of offshore wind farms. A simulation model of a typical power system integrated with offshore wind farms was established using PSAT. Relevant transient voltage stability indices were calculated, and the impact of varying wind speeds on the transient voltage stability of the grid-connected system was investigated. Static Var Compensators (SVC) and Static Synchronous Compensators (STATCOM) were installed for comparative analysis. The results demonstrate that excessively low wind speeds degrade the system's transient stability, and the installation of SVC provides superior improvement in transient voltage stability compared to STATCOM.

In recent years, power systems have become larger and more complicated. Deregulation of electric power industries has begun in some countries, and power system operations have become more difficult. However, the stability of the power supply is the most important consideration for customers. Therefore, power system operators should consider not only economic load dispatch but also on-line stability aspects. In this paper, the authors pay attention to one of the most important transient stability indices [critical clearing time (CCT)] and propose a new transient stability preventive control method using linear relationships between CCTs and generator rotor angles. At first, CCT calculations, as contingency screenings, are carried out to find contingencies that have smaller CCTs than predefined target values. The target values are taken as larger values than the actual circuit breaker operating times in the power system. A preventive control to achieve a more stable power system operating point is carried out by generator output rescheduling and generator terminal voltage control, which are determined using the relationships between CCTs and generator rotor angles. This proposed method is demonstrated using a Japanese standard power system called the IEE of Japan (IEEJ) EAST 10 machine system.

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.

Lower-limb exoskeletons are widely researched to improve walking performance and mobility. Low-level sensor-less exoskeleton motor control is attractive for consumer applications due to reduced device complexity and cost, but complex and variable transmission system configurations make the development of effective open-loop motor controllers that are responsive to user input challenging. The objective of this study was to develop and validate an open-loop motor control framework resulting in similar or greater performance vs. closed-loop torque control. We used generalized linear regression to develop two open-loop controllers by modeling motor current during exoskeleton-assisted walking; a "complex" model used desired torque and estimated ankle angular velocity as inputs, while a "simple" model used desired torque alone. Five participants walked at 1.0-1.3 m/s on a treadmill with closed-loop and both open-loop controllers providing ankle exoskeleton assistance. Both open-loop current controllers had similar root-mean-squared torque tracking error (p=0.23) compared to the closed-loop torque-feedback controller. Both open-loop controllers had improved relative average torque production (p<0.001 complex, p=0.022 simple), lower power consumption (p<0.001 for both), and reduced operating noise (p=0.002 complex, p<0.001 simple) over the closed-loop controller. New control models developed for a different ankle exoskeleton configuration showed similar improvements (lower torque error, greater average and peak torque production, lower power consumption) over closed-loop control during over-ground walking. These results demonstrate that our framework can produce open-loop motor controllers that match closed-loop control performance during exoskeleton operation.

This paper presents a novel global hierarchical decision-making transient stability control system (HDTSC system) of large scale power systems based on online stability calculation. When a serious fault occurs, the HDTSC system can prevent wide-area power system outages by instantly taking remedial action, which has been calculated in advance based on online transient stability calculation using quasi-time measurements with a high degree of accuracy. By periodically estimating, online, the stability of the power system against contingencies, the control system can keep pace with various operation conditions, considerably enhancing the stability of the power system. A decentralized decision-making control method is used to improve the speed, reliability and capacity of the power system in coping with contingencies. Although it is developed for the large-scale power system of the Central China Power Group (CCPG), it also can be applied to any power system configuration of other power systems.

The development of the modern power system has led to an increasing complexity in the study of power systems, and also presents new challenges to power system stability, and in particular, to the aspects of transient stability (TS) and small- signal stability (SS). Transient stability control plays a significant role in ensuring the stable operation of power systems in the event of large disturbances and faults, and is thus a significant area of research. In this paper, the applications of nonlinear control theories in power system stability control are presented together with a brief description of the various control objectives in power systems. Issues regarding coordination of controls and new technologies are also discussed, and the implications of such issues on further research are also provided. (7 pages)

Chapter 18 - Power System Stability

FAST TRANSIENT SECURITY ASSESSMENT AND ENHANCEMENT OF ELECTRIC POWER SYSTEMS USING PATTERN RECOGNITION

The reduction of drag force and suppression of vortex shedding on a circular cylinder in electrically poorly-conducting water can be realized by utilizing the Lorentz forces. In order to control the cylinder wake effectively, three kinds of control approach, i.e. open-loop, closed-loop and optimal control methods, are proposed and discussed in the present paper. Constant Lorentz forces are used in the open-loop control process, and Lorentz forces are varied to target the zero total drag or the zero pressure drag coefficients during the control process in two closed-loop control procedures. The purpose of optimal flow control is to determine controls that effectively tailor the flow, and the key element of an optimal flow control is the minimization of a cost functional that provides a quantitative measure of the objective. Numerical simulation of the theoretical models is performed in the exponential–polar coordinates system, from which it has been shown that the above three methods can suppress the vortex shedding successfully; moreover, during the control process, the optimal control needs less energy than the closed-loop control method targeting a zero total drag or pressure drag coefficients.

Nowadays, integration of large-scale wind farms (WFs) into power systems is experiencing rapid growth. As this rapid integration can affect transient stability significantly, employing doubly fed induction generator (DFIG)-based wind turbines, which have shown better behavior regarding system stability, has attracted much attention. This research contributes to the literature by investigating the transient stability of the power system with increasing penetration of DFIG-based WFs. In the proposed framework, the current-balance form is utilized for the network equations, and in this way, transient stability is performed using time-domain simulation. According to the simulation results, when the rate of wind power generation exceeds 0.7 per-unit, the increasing trend of the critical clearing time (CCT) is reversed and the CCT decreases greatly with the increased wind power penetration. In addition, the reactive power compensation by DFIG, the gearbox ratio, the power system strength, and DFIG parameters are comprehensively investigated as effective parameters on transient stability. Since the rated rotor speed of DFIG significantly impacts the electrical torque and machine currents, the reduction of the rated rotor speed due to the change of the gearbox ratio has been investigated as one of the effective factors to improve the transient stability. The simulation results demonstrate the effectiveness of the proposed approach in improving power system transient stability.

An optimized algorithm for optimal power flow based on deep learning

This paper presents a closed-loop stator flux control method for a soft starter to prevent speed oscillations. A soft starter controls starting current of an induction motor by gradually decreasing the firing angle. However, when the conventional firing angle control method is applied to the induction motor with a low inertia rotor and load, the stator flux magnitude rapidly increases as the rotor speed reaches the rated speed. This rapid increase in stator flux magnitude results in speed oscillations. To overcome the issues, in this paper, a stator flux magnitude closed-loop control method is proposed to prevent the rapid increases of the stator flux. First, the situation where speed oscillations can occur with low inertia is analyzed, using the torque-speed curve and the equations of the stator flux and rotor current in the transient state. Then, the closed-loop controller is designed by considering the relationship between the stator flux magnitude and the input voltage. The proposed stator flux control method allows the stator flux magnitude to increase at a moderate speed and prevents speed oscillation at start-up. The MATLAB/SIMULINK simulations using the model of a lightly loaded induction motor are performed and show the effectiveness of the proposed method.

To ensure stable operation of a power system, it is necessary to analyse the power system performance under various operating conditions. Analysis includes studies such as power flow and both steady-state and transient stability. To perform such studies requires knowledge about the models used to represent the various components that constitute an integrated power system. In situations where there is a risk of loss of stability, it is necessary to apply controls that can ensure stable and uninterrupted supply of electricity following a disturbance. The subject of stability thus encompasses modelling, computation of load flow in the transmission grid, stability analysis under both steady-state and disturbed conditions, and appropriate controls to enhance stability. All these topics are covered in this book, providing a comprehensive treatment of the overall subject of stability of power systems. Topics covered include modelling of the synchronous machine; the synchronous machine connected to power systems; modelling of transformers, transmission lines and loads; power flow analysis; optimal power flow; small signal stability; transient stability; transient energy function methods; artificial intelligent techniques; power system stabilizers; series compensation; shunt compensation; compensation devices; and recent technologies. The subject matter is covered at a level that is suitable for students, scientists and engineers involved in the study, design, analysis and control of power systems.