Intelligent Systems for Power System Dynamic Security Assessment: Review and Classification Rui Zhang Hong Kong Polytechnic University Hong Kong rachelzhang.au@gmail.com Yan Xu Hong Kong Polytechnic University Hong Kong ee.yanxu@polyu.edu.hk Zhao Yang Dong University of Newcastle Australia zydong@ieee.org Abstract—Intelligent System (IS) technologies have shown encouraging potential in facilitating very fast power system dynamic security assessment (DSA) in recent years. In general, the development of the IS consists of four stages. This paper conducts a comprehensive and detailed review on each stage and classifies the methods involved in the stages. The classification highlights the differences between the methods and show their advantages and deficiencies. The value of this paper is that it can serve as a useful and practical guideline to assist in developing ISs for very fast DSA. Moreover, the proposed classifications can provide deep insight into the enhancement of the IS towards improved DSA performance. Zhao Xu Hong Kong Polytechnic University Hong Kong eezhaoxu@polyu.edu.hk techniques have also shown encouraging application potential to very fast DSA [6]. The initial idea that uses pattern recognition for security assessment dates back to as early as 1968 [7], while the first systematic investigation of artificial intelligence (AI) to this area was performed in late 80s [8], since then, various advanced AI techniques as well as machine learning and data mining approaches have been tried to develop very fast and “intelligent” DSA systems and encouraging demonstration results have been obtained [7]-[31]. By directly capturing the mapping relationship between system parameters (input) and the dynamic security status (output) from a database, the DSA can be accomplished by the ISs within second level. Additionally, the ISs can also extract useful knowledge on system dynamic security characteristics which can be used for security controls. Generally, the development of the IS for DSA consists of four stages: 1) Dynamic security information database generation, 2) Input-output specification, 3) Knowledge extraction, and 4) Model validation. The objective of this paper is to conduct a detailed review on the four stages, and provide clear classifications to distinguish the techniques/algorithms used in the stages. The value of this work is that it can serve as a useful and practical reference to assist in developing ISs for fast DSA, and to provide insight into the enhancement of the IS scheme towards improved performance. The remaining part of the paper is organized as follows: section II gives a general introduction of IS scheme for DSA, section III to VI review the four stages in detail and proposes classifications for each stage, section VII presents some comments and discussions on the review and classification results, and section VIII concludes the whole paper. Keywords- power system, dynamic security assessment (DSA), intelligent system (IS), review, guideline I. Ke Meng Hong Kong Polytechnic University Hong Kong eekemeng@polyu.edu.hk INTRODUCTION Dynamic security assessment (DSA) is the procedure to determine if the power system can withstand imminent disturbances (contingencies) without losing stability [1]. It’s of great importance to the secure and reliable operations of the power system. Historically, DSA is performed in off-line based on enumerative forecasting and extensive contingency simulations. However, since the early 90s, power systems have seen many drastic changes, highlighted by open power market and large connection to renewable energies which, both have driven power system into an erratic and highly unpredictable operating pattern and thereby made the conventional off-line DSA practice inadequate and noneconomical. As a result, there has been a pressing need to move DSA to real-time (or at least on-line), for continuously monitoring system dynamic security conditions so as to reduce the risk of dynamic insecurity which could lead to catastrophic blackout. At present, state-of-the-art technologies that enable realtime (or on-line) DSA can be divided into two categories: i) Speeding up time-domain simulations by distributed computing architecture and earlier termination [2], [3]; ii) Direct or hybrid DSA approaches, such as Transient Energy Function (TEF)-based method [4], Extended Equal Area Criterion (EEAC) method [5]. As a promising alternative, Intelligent System (IS) II. GENERAL INTRODUCTION OF INTELLIGENT SYSTEM Power system dynamic security assessment (DSA) involves the evaluation of the system dynamic behavior with respect to the criteria of three stability categories, i.e. rotor angle stability, voltage stability, and frequency stability, [1]. Since power system in essence is a non-linear and time-varying system, the exact evaluation of it dynamic behavior requires This work is partially supported by an open grant project of the Intelligent Electric Power Grid Key Laboratory of Sichuan Province of China. 