Showing papers in "IEEE Transactions on Power Delivery in 1994"

Dynamic compensation of AC transmission lines by solid-state synchronous voltage sources

Abstract: This paper describes a novel approach in which solid-state synchronous voltage sources are employed for the dynamic compensation and real time control of power flow in transmission systems. The synchronous voltage source is implemented by a multi-pulse inverter using gate turn-off (GTO) thyristors. It is capable of generating internally the reactive power necessary for network compensation, and is also able to interface with an appropriate energy storage device to negotiate real power exchange with the AC system. The paper develops a comprehensive treatment of power flow control using solid-state synchronous voltage sources for shunt compensation, series and phase angle control. It also describes the unique unified power flow controller that is able to control concurrently or selectively all three network parameters (voltage, impedance, transmission angle) determining power transmission. Comparison of the synchronous voltage source approach with the more conventional compensation method of employing thyristor-switched capacitors and reactors shows its superior performance (including the unmatched capability of using both reactive and real power compensation to counteract dynamic disturbances), uniform applicability, smaller physical size, and potentially lower overall cost. >

Voltage phasor and local system frequency estimation using Newton type algorithm

Abstract: A new approach to the design of a digital algorithm for voltage phasor and local system frequency estimation is presented. The estimation problem is considered as an unconstrained optimization problem. The algorithm is derived using Newton's iterative method, very commonly used in load-flow studies. The algorithm showed a very high level of robustness as well as high measurement accuracy over a wide range of frequency changes. The algorithm convergence of order two provided fast response and adaptability. To demonstrate the performance of the algorithm developed, computer simulated, experimentally obtained and real-life data records are processed. The presented work is a part of a project concerning the application of microprocessors in frequency relaying. >

Complete transformer model for electromagnetic transients

Abstract: A complete, three phase transformer model for the calculation of electromagnetic transients is presented. The model consists of a set of state equations solved with the trapezoidal rule of integration in order to obtain an equivalent Norton circuit at the transformer terminals. Thus the transformer model can be easily interfaced with an electromagnetic transients program. Its main features are: (a) the basic elements for the winding model are the turns; (b) the complete model includes the losses due to eddy currents in the windings and in the iron core; and (c) the solution of the state equations is obtained in decoupled iterations. For validation, the frequency response of the model is compared with tests on several transformers. Applications to the calculation of transients are given for illustration. >

Synchronized sampling and phasor measurements for relaying and control

Abstract: This paper describes the concept of utilizing time synchronized sampling over an entire power system to simultaneously obtain the phasor values of voltages and currents at particular instants of time. Uses of the phasors are reviewed and the necessary accuracy of synchronization for several applications is established for magnitude and angle of the phasors. Various methods of providing synchronizing signals are examined, and a possible format for transmitting the phasor measurements to remote locations is described. Finally, some possibilities for applications of this technique in protection and control tasks of the future are explored. >

The soil ionization gradient associated with discharge of high currents into concentrated electrodes

Abstract: It has been known for many years that the grounding resistance of a concentrated electrode drops when it is subjected to a high current charge. This helps reduce the ground potential rise. The degree of the resistance depends on the magnitude of the ionization gradient of the soil E/sub o/. Based on both a theoretical analysis and a critical review of the large number of available measurements, this paper recommends that E/sub o/ be taken equal to 300 kV/m for typical soils. This is significantly less than the 1000 kV/m value used by some authors. Graphs are also given describing the behaviour of the rod electrodes which are used in many field installations. >

An investigation of harmonics attenuation and diversity among distributed single-phase power electronic loads

Abstract: Widely distributed single-phase power electronic loads are an increasingly important source of harmonics in power distribution systems. The objective of this paper is to investigate the cumulative harmonic current characteristics of a large number of such loads. A complete analytical model for the most common load type is derived. This model is then used to investigate the impact of: (1) interaction due to a shared source impedance; (2) variation in power level; and (3) variations in circuit parameters, on individual and cumulative current harmonics. The key findings of the paper are that diversity and attenuation are very important factors in predicting the behavior of distributed single-phase power electronic loads, especially for the higher-order harmonics, and that due to these two factors, the commonly-used fixed current injection method, using arithmetic sums of harmonic current magnitudes, can significantly overestimate the cumulative harmonic currents produced by these loads. >

