Showing papers on "Power-system protection published in 1997"

Use of local measurements to estimate voltage-stability margin

Abstract: Estimating the proximity of power systems to voltage collapse in real-time still faces difficulties. Beside the data management and computational issues, any central-control method is subject to the reliability of long-distance data communications. In the paper, the authors describe a new data-processing method to estimate the proximity to voltage collapse. The method (code-named SMARTDevice, for Stability Monitoring And Reference Tuning Device) employs only local measurements-bus voltage and load current-and calculates the strength of the transmission system relative to the bus. The collapse occurs when the local load approaches this value. The method is simple enough so that it can be implemented in a numerical relay. The performance of SMARTDevice is compared against the conventional undervoltage relays. It is shown that the latter can misoperate while the new device does not. SMARTDevice is in fact a new breed of voltage relay whose setpoint is automatically tuned to the power system condition.

Automated fault location and diagnosis on electric power distribution feeders

Abstract: This paper presents new techniques for locating and diagnosing faults on electric power distribution feeders. The proposed fault location and diagnosis scheme is capable of accurately identifying the location of a fault upon its occurrence, based on the integration of information available from disturbance recording devices with knowledge contained in a distribution feeder database. The developed fault location and diagnosis system can also be applied to the investigation of temporary faults that may not result in a blown fuse. The proposed fault location algorithm is based on the steady-state analysis of the faulted distribution network. To deal with the uncertainties inherent in the system modeling and the phasor estimation, the fault location algorithm has been adapted to estimate fault regions based on probabilistic modeling and analysis. Since the distribution feeder is a radial network, multiple possibilities of fault locations could be computed with measurements available only at the substation. To identify the actual fault location, a fault diagnosis algorithm has been developed to prune down and rank the possible fault locations by integrating the available pieces of evidence. Testing of the developed fault location and diagnosis system using field data has demonstrated its potential for practical use.

Loss of mains detection for embedded generation by system impedance monitoring

Abstract: Embedded generators operating in parallel with the electricity supply network are required by law to fit 'loss of mains' protection. This paper presents a new technique for detecting loss of mains based on the measurement of power system impedance and overcomes the deficiencies of more popular relays. The unit's decision to trip is based on the identification of the change in impedance that occurs at a private generator's site when it becomes disconnected from the mains supply.

An adaptive out-of-step relay [for power system protection]

Abstract: The paper describes the development of an adaptive out-of-step relay, from the formulation of its concept to its field implementation at the Florida-Georgia interface. Adaptive relaying seeks to make adjustments to the relay characteristics as power system conditions change, thereby making the relay more attuned to the prevailing power system conditions. The work described here began as a theoretical examination of the out-of-step relaying application, its possible shortcomings, and the opportunities to make adaptive improvements. It is shown that for a system that behaves primarily as a two-machine power system, the out-of-step relay could be made more secure by applying the principle of equal area criterion. The paper describes the theory of such a relay, its hardware configuration, and the system as it is installed in the field. The newly developed technology of synchronized phasor measurements plays an important role in the realization of this relay. The paper includes a summary of the field experience gathered over a period of one year.

An Algorithm for Compensating Secondary Currents of Current Transformers

Abstract: Current transformer (CT) saturation may cause power system relays to malfunction. The conventional method used to deal with the problem is overdimensioning of the transformer core so that CTs can carry up to 20 times the rated current without exceeding 10% ratio correction. However, this not only reduces the sensitivity of power system relays, but also increases the CT core size. This paper presents a technique of estimating the secondary current corresponding to the CT ratio under CT saturation. The proposed algorithm can improve the sensitivity of relays to low level internal faults, minimize the instability of relays for external faults, and might ultimately assist in reducing the dimension of the required CT core cross-section.

A new type of differential feeder protection relay using the Global Positioning System for data synchronization

Abstract: This paper describes a new technique for sampling synchronisation in a numerical differential feeder protection relay. The protection uses a Global Positioning System satellite receiver for sampling synchronization at each end of the protected feeder. The EMTP/ATP power system simulator was used to simulate a typical distribution feeder with faults at various locations. The current signals generated at both ends of feeder were used to evaluate the performance of the protection. These tests were carried out using a protection simulator. The results illustrate the effect of current transformer saturation on the sensitivity and stability of the protection. Finally it is shown that a precisely synchronized differential protection is fast, selective and accurate. On an internal fault it provides a high operating sensitivity, whilst it remains stable on all external faults.

