Showing papers in "Electric Machines and Power Systems in 1997"
TL;DR: In this article, the authors reviewed literature on using high speed thyristor based control of HVAC power system elements to enhance the power carrying capacity of existing transmission circuits without compromising reliability.
Abstract: The paper reviews literature which addresses the application of Flexible AC Transmission System (FACTS) concepts to the improvement of Power System utilisation and performance. It summarises literature on using high speed thyristor based control of HVAC power system elements to enhance the power carrying capacity of existing transmission circuits without compromising reliability. It describes a study system representative of existing power systems that has been developed to evaluate the economic and technical issues of loading transmission lines to their thermal limits. Considered are two scenarios, a multi-line corridor and a long radial interconnection, where the issues addressed are transient and dynamic stability, power flow control, reactive support and voltage stability. A benchmark system is developed to validate performance of die more simple devices. It is concluded that FACTS devices have the potential to significantly increase system stability margins thereby increasing loading capabil...
544 citations
TL;DR: In this article, the radial basis function neural networks (RBFN) was used to detect die feature signals in a certain frequency range under fault conditions, which is different from conventional schemes that are based on deriving implicit mathematical equations.
Abstract: Since die complex variation of line impedance measured is controlled by thyristors and is accentuated as the capacitor's own protection equipment operates randomly under fault conditions in controllable series compensated transmission systems (CSC), conventional distance protection schemes are limited to certain applications. The authors have extensively addressed the development of new protection techniques for such systems using multilayer percetrons. The basic idea of the method is to design a protection scheme using a neural network approach by catching die feature signals in a certain frequency range under fault conditions. This is different from conventional schemes that are based on deriving implicit mathematical equations based on the infoimation obtained by complex filtering techniques. This paper presents some recent results of employing radial basis function neural networks (RBFN) for this particular application. The use of RBFN is because it has a number of advantages over multilayer ...
67 citations
TL;DR: In this article, a method for removing the load effects from the monitored quantity of the machine is presented by comparing the actual stator current to a model reference value which includes the load effect Simulation results illustrating the effects on the spectrum of monitored quantity are given for both constant and eccentric air gaps when in the presence of an oscillating load.
Abstract: Fault conditions in induction machines such as eccentric air gaps and broken rotor bars result in harmonics in the stator current of the motor which can be used to monitor the health of the machine. However, variations in the load torque at multiples of rotational speed typically have the same effect on the motor current spectrum. When monitoring a single phase of the stator current, this load effect can prevent the detection of a fault condition by producing current components that obscure those harmonics related to the condition of the machine. This paper presents a new method for removing the load effects from the monitored quantity of the machine. This is accomplished by comparing the actual stator current to a model reference value which includes the load effect Simulation results illustrating the effects on the spectrum of the monitored quantity are given for both constant and eccentric air gaps when in the presence of an oscillating load.
56 citations
TL;DR: In this paper, a modified criterion for the determination of the critical value of the inertia which can be synchronized against a given load is presented, and parameters affecting the synchronization capability of permanent magnet synchronous motors are examined.
Abstract: Recently, permanent magnet synchronous motors are attracting growing attention for a wide variety of industrial applications. In this paper the behaviour of a line-start permanent magnet synchronous motor during run-up and synchronization is presented. A modified criterion which accounts for the variations of the pulsating torque components during the synchronization process is given. An accurate criterion for the determination of the critical value of the inertia which can be synchronized against a given load is also developed. Further, parameters affecting the synchronization capability of permanent magnet synchronous motors are examined. The computed results have been verified with the experimental ones, thus confirming the validity and accuracy of the criterion developed.
33 citations
TL;DR: In this article, a disc-type arrangement, with slotless winding and axially-directed flux in the air gap, has particularly short axial length and is well suited for system integration as either generator or motor.
Abstract: The availability of new high-field permanent-magnet materials has created opportunities for novel topologies of electrical machines with advantageous characteristics such as high efficiency and good power/weight ratio. A disc-type arrangement, with slotless winding and axially-directed flux in the air gap, has particularly short axial length and is well suited for system integration as either generator or motor. Applications include auxiliary power units, electric vehicle drives and wind power generation.