978-1-4577-0365-2/11/$26.00 ©2011 IEEE 134 Authorized licensed use limited to: NATIONAL INSTITUTE OF TECHNOLOGY WARANGAL. Downloaded on April 29,2022 at 13:25:50 UTC from IEEE Xplore. Restrictions apply. the calculation of a large set of differential-algebraic equations in the dynamic time frame (normally up to 10 s after the disturbance(s) cleared). This process can be computationally demanding since modern power system is highly dimensional and the number of contingency to be considered is always considerable. As already mentioned, analytical approaches for DSA include time-domain simulation and direct or hybrid methods, they all rely on the mathematical formulation of the system dynamic behavior, however, they can suffer from insufficiently fast computation speed, conservative results, and limited provision of additional “interesting” information on system dynamic security characteristics. The advance of Artificial Intelligence (AI), Machine Learning (ML), and Data Mining (DM) techniques has provided another promising scheme for very fast DSA. Typically, an Intelligent System (IS) can be developed with these techniques, to directly capture the mapping relationship between system parameters (input) and the dynamic security conditions (output), and extract the additional useful knowledge for extended dynamic security analysis (e.g. preventive and corrective controls). During the on-line application phase, once the input is fed, the output, i.e. dynamic security status, can be determined within split second. Consequently, it can significantly facilitate real-time DSA in practice. In addition to the very fast computation speed, its other advantages over analytical approaches include: y Knowledge discovery, the IS can discover and extract useful information on system dynamic security characteristic, which are unseen by conventional analytical approaches. y Less data requirement, unlike analytical methods which require an accurate and detailed description of the power system, the IS only needs significant and/or available input parameters to determine the stability condition of a power system. y Unified DSA capability, if properly developed, the IS can be versatile to the three stability analysis tasks (rotor angle, voltage, and frequency stability), provided the problem complies with appropriate format. Typically, to develop an IS for DSA, there are four stages as shown in Fig.1. characteristics is generated, the database should be accurate and unbiased, and it should also be able to match practical operating conditions which need to be evaluated. Secondly, the input and the output of the IS is defined, i.e. which system parameters should be used to determine the system security, and how the security is described. Thirdly, the knowledge is extracted from the database and is reformulated for on-line application. Finally, a validation is performed on the developed IS, to examine its accuracy, reliability, and robustness, usually, refinement and/or correction are needed if the IS is shown insufficiently good. For each stage, in particular stage 2 and 3, there are different techniques and algorithms can be adopted, which however can lead to different DSA model and the practical applications, this in turn will result in accuracy and reliability requirements. In the subsequent sections of the paper, a comprehensive review on each stage will be presented, and the classification of distinct methods will be also given. III. DYNAMIC SECURITY INFORMATION DATABASE GENERATION The first stage of developing the IS is to generate a sufficient database that contains the dynamic security characteristic of the power system, i.e. dynamic security information database. It is critical to the success of the IS since it not only determines the DSA accuracy, but also has a significant impact on the robustness of the IS. Normally, the database can be constructed by historical DSA archives and simulation, and it should cover a comprehensive range of operating points (OPs), both typical and untypical conditions. Generally, it can be classified into off-line and on-line classes depending on the timing that database is generated. In literature, most of the databases are generated in off-line, based on a widely ranged set of operating conditions (varied power flow profiles and network topologies) and contingency simulations. By contrast, in [18], the authors proposed a periodic updating scheme which is associated with an on-line database generation. The idea is to produce database using online operating information, once the IS is found not perform well on the new instances, the new database together with the old one (off-line generated) will be used to update the IS. It should recognize that the on-line database generation combined with updating is advantageous, since it can catch the real operating conditions, by which the robustness and accuracy