A transformer model for winding fault studies

Abstract: This paper deals with a method of modeling internal faults in a power transformer. The method leads to a model which is entirely compatible with EMTP software. It enables simulation of faults between any turn and the earth or between any two turns of the transformer windings. Implementation of the proposed method assumes knowledge of how to evaluate the leakage factors between the various coils of the transformer. A very simple method is proposed to evaluate these leakage factors. At last, an experimental validation of the model allows the estimation of its accuracy. >

Arc-furnace model for the study of flicker compensation in electrical networks

Abstract: This paper presents an arc-furnace model consisting of nonlinear, time varying resistance where time-variation laws of arc length are considered. One consists of a periodic, sinusoidal law, the other of a band-limited white-noise law. The arc-furnace model is implemented by EMTP, referring to actual electric-plant configurations. Simulations are reported where the values of flicker sensation and short-term flicker severity P/sub ST/, are determined according to UIE specifications. The results show that the model based on the sinusoidal time-variation law can be useful for worst-case approximations, while the model using white-noise law is able to fit flicker measurements made in electric plants supplying arc furnaces. The models are used to investigate the effect on flicker compensation of the insertion of series inductors at the supply side of the furnace transformer. It is shown that considerable reduction of P/sub ST/ is obtained at the point of common coupling by series inductor installation at constant furnace active power. >

Adaptive relay setting for stand-alone digital distance protection

Abstract: Reach accuracy of a distance relay on transmission lines is adversely affected by fault resistance combined with remote-end infeed which is not measurable at the relaying point. Different network conditions correspond to different remote-end infeed behaviour and in conventional setting a safety margin is necessary so as to avoid maloperation. In this paper an adaptive setting concept which can overcome this disadvantage is proposed. A microprocessor based distance relay with this new technique can respond to network conditions that change from time to time and computer simulation has confirmed the validity of this new concept. >

Training an artificial neural network to discriminate between magnetizing inrush and internal faults

Abstract: A feedforward neural network (FFNN) has been trained to discriminate between power transformer magnetizing inrush and fault currents The training algorithm used was backpropagation, assuming initially a sigmoid transfer function for the network's processing units ("neurons") Once the network was trained the units' transfer function was changed to hard limiters with thresholds equal to the biases obtained for the sigmoids during training The off-line experimental results presented in this paper show that a FFNN may be considered as an alternative method to make the discrimination between inrush and fault currents in a digital relay implementation >

Damping of power system oscillations by use of controllable components

Abstract: This paper deals with damping of power system electromechanical oscillations using controllable series compensators (series capacitors and phase shifters) and controllable shunt capacitors. The study is based on investigation of the eigenvalues of a simple linearized power system model. The controllable series components are located on the transmission line connecting two areas and the controllable shunt capacitor is located close to a load. It is shown that the level of transmission line loading, load characteristics and inertia of the two areas influence the damping achieved by the controllable components to different degrees. The contributions of the controllable series components to the enhancement of power system damping for the same rating of the controllable components are compared. Numerical examples are provided to verify the theoretical findings. The purpose of this paper is to provide an insight and understandings in the basic characteristics of the damping effects of the studied devices. This insight is obtained by study of a simple power system that exhibits power oscillations. The understandings and findings can facilitate the analysis of realistic, and more complex power systems. >

Power system harmonic state estimation

Abstract: A power system state estimation based on (a) multiphase model, (b) voltage and current waveform measurements, (c) synchronized measurements and (d) multifrequency model (i.e. the approach accounts for waveform distortion or harmonics) is formulated. The paper focuses on the following: (a) modeling, (b) implementation, (c) observability and (d) performance. Sensitivity analysis is used to show how transmission line modeling and measurement schemes affect the performance of harmonic state estimation. The overall performance of the system is described in terms of confidence level versus error. These concepts are illustrated with simple systems. The overall harmonic measurement system is scheduled for installation and field evaluation on the NYPA/New York Power Pool transmission system by the end of 1993. >

