Synchronous generator stability due to multiple faults on unbalanced power systems

This paper investigates the dynamic behavior of a synchronous generator connected to an unbalanced three-phase power system which is subjected to a shunt fault at one location and blown fuses or open conductors at another location. The method involves phase frame representation of the line. The imbalance created by large single-phase loads, untransposed lines, conductor bundles, etc., is reflected in polyphase impedance matrices. Consequently, line removals, additions, impedance changes, conductor and faulted phase openings can be simulated based on the boundary conditions. Thus, the proposed method makes it possible to study the synchronous generator stability due to any combination of single or multiple asymmetrical series and shunt faults occurring anywhere in the system and the results of several case studies are presented. A significant finding is that certain faults cause the generator to lose synchronism only after several cycles of the rotor angle swing curve. In such cases it is therefore no longer correct to judge the system's stability only on the ‘first swing cycle’ of the rotor angle.     

Induction motor drive behavior during unbalanced faults

The ability of an induction motor drive to ‘ride through’ short-term power system faults or voltage dips of several cycles is of interest in processing plants where shutdowns are expensive. In this paper, the rectifier/inverter induction motor drive is modeled to include the effects of an unbalanced supply circuit, such as that caused by a fault in the supply. A three-phase diode bridge rectifier is used to simulate the effects of the overlap angle caused by the unbalanced source reactance and associated harmonics. The filter circuit between the rectifier and inverter is included. A detailed switching function model is chosen for the inverter so that generated harmonics are included in the motor voltage. Experimental results are included.     

Induction motor response to voltage dips

This paper presents a study using the EMTP to investigate the dynamic response of induction motors to voltage dips. The machine response is related to the voltage dip magnitude and duration. Dips represented include single and multi phase dips as well as sequential dips due to re-closing operations. The survivability of the motor operation is assessed against typical induction machine protection settings. It is demonstrated that protection settings can be adjusted to improve the motor operation survivability of voltage dips without endangering the safety of the motor.     

基于临界惯量和预想故障的含风电电力系统暂态功角稳定在线预警

大规模风电接入使得电力系统惯量水平降低,暂态功角失稳风险加剧,开展含风电电力系统故障前的暂态功角稳定在线预警对保障其安全稳定至关重要。提出了基于临界惯量和预想故障的含风电电力系统暂态功角稳定在线预警方法。首先,通过理论分析得到风电场在故障清除后短时间的功率恢复过程不能忽略的推论,并建立等效外特性模型,进而基于扩展等面积定则构建了计及风电场多阶段外特性的暂稳数学模型。其次,定义了含风电系统暂态功角稳定的临界等效惯量,并通过匀加速分段等效提出了实用化的求解方法,进一步提出了暂态临界惯量指数,实现稳定裕度量化分析并给出风险等级。最后,以含风电接入的四机两区域系统和IEEE39节点系统为例,仿真验证了所提方法的有效性和适用性。     

Improved phase selector for unbalanced faults during power swings using morphological technique

In order to prevent distance protection from tripping during power swing conditions, a power swing blocking device is often utilized. On the other hand, it has been an increasing requirement to achieve rapid clearance of internal faults during power swings. Accurate phase selection is a prerequisite to selective clearance of faults. An improved unbalanced fault phase selector during a power swing based on series multiresolution morphological gradient (SMMG) transform is proposed in this paper. As a feature extractor from raw signals, SMMG is employed to extract superimposed (fault component) current under the power swing condition in this paper. First, the operating characteristic of the sequence component fault phase selector during power swings is discussed. In order to overcome the disadvantage of the above selector, an improved fault phase selector for unbalanced faults during power swings is then proposed by using SMMG to extract superimposed components of modular current. The efficiency and feasibility of the proposed schemes are proven using a Power Systems Computer-Aided Engineering/Electromagnetic Transients including DC-based simulation on a sample power system.     

Induction motor drives with minimal power losses

The goal is to synthesize a control system for an induction motor such as to ensure operation with minimal power losses. Standard nonlinearity and losses in the steel of the motor’s stator are taken into account in MatLab simulation. The proposed control system reduces the losses by up to 25% in comparison with classical scalar control.     