can be enhanced/maintained. However, it should note that the on-line dynamic simulation remains computation expensive, and the updating of IS can be also time-consuming depending on the knowledge extraction algorithms, if the computation time is too long there can hardly harvest practical value [24]. IV. Fig.1. Four stages of development of the IS for DSA. INPUT AND OUTPUT SPECIFICATION The second stage is to specify the input and output of the IS, the use of different input-output can lead to distinct ISs in terms of training and application. Firstly, a data base that contains system dynamic security 135 Authorized licensed use limited to: NATIONAL INSTITUTE OF TECHNOLOGY WARANGAL. Downloaded on April 29,2022 at 13:25:50 UTC from IEEE Xplore. Restrictions apply. The input is the vector of system state parameters that characterize the current system state, usually called feature, they can be classified into pre- and post- fault features. Pre-fault features are usually the steady-state parameters, in general are power flow quantities [8]-[24]. They are basically available in EMS, SCADA and PMUs. Post-fault features are dynamic variables during/after the contingency. They can be relative rotor angle, angular velocity, dynamic voltage trajectory, and even synthetic parameters [25]-[31]. Besides, the contingency can also be as one dimension of the input. With pre-fault features, dynamic security can be assessed before contingency comes, if current operating point is shown unstable, operators can arm preventive controls, such as generation rescheduling, to modify the current operating point to avoid the risk of insecurity. This can ensure the power system operates at a preventive state against anticipated contingencies. However, it will incur expensive cost due to the usually required shifting of power generation among generators. By contrast, when using post-fault features as input, DSA can only be performed after contingency really occurs; and if security is evaluated to be lost shortly, emergency controls such as generator tripping and/or load shedding should be activated immediately. This countermeasure brings no prepaid cost, but will lead to tremendous economic and social cost if the removal of generators and/or load buses is unavoidable. Besides, the use post-fault features requires a certain response time to predict the stability, which has been shown at least 1s after the fault is cleared [31]. This critical time can be too long for operators to take timely remedial actions to stop the very fast instability development (loss of synchronism may arise within 150ms). Besides, another difference between the two types of features is their dependence on network topology, which determines the flexibility of the database. Generally, pre-fault features are much more dependent on the network structure than that of post-fault topology. However, no matter pre- or post- fault features, they can be very large in dimension, especially for today’s increasingly large-scale power systems. Consequently, a usual sub-step in this stage is the feature selection, i.e. to select critical features among the candidates as the input of the IS. Generally, feature selection consists of selecting a subset from the whole input set that are highly related to the output, by which the irrelevant/redundant features can be removed and the size of database can be significantly reduced, leading to a faster training speed and higher accuracy [10]. Besides, by feature selection one can identify significantly relevant features to system dynamic security, and hence obtain insight on system weak point, based on which the control strategies can be designed to enhance system stability [21], [23]. In literature, fisher discrimination [10], divergence criterion [11], principle component analysis (PCA) [20] and wrapper approaches [14], [21] have been adopted for systematically selecting features. A comprehensive study and review on these feature selection techniques and introduction of two alternative approaches is presented in [35]. For the output, which is the parametric values representing the dynamic security conditions, they can be classified into numerical and nominal categories. The numerical output can indicate the continuous security degree [20], so that the DSA will be a regression problem. Alternatively, nominal output can only represent the discrete security status, e.g. secure/insecure [13], or levels of security [23], it makes the DSA as a classification problem. Besides, it’s also noted that the control actions can also be used as the output, to develop the IS for dynamic security controls [32]. In [32], the corrective control actions including load shedding, transformer tap changer blocking, generator tripping and fast valving are coded as the output. V. KNOWLEDGE EXTRACTION The third and usually the core part in developing the IS is the knowledge extraction stage. In this stage, the learning/mining algorithms