The classification of power system disturbance waveforms using a neural network approach

Abstract: Owing to the rise in power quality problems, the use of transient recorders to monitor power systems has increased steadily. The triggering strategies used by these transient recorders to capture disturbance waveforms are usually based on the violation of a set of predetermined measurement thresholds. Unfortunately, threshold based triggering strategies are difficult to apply in situations when only waveforms corresponding to a given class of disturbances need to be recorded. This inability of the recorder to automatically discriminate between waveform types tends to burden the user with the task of manually sifting and sorting through a large number of captured waveforms. This paper describes an artificial neural network methodology for the classification of waveforms that are captured, as part of a larger scheme to automate the data collection process of recorders. Two different neural network paradigms are investigated: the more common feedforward network (TDNN), and a modification of that, the time-delay network (TDNN), which has the ability to encode temporal relationships found in the input data and exhibits a translation-shift invariance property. Comparisons of both network paradigms, based on a typical distribution circuit configuration, are also presented. >

An experimental analysis of DC excitation of transformers by geomagnetically induced currents

Abstract: DC excitation of transformers due to geomagnetically induced currents was studied by model experiments. First, differences in the DC excitation phenomenon due to the iron core structure were studied using three typical small-scale models. The results verified that single-phase three-legged cores were most susceptible and three-phase three-legged cores least susceptible to such excitation effects. Secondly, the local heating due to DC excitation was quantitatively assessed using large-scale core form and shell form models with the most susceptible single-phase three-legged cores. The results demonstrated that the maximum temperature rise was approximately 110/spl deg/C due to the GIC (200 A/3 phases). >

A heuristic based fuzzy reasoning approach for distribution system service restoration

Abstract: A fuzzy reasoning approach is proposed for the service restoration of a distribution system. After the location of a fault has been identified and the faulted zone has been isolated, it is important for the operators to reach a proper service restoration plan in order to restore the electricity service outside the faulted zone. The operators tend to use their past experience and heuristic rules to devise such a restoration plan because it must satisfy a lot of practical needs and objectives. In addition, the operators' needs and heuristic rules are often expressed in imprecise linguistic terms. In this paper, fuzzy set notation is employed to deal with these imprecise linguistic variables and a set of fuzzy reasoning procedures are developed to implement the operators' heuristic rules. These procedures can be employed to solve the multiple-objective problem of service restoration described in imprecise linguistic variables. To demonstrate the effectiveness of the proposed fuzzy reasoning approach, service restoration on a distribution system within the service area of Taipei West District Office of Taiwan Power Company is examined. It is found that a proper restoration plan can be reached very efficiently by the proposed approach. >

Experimental evaluation of EMTP-based current transformer models for protective relay transient study

Abstract: This paper describes an EPRI study of current transformer (CT) digital models intended for protective relay transient performance analysis. Experimental evaluation of CT models implemented using the Electromagnetic Transient Program (EMTP) was carried out. Two relaying CTs with 600/5 and 2000/5 ratios were used in the study. Experiments in a high power laboratory were performed to obtain transient responses. Simulation of the CT response to the same transient events was set up using three different CT models. They were implemented based on the saturable transformer and nonlinear reactor models available in an EMTP. Comparison of laboratory and simulation results indicates that CT models developed based on the EMTP program give satisfactory results for most of the cases. It has also been discovered that in some instances EMTP models need further improvements. >

Characteristics and rating considerations of thyristor controlled series compensation

Abstract: Thyristor-controlled series compensation systems are expected to provide benefits to utility transmission systems, with applications forthcoming over the next several years. This paper provides basic information on the characteristics and key rating issues associated with this new apparatus, intended to support the anticipated future applications. >

EMTP-based model for grounding system analysis

Abstract: EMC and lightning protection analyses of large power systems require the knowledge of the dynamic behavior of extended grounding systems. They cannot be regarded as equipotential planes, but must be treated as coupling paths for transient overvoltages. This contribution presents a model for linear earth conductors based on the transmission line approach and outlines its integration in the transients program EMTP. Validation of the presented model is achieved by comparison with field measurements and with a rigorous electromagnetic model. Overvoltages and electrical fields throughout electrical power systems can thus be computed. >