Rescheduling of power systems constrained with transient stability limits in restructured power systems

This paper investigates various approaches to relieve the transient stability constraint in restructured power systems. The approaches adopted fall into two broad categories: those based on eliminating the constraint in the least-cost way and those based on eliminating with the least possible rescheduling. The latter group can, on the other hand, emerge in the form of a pool-protected policy in which the bilateral contracts are rescheduled to maintain the stability or in the form of a contract-protected policy in which the realizable bilateral contracts are maximized while minimizing the rescheduling in pool market. Transient energy function (TEF) method is used as a tool to calculate the sensitivity of energy margin to the variations in the magnitude of generation and load. The effectiveness of the method is illustrated by case studies on Western System Coordinating Council (WSCC) 3-machine 9-bus power system and on the 10-machine 39-bus New England test system and the results are compared. The results are also verified by time domain simulations.     

Development of unbalanced load flow program for large power systems

The idea of unbalanced load flow calculation was proposed many years ago. At that time, however, the needs for such techniques was not urgent. Modern power system networks are comprised of long untransposed transmission lines. Therefore, for some kinds of analysis, it is now almost impossible to treat a system as though it were a symmetrical network. The aims of most previous studies were oriented to solve voltage/current imbalance in local or small systems, as local imbalance was a serious concern. This is still an important issue, but more recently needs have become concentrated on practical bulk power systems, since principal EHV lines are entirely untransposed. In this paper, the development of practical unbalanced load program and practical experience with it are reported. This program was developed for steady-state analysis of large-scale practical networks under many possible unbalanced conditions. The Newton-Raphson method in polar coordinates is used, since fast and moderate convergent characteristics are suitable for solving solutions. The program was applied to practical case studies. The models contain all of the 500-kV and most of the 275-kV overhead transmission lines in TEPCO. Since the density of the Jacobian matrix is higher than in single-phase load flows, the number of nonzero elements is extremely large. Convergent characteristics are excellent in all cases. The fear of numerical instability has been allayed.     

A multiphase optimal power flow algorithm for unbalanced distribution systems

This paper describes a new methodology for the optimization of an n-conductor electrical system, in which the phase imbalances, different types of loads, neutral cables, groundings and other inherent characteristics of distribution systems are taken into account. In addition, the methodology is useful for the detailed analysis required for smart grids. A formulation for the optimal power flow of an n-conductor system was developed using a primal–dual interior point method and the n-conductor current injection method in rectangular coordinates. Distribution and transmission systems were analyzed to verify the generality and efficiency of the proposed methodology.     

An approach to the evaluation of electromechanical transient processes in power systems

The paper presents an approach to the evaluation of electromechanical transient processes in power systems. On the basis of identification of the physical properties of the system considered, for an adequately chosen time interval, quasilinearization and decomposition of the original system of nonlinear differential equations into subsystems is performed. A linear transformation is introduced which transforms the state matrix of the sub-systems into diagonal form. These simplifications enable the use of numerical integration schemes based on difference state equations, which have an elementary form. As a result, a model is formed which is very suitable for numerical treatment. The integration is performed without numerical instability, quickly and effectively, with the desired simulation accuracy, and with low memory requirements.     

Optimal installation of three-phase active power line conditioners in unbalanced distribution systems

In this paper, a new solution algorithm based on a multiple gradient summation (MGS) and differential evolution (DE) approach for optimal three-phase active power line conditioners (APLCs) installation in unbalanced distribution systems is proposed. The active power line conditioners installation problem considers the individual and total harmonic voltage distortions as well as the commercially available discrete sizes of the APLCs limits to minimize the total sizes of three-phase APLCs. The imbalance of systems resulting from using asymmetrical connection transformers was taken into account. The effectiveness of the proposed method was demonstrated by its application to a 23-bus unbalanced radial distribution system.     

k-means algorithm and mixture distributions for locating faults in power systems

Enhancement of power distribution system reliability requires of a considerable investment in studies and equipment, however, not all the utilities have the capability to spend time and money to assume it. Therefore, any strategy that allows the improvement of reliability should be reflected directly in the reduction of the duration and frequency interruption indexes (SAIFI and SAIDI).     