are used to extract the knowledge on the mapping relationship between input and output, and formulate the knowledge as a classifier (for classification) or predictor (for regression). According to different algorithms, the classifier/predictor can be divided into rule- and blackbox- based models. For rule-based IS, the input-output relation formulated by the IS is interpretable by human, a famous example is Decision Tree (DT), which represent the knowledge as the decision-making splits and branches [8]. Besides, data mining approaches can also provide transparent rules [16]-[17], [22][23], [29], Fuzzy techniques [29] can represent the rules with complying human linguistic habits. For blackbox-based IS, the input-output can’t be (or at least very difficult to be) interpreted by human, the representative example is Artificial Neural Network (ANN) [9], which formulate the knowledge as a set of weights on the links of the ANN nodes. The recently popular Supporting Vector Machine (SVM) [13] may also belong to blackbox, although it makes classification decisions according to the maximum-margin hyperplane (classification boundary), the boundary is hardly to be explicitly interpreted and utilized by human. In terms of transparency and extended IS usage, the rulebased IS is evidently advantageous over blackbox-based IS, and more important is that the interpretable knowledge can be utilized to design dynamic security controls. In [33], the DT is used to identify the stability region, based on which a generation rescheduling scheme is developed to restore system dynamic security. In [15], the decision-making branches of DT are used to determine load shedding actions to restore stability. In addition to the interpretability of the IS, some other factors in evaluating the intelligent algorithms are even more important, including the accuracy, robustness, and training time, etc. The accuracy and robustness basically depends on the 136 Authorized licensed use limited to: NATIONAL INSTITUTE OF TECHNOLOGY WARANGAL. Downloaded on April 29,2022 at 13:25:50 UTC from IEEE Xplore. Restrictions apply. quality of the algorithms, generally, ANN has the excellent non-linear modeling capability, but it can suffer from overfitting as the training samples increase; SVM, DT and data mining approach on the other hand, may be limited to ceiling accuracy. Currently, there is a trend to develop hybrid models which make use of more than one algorithm/classifier to improve the accuracy and robustness [28]-[30]. The training time/speed is another critical dimension of the algorithms, since it determine the feasibility of the on-line updating of the IS which is important to the on-line DSA performance. Also, if the training speed is too low, the whole computation efficiency of the IS can be reduced. Generally, the training time depends on the size of the training data. And typically, ANN and SVM is reported to need considerable training time, DT is shown much faster than them [24]. In [24], a superiorly fast IS for transient stability assessment is developed using the recently proposed Extreme Learning Machine (ELM) approaches [34], and it’s shown the IS needs far less training time which enable efficiently on-line selfupdating. It’s worth mentioning that in AI area, there is a popular classification criterion known as supervised (such as ANN, SVM, DT and the alike) and unsupervised learning algorithms [36], depending on the use of prior knowledge (e.g. class labels) during the knowledge extraction process. And the difference between them is that unsupervised learning is unbiased and objective in extracting the knowledge. Under this criterion, it’s important to mention some data mining approaches, such as Pattern Discovery (PD) method as utilized in [21] and [22] belongs to unsupervised learning. VI. performance and robustness of the IS, on-line database generation is preferable since it allows the on-line updating of the IS. However, it’s important to point that the feasibility of on-line updating also heavily depends on the computation speed of the database generation and the (re)training of the IS. For input definition, if the preventive DSA is required, prefault features should be used; if emergency DSA is needed, post-fault features should be used. However, under the latter situation, the requirement of accuracy and speed is much more stringent since any misclassification or excessive response time can lead to unable to predict the stability timely and/or severe consequence. For the output definition, the numerical values make the knowledge extraction as a regression process, while nominal values make it classification process. For the knowledge extraction algorithms/techniques, the accuracy, extrapolability, robustness, as well as training time are critical factors to be considered. It has been shown that hybrid models tend to result in higher quality performance [28]-[30]. In practical development of DSA