A fuzzy-set approach to fault-type identification in digital relaying

Abstract: Sometime the operations that a relay is required to perform cannot be easily described in a deterministic way. A significant example of this situation is given by the operations a line relay must perform in order to detect the type of fault (line-to-ground, line-to-line, line-to-line involving ground). Recently, the mathematical theory of fuzzy sets featured many practical applications, mainly in industrial controls, and fuzzy-set processors are available to allow real time applications. This paper shows that a fuzzy-set approach can be useful even in digital relaying, whenever "fuzzy" decisions have to be under-taken. A possible application to the detection of the type of fault when symmetrical component relaying techniques are adopted is proposed and the results of simulation tests are given. >

Modeling of lightning incidence to tall structures. I: Theory. Discussion. Author' reply

Abstract: The paper generalizes a recent physical approach to assess negative downward lightning incidence to apply to tall masts and hilly regions. Criteria for occurrence of an upward flash from a tall structure under negative cloud are formulated, both for flat and hilly terrain. Finally the effect of structure on statistical stroke current distribution is analytically investigated. Extensive computer investigation to apply the theory as well as comparison with field observations are reported in a companion paper

A new technique for loss reduction using compensating capacitors applied to distribution systems with varying load condition

Abstract: A new method based on a heuristic technique for reactive loss reduction in distribution network is presented. This method allocates capacitors to certain nodes (sensitive nodes) which are selected by first identifying the branch which has the largest losses due to reactive power. Then, the node therein, which has the largest reactive power is selected. The capacitor rating is determined by differentiating the system losses with respect to the load connected to that node. The compensating capacitors are placed at these optimal locations with appropriate VAr ratings to achieve maximum benefits in dollar savings. The variation of the load during the year is considered. The capital and installation costs of the capacitors are also taken into account. This method is applied to a 38 feeder distribution system of 27.6 kV, 560 MVA of the city of Windsor, Ontario, resulting in annual saving of about CAN $145000 after amortizing the capital and installation costs of applying the compensating capacitors, and using a figure for the cost of energy of 1.986 cents per kWh. >

Modeling of lightning incidence to tall structures. I. Theory

Abstract: The paper generalizes a physical approach to assess negative downward lightning incidence to apply to tall masts and hilly regions. Criteria for occurrence of an upward flash from a tall structure under negative cloud are formulated, both for flat and hilly terrain. Finally the effect of structure on statistical stroke current distribution is analytically investigated. >

Detecting arcing downed-wires using fault current flicker and half-cycle asymmetry

Abstract: The downed-wires problem, known as high impedance faults, is described. A high voltage laboratory setup was devised to investigate the phenomenon. The laboratory model results agreed with field test results, and previous research efforts. The arcing fault model was justified. The setup was used as a source of fault current signal. A simple approach was taken to design an arcing fault detector. The algorithm utilizes the random behavior of the fault current. It compares the positive and negative current peaks in one cycle to those in the next cycle to measure the flicker in the current signal. The asymmetry of the current is calculated by comparing the positive peak to the negative peak, for each cycle: the moving window length is half a cycle. Both values are used as a signature of arcing. The result is filtered and compared with a suitable detection threshold. The algorithm was tested by traces of normal load, and no-load current disturbed by currents of faults on dry and wet soil, arc welders, computers, and fluorescent light loads, as well as short circuit currents. The algorithm performed well under the test conditions, except for the arc welder load. This load is also a source of insecurity for other algorithms. The detection criterion will be integrated with another detection method to improve the security during arcing load events. >

Using fuzzy sets to model the uncertainty in the fault location process of distribution networks

Abstract: In the computerized fault diagnosis of distribution networks the heuristic knowledge of the control center operators can be combined with the information obtained from the network database and SCADA system. However, the nature of the heuristic knowledge is inexact and uncertain. Also the information obtained from the remote control system contains uncertainty and may be incorrect, conflicting or inadequate. This paper proposes a method based on fuzzy set theory to deal with the uncertainty involved in the process of locating faults in distribution networks. The method is implemented in a prototype version of the distribution network operation support system. >

