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Showing papers on "Power-system protection published in 1991"


Journal ArticleDOI
TL;DR: In this paper, three adaptive methods are suggested to deal with the following problems: (1) the effect of frequency variation, (2) fault resistance in single-phase-to-ground faults, and (3) power swings in power systems.
Abstract: Adaptive distance protection can be obtained by forming an adaptive control system, and rules about information, functions, and time sequence must followed in its design. Three adaptive methods are suggested to deal with the following problems: (1) the effect of frequency variation, (2) the effect of fault resistance in single-phase-to-ground faults, and (3) the effect of power swings in power systems. A prototype of digital distance protection has been designed and tested on a 500 kV dynamic power system model. Tests results show that the performance of distance protection can be improved by the adaptive methods. >

149 citations


Proceedings ArticleDOI
11 Jun 1991
TL;DR: In this article, a small size emergency power supply was investigated by adopting a quick detector of momentary power failure and a compact DC-DC converter, which can detect power failures at its earliest stage.
Abstract: A small size emergency power supply was investigated. This was realized by adopting a quick detector of momentary power failure and a compact DC-DC converter. The power failure detector has a novel type of T-V (time-voltage) conversion using an SC (switched capacitor) integrator and can detect power failures at its earliest stage. The DC-DC converter was constructed by an SC transformer in which the voltage of each stage increases according to the form of the Fibonacci sequence. The proposed power supply is supplied with power from chargeable batteries for several minutes, while the computer system saves important data to an auxiliary memory such as a disk and can complete its emergency quit operation. The circuit configurations, operations and experimental results are presented. >

101 citations


Journal ArticleDOI
TL;DR: In this paper, a knowledge-based system for fault analysis in the area of medium-voltage distribution networks is described, based on an expert system shell developed for applications in the field of power supply.
Abstract: A knowledge-based system for fault analysis in the area of medium-voltage distribution networks is described. It is based on an expert system shell developed for applications in the field of power supply. Applications like contingency analysis or corrective switching have been realized. The solution for fault analysis attaches great importance to portability, simple integration in different telemetry and control systems, integration of different protection relay types, and capability of working with little information about the electrical network and the triggering of the protection devices. >

47 citations


Journal ArticleDOI
TL;DR: In this article, the authors describe a new digital dynamic testing (DYNA-TEST) simulator developed for protection relay applications, which is capable of producing voltage and current transients that correspond to actual fault events in the power systems.
Abstract: The authors describe a new digital dynamic testing (DYNA-TEST) simulator developed for protection relay applications. This simulator is capable of producing voltage and current transients that correspond to actual fault events in the power systems. Fault transients are generated either by simulating power system faults using an electromagnetic transient program or by replaying records of fault signals captured in substations by digital fault recorders. These signals are used to test protection relays. Such an approach to relay testing represents a major improvement over steady-state testing. >

45 citations


05 Nov 1991
TL;DR: In this paper, the authors present a power swing block principle based on a microcomputer, which can reliably distinguish short circuits from swings, so that fast tripping is obtained for in-zone faults any time.
Abstract: The authors present a new power swing block principle based on a microcomputer. This principle can reliably distinguish short circuits from swings, so that fast tripping is obtained for in-zone faults any time. The authors also analyse and calculate the performance of this principle in the case of different fault locations, fault resistance and power swing cycles. The program is incorporated into microprocessor based distance protection hardware, and tested on a power system dynamic model. The results are satisfactory. Distance protection with this power swing block has been installed on 220 kV transmission lines since 1986, and has achieved excellent performance.

39 citations


Proceedings ArticleDOI
28 Sep 1991
TL;DR: In this paper, an experimental verification of the energy deposited in the devices for a matrix of combinations of these three parameters is provided, which is a finding that should reconcile contradictory conclusions reported by different authors making different assumptions.
Abstract: Cascading surge protection devices located at the service entrance of a building and near the sensitive equipment are intended to ensure that each device shares the surge stress in an optimum manner to achieve reliable protection of equipment against surges impinging from the utility supply. However, depending on the relative clamping voltages of the two devices, their separation distances, and the waveform of the impinging surges, the coordination may or may not be effective. Computations with experimental verification of the energy deposited in the devices for a matrix of combinations of these three parameters is provided. Results show coordination to be effective for some combinations and ineffective for some others, which is a finding that should reconcile contradictory conclusions reported by different authors making different assumptions. From these results, improved coordination can be developed by application standards writers and system designers. >