ISs, the researchers should integrate all the factors into selecting the techniques for each stage. TABLE I CLASSIFICATION OF DYNAMIC SECURITY ASSESSMENT INTELLIGENT SYSTEMS Stage Classification Database generation Off-line On-line Input definition Pre-fault features Post-fault features Output definition Numerical Nominal Knowledge extraction Rules Blackbox MODEL VALIDATION The last stage, validation, is to test the developed IS in terms of accuracy, extrapolability, etc. Depending on the output (nominal or numerical), the error rate or misclassification rate can be used. In literature, some more informative indices are also proposed to be used to test the accuracy of ISs. For example, in [15], the so called “false alarm rate” and “missed alarm rate” are employed; in [29], the so called “misdetection”, “reliability”, “yield” are used. We would like not to propose a classification for this stage since it’s hard and meaningless to compare the performance indices, but we herein would like to mention the testing method, which is important to evaluate the IS comprehensively. In literature, most of the researches examine their ISs using only one testing data set, however, it may not be sufficient. In [19], the authors propose to adopt v-folder cross validation to measure accuracy and the over-fitting performance of the IS, and it tends to be able to more comprehensively grasp the IS accuracy. VIII. CONCLUSION Intelligent Systems have been identified as a promising alternative of conventional dynamic security assessment methods for large power system. In general, the development of the IS for DSA comprises four stages, known as 1) Dynamic security information database generation, 2) Input-output definition, 3) Knowledge extraction, and 4) Model validation. This paper conducts a comprehensive and detailed review on each stage and classifies the methods used in the stages. The classification highlights the differences between the methods and show their advantages and deficiencies. 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Frank, Data Mining-Practical Machine Learning Tools and Techniques. San Francisco, CA: Elsevier, 2005. BIOGRAPHY Rui Zhang obtained her diploma from Ngee Ann Polytechnic, Singapore, in 2005 and Bachelor degree from University of Queensland, Australia, in 2009, both in electrical engineering. She was previously with Mawan Electric Power Co. Ltd., China. Currently, she is with the “Intelligent Electric Power Grid Key Laboratory of Sichuan Province” and Hong Kong Polytechnic University. Her research interests include power system operation and control, intelligent system applications to power engineering. Yan Xu (S’10) obtained his B.Eng. degree from South China University of Technology, Guangzhou, China, in 2008. Currently, He is a Ph.D. student at Hong Kong Polytechnic University. His research interests include power system stability and control, power system planning, renewable energy, and intelligent system applications to power engineering. Zhao Yang Dong (M’99, SM’06) obtained his Ph.D. degree from University of Sydney, Australia in 1999. He is now Ausgrid(EnergyAustralia) Chair 138 Authorized licensed use limited to: NATIONAL INSTITUTE OF TECHNOLOGY WARANGAL. Downloaded on April 29,2022 at 13:25:50 UTC from IEEE Xplore. Restrictions apply. Professor and Director of the Centre for Intelligent Electricity Networks, University of Newcastle, Australia. He also held academic and industrial positions with Hong Kong Polytechnic University, University of Queensland, Australia and Transend Networks, Tasmania, Australia. His research interest includes power system planning, power system security, stability and control, load modeling, electricity market, and computational intelligence and its application in power engineering. Zhao Xu (S’00-M’06) received his B.Eng., M.Eng, and Ph.D. degree from Zhejiang University, China, in 1996, National University of Singapore, Singapore, in 2002, and University of Queensland, Australia, in 2006, respectively. From 2006–2009, he was an Assistant Professor and later an Associate Professor at the Centre for Electric Technology, Technical University of Denmark, Lyngby. Since 2010, he has been with the Department of Electrical Engineering, Hong Kong Polytechnic University. His research interests include demand side, electric vehicles to grid, grid integration of wind power, electricity market planning and management, and AI applications. Ke Meng (M’10) received his Ph.D. degree from University of Queensland, St. Lucia, Australia, in 2009. He is now a research fellow at the Department of Electrical Engineering, Hong Kong Polytechnic University, Kowloon, Hong Kong. His research interest includes wind power, intelligent algorithms, power system stability analysis and control. 139 Authorized licensed use limited to: NATIONAL INSTITUTE OF TECHNOLOGY WARANGAL. Downloaded on April 29,2022 at 13:25:50 UTC from IEEE Xplore. Restrictions apply.