Application of nonlinear dynamics and chaos to ferroresonance in distribution systems

Abstract: Ferroresonant overvoltages or undervoltages can occur in cable-fed power transformer installations when single phase switching or interrupting is practiced. This paper identifies the ferroresonant circuit as a nonlinear dynamical system. Analysis and classification methods are presented which provide new insight into the global behavior of ferroresonance. The concepts presented offer potential for progress in the areas of transformer model development and evaluation, analysis and prediction of ferroresonance, and distribution system design and operation. Measurements from a typical five-legged core transformer installation are used to illustrate the application of nonlinear dynamics and chaotic systems to the problem of ferroresonance. >

Discussion of power definitions contained in the IEEE Dictionary

Abstract: This tutorial paper discusses the drawbacks of the definitions of various types of powers found in the IEEE Standard Dictionary of Electrical and Electronics Terms (IEEE Std. 100-88). With the exceptions of instantaneous power and active power, all remaining kinds of "powers" are nonphysical. The concept of power factor in polyphase circuits is ambiguous. Examples that illustrate the shortcomings of many power definitions are included. The impact of these definitions on current power/energy metering practices are discussed. It is recommended that some definitions be either changed or eliminated from the IEEE Dictionary. >

Transient electromagnetic interference in substations

Abstract: Electromagnetic interference levels on sensitive electronic equipment are quantified experimentally and theoretically in air and gas insulated substations of different voltages. Measurement techniques for recording interference voltages and currents and electric and magnetic fields are reviewed and actual interference data are summarized. Conducted and radiated interference coupling mechanisms and levels in substation control wiring are described using both measurement results and electromagnetic models validated against measurements. The nominal maximum field and control wire interference levels expected in the switchyard and inside the control house from switching operations, faults, and an average lightning strike are estimated using high frequency transient coupling models. Comparisons with standards are made and recommendations given concerning equipment shielding and surge protection. >

Maximum likelihood estimation of fault location on transmission lines using travelling waves

Abstract: An improved method for fault location on transmission lines using the maximum likelihood estimate of the arrival times of reflected travelling waves is presented. The method is compared with previously proposed correlation based methods using results determined by ATP. The effectiveness of the new method for small fault angles and close in faults has been detailed. >

Adaptive distance protection of a double-circuit line

Abstract: Due to changes in the power system, such as generator and line outages and changes in load and generation, the performance of distance relays can vary. In the case of a distance relay protecting a phase of a double-circuit line, the state of the parallel circuit is of major importance. Simulations show that, depending on the power system state, a distance relay can cover from less than 50% up to far more than 100% of the total line length. This is demonstrated with a double-circuit line under the single-line-to-ground fault (SLG) fault condition, since this is the most common type of fault. In this paper the distance protection of a double-circuit line under the SLG fault condition is formulated. To achieve correct operation, the relay does not only use the measured quantities of the circuit-to-be-protected, but also the zero sequence current of the parallel circuit. Such a relay requires extra measuring equipment, and, moreover, the zero sequence current of the parallel circuit cannot always be measured. Therefore, another approach is chosen. A correction factor is introduced, set adaptively according to the actual power system state. In this way, the appropriate setting of the relay is provided, in relation with the actual power system state. A side-effect of the adaptive setting of the relay is that the safety margin in the relay settings is decreased, due to the uncertainty in the power system state. By adapting the relay to the actual power system state, maximum selectivity is achieved, and the protection system as such will be more reliable. >

A new neural networks approach to on-line fault section estimation using information of protective relays and circuit breakers

Abstract: This paper proposes a new neural network diagnostic system for online power system fault section estimation using information of relays and circuit breakers. This system has a similar profile of an expert system, but can be constructed much more easily from elemental samples. These samples associate fault section with its primary, local and/or remote protective relays and breakers. The diagnostic system can be applicable to the power system control center for single or multiple fault sections estimation, even in the cases of failure operation of relays and breakers, or error-existent data transmission. The proposed approach has been practically verified by testing on a model power system. The test results, although preliminary, suggest this system can be implemented by various electric utilities with relatively low customization effort. >