27 citations
TL;DR: In this paper, the uniqueness of feasible voltage solution and its stability limit of radial distribution networks are analyzed using the DistFlow method to find the load flow solutions for radial power networks.
Abstract: The planning and operation of a distribution system depend heavily on the load flow solutions obtained In this paper, the uniqueness of feasible voltage solution and its stability limit of radial distribution networks is analyzed. The DistFlow method is employed to find the load flow solutions for radial power networks. By using this method, an equivalent 2-bus network can be obtained during the solving process. It is proposed that only one feasible voltage solution exists for a radial power network. Moreover, the feasibility can be judged directly from the sign of the Jacobian determinant of the equivalent 2-bus network obtained. A 22-bus practical system was tested to justify the approach
17 citations
TL;DR: In this paper, an artificial neural network based method for on-line voltage collapse margin estimation is presented, in which the distance of operating point from critical point, measured in terms of system loading may be regarded as margin to voltage collapse.
Abstract: Owing to frequent black-outs world wide, voltage collapse has received much attention in the electric utilities industry. This paper presents an artificial neural network based method for on-line voltage collapse margin estimation. Homotopy Continuation based Newton-Raphson method is used to drive system operating point to knee of nose curve. The distance of operating point from critical point, measured in terms of system loading may be regarded as margin to voltage collapse. This paper utilizes Kohonen classifier to estimate margin so that computational efforts are reduced compared to conventional methods. Also there is ample of saving in training time compared to error back propagation for parameter estimation. Kohonen neural network classifier transforms input patterns into neurons on the 2-dimensional grid. Power system conditions are assigned to neurons on the grid based on self-organized feature mapping. Finally these neurons are allocated voltage collapse margins corresponding to their sys...
17 citations
TL;DR: In this paper, the effects of non-negligible off diagonal submatrices through a subiteration scheme are considered for determining voltage stability limit of interconnected power systems, and the load parameter is included in both sets of equations.
Abstract: This paper presents an efficient continuation power flow-method for determining voltage stability limit of interconnected power systems. Decoupled power-flow solutions are extremely popular because of lesser storage and faster solution speed as compared to coupled power flow solution. However, it is generally believed that decoupling cannot be used near critical loading conditions. This paper shows that decoupled power-flow solution is possible even under such adverse operating conditions, it is achieved by judiciously incorporating the effects of non-negligible off diagonal submatrices through a subiteration scheme. The load parameter is included in both sets of equations. Bus angle (other than slack bus) and load parameter are used as continuation parameters for solving real & reactive power equations respectively. Results for 5-bus, 14-bus and a practical 91 -bus test systems have been obtained with the coupled and proposed decoupled versions of continuation power-flow to demonstrate the valid...
16 citations
TL;DR: In this article, an algorithm is presented to obtain the optimal schedule for hydro, thermal plants including the pumped-hydro unit by proper selection of initial feasible trajectory for the pumpedhydro plant.
Abstract: The system demand, during some intervals of a day, may exceed the available system generation (excluding the pumped-hydro generation). A pumped-hydro plant may then, be used as a peak-load management unit to safeguard the power system by minimizing the power blackout in order to avoid large deviation in system frequency. In this paper, an algorithm is presented to obtain the optimal schedule for hydro, thermal plants including the pumped-hydro unit by proper selection of initial feasible trajectory for the pumped-hydro plant. An additional constraint is introduced to ensure the power balance in each time interval during the perturbation of water storage trajectory. The proposed method decomposes the problem into hydro and thermal subproblems and solves them alternately. The hydro subproblem is solved using a search procedure namely, the local variation method and the thermal subproblem is solved using a judicious combination of participation factors/linear programming method. Optimal scheduling w...
15 citations
TL;DR: In this paper, the problem of designing load frequency control CLFO system which reduces frequency oscillation based on static Var compensation (SVC) has been investigated, and a new feedback control signal sent from SVC to power houses is suggested.
Abstract: In this paper the problem of designing load frequency control CLFO system which reduces frequency oscillation based on static Var compensation (SVC) has been investigated. A linear model of a power system area has been used, and a new feedback control signal sent from SVC to power houses is suggested. The new feedback controller includes the frequency deviation and the change of reactive power due to SVC. The dynamic response of the system is determined in the presence of the suggested control system and compared with the case of no control action to evaluate the capabilities of the proposed control system.