34 citations


Proceedings ArticleDOI
11 Jun 1991
TL;DR: In this paper, the authors present a comprehensive study of fault modeling of the class of sample-and-hold circuits frequently used in mixed analog/digital signal processors, and validate the concept of fault equivalence for analog circuits.
Abstract: The author presents the first comprehensive study of fault modeling of the class of sample-and-hold circuits frequently used in mixed analog/digital signal processors. The faults under study consist of catastrophic faults and out-of-specification faults. Even if the faults are restricted to the passive components and MOS switches (i.e. the operational amplifiers are assumed fault-free), the effects of these faults are quite complex, especially the out-of-specification faults. For example, an incorrect value of the resistor R/sub on/ of an MOS switch and an incorrect value of the capacitor in some cases have the same faulty manifestations at the output, and may be thought of as equivalent faults. The concept of fault equivalence is validated for analog circuits. The results show that various types of faults are distinguishable, thus reducing the size of the analog fault dictionary used in further diagnosis. >

30 citations


Journal ArticleDOI
TL;DR: In this paper, the theory of three-phase symmetrical components is used to develop a method for fault analysis in six-phase power systems. But the method is not applicable to all significant faults that occur on 6-phase systems.
Abstract: The use of six-phase (and other higher order phase) systems for power transmission was proposed as an alternative for using electrical rights-of-way more effectively and efficiently. One of the important items in applying the six-phase alternative in transmission planning is the design of an adequate protective scheme. This requires a detailed and realistic fault analysis. The 23 significant fault types (symmetrical or asymmetrical) that can occur in six-phase power systems are analyzed. The theory of three-phase symmetrical components is used to develop a method for such analysis. The validity of the proposed method is checked by application to a simple system under a single line-to-ground fault, and three line-to-ground, five line-to-ground, and six line-to-ground faults. The advantage of the method is its applicability to all significant faults that occur on six-phase power systems. >

29 citations


Book
01 Jan 1991
TL;DR: In this paper, the authors define a set of basic principles of the Transformer Transformer Failure Modes (TFM) of the AC Power System (PPS) and apply them to the Power Generation System (PGS) and Power Control System (PCS).
Abstract: AC POWER SYSTEMS Introduction AC Circuit Analysis Elements of the AC Power System Utility AC Power System Architecture References Bibliography POWER-GENERATION SYSTEMS Introduction Fundamental Concepts Power-Generating Systems References Bibliography POWER FACTOR Introduction Fundamental Principles PF Correction Techniques References Bibliography POWER TRANSFORMERS Introduction Inductive Properties Basic Principles of the Transformer Transformer Failure Modes References Bibliography CAPACITORS Introduction Basic Principles Capacitor Failure Modes References Bibliography SEMICONDUCTORS Introduction Semiconductor Failure Modes MOSFET Devices Thyristor Components ESD Failure Modes Semiconductor Development Effects of Arcing References Bibliography RECTIFIER AND FILTER CIRCUITS Introduction Power Rectifiers Power Supply Filter Circuits References Bibliography POWER ELECTRONICS Introduction Thyristor Devices Power Transistors References Bibliography ORIGINS OF AC LINE DISTURBANCES Introduction Naturally Occurring Disturbances Equipment-Caused Transient Disturbances References Bibliography POWER DISTURBANCE CHARACTERIZATION Introduction Standards of Measurement Reliability Considerations References Bibliography POWER SYSTEM PROTECTION METHODS Introduction The Key Tolerance Envelope Assessing the Lightning Hazard FIPS Publication 94 Protection Alternatives References Bibliography MOTOR-GENERATOR SET Introduction System Configuration References Bibliography UNINTERRUPTIBLE POWER SYSTEMS Introduction UPS Configuration References Bibliography POWER CONDITIONING DEVICES Introduction Ferroresonant Transformer Isolation Transformer Line Conditioner References Bibliography TRANSIENT-SUPPRESSION DEVICES Introduction Filter Devices Crowbar Devices Voltage-Clamping Devices Selecting Protection Components References Bibliography FACILITY WIRING AND TRANSIENT PROTECTION Introduction Facility Wiring Power-System Protection References Bibliography CIRCUIT-LEVEL TRANSIENT SUPPRESSION Introduction Protecting Low-Voltage Supplies Protecting High-Voltage Supplies RF System Protection Protecting Logic Circuits Protecting Telco Lines Inductive Load Switching Bibliography GROUNDING PRACTICES Introduction The Need for Grounding Establishing an Earth Ground References Bibliography GROUNDING TOWER ELEMENTS Introduction Ground-Wire Dressing Facility Ground Interconnection Grounding on Bare Rock Transmission-System Grounding References FACILITY GROUND-SYSTEM DESIGN Introduction Bulkhead Grounding AC System Grounding Practices Grounding Equipment Racks Grounding Signal Cables Patch-Bay Grounding Cable Routing References Bibliography STANDBY POWER SYSTEMS Introduction Standby Power Options References Bibliography DESIGNING FOR FAULT-TOLERANCE Introduction Critical System Bus Plant Maintenance References Bibliography THE EFFICIENT USE OF ENERGY Introduction Energy Usage Peak Demand Load Factor Power Factor References Bibliography SAFETY AND PROTECTION SYSTEMS Introduction Electric Shock Polychlorinated Biphenyls OSHA Safety Requirements References Bibliography REFERENCE DATA AND TABLES Standard Electrical Units Reference Tables UNITS CONVERSION TABLE Quantity Conversion INDEX