A canonical model for the study of faults in power systems

The authors introduce a general class of fault conditions encompassing all possible faults in a power system, e.g., simultaneous symmetrical and unsymmetrical short circuits, line interruptions, and predicted breaker operations at different locations. The influence of mutually coupled lines, static VAR systems and/or faults at several busbar systems are also considered. The fault conditions are described in both the (three) phase and sequence domains, with the same simplicity. The proposed model of a faulted power system, is in a canonical form, over the general class of fault conditions. It is found that its effectiveness is equal in both domains. The authors also present the model solution procedure for various fault conditions in the general frame of the study of faults. They present the derivation of the canonical model and numerical examples. The application of the canonical model to faults that include buses of distinct voltage levels is stressed     

Sensitivity analysis by neural networks applied to power systems transient stability

This work presents a procedure for transient stability analysis and preventive control of electric power systems, which is formulated by a multilayer feedforward neural network. The neural network training is realized by using the back-propagation algorithm with fuzzy controller and adaptation of the inclination and translation parameters of the nonlinear function. These procedures provide a faster convergence and more precise results, if compared to the traditional back-propagation algorithm. The adaptation of the training rate is effectuated by using the information of the global error and global error variation. After finishing the training, the neural network is capable of estimating the security margin and the sensitivity analysis. Considering this information, it is possible to develop a method for the realization of the security correction (preventive control) for levels considered appropriate to the system, based on generation reallocation and load shedding. An application for a multimachine power system is presented to illustrate the proposed methodology.     

Hybrid transient stability analysis [power systems]

The hybrid method of power system transient stability analysis, which combines the desirable features of both the time-domain simulation technique and the direct method of transient stability analysis, is presented. The hybrid method first computes the actual system trajectory using time-domain simulation, then evaluates the transient energy function in order to derive a stability index for fast derivation of transient stability limits. Proper criteria are proposed to stop the time-domain simulation of the system trajectory to reduce CPU time, once the status of the system has been identified. The method was successfully applied to three test systems varying in size from four generators to 50 generators. The method is shown to be a potential tool for online calculation of transient stability limits     

A parallel transient stability simulation for power systems

As power systems continue to develop, online dynamic security analysis and real-time simulation using parallel computing are becoming increasingly important. This paper presents a novel multilevel partition scheme for parallel computing based on power network regional characteristics and describes the design and implementation of a hierarchical block bordered diagonal form (BBDF) algorithm for power network computation. Some optimization schemes are further proposed to reduce the computation and communication time and to improve the scalability of the program. The simulation results show that, for a large network with 2115 nodes, 2614 branches, 248 generators, and 544 loads, the proposed algorithms and schemes run ten times faster on a cluster system with eight CPUs than on a single CPU. Thus, they satisfy the real-time simulation requirement for large-scale power grids.     

Current signature analysis to detect induction motor faults

Three-phase induction motors are the “workhorses” of industry and are the most widely used electrical machines. In an industrialized nation, they can typically consume between 40 to 50% of all the generated capacity of that country. This article focuses on the industrial application of motor current signature analysis (MCSA) to diagnose faults in three-phase induction motor drives. MCSA is a noninvasive, online monitoring technique for the diagnosis of problems in induction motors. Reliability-based maintenance (RBM) and condition-based maintenance (CBM) strategies are now widely used by industry, and health monitoring of electrical drives is a major feature in such programs     

Air conditioner response to transmission faults

This paper describes two multi-phase faults events which occurred during periods of high air conditioning use. There was a significant loss of load in these events which is attributed to air conditioner motor protection. The overall response of the transmission system is simulated using induction motor models based on the characteristics of a typical residential air conditioner compressor motor. The sensitivity of factors such as fault location, fault duration and excitation system performance is also investigated     

Optimum transient security constrained dispatch in competitive deregulated power systems

The ongoing deregulation of the energy market needs to consider commercial implications when generators are re-dispatched to eliminate potential transient instability after a contingency to ensure transient security. A further complication is that different candidate contingencies may motivate different re-dispatch strategies because the advanced (critical) generator group is different in each case, or the sensitivity of the stability margin to power exchanges between generators is different. This paper develops a generalized approach that attempts to address all of these important issues in competitive deregulated power systems.