14 citations
TL;DR: In this article, a torque control associated with an appropriate strategy for phase commutation is proposed to reduce the torque ripple in the Switched Reluctance Machine (SRM) in order to compensate for its nonlinear electromagnetic characteristics.
Abstract: The Switched Reluctance Machine (SRM) presents simple structure with passive rotor without windings nor any magnets. That enables applications in mass production (low production and material costs) or in harsh environments (high temperature...). Unfortunately, its polyphase torque is naturally pulsed because of the highly nonlinear electromagnetic characteristics and because the phases have to be successively commutated in order to maintain a continuous rotation. A torque control associated with an appropriate strategy for the phase commutation is thus sought in order to reduce the torque ripple. Initially, the limits of the classical linear control (Proportional Integral controller) are determined. Then, the performances of the PI controller are improved by means of electromotive forces compensation. Nevertheless, the torque ripple remains significant. In order to compensate for SRM nonlinearities, we propose a nonlinear torque control associated with a suitable commutation strategy. This contro...
TL;DR: In this paper, the harmonic contents of the load voltage and current waveforms are assumed to be known in advance, and the least errors square parameter estimation algorithm is used to estimate the load resistance, inductance and capacitance.
Abstract: In this paper we present two novel time domain techniques for load modelling in the presence of harmonics In the first technique, the harmonic contents of the load voltage and current waveforms are assumed to be known in advance The second technique uses samples of the load voltage and current waveforms directly The least errors square parameter estimation algorithm is used to estimate the load resistance, inductance and capacitance The residual current associated with incomplete extraction of the parameters for all significant signal frequencies is also estimated We also identify harmonics that are not common between the load voltage and current waveforms The proposed techniques are applied to synthetic and actual recorded data The paper's conclusions are based on comparing the results obtained using both techniques
TL;DR: In this article, the authors extended the analysis of the acoustic noise and vibration produced by single-phase induction motors to a 4-pole PSC motor and fan assembly, with particular emphasis on the effects of rotor eccentricity, magnetic circuit saturation and stator and rotor resonances.
Abstract: Existing theory for the analysis of the acoustic noise and vibration produced by single-phase induction motors is restricted to 2-pole machines (Yang, 1975). In this paper it is extended to cater for motors having any number of poles, and is then used to analyse the noise and vibration spectra of a typical 4-pole PSC motor and fan assembly. Both electromagnetically and aerodynamically generated noise and vibration are studied, with particular emphasis on the effects of rotor eccentricity, magnetic circuit saturation, and stator and rotor resonances.
TL;DR: In this paper, a new technique for optimizing the power generated by a double output induction generator (DOIG) is established, which employed a controllable rectifier, a d. c. link, an inverter, and an isolating transformer in the rotor circuit.
Abstract: In this paper a new technique for optimizing the power generated by a double output induction generator (DOIG) is established. The proposed control strategy ensures stable operation of the DOIG within a wide speed range without overheating the machine. The proposed scheme employed a controllable rectifier, a d. c. link, an inverter, and an isolating transformer in the rotor circuit. Variation of firing angles, overlap angle, voltages, and powers with speed are deduced. Experimental results are given. Results showed that twice the rated machine power is generated at S=-l.
TL;DR: In this article, an efficient and simple fuzzy logic power system stabilizer (FLPSS) was proposed for damping electromechanical modes of oscillations and enhancing power system synchronous stability.
Abstract: The paper presents an efficient and simple fuzzy logic power system stabilizer (FLPSS) for damping electromechanical modes of oscillations and enhancing power system synchronous stability. The essential feature of the proposed controller is that the pole placement approach is used to assign the open-loop poles at certain specified locations at nominal operating point. Then the fuzzy logic controller is employed to excite the synchronous machine excitation system by an incremental signal for enhancing the power system stability over a wide range of operating condition and exciter gain. The input signals to the FLPSS is the speed deviation and speed deviation change. The simulation results using the proposed controller are carried out on an synchronous machine connected to infinite bus through transmission line.Moreover,a comparison between the conventional fixed-gain PI controller and the proposed FLPSS is presented. The results validate the effectiveness of the proposed FLPSS in terms of less ove...