29 citations


Journal ArticleDOI
TL;DR: In this article, a relay under evaluation with current and voltage input similar to that encountered under service and fault conditions was used to evaluate a new EHV teed feeder relaying system.
Abstract: The power system simulation system described provides a relay under evaluation with current and voltage input similar to that encountered under service and fault conditions. Using digital transient simulation programs to model power system behaviour, the facility provides an effective bandwidth of from DC to 4 kHz for both the current and voltage relay input. The relay's output is automatically monitored during the test, enabling a detailed report of its response to be produced. The application of the system is illustrated by the evaluation of a new EHV teed feeder relaying system. >

27 citations


Journal ArticleDOI
TL;DR: A protection system design algorithm that encompasses all possible circuit configurations of a radial distribution system is presented and a predetermined set of coordination rules is incorporated to develop a relational database type managed coordination table.
Abstract: A protection system design algorithm that encompasses all possible circuit configurations of a radial distribution system is presented. A predetermined set of coordination rules is incorporated to develop a relational database type managed coordination table. The algorithm performs both computerized placement and selection of devices which coordinate. Automated placement rules determine the location of any new devices required for circuit protection. Automated selection rules work with the coordination table in selecting groups of protection devices which mutually coordinate for the chosen circuit design. A design example is presented depicting a radial distribution system composed of three individual circuits. >

Journal ArticleDOI
TL;DR: It is found that, even when it met very complicated situations, the learning detection system made smart decisions and evolved to a new situation with a newly derived decision rule.
Abstract: An attempt is made to find an intelligent detection system. This intelligent system has two specific properties. First, it is capable of taking advantage of multiple detection parameters or variables. Second, it provides adaptability using inductive reasoning, incorporated with some critic modules in a detection system. Inductive reasoning which will minimize entropy was used to acquire the knowledge of fault class and nonfault class. This knowledge was used to make the intelligent system adaptive to surrounding environments. A learning detection system was implemented with inductive reasoning and an event detector. An example execution is shown with a decision rule which was derived using training sample data consisting of high impedance faults, switching events, and normal status. A complicated set of test data is used to test the performance of the learning detection system. It is found that, even when it met very complicated situations, the learning detection system made smart decisions and evolved to a new situation with a newly derived decision rule. >

Journal ArticleDOI
TL;DR: In this article, four fundamental algorithms that make it possible to simulate numerous types of protective relay systems are identified and the characteristics of these fundamental modules necessary for building an embedded relay model within an EMTP (electromagnetic transient program) are discussed.
Abstract: Four fundamental algorithms that make it possible to simulate numerous types of protective relay systems are identified. The characteristics of these fundamental modules necessary for building an embedded relay model within an EMTP (electromagnetic transient program) are discussed. A directional time overcurrent relay model is used to provide a detailed demonstration of relay model development and to illustrate the utilization of the fundamental modules. The results of using this relay model in a modeled power system reveal the power of this concept to enhance power systems education, applications engineering, and relay design procedures. >