A New Approach to Phase Selection Using Fault Generated High Frequency Noise and Neural Networks

Abstract: Single-pole autoreclosure is quite extensively used in long-line applications and involves tripping only the faulted phase for single-phase earth faults. Reliable and fast phase selection is thus imperative in order to avoid potential problems of system insecurity and instability. Conventional phase selectors, primarily based on power frequency measurands, can suffer some impairment in performance because of their heavy dependency on varying system and fault conditions. However, the advent of artificial neural networks (ANNs), with their ability to map complex and highly nonlinear input/output patterns, provides an attractive potential solution to the long-standing problems of accurate and fast phase selection. This paper describes the design of a novel phase selector using ANNs. The technique is based on utilising fault generated high frequency noise (captured through the high voltage coupling capacitor of a conventional capacitor voltage transformer) to essentially recognise the various patterns generated within the frequency spectra of the fault generated noise signals on the three phases, for the purposes of accurately deducing the faulted phase. The paper demonstrates a new concept and methodology in phase selection which will facilitate single-pole autoreclosure applications in power systems.

Grounding System Analysis in Transients Programs Applying Electromagnetic Field Approach

Abstract: Lightning protection studies of substations and power systems require knowledge of the dynamic behavior of large grounding grids during electromagnetic transients. This paper presents strategies which allow incorporation of complex grounding structures computed using a rigorous electromagnetic model in a transients programs. A novel technique for rational function representation of frequency-dependent grounding system impedances in the EMTP is described. An arbitrary number of feeding points can be modeled as mutual coupling is taken into account. Overvoltages throughout electrical power systems and the transient ground potential rise in the surroundings of grounding structures can be computed.

Problems and solutions for microprocessor protection of series compensated lines

Abstract: Protection of systems with series compensated lines is considered to be one of the most difficult tasks for relay manufacturers and utility engineers. Protection and control of surrounding circuit elements, particularly transmission line protection, needs to be adapted to the variations introduced by these devices. Fixed series capacitors and thyristor controlled devices introduce harmonics and nonlinearities which adversely impact the protection function. To take full advantage of the series capacitor installation in a utility network, it is necessary to explore the impact of series capacitors on protection and implement appropriate schemes. This paper describes practical issues related to microprocessor based protection of power networks with fixed and controllable series compensating devices. Phenomena that affect the protection of a network with series compensated lines are explored. The paper also presents solutions and new concepts in protection and fault location for series compensated power networks.

Electrical application of high T/sub c/ superconducting saturable magnetic core fault current limiter

Abstract: A saturable magnetic core electrical fault current limiter (FCT) designed with a high T/sub c/ superconducting (HTS) DC bias winding is described. The HTS winding is prepared by using a Ag-clad (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub O10+x/ HTS wire. The limiting behaviour of this FCL is investigated, and its electrical application is considered in an electrical power transmission system. The results show that this FCL limits fault currents effectively and is a possible solution for reducing power system fault currents.

An improved reliability model for redundant protective systems-Markov models

Abstract: A number of reliability models of power system components including protective-system failures have been proposed. These models have concentrated on modeling a single protective device. When this protections fails, it is usually assumed that backup protection will isolate the faulted component with 100% reliability. In reality, power system components, have redundant protection schemes with two or more protective devices operating in parallel. Existing reliability models have not dealt with redundant protective systems adequately. This paper presents an improved reliability model capable of modeling these systems. The proposed Markov model and the equations for a number of important performance indices are derived and illustrated in this paper using the data for a typical transmission system.

Rogowski coils suit relay protection and measurement ~of power systems\

Abstract: Microprocessor-based technology has been accepted today in virtually all areas of electrical engineering, including power. Input signals that accurately replicate events are critical for the proper operation of microprocessor-based equipment. Therefore, current sensors are an integral element of relay protection and current measuring systems. Current transformers (CTs) have been used exclusively for protection and measurement applications in part because of their ability to produce the high power output needed by electromechanical equipment. The introduction of microprocessor-based equipment has made the high-power output of CTs unnecessary and made the use of other measurement techniques possible. One such measuring device that does not produce high-power output but offers many advantages over CTs is the Rogowski coil. This article describes Rogowski coil operational characteristics and it applications for current measurements.

A new principle for high resistance earth fault detection during fast power swings for distance protection

Abstract: The combined effects of a high impedance ground fault condition and a fast power swing condition cause the apparent impedance 'seen' by power system distance protection schemes to vary over a wide range. Previous research work has shown that the detection and location of high impedance faults during power swings is very difficult. A new principle for high impedance fault detection during fast power swings is presented which overcomes these deficiencies. This method forms a part of a complete power swing relaying scheme and can be implemented on standard relaying hardware. Results are presented based on extensive simulation studies carried out on a typical 400 kV transmission system, using the Electromagnetic Transient program (EMTP).

Grids get smart protection and control

Abstract: As open transmission access is becoming a reality, a major concern of electric power utilities is to maintain the reliability of the grid. Increased power transfers raise concerns about steady-state overloads, increased risks of voltage collapse and potential stability problems. Strengthening the protection and control strategies is what utilities must do to prevent a local problem from spreading to other parts of the grid. This article defines the framework and motivation for the development of a multilayered protection and control scheme that starts with local measurement devices and integrates higher-level control schemes into an overall control strategy.