TL;DR: In this article, the authors make an indepth analysis of shaft torsional vibrations in steam turbine-generator-exciter shafts by variable-frequency ripple currents superimposed on DC currents in asynchronous Links.
Abstract: This paper makes an indepth analysis of excitation of shaft torsional vibrations in steam turbine-generator-exciter shafts by variable-frequency ripple currents superimposed on DC currents in asynchronous Links. First, frequencies at which sympathetic shaft torsional vibrations would be excited by modulation product harmonics in 50Hz/50Hz asynchronous Links as a function of deviation in system frequency is illustrated. Then steady-state torque excitation due to injected harmonic stator currents due to the DC ripple currents is analysed. Time constants for decay of shaft torsional vibrations based on electrical and steam viscous damping for generators operating at full-load and no-load for reduced shaft models which are used with the torque excitation to give amplitude of modal torque are also evaluated. Amplitude of shaft torque for a given torque excitation using reduced shaft models is illustated. The effect of generator load on shaft torque which results from resonant excitation for reduced sh...
TL;DR: In this paper, the design problems of high frequency machines are analyzed by the electromagnetic, the mechanical and the thermal points of view, starting from both the simulation and the experimental results, some specific criteria for the electromagnetic design are well focused.
Abstract: In the paper the authors deal with induction motors used in electrical spindles and fed by high frequency inverters. The design problems of high frequency machines are analyzed by the electromagnetic, the mechanical and the thermal points of view. The possibility to use the classical relations adopted to calculate the equivalent circuit parameters for standard motors at 50 Hz is carried out and the results are compared with the experimental ones. A brief overview of problems related to the mechanical design (bearings, lubrication, shaft stiffness) and to the thermal design is reported. Finally, starting from both the simulation and the experimental results, some specific criteria for the electromagnetic design are well focused.
TL;DR: In this paper, a methodology is presented to compute the optimal inter-utility power interchange in deregulated power systems, where each utility defines a price curve for the import and export interchange and the problem is formulated as an optimization approach with a nonlinear objective function and linear constraints.
Abstract: In this paper a methodology is presented to compute the optimal inter-utility power interchange in deregulated power systems. Cost functions in utilities are considered proprietary information. Each utility defines a price curve for the import and export interchange. The problem is formulated as an optimization approach with a nonlinear objective function and linear constraints. Electric losses are considered in the solution using penalty factors. The values of the penalty factors are calculated from a DC power flow. The results for a three-area test system are presented.
TL;DR: In this article, a position control scheme for ultrasonic motor is presented, it consists of a PI controller and an adaptive controller which compensates the speed characteristic variations with online parameter identification.
Abstract: Ultrasonic motor (USM) has an excellent performance and many useful features. Since this motor is superior in holding torque and high response characteristic, it has been expected to use as position servomotor for small motors. There have been reported some mathematical models of the ultrasonic motor. however, these models are too complex to apply for control of the motor. Therefore position controllers based on PI control or fuzzy control have been proposed in recent years. It is difficult to control the ultrasonic motor with high-performance using such controllers, thus simple and convenient mathematical model is necessary for precise control. This paper presents a new position control scheme of ultrasonic motor, it consists of a PI controller and an adaptive controller which compensates the speed characteristic variations with online parameter identification. Moreover, this system controls both driving frequency and phase difference in order to achieve a quick and precise position control. The effectiveness of proposed control scheme is demonstrated by experiments.
TL;DR: In this article, a fuzzy logic model for an emissions constrained OPF is presented, where fuzzy membership functions for measuring the degree of satisfaction with each objective are defined and the max-min criterion is used to convert the fuzzy multi-objective formulation to a "crisp" model that can be solved using traditional OPF techniques.
Abstract: Curtailing S02 emissions and minimizing production cost are two conflicting objectives that have to be reconciled while using an Optimal Power Flow (OPF) to dispatch generating units on a short-term (hourly/half-hourly) basis. In this paper, we present a fuzzy logic model for such an emissions constrained OPF. First, we define fuzzy membership functions for measuring the degree of satisfaction with each objective. Then, we use the max-min criterion to convert the fuzzy multi-objective formulation to a “crisp” model that can be solved using traditional OPF techniques. We illustrate, using the IEEE 14-bus system, how the proposed approach facilitates the arbitration of these conflicting objectives. The resulting Pareto curve can be used by the system operator to pick a suitable tradeoff point.