Proceedings ArticleDOI
06 May 1991
TL;DR: A microcontroller based intelligent under/over frequency relay is introduced, which has the advantages of high flexibility, less cost, and high communication ability.
Abstract: Several practical intelligent setting schemes that consider the effect of df/dt are suggested and simulated for a power system operating in an underfrequency situation. Considerations for df/dt relay setting are discussed. These intelligent setting schemes have made a positive improvement in the underfrequency condition. As an extension of these new intelligent control schemes, a microcontroller based intelligent under/over frequency relay is introduced. With its advanced hardware and intelligent software, this microcontroller has the advantages of high flexibility, less cost, and high communication ability. >

Journal ArticleDOI
TL;DR: A number of papers have been published on the subject with an emphasis on applications that relate to the overall power system monitoring, operation and planning, however, less emphasis was placed on protective relaying, substation control and related monitoring functions.

Proceedings ArticleDOI
07 May 1991
TL;DR: In this paper, the choice and adjustments of the most suitable models as well as determinations of their limitations in the digital simulation of very fast transients in GIS are investigated.
Abstract: Very fast electromagnetic transients caused by switching operations in gas-insulated substations (GIS) cannot be calculated if conventional techniques of modeling and simulations are used. The choice and adjustments of the most suitable models as well as determinations of their limitations in the digital simulation of very fast transients in GIS are investigated. Results obtained from very extensive field tests as well as from digital simulations for different types of GIS have been used for the development of more accurate models for GIS components and GIS as a whole. A comparison between field test and calculation results enables the development of a model which takes into account the most decisive physical phenomena inherent in the very fast transients in the GIS. This is a prerequisite for designing and manufacturing power system components as well as for control and protection of the system. >

Journal ArticleDOI
TL;DR: In this paper, a full-scale model of a residential service has been set up in a laboratory and subjected to impulses approximating lightning strokes, and tests were made to determine the impulse characteristics of the secondary system and to test the validity of previous analyses.
Abstract: Low-side surges are known to cause failures of distribution transformers. They also subject load devices to overvoltages. A full-scale model of a residential service has been set up in a laboratory and subjected to impulses approximating lightning strokes. The tests were made to determine the impulse characteristics of the secondary system and to test the validity of previous analyses. Among the variables investigated were stroke location, the balance of the surges in the service cable, and the effectiveness of arrester protection. Low-side surges were found to consist of two basic components: the natural frequency of the system and the inductive response of the system to the stroke current. The latter component is responsible for transformer failures while the former may be responsible for discharge spots often found around secondary bushings. Arresters at the service entrance are effective in diverting most of the energy from a lightning strike, but may not protect sensitive loads. Additional local protection is also needed. The tests affirmed previous simulations and uncovered additional phenomena as well. >

Proceedings ArticleDOI
29 May 1991
TL;DR: In this paper, the feasibility of an adaptive relaying system for a distribution network is examined and a reduced version of the City of Saskatoon distribution network has been selected for this study.
Abstract: The feasibility of an adaptive relaying system for a distribution network is examined. A reduced version of the City of Saskatoon distribution network has been selected for this study. Overcurrent and directional overcurrent relays are used to protect the system. The scope of the work includes changing relay settings and relay pick-up currents in an online mode as operating conditions of the system change. The system changes considered are the loads and generation level changes and line and transformer outages over a period of several days. Relay coordination and communication software which are essential for implementing an adaptive relaying system are discussed. A laboratory model for an adaptive relaying system is presented. It is shown that by using microprocessor-based relays and appropriate software, as well as communication aids, an adaptive relaying concept can be implemented. >

Journal ArticleDOI
TL;DR: The MARS project as discussed by the authors is a second-generation expert system which takes into account the man-machine cooperation problematic and constructs restoration plans after a local failure on a power transmission network.
Abstract: Power transmission networks are protected from local failures by automatic devices that disconnect faulted areas after a failure to suppress short circuits or overloads on lines. To construct power restoration plans, operators must take into account general restoration guidelines or local constraints, but most of the procedure relies on their experience. The aim of the MARS project is to provide the operator with an aid during these delicate phases of network monitoring. MARS is a second-generation expert system which takes into account the man-machine cooperation problematic. MARS constructs restoration plans after a local failure on a power transmission network. Organized around a blackboard architecture, it integrates planning knowledge sources containing the restoration expertise, a qualitative model to predict the results of the considered actions, and a quantitative model to verify the correctness of the plans towards numerical constraints (load flow). >

