Disturbance Analysis for Power Systems

Mohamed A. Ibrahim

• Publisher: Wiley-IEEE Press
• Year: 2011
• Print ISBN: 9780470916810
• eBook ISBN: 9781118172100
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Power Resources
• Index: Technology & Engineering

More than ninety case studies shed new light on power system phenomena and power system disturbances

Based on the author's four decades of experience, this book enables readers to implement systems in order to monitor and perform comprehensive analyses of power system disturbances. Most importantly, readers will discover the latest strategies and techniques needed to detect and resolve problems that could lead to blackouts to ensure the smooth operation and reliability of any power system.

Logically organized, Disturbance Analysis for Power Systems begins with an introduction to the power system disturbance analysis function and its implementation. The book then guides readers through the causes and modes of clearing of phase and ground faults occurring within power systems as well as power system phenomena and their impact on relay system performance. The next series of chapters presents more than ninety actual case studies that demonstrate how protection systems have performed in detecting and isolating power system disturbances in:

• Generators

• Transformers

• Overhead transmission lines

• Cable transmission line feeders

• Circuit breaker failures

Throughout these case studies, actual digital fault recording (DFR) records, oscillograms, and numerical relay fault records are presented and analyzed to demonstrate why power system disturbances happen and how the sequence of events are deduced. The final chapter of the book is dedicated to practice problems, encouraging readers to apply what they've learned to perform their own system disturbance analyses.

This book makes it possible for engineers, technicians, and power system operators to perform expert power system disturbance analyses using the latest tested and proven methods. Moreover, the book's many cases studies and practice problems make it ideal for students studying power systems.

Chapter 1

Power System Disturbance Analysis Function

An analysis of system disturbances provides a wealth of valuable information regarding power system phenomena and the behavior of protection systems. Experience can be enhanced and knowledge can be gained from the analysis function. This book is organized, first, to cover the analysis function and how it can be implemented. Then, in the following sections, phenomena related to system faults and the clearing process of faults from the power system are described. Power system phenomena derived from an analysis of system disturbances are stated. In addition, case studies of actual system disturbances involving the performance of protection systems for generators, transformers, overhead transmission lines, cable feeders, and breaker failures are provided. A section is devoted to problems that enhance an understanding of the system disturbance analysis function.

Analysis of system disturbance is based on 60-Hz phenomena associated with power system faults. Therefore, sampling rates of digital fault recorders (DFRs) are designed to fulfill this requirement. High-frequency power system transient analysis requires special devices other than conventional DFRs and numerical relays, with unique requirements different from those of a traditional power system disturbance analysis function.

To analyze the performance of protective relaying systems, high-speed digital fault and disturbance recording devices need to be employed properly. Equipