Protective relays using electrical quantities are connected to the power system through current transformer CT or voltage transformer VT. These input devices or instrument transformers provide insulation from the high-power system voltages and reduce the magnitudes to practical secondary levels for the relays. The protective relay system is connected to the AC power system through the CTs commonly associated with the circuit breaker and, if necessary, to the VTs. These are shown connected to the station AC bus, but often at the higher voltages the voltage devices are connected to the transmission line.

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One is the fact that the requirements imposed by power systems are in a constant state of change, and our understanding of the basic concepts has sharpened considerably over the years. The other is that the means of implementing the fundamental concepts of fault location and removal and system restoration are constantly growing more sophisticated.

It is primarily because of these changing constraints that this text has been revised and expanded. It began with contributions from two giants of the industry, J. Lewis Blackburn and George D. From the nucleus of their extensive analyses and writings, and the desire to cover each new contingency with new relaying concepts, this volume has evolved. New solutions to age-old problems have become apparent as greater experience has been gained.

This new edition weeds out those relaying concepts that have run their course and have been replaced by more perceptive methods of implementation using new solid-state or microprocessor-based devices. Initially, the methods of translating a collection of instantaneous samples of sine waves into useful current, direction, and impedance measurements were not obvious.

Diligent analysis and extensive testing allowed these useful functions to be obtained and to be applied to the desired protective functions. This text attempts to describe, in the simplest possible terms, the manner in which these digital measurements are accomplished in present-day devices. Some of these names may not be immediately recognizable, but all have made an impact with their thoughtful, accurate, well-reasoned writings, and they all deserve the gratitude of the industry for the wealth of knowledge they have contributed to this book.

I am keenly aware of the high quality of the technical offerings of these people, and I am particularly grateful for the warmth and depth of their friendship.

Walter A. Elmore 1 Introduction 11 2 Phasors 11 2. Phasor Rotation 15 3 Polarity in Relay Circuits 15 3. Elmore 1 Introduction 43 2 Electromechanical Units 43 2.

Elmore 1 Introduction 71 1. Elmore 1 Introduction 81 2 Current Transformers 81 2. Elmore 1 Introduction 2 Ungrounded Systems 2. Downs 1 Introduction 1. McGowan 1 Introduction 2 Magnetizing Inrush 2.

Price 1 Introduction 2 Remote vs. Local Backup 2. Wang 1 Introduction 1. Multiple-Shot Reclosing Relays 3. Are activated when an operat- ing parameter deviates from predetermined Relays are compact analog, digital, and numerical limits. Regulating relays function through sup- devices that are connected throughout the power plementary equipment to restore the quantity to system to detect intolerable or unwanted conditions the prescribed limits.

They are, in effect, a form of Auxiliary relays. Operate in response to the open- active insurance designed to maintain a high degree of ing or closing of the operating circuit to service continuity and limit equipment damage. They supplement another relay or device. Synchronizing or synchronism check relays. As- sure that proper conditions exist for intercon- necting two sections of a power system. Detect defective lines, defective structure, and performance characteristic.

Verify conditions on the power system or in the protection system. Frequency Reclosing relays. Establish a closing sequence for a Temperature circuit breaker following tripping by protective Flow relays. Operating Principle or Structures 2. There High or low speed is little or no value in applying one without the other. Pilot Protective relays or systems are not required to Phase comparison function during normal power system operation, but Directional comparison must be immediately available to handle intolerable Current differential system conditions and avoid serious outages and damage.

Thus, the true operating life of these relays A separate volume, Pilot Protective Relaying, covers can be on the order of a few seconds, even though they pilot systems those relaying functions that involve a are connected in a system for many years.

In practice, communications channel between stations. In practice, under one of the following designations. Initial, operating, and maintenance Available measures of fault or troubles. Fault Analog relays are those in which the measured magnitudes and location of current transformers quantities are converted into lower voltage but similar and voltage transformers signals, which are then combined or compared directly Operating practices.