Proceedings ArticleDOI
23 Sep 1991
TL;DR: Numerical results indicate that the structure of the optimum algorithm shows that efficient power control and dynamic channel assignment algorithms should be closely related.
Abstract: Transmitter power control as a method to reduce cochannel interference for a given channel allocation is investigated. The performance is analyzed in terms of interference. (outage) probabilities. Qpthm and nearoptimum global power control schemes are proposed. Using these schemes upper bounds on the performance of power control schemes are derived. Numerical results indicate that these upper bounds exceed the performance of conventional systems by an order of magnitude in interference suppression and by a factor of 5 in capacity. The structure of the optimum algorithm shows that efficient power control and dynamic channel assignment algorithms should be closely related.

Journal ArticleDOI
28 Sep 1991
TL;DR: The protection of power factor correction capacitors used to enhance electrical system efficiency is addressed in this article, where different considerations in the selection of capacitor fuse applications and the philosophies behind them are reviewed.
Abstract: The protection of power factor correction capacitors used to enhance electrical system efficiency is addressed. Different considerations in the selection of capacitor fuse applications and the philosophies behind them are reviewed. Individual and group fusing are discussed. Continuous current, transient current, fault current, tank rupture curve coordination, voltage on good capacitors during a fault, energy discharged into a failed unit, outrush current, and coordination with unbalance detection schemes are topics covered. Examples of actual applications of capacitors in industrial distribution systems are presented. >

Proceedings ArticleDOI
L. Yu1, R.L. Henriks
09 Sep 1991
TL;DR: In this article, a modified high-resistance grounding (MHRG) with ground fault tripping may be the better practice in some industrial applications, and the selection of neutral grounding resistor values and ground fault protection devices are discussed.
Abstract: Low-voltage (LV) and medium-voltage (MV) industrial power distribution systems below 15 kV are discussed. The method chosen for system neutral grounding is significant to system performance, operation, maintenance, safety, etc. Solidly grounded (SG), low-resistance grounded (LRG), and high-resistance grounded (HRG) systems are discussed and compared. Selection of neutral grounding resistor values and ground fault protection devices are discussed. It is proposed that a modified high-resistance grounding (MHRG) with ground fault tripping may be the better practice in some industrial applications. >

05 Nov 1991
TL;DR: The systematic approach allows regular reviews to be conducted as required, such as after the introduction of new generation plants, new circuits or transformers in the system.
Abstract: Distance protection has been widely used in the protection of EHV and UHV transmission lines. One of the advantages of using distance protection is that its built-in zone 2 and zone 3 elements provide local relay back-up as well as remote system back-up protection. Typically the reach settings of zone 2 and zone 3 are taken respectively as the protected line plus 25% of the shortest second line and the protected line plus the longest second line plus 25% of the longest third line. Distance reach setting determination can be a relatively complicated procedure in view of the large volume of data that has to be processed. With the help of computer, the impedance seen by a distance relay at a particular fault location under various generation and system configuration can be calculated. The use of a computer allows the protection engineer to concentrate on judgment and evaluation of the performance of the distance relay at the chosen reach setting, such as the zone 2 or 3 coverage, the load encroachment and the power swing problem. With computerization, the time for distance setting review is shortened. The systematic approach allows regular reviews to be conducted as required, such as after the introduction of new generation plants, new circuits or transformers in the system.

Proceedings ArticleDOI
23 Jul 1991
TL;DR: The test showed that the application of a neural network to this issue is possible in principal and that the signal quality is improved with respect to the needs of a busbar protection system, respectively.
Abstract: During the development of a (conventional) busbar protection algorithm which is able to cope with current signals distorted by current transducer saturation, the question came up, whether it would be possible to use a neural network for preprocessing the data and restoring the distorted signals. A training tool for neural networks and a set of typical distorted and undistorted current signals was selected for a verification of the idea. The test showed that the application of a neural network to this issue is possible in principal and that the signal quality is improved with respect to the needs of a busbar protection system, respectively. The ability of the neural networks to map an increasing number of input signals to reasonable output signals is investigated. Furthermore some studies were made for implementing the trained neural network in hardware. >