Application of genetic algorithm to overcurrent relay grading coordination

Abstract: To guard against the possible failure of main protection devices in power system protection, the overcurrent (OC) relay is usually employed as backup protection to prevent the system from suffering catastrophic consequences. OC protection devices and their variants represent the largest installed base of protective equipment on any distribution system and may be considered as the backbone of any protection strategy. Their individual operation is a standalone tripping action made without other relay information except the current transformer (CT) input only. This paper describes the grading coordination of OC relays with normal IDMTL (inverse definite minimum time lag) characteristic complying with the IEC 255-4 standards.

Protection scheme for ehv transmission systems with thyristor controlled series compensation using radial basis function neural networks

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 ...

Adaptive Distance Protection of Double-Circuit Lines using Artificial Neural Networks

Abstract: Because of the zero sequence mutual coupling of parallel transmission circuits, the distance calculation performed by a ground distance relay is incorrect. This error is influenced by the actual power system condition. Although accounted for by using a large safety margin in the zone boundaries, unexpected overreach can still occur and the operation speed is decreased. Adaptive protection offers an approach to compensate for the influence of the variable power system conditions. By adapting the relay settings to the actual power system condition, the relay will respond more accurately to power system faults. The selectivity of the protection system is increased, as is the power system reliability. In this paper, an adaptive distance relaying concept is presented. In order to minimize the required communication, local measurements are used to estimate the entire power system condition. An artificial neural network is used to estimate the actual power system condition and to calculate the appropriate tripping impedance. Application of this concept to the model of the Dutch 380 kV power system has resulted in an enormous increase in relaying accuracy. The relaying error is reduced substantially. Most importantly, the standard deviation, indicating the relay's sensitivity to power system condition variations, is reduced to nearly zero. The zone boundary is kept nearly constant, which facilitates the relay coordination. The selectivity of the entire power system protection system is improved. It is shown that adaptive protection improves the protection system selectivity, and that artificial neural networks can very well be used to estimate the actual power system condition.

Surge protection for power grids

Abstract: The new high temperature superconductors are well-suited for fault-current limiters, thanks to stable thermal properties and high operating temperatures. This article surveys efforts worldwide to develop a commercially viable superconducting fault-current limiter, but particularly focuses on one project, with which the author is associated, which developed a 2.4 kW, 3 kA prototype.

Wide-area protection against voltage collapse

Abstract: A wide-area protection system against long-term voltage collapse was designed, implemented, tested, modified, and commissioned. The system is based on the present SCADA system in southern Sweden, which has been complemented with input signals and equipment to execute action orders from the protection system. Bus voltages from the transmission system, reactive power output from generators connected to the transmission system, and current limiter information from main generators were used as input signals to the wide-area protection system. Actions from the protection system are shunt reactor disconnection, shunt capacitor connection, start of gas turbines, emergency power request from the HVDC connection to Germany, low priority load disconnection, and finally shedding of high priority load.

Enhanced double circuit line protection

Abstract: This paper describes a double circuit line protection relay which has all 6 line currents and voltages available for processing. This allows several alterations to the standard 3-phase distance package which enhance the performance of the protection system. Results are presented for the relay fed with simulated waveforms from a Playback Simulator. The system used in the simulations is modelled on a 69 kV parallel line in the Winnipeg Hydro network.

A new summary on the iec protection against electric shock

Abstract: This paper provides a summary of the IEC protection against electric shock. This protection is provided by appropriate basic measures as follows: (1) for protection both in normal service and in case of a fault (against both direct and indirect contact), use low and safe voltage of 50 V and below; (2) for protection in normal service (against direct contact), use insulation and/or enclose live parts or use isolation distance; and (3) for protection in case of a fault (against indirect contact), prevent conducting parts not normally energized from becoming live. This is accomplished by grounding and automatic disconnection of the supply, by use of Class II equipment (as double or equivalent insulation), or by separating the supply from ground, IEC publication 364-4-41 "Electrical installations of buildings", (Part 4, Chapter 41) classifies types of system grounding as TN-system, TT-system, and IT-system. Development of this summary is based on actual hazard risk analysis of potential incidents to suggest criterion by which the appropriate measures can be applied to avoid or mitigate injury or damage.

Application of current limiting fuses in distribution systems for improved power quality and protection

Abstract: This paper presents a comparative analysis of the effects of distribution system expulsion and current limiting fuse operations on power quality. To perform this analysis, digital fuse models were developed for use with the EMTP/ATP program. Expulsion fuses interrupt current at current zeros and are easily modeled. Current limiting fuses (CLFs) interrupt current by forcing a current zero and therefore, require a special modeling technique. CLFs were modeled by representing them as nonlinear resistances. The nonlinear resistances were calculated using laboratory test results of actual CLF operations. This technique is very convenient for simulating CLF operations and analyzing their interaction with the distribution system. This paper shows that CLFs improve power quality by supporting system voltage during faults and reducing voltage dip duration. Additionally, CLFs reduce the fault let-through I/sup 2/t.

Testing and modelling of inductive superconducting fault current limiters

Abstract: Fault current limiters are expected to be amongst the first applications of high-temperature superconductors in power engineering. The use of the intrinsic property of a superconductor to transit from a zero (or near zero in the case of AC currents) resistance state to a highly resistive state when the critical current is exceeded is the basis for a fast acting fault current limiter. In this article, the authors report test results (43 kVA nominal power: 450 V RMS and 95 A RMS) and comparisons to theoretical simulations. Circuit analysis is used to study design issues and to extract the superconducting material's (BSCCO 2212) properties during the application of the short-circuit. EMTP simulation is employed to predict fault current limiter behaviour in the utility network in order to coordinate with other equipment.

Frequency and transient behavior of grounding systems. I. Physical and methodological aspects

Abstract: In the paper, the author presents a soil electrical model, based on extensive measurements, from very low frequency to 1 MHz, and in a physical criterion to validate measurement results. A methodology and a computational procedure to determine the frequency and the transient behavior of grounding systems, and the effects of such behavior on safety of people and equipment are also presented. This procedure considers, in correct way, soil electrical characteristics and the treatment of electromagnetic behavior of grounding systems, including propagation and attenuation in dissipative media, as it is the case of soil.

Modeling of a transmission line protection relaying scheme using Petri nets

Abstract: This paper proposes a transmission line protection relaying scheme model using Petri nets (PNs), including three types of relays as well as an automatic reclosing device. By analyzing the properties of the PN model, the dynamic behaviors of the modeled system are evaluated and the drawback of the system is detected. This model may also be extended to other similar systems to evaluate their performances.

Frequency and transient behavior of grounding systems. II. Practical application examples

Abstract: In this paper, the authors present the application of the methodology and computational procedures described in part I of this paper to basic practical problems of electromagnetic compatibility, including: the effects of lightning discharges in electrical grounding systems and the definition of risks for human safety; for equipment damage and of noncorrect operation of control and protection schemes; procedures to limit the consequences of lightning; interference, through ground systems, in sensitive circuits, considering frequency dependence of interaction of power equipment with sensitive elements and surge type phenomena; and procedures to limit interference.

Vacuum interrupters, the new technology for switching and protecting distribution circuits

Abstract: This paper provides application guidelines for, and shows the advantages of, applying vacuum interrupters to the switching and to the protection of power distribution circuits. The performance of vacuum interrupters for load switching and for short circuit interruption is discussed with reference to long-term switching life and low maintenance costs. During this discussion, the unique ability of the vacuum interrupter to handle developing faults is analyzed. The effects of current chopping and voltage escalation on the components in a distribution circuit are examined, and straightforward methods to minimize their effects are presented. The advantages of using vacuum interrupters for long-life, maintenance-free performance for capacitor switching and for motor switching are discussed. The paper discusses the very fast recovery characteristics of vacuum interrupters, and shows how these characteristics can be used to produce a circuit breaker that will reliably protect distribution transformers. Finally, the use of vacuum interrupters for other switching duties is examined.

Transmission Network Protection: Theory and Practice

Abstract: From the basic fundamentals and principles of protective relaying to current research areas in protective systems and future developments in the field, this work covers all aspects of power system protection. It includes the implementation of relays using electromechanical devices, static devices and microprocessors; distance protection of high voltage and extra high voltage lines, including distance relay errors; and adaptive, dynamic, travelling wave and noise-based relays.

The Influence of Protection System Failures and Preventive Maintenance on Protection Systems in Distribution Systems

Abstract: Protection systems in power networks can fail either by not responding when they should (failure to operate) or by operating when they should not (false tripping). The former type of failure is particularly serious since it may result in the isolation of large sections of the power network. However, the probability of a failure to operate can be reduced by carrying out preventive maintenance on protection systems. This paper describes an approach to determine the impact of preventive maintenance on distribution network protection systems on the reliability of the power supply to customers. The proposed approach is based on Markov models.

A new loss of grid protection based on power measurements

Abstract: The growth in the use of embedded generation operating in parallel with electric utility distribution systems has increased the importance of loss of grid protection. A new loss of grid protection algorithm is described in this paper which has been designed to be amenable to inclusion in a multi-function numeric protection package. In particular, it has been designed to detect, reliably, a loss of grid within 120 milliseconds, remain stable during other power system disturbances and demand a minimum of processing resources of the microcomputer host relay.