TL;DR: In this paper, two ANN models have been used to classify the power system as to whether it is dynamically stable or unstable, and then the second ANN model is used for the dynamically stable system to predict the voltage magnitudes at load busbars.
Abstract: The application of artificial neural network (ANN) on dynamic voltage stability analysis is presented. Two ANN models have been utilized, in which the first ANN model is used to classify the power system as to whether it is dynamically stable or unstable. Then the second ANN model is used for the dynamically stable system to predict the voltage magnitudes at load busbars. Both ANN models are based on the multiperceptron model, and the training is done using the error back propagation scheme. The training set patterns are generated by carrying out dynamic simulations, using induction motor and constant P-Q load models. This paper highlights the method for selection of the optimum number of training sets so as to minimise the time taken in the ANN learning process. The performance of the ANN models have been tested and shown to give good results.
TL;DR: In this paper, the problem of two-dimensional steady state heat flow in an induction motor is solved using finite-element formulation and employing simple triangular elements in the xy plane of the rectangular co-ordinate system.
Abstract: In this paper the problem of two-dimensional steady state heat flow in an induction motor is solved using finite-element formulation and employing simple triangular elements in the xy plane of the rectangular co-ordinate system. The shape functions and exact solution matrices are derived algebraically for utmost economy in computation. The global system of equations is determined by a suitable algorithm to avoid repeated calculations of the elemental stiffness matrix. The temperature distribution has been computed considering convection form the outer boundary surfaces of the machine.
TL;DR: In this paper, a variable-bias digital differential relay for power transformers protection is proposed, where the amount of bias is varied linearly with the value of through current (mean of the currents entering and leaving the transformer).
Abstract: In the present paper a variable-bias digital differential relay for power transformers protection is proposed. The amount of bias is varied linearly with the value of through current (mean of the currents entering and leaving the transformer). To take care of mismatch between CT ratios on primary and secondary sides of the power transformer, secondary currents of CT are corrected using a precalculated correction factor for the power transformer tap in operation. In the event of an abnormal operation, a fault discriminant based on fundamental, second and fifth harmonic components of differential current is used. It provides discrimination between faults and magnetizing inrush /overexcitation copditions. Trip and indication signals arc issued in the former case, while only an indication is given in the latter case. The relay scheme is simulated on a personal computer. The advantage of the proposed scheme is its ability to detect even low-level internal faults, which may not be detected by a fixed-b...
TL;DR: In this paper, a static var compensator (SVC), constructed by fixed capacitor (FC) and thyristor controlled reactor (TCR), is designed and implemented to improve the damping of a synchronous generator, as well as controlling the system voltage.
Abstract: As power systems have increased in size and complexity, and as the number of power ties between different systems has increased, the nature of the system stability has changed. A static var compensator (SVC), constructed by fixed capacitor (FC) and thyristor controlled reactor (TCR), is designed and implemented to improve the damping of a synchronous generator, as well as controlling the system voltage. The SVC is placed at the generator bus terminal, with the speed and voltage deviation as the feedback signals. In this paper, a formula has been developed showing the effect of SVC upon synchronizing torque coefficient, such that damping is improved and the system voltage is controlled. A simplified analysis of the effect of SVC on the stability of a machine connected to an infinite bus, as well as 21 bus real system is presented. It is observed that the SVC is capable of increasing damping and controlling the system voltage.
TL;DR: In this article, the authors presented the analysis of automatic generation control of a two-equal area reheat thermal system in the presence of generation rate constraints considering a discrete-continuous time mathematical model.
Abstract: The paper presents the analysis of automatic generation control (AGC) of a two-equal area reheat thermal system in the presence of generation rate constraints considering a discrete-continuous time mathematical model. The effect of variation of sampling period on optimum integral gain setting and system dynamic performance has been analyzed considering supplementary controllers based on conventional area control errors (ACEs) and new area control errors (ACENs). Investigations reveal that the optimum integral gain setting and system dynamic performance are hardly affected over a wide range of sampling period T for controllers based on conventional ACEs. Studies also reveal that the permissible sampling period is further enhanced with integral controllers based on new area control errors (ACENs).