Previous experience. History and anticipation of the types of trouble likely to be encountered 2. Logic circuits or microprocessors compare Since it is simply not feasible to design a protective the phase relationships of the square waves to make a relaying system capable of handling any potential trip decision e. Natu- System reliability consists of two elements: depend- rally, this makes relaying somewhat of an art. Different ability and security.

As a result, there is little a relay will not operate incorrectly. Unfortunately, standardization in protective relaying. Not only may these two aspects of reliability tend to counter one the type of relaying system vary, but so will the extent another; increasing security tends to decrease depend- of the protective coverage. Too much protection is ability and vice versa.

In general, however, modern almost as bad as too little. The continuous supervision made possible by of the power system as a whole.

The relay engineer numerical techniques affords improvement in both must know not only the technology of the abnormal, dependability and security.

Protective relay systems but have a basic understanding of all the system must perform correctly under adverse system and components and their operation in the system. Relay- environmental conditions. This horizontal, or total laboratory or during installation by simulated tests or system, concept of relaying includes fault protection a staged fault.

Although these areas are vitally important to the indications in the power system and its environment. A relay engineer, his or her concern has not always been secure system is usually the result of a good back- fully appreciated or shared by colleagues. Frequent reviews of protective systems power system itself and its environment. A complex relaying system may result from poor 3. But, breakers.

Considerable savings may be realized by even if available, they would doubtlessly raise the using fewer circuit breakers and a more complex relay question of whether or not the fault or trouble really system.

Such systems usually involve design compro- required a trip-out. The development of faster relays mises requiring careful evaluation if acceptable protec- must always be measured against the increased tion is to be maintained.

It should be recognized that probability of more unwanted or unexplained opera- the exercise of the very best relaying application tions. Time is an excellent criterion for distinguishing principles can never compensate for the absence of a between real and counterfeit trouble.

Applied to a relay, high speed indicates that the operating time usually does not exceed 50 ms three cycles on a Hz base. The term instantaneous 3. In practice, the terms high speed and The application logic of protective relays divides the instantaneous are frequently used interchangeably. In all cases, the four design criteria 3. Consequently, both low- and high-speed relays are used to protect power systems.

Both types have high reliability records. Records on protective relay operations con- sistently show The simplest relay system, however, is not always the most economical. As previously indicated, major economies may be possible with a complex relay system that uses a minimum number of circuit breakers. Other factors being equal, simplicity of design improves system reliability—if only because there are fewer elements that can Figure A typical system and its zones of protection.

Transmission and distribution circuits 3. Relay performance is generally classed as 1 correct, A typical power system and its zones of protection are 2 no conclusion, or 3 incorrect. Incorrect operation shown in Figure The location of the current may be either failure to trip or false tripping.

Equipment that can cause an incorrect protection within the guidelines outlined above. Since operation includes current transformers, voltage trans- failures do occur, however, some form of backup formers, breakers, cable and wiring, relays, channels, protection is provided to trip out the adjacent breakers or station batteries.

Incorrect tripping of circuit breakers not associated Protection in each zone is overlapped to avoid the with the trouble area is often as disastrous as a failure possibility of unprotected areas. This overlap is to trip. Hence, special care must be taken in both accomplished by connecting the relays to current application and installation to ensure against this. Any trouble in the small area between the current transformers will operate both zone A and B relays and trip all breakers in the two zones.

Although develop- 1. Generators ing a clear, accurate statement of the problem can 2. Transformer connections are particularly impor- tant. The existing protective equipment and reasons for the desired change s should be outlined. As new relay systems will often be required to operate with or utilize parts of the existing relaying, details on these existing systems are important.

Whenever possible, changes in system protection should conform with existing operating procedures and practices. Figure The principle of overlapping protection around a circuit breaker. These data will provide answers to the Existing operating procedures and practices and following types of questions. Is pilot, high-, medium-, possible future expansions or slow-speed relaying required?


Protective Relaying Theory and Applications.pdf








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