Discriminant Analysis in Security Assessment of Power Systems by Statistical Pattern Recognition

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

Braking resistor is a very powerful tool to improve transient stability in a power system. Usually, a fixed value of the braking resistor is considered for the transient stability analysis. However, when the braking resistor is in operation, temperature of the resistor material rises above ambient temperature which ultimately causes the resistance value to increase. This paper analyzes the effect of the temperature rise of the fuzzy logic-controlled braking resistor on the transient stability in a multimachine power system. The performance of the braking resistor scheme with fuzzy controller is compared to that of with conventional proportional-integral-derivative (PID) controller. Simulation results of both balanced and unbalanced faults at different points in the system indicate that the temperature rise of the fuzzy logic-controlled braking resistors has little or almost no effect on the transient stability of the multimachine power system. Moreover, it is found that the performance of fuzzy logic controller is better than that of conventional PID controller. Thus, the proposed fuzzy control strategy provides a simple and effective method of transient stability enhancement.

This paper aims at improving the excitation control system for the doubly fed induction generator-based wind farm (WF). The proposed decentralized coordinated neural control is consisted of two controllers, i.e., a multiple model predictive controller (MPC) and an artificial neural network (ANN) controller. The MPC is based on the interaction measurement model and designed to guarantee the performance of system-wide optimization around the chosen operating points, while the ANN controller is used to calculate the weights of MPC controllers and trained to guarantee the control performance over the full operating region. The proposed control strategy allows to reduce the computational cost, and at the same time to take advantage of the distributed nature of the power system. Moreover, a simple, generic hybrid power system is used to demonstrate transient stability contributions. The problems of large-scale WF integration and its impact on transient stabilities of power systems are discussed through the eigenvalue analysis and time domain simulation. The results show that system damping and transient stability of hybrid power system are considerably improved.

This paper introduces the working principle and the mathematical model of additional power system excitation control-Power System Stabilizer (PSS). Through established a typical single machine-infinite bus power system simulation model, we simulate the synchronous generator’s transient operational characteristics following a severe disturbance. The simulation results show that the PSS can not only effectively increase the system damping, but also improve operational characteristics of the generator, considerably enhance power system dynamic and transient stability.

DISCRIMINANT ANALYSIS IN SECURITY ASSESSMENT OF POWER SYSTEMS BY STATISTICAL PATTERN RECOGNITION

Recent efforts to apply direct methods of transient stability analysis to multimachine power systems have used the so-called "energy functions." These functions describe the system transient energy causing the synchronous generators to depart from the initial equilibrium state, and the power network's ability to absorb this energy so that the synchronous machines may reach a new post-disturbance equilibrium state. Recent results have shown that not all the transient energy contributes to system separation. Indeed it has become increasingly evident that system separation depends on the energy of certain individual machines or groups of machines, which comprise the critical group, and which tend to separate from the rest of the machines (which make up the noncritical group). Thus there is a need for generating energy functions for individual machines (or for groups of machines). Using a center of inertia frame of reference, the energy function <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V_{i}</tex> for machine <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</tex> is derived. A procedure for first swing transient stability assessment is developed using the energy function of individual machines and groups of machines. The method is tested extensively on two realistic power networks (the 20-generator IEEE System and the 17-generator reduced Iowa System). An analytical justification for using the critical energy of individual machines in stability assessment is provided using an invariance principle for ordinary differential equations. Power system transient stability is analyzed using the energy function of the critical group of machines. This energy function is dependent on all state variables of the power system, and satisfies the hypotheses of the invariance theorem of La Salle, enabling us to deduce the asymptotic stability of the post-disturbance equilibrium of the entire power system. It also enables us to obtain an estimate of the domain of attraction of this equilibrium of the entire power system. The methodology advanced herein, which combines computer-aided techniques with analytical tools, yields less conservative results than were obtained in previous works that used total system energy. It is to be noted that the present results are preliminary in the sense that the mechanism of separation of the critical group of machines from the rest of the system needs further investigation.

