Protective Relaying

Theory and Applications

Walter A. Elmore

• Publisher: CRC Press
• Year: 2003
• ISBN:
  9781135526900
• Edition: 2
• Topic: Computer Science
• Subtopic: Hardware
• Index: Computer Science

Targeting the latest microprocessor technologies for more sophisticated applications in the field of power system short circuit detection, this revised and updated source imparts fundamental concepts and breakthrough science for the isolation of faulty equipment and minimization of damage in power system apparatus. The Second Edition clearly descri

1
Introduction and General Philosophies

Revised by: W.A.ELMORE

1 INTRODUCTION

Relays are compact analog, digital, and numerical devices that are connected throughout the power system to detect intolerable or unwanted conditions within an assigned area. They are, in effect, a form of active insurance designed to maintain a high degree of service continuity and limit equipment damage. They are "silent sentinels." Although protective relays will be the main emphasis of this book, other types of relays applied on a more limited basis or used as part of a total protective relay system will also be covered.

2 CLASSIFICATION OF RELAYS

Relays can be divided into six functional categories:

Protective relays. Detect defective lines, defective apparatus, or other dangerous or intolerable conditions. These relays generally trip one or more circuit breaker, but may also be used to sound an alarm.

Monitoring relays. Verify conditions on the power system or in the protection system. These relays include fault detectors, alarm units, channel-monitoring relays, synchronism verification, and network phasing. Power system conditions that do not involve opening circuit breakers during faults can be monitored by verification relays.

Reclosing relays. Establish a closing sequence for a circuit breaker following tripping by protective relays.

Regulating relays. Are activated when an operating parameter deviates from predetermined limits. Regulating relays function through supplementary equipment to restore the quantity to the prescribed limits.

Auxiliary relays. Operate in response to the opening or closing of the operating circuit to supplement another relay or device. These include timers, contact-multiplier relays, sealing units, isolating relays, lockout relays, closing relays, and trip relays.

Synchronizing (or synchronism check) relays. Assure that proper conditions exist for interconnecting two sections of a power system.

Many modern relays contain several varieties of these functions. In addition to these functional categories, relays may be classified by input, operating principle or structure, and performance characteristic. The following are some of the classifications and definitions described in ANSI/IEEE Standard C37.90 (see also ANSI/IEEE C37.100 "Definitions for Power Switch-gear"):

Inputs

Current

Voltage

Power

Pressure

Frequency

Temperature

Flow

Vibration

Operating Principle or Structures

Current balance

Percentage

Multirestraint

Product

Solid state

Static

Microprocessor

Electromechanical

Thermal

Performance Characteristics

Differential

Distance

Directional overcurrent

Inverse time

Definite time

Undervoltage

Overvoltage

Ground or phase

High or low speed

Pilot

Phase comparison

Directional comparison

Current differential

A separate volume. Pilot Protective Relaying, covers pilot systems (those relaying functions that involve a communications channel between stations.

2.1 Analog/Digital/Numerical

Solid-state (and static) relays are further categorized under one of the following designations.

2.1.1 Analog

Analog relays are those in which the measured quantities are converted into lower voltage but similar signals, which are then combined or compared directly to reference values in level detectors to produce the desired output (e.g., SA-1 SOQ, SI-T, LCB, circuit shield relays).

2.1.2 Digital

Digital relays are those in which the measured ac quantities are manipulated in analog form and subsequently converted into square-wave (binary) voltages. Logic circuits or microprocessors compare the phase relationships of the square waves to make a trip decision (e.g., SKD-T, REZ-1).

2.1.3 Numerical

Numerical relays are those in which the measured ac quantities are sequentially sampled and converted into numeric data form. A microprocessor performs mathematical and/or logical operations on the data to make trip decisions (e.g., MDAR, MSOC, DPU, TPU, REL-356, REL-350, REL-512).

3 PROTECTIVE RELAYING SYSTEMS AND THEIR DESIGN

Technically, most relays are small systems within themselves. Throughout this book, however, the term system will be used to indicate a combination of relays of the same or different types. Properly speaking, the protective relaying system includes circuit breakers and current transformers (ct's) as well as relays. Relays, ct's, and circuit breakers must function together. There is little or no value in applying one without the other.

Protective relays or systems are not required to function during normal power system operation, but must be immediately available to handle intolerable system conditions and avoid serious outages and damage. Thus, the true operating life of these relays can be on the order of a few seconds, even though they are connected in a system for many years. In practice, the relays operate far more during testing and maintenance than in response to adverse sendee conditions.

In theory, a relay system should be able to respond to an infinite number of abnormalities that can possibly occur within the power system. In practice, the relay engineer must arrive at a compromise based on the four factors that influence any relay application:

Economics. Initial, operating, and maintenance

Available measures of fault or troubles. Fault magnitudes and location of current transformers and voltage transformers

Operating practices. Conformity to standards and accepted practices, ensuring efficient system operation

Previous experience. History and anticipation of the types of trouble likely to be encountered within the system

The third and fourth considerations are perhaps better expressed as the "personality of the system and the relay engineer."

Since it is simply not feasible to design a protective relaying system capable of handling any potential problem, compromises must be made. In general, only those problems that, according to past experience, are likely to occur receive primary consideration. Naturally, this makes relaying somewhat of an art. Different relay engineers will, using sound logic, design significantly different protective systems for essentially the same power system. As a result, there is little standardization in protective relaying. Not only may the type of relaying system vary, but so will the extent of the protective coverage. Too much protection is almost as bad as too little.

Nonetheless, protective relaying is a highly specialized technology requiring an indepth understanding of the power system as a whole. The relay engineer must know not only the technology of the abnormal, but have a basic understanding of all the system components and their operation in the system. Relaying, then, is a "vertical" speciality requiring a "horizontal" viewpoint. This horizontal, or total system, concept of relaying includes fault protection and the performance of the protection system during abnormal system operation such as severe overloads, generation deficiency, out-of-step conditions, and so forth. Although these areas are vitally important to the relay engineer, his or her concern has not always been fully appreciated or shared by colleagues. For this reason, close and continued communication between the planning, relay design, and operation departments is essential. Frequent reviews of protective systems should be mandatory, since power systems grow and operating conditions change.

A complex relaying system may result from poor system design or the economic need to use fewer circuit breakers. Considerable savings may be realized by using fewer circuit breakers and a more complex relay system. Such systems usually involve design compromises requiring careful evaluation if acceptable protection is to be maintained. It should be recognized that the exercise of the very best relaying application principles can never compensate for the absence of a needed circuit breaker.

3.1 Design Criteria

The application logic of protective relays divides the power system into several zones, each requiring its own group of relays. In all cases, the four design criteria listed below are common to any well-designed and efficient protective system or system segment. Since it is impractical to satisfy fully all these design criteria simultaneously, the necessary compromises must be evaluated on the basis of comparative risks.

3.1.1 Reliability

System reliability consists of two elements: dependabili...