Power System Protective Relaying

J. C. Das

• Publisher: CRC Press
• Year: 2017
• eBook ISBN: 9781351230797
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

This book focuses on protective relaying, which is an indispensable part of electrical power systems. The recent advancements in protective relaying are being dictated by MMPRs (microprocessor-based multifunction relays). The text covers smart grids, integration of wind and solar generation, microgrids, and MMPRs as the driving aspects of innovations in protective relaying. Topics such as cybersecurity and instrument transformers are also explored. Many case studies and practical examples are included to emphasize real-world applications.

1 Modern Protective Relaying: An Overview

Protective relaying has been called an “art” and also a “science.” This is so because there is a judgment involved in making selections, which require compromises between conflicting objectives, such as maximum protection, reliability, fast fault clearance times, economics, and selectivity. A fault in the system should be detected fast, and only the faulty section isolated without impacting the unfaulted system. Protective relaying is an essential feature of the electrical system which is considered concurrently with the system design. Protection is not a substitute for poorly designed systems; that is, protecting a poorly designed system will be more complex and less satisfactory than a properly designed system.

In many continuous processes industrial plant distribution systems, a single nuisance trip can result in colossal loss of revenue and it may take many hours to days to restore the processes to full-stream production.

In terms of modern technology, a revolution has taken place in the development and application of microprocessor-based multifunction relays (MMPRs). The single-function electromechanical relays are now outdated, and these are being replaced with MMPRs in many industrial and utility systems. Chapter 4 describes the functionality of a feeder relay.

1.1 Design Aspects and Reliability

Protective relaying must be considered alongside the design of power systems, large or small. It is difficult and even unsatisfactory to protect a badly designed system, and protective relaying, which sometimes comes in last, cannot cover the lapses of the inadequate system designs. See Chapter 1 of Volume 1 for the fundamental concepts of design and planning of electrical power systems.

The safety and reliability of a power system cannot be considered based on only one aspect. It is a chain where the weakest link can jeopardize the reliability and security. The basic concepts of reliability are discussed in Chapter 1 of Volume 1 and are not repeated here.

Many utilities establish standards of quality of service based on a number and duration of outages on a given type of circuit on a yearly basis. In continuous process plants, a single loss of critical equipment due to nuisance trip may result in colossal loss of revenue. This returns us to system designs, redundant sources of power for the critical equipment, standby generators or tie lines, UPS, etc.

A number of ANSI/IEEE standards have been developed for protection and relaying and are continuously updated. These standards cover the application of protective devices, that is, the manner in which these need to be applied for specific protection systems. Protection standards for bulk power facilities require that redundancy exists within protection system designs. Redundancy requires that the failure of a protection component, protective relay, circuit breaker, or communication channel will not result in failure to detect and isolate faults.

The North American Electric Reliability Corporation (NERC) has the statuary responsibility to regulate bulk power system users and producers through adoption and enforcement of their standards. In 2007, the Federal Energy Regulatory Commission (FERC) which is the U.S. federal agency granted NERC the legal authority to enforce reliability standards for bulk power systems in the United States and made compliance with these standards mandatory and enforceable.

1.2 Fundamental Power System Knowledge

A number of power system design concepts are covered in Volumes 1–3 of this series and some data are transparent with respect to protective relaying. The following underlying concepts are not repeated in this volume:

• Nature of modern power systems, generation, distributed generation, transmission and subtransmission systems, industrial and commercial systems (see Volume 1)
• Renewable energy sources, solar and wind power plants (see Volume 1)
• Short-circuit calculations, three-phase and unsymmetrical faults like single-line- to ground faults, double-line-to-ground faults, line-to-line faults, open conductor faults, and 30-cycle faults for protective relaying (see Volume 1)
• Symmetrical components theory and its applications (see Volume 1)
• Rating structures of high-voltage circuit breakers, fundamental characteristics of high- and low-voltage power fuses, low-voltage power circuit breakers, molde case and insulated case circuit breakers (see Volume 1)
• Calculations of transmission line and cable parameters (see Volume 1)
• Fundamental concepts of AC current interruption, rating structure of circuit breakers, fuses, low-voltage circuit breakers, MCCBs (Molded Case Circuit Breakers), etc. (see Volume 1)
• Shunt reactors, their switching, application considerations, and transients (see Volume 1)
• ANSI/IEEE standards of system voltages (see Volume 2)
• Power transformers, synchronous generators, and motors, their models, and their operations (see Volumes 1 and 2)
• Load flow and reactive power compensation (see Volume 2)
• Starting of motors (see Volume 2)
• FACTs devices (see Volume 2)
• Effect of harmonics on power system equipment (see Volume 3)
• Protection of shunt capacitor banks is covered in Volume 3 and not repeated in this volume

In addition, familiarity with phasors, vectors, per unit system, electrical circuit concepts, and matrix algebra are required.

In particular, the protective relaying demands knowledge of calculations of short-circuit current in the systems and also symmetrical components. See [1–3] for Volumes 1–3 referred here. Research works [4–10] list some popular books on protective relaying.

1.3 Design Criteria of Protective Systems

The logic of protective relaying looks at a complex distribution system as an integration of subsystems. In all cases, some common criteria are applicable. These are as follows:

• Selectivity
• Speed
• Reliability
• Simplicity
• Economics
• Maintainability (sometimes)

1.3.1 Selectivity

A protection system must operate so as to isolate the faulty section only. In a radial distribution system, which is a common system configuration in the industrial power distribution systems, inverse time overcurrent relays are used as the primary protection devices. The desired selectivity is attained by coordinating upstream relays with the downstream relays, so that the upstream relay is slower than the downstream relay. A proper time delay should be selected between two overcurrent relays in series by providing either a certain appropriate time delay, called coordinating time interval, or variations of the inverse time–current characteristics, not forgetting the definite time–current characteristics. This coordination is discussed in Chapter 6. This increases the time delay for fault clearance toward the source, which is not desirable from arc flash hazard limitation and equipment damage. Separate zones of protection can be established around each equipment, which are called unit protection systems (Section 1.5). The unit protection systems are discussed throughout this volume; for example, a differential system is a unit protection system.

1.3.2 Speed

Fault damage to the system components and the stability between synchronous machines and interconnected systems are related to the speed of operation of the protective systems. In case all faults could be cleared instantaneously, the equipment damage as well as the arc flash hazard will be a minimum. Thus, there is a direct relation between limiting the arc flash hazard and equipment damage. Unit protection systems, with overlapping zones of protection, can limit equipment damage and reduce arc flash hazard.

Practically, unit protection systems are not applied throughout an industrial distribution, primarily because of cost. However, the concepts are changing; for example, commercial low-voltage switchgear is available with differential and zone interlocking protection (see Chapter 6).

From transient stability considerations, there is a critical fault clearing time and even a slight delay of one-fourth of cycle exceeding this time can result in system separation, see Chapter 15. Single-pole closing, fast load shedding, bundle conductors, fast excitation systems...

Table of contents

• Cover Page
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Series Preface
• Preface to Volume 4: Power System Protective Relaying
• Author
• 1. Modern Protective Relaying: An Overview
• 2. Protective Relays
• 3. Instrument Transformers
• 4. Microprocessor-Based Multifunction Relays
• 5. Current Interruption Devices and Battery Systems
• 6. Overcurrent Protection: Ideal and Practical
• 7. System Grounding
• 8. Ground Fault Protection
• 9. Bus-Bar Protection and Autotransfer of Loads
• 10. Motor Protection
• 11. Generator Protection
• 12. Transformer Reactor and Shunt Capacitor Bank Protection
• 13. Protection of Lines
• 14. Pilot Protection
• 15. Power System Stability
• 16. Substation Automation and Communication Protocols Including IEC 61850
• 17. Protective Relaying for Arc-Flash Reduction
• Appendix A: Device Numbers according to IEEE C37.2
• Index