Journal ArticleDOI
TL;DR: The capabilities of CRAFT are extended by adding two new knowledge bases with rules to determine the status of unsupervised automatic switches and to detect the presence and location of a manual switch with an incorrect status following a fault.
Abstract: The implementation and operation of an expert system in a control center environment is discussed. CRAFT (customer restoration and fault testing) is a rule-based expert system which is capable of locating the faulted section of a transmission line equipped with automatic switches and circuit breakers. In this study, the capabilities of CRAFT are extended by adding two new knowledge bases with rules to determine the status of unsupervised automatic switches and to detect the presence and location of a manual switch with an incorrect status, following a fault. An approach to appending a separate expert system computer to the SCADA system is developed. Software and hardware interfaces are designed and implemented. The completed implementation is operating online. The implementation represents a step toward practical application of expert systems for real time operation of power systems. >

Proceedings ArticleDOI
25 Jun 1991
TL;DR: A method for analyzing multiple faults in gate-level combinational circuits that does not explicitly enumerate all the multiple stuck-at faults that may be present in a circuit is presented, and significant speedup is observed.
Abstract: A method for analyzing multiple faults in gate-level combinational circuits that does not explicitly enumerate all the multiple stuck-at faults that may be present in a circuit is presented. First, a fault collapsing phase is applied to the network, so that equivalent faults are eliminated. During the analysis, frontier faults where there is at least a normal path from each faulty line to a primary output are considered. It is shown that the set of frontier faults is equivalent to the set of multiple faults. Given an input vector, the normal circuit is evaluated and the fault effects propagated. A fault dropping procedure is then applied to eliminate faulty conditions on specific lines that are either absent or permanently masked by other faulty conditions. The method is applied to some benchmark circuits, and significant speedup is observed. >

Journal ArticleDOI
TL;DR: V-PRO as discussed by the authors is a computer-based coordination program that can significantly reduce the time that it takes the protection engineer to evaluate alternative schemes, and some application examples are presented. But it is not suitable for large-scale applications.
Abstract: Transformer protection considerations are discussed, and V-PRO, a computer-based coordination program that can significantly reduce the time that it takes the protection engineer to evaluate alternative schemes, is discussed. An overview of the package is given, and some application examples are presented. >

Journal ArticleDOI
TL;DR: In this paper, a system-wide, systematic, analytical solution to protection system coordination has been achieved. But this solution is not suitable for all aspects of the power system, and it requires the use of every tool at its disposal.
Abstract: Protection system coordination is a strategic problem for utilities in that it affects most aspects of transmission and distribution and has a dramatic impact on transmission and distribution reliability. The ramifications of protection system coordination affect planning functions as well as system operations, and the lack of coordination can have far reaching effects, even to the point of grid disturbance. For true transmission and distribution reliability, protection engineering must continue to serve an active, integrated, proactive role in all facets of the power system industry and must make use of every tool at its disposal. In this regard, computer technology has evolved such that the coordination process has been replicated, and the means for a system-wide, systematic, analytical solution to protection system coordination has been achieved. >

Journal ArticleDOI
TL;DR: A coincident lightning events detector (CLED) was developed, tested, and installed in a test area on the Florida Power Company distribution system in St. Petersburg, Florida.
Abstract: As a part of EPRI Research Project 2542-1, Characteristics of Lightning Surges on Distribution Lines, a coincident lightning events detector (CLED) was developed, tested, and installed in a test area on the Florida Power Company distribution system in St. Petersburg, Florida. The CLED provided the data to determine the number of lightning-caused circuit breaker operations for the first time. During the 3-year recording period, a total of 546 faults causing circuit breaker operations occurred on the seven circuits in the test area. Of these faults, 415 (or 76%) were temporary faults. A lightning strike was coincident with 42% of the temporary faults. Improper operation of line fuses was traced to the absence of lightning arresters at line fuse locations. >

Journal ArticleDOI
TL;DR: In this paper, the reliability of a power system substation taking into account its associated relaying system is evaluated using minimum cutsets to compute the frequency and duration of a fault with respect to the criterion of continuity of service.
Abstract: The authors present an approach for evaluating the reliability of a power system substation taking into account its associated protective relaying system. The method employs the technique of minimum cutsets to compute the frequency and duration of a fault with respect to the criterion of continuity of electric service. The protective relaying system is accounted for by explicitly incorporating its elements into a new functional model developed for their associated circuit breakers. The approach was implemented in the form of a computer program and used to compare the reliability of several substation arrangements. These comparisons as well as some details on the practical implementation of the approach are also presented. >