TL;DR: In this article, a computer approach to evaluate the power system reliability considering generation/transmission capacities is presented, and a reinforcement strategy to improve the system reliability is suggested, where the strategy is to select the important link and reinforce it.
Abstract: This paper presents a computer approach to evaluate the power system reliability considering generation/transmission capacities. The proposed technique is a 2-step approach. Step 1 is to modify the system minimal cutsets to a capacity related cutsets CRC. CRC is defined as a minimum set of system elements, which if failed, the rest of the elements are not enough for the system to survive, and this set is a subset of any minimal cutset of the system. Step 2 deals with the evaluation of the power system reliability based on CRC of the system, enumerated in step 1, and a symbolic reliability computation algorithm based on journal article(Fong, et al.,1987). The above method is illustrated by numerical examples and compared with the results tabulated in journal article(Rai, et at.,1991). A reinforcement strategy to improve the system reliability is suggested. The strategy is to select the important link and reinforce it.
TL;DR: A neural network based adaptive autoreclosure scheme is developed for compensated systems and special emphasis is placed on the selection of an appropriate neural network topology and its training.
Abstract: This paper discusses the application of neural networks to adaptive autoreclosure technique for long-distance compensated systems Firstly, the detailed simulation studies on a long-distance 500kV compensated systems are presented From the simulation studies, some salient features of the faulted responses are revealed and summarized Based on the findings of investigations, a neural network based adaptive autoreclosure scheme is developed for compensated systems Special emphasis is placed on the selection of an appropriate neural network topology and its training
TL;DR: In this article, a multi-loop model for analysing the torque of a synchronous machine is presented, in which the electro-magnetic torque of the machine which is very difficult to calculate in some operation conditions by other methods, can be obtained from the inductances of the machines and the currents of every loop of the system, then the average torque and the pulse torque by Fast Fourier Technique.
Abstract: By using a multi-loop model, a new technique for analysing the torque of a synchronous machine is presented in this paper. The electro-magnetic torque of the machine which is very difficult to calculate in some operation conditions by other methods, can be obtained from the inductances of the machine and the currents of every loop of the machine, then the average torque and the pulse torque by Fast Fourier Technique. Comparing the results obtained by the new technique with an established method, the new method is justified.
TL;DR: In this paper, the authors proposed a simple, efficient and fast technique for the alleviation of line overloads by corrective generation and/or load shedding, which is superior to existing methods and, in some cases, significant smaller amount of load shedding is required.
Abstract: This paper presents simple, efficient and fast technique for the alleviation of line overloads by corrective generation and/or load shedding. The developed method ensures that alleviation of the existing violations does not create any new violations. The proposed algorithm is superior to existing methods and, in some cases, significant smaller amount of load shedding is required. The sensitivity matrix and the Lagrangian function, which are needed in almost all of the existing methods, it is not used in the proposed method. The reconstruction of Jacobian-like matrices, and determination of their inverse at each new operating point during the optimization procedure are not required. It gives less computation time for the proposed method as compared to the existing methods. The proposed technique will find extensive applications in operational planning, security and contingencies studies, and the reliability evaluation of power systems. To validate the proposed method results for 6-bus and 14-bus s...
TL;DR: In this article, a novel ANN-based coordinated excitation/governor artificial neural network (ANN) based controller for AC synchronous generators is presented. And the detailed design methodology of the ANN based controller, the training and validation results based on the performance of the best benchmark conventional excitation and speed controls are presented.
Abstract: The paper presents a novel coordinated excitation/governor artificial neural network (ANN) based controller for AC synchronous generators. The proposed ANN based controller replaces the full global action of the voltage regulator, power system stabilizer and speed governor controls. The detailed design methodology of the ANN based controller, the training and validation results based on the performance of the best benchmark conventional excitation and speed controls are presented in the paper. The dynamic simulation results and study of the performance of ANN based coordinated control scheme in comparison with the conventional excitation/governor action are also presented in the paper.