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.

In this paper we present a robust nonlinear excitation controller of synchronous generator connected through one transmission line to an infinite bus (SMIB) to enhance transient stability. Simulation results show that transient stability of a single machine infinite bus power system under a large sudden fault can be improved.

An Efficient Methodology for Study Mode Security Assessment of Power Systems for Large Disturbances

It is becoming increasingly difficult to conduct standard stability studies on modern power systems due to the growing use of power electronic devices. Power electronic converters, regardless of their power or voltage, must contend with a rising challenge known as electromagnetic transient stability. For power system stability analysis and control, transient stability assessment (TSA) is essential. On the basis of numerical integration of the dynamics equations for an EPS under consideration at a particular disturbance, a method for assessing transient stability reserves is described. In order to improve the transient stability of hybrid AC/DC power systems, a multi-HVDC emergency coordinated modulating method has been suggested in this study. Multi-DC lines are used to build and test the AC/DC hybrid power system. Machine learning models that forecast power system transient stability are more reliable, versatile, and robust if they incorporate knowledge of power system dynamics into their data gathering process. Fault location and how disturbance effects spread over the network are critical for determining unstable regions of operation, thus this determination is made using domain knowledge. That’s why traditional machine learning models may now be used in an instance transfer learning framework thanks to a whole new data gathering technique. Therefore, the suggested solution does not rely primarily on the collected data, but also incorporates system insights.

Wind generation is increasing rapidly in the grid, which has greater impacts on the power system than ever. The impacts are more serious under grid fault. The low‐voltage ride through (LVRT) controls of doubly fed induction generators (DFIGs) may change characteristics of synchronous generators (SGs) and threaten the transient stability of power system. However, the influence on the transient stability and even the controllable powers of a DFIG has not been considered in the existing LVRT method. The contributions of the reactive power supplied by a DFIG in the oscillation process of power angle of SG were analysed. The controllable range of powers of a DFIG was researched with consideration of the rotor speed limit. An LVRT control method of a DFIG based on the controllable range was proposed to improve the transient stability of power system. Simulations show that the proposed method could enhance the power angle stability of power system under the condition of avoiding overspeed and out of service of DFIG.

Comparison between inductive and resistive SFCL in terms of current limitation and power system transient stability

Flexible DC transmission (VSC-HVDC) after a large number of access to the power system, the flexible control and unique topological structure will greatly change the system fault response characteristics. This paper aims at studying the effect of VSC-MTDC system of different topological structures and different control modes on power system transient stability. In this paper, firstly, a four terminal flexible HVDC transmission system is constructed. The statistical analysis of clearing time critical with different topologies in three phase short circuit fault is carried out. Through statistical analysis, the relationship between VSC-MTDC topology and transient stability is obtained. Based on that, studying the influence of control mode on transient stability, in turn, four converters are set to the constant DC voltage control, analysis of the impact of control on the stability of the power grid integrated proximal and distal line fault. Finally combined with extended equal area criterion (EEAC) analyze qualitatively the influence of VSC-MTDC accessing on the transient stability of power system.

Approximate dynamic programming based supplementary reactive power control for DFIG wind farm to enhance power system stability

In this paper, the doubly-fed induction generator (DFIG) for transient stability evaluations of power systems was systematically discussed, and the impacts of grid-connected large DFIG based wind farms on power system transient stability were then studied in detail. In the modeling of DFIG, an equivalent generator model was employed to reflect the combined behavior of the DFIG and its rotor converter. Furthermore, the control strategy of rotor-side converter, which was for the decoupled controls of the stator active and reactive power, was considered. The corresponding numerical simulations were carried out to explore and exploit the influences and interactions between synchronous generators and wind farm respectively on power system transient stability. In particular, different synchronous generator dynamic models and load models are considered during the simulation comparisons. Finally, some meaningful conclusions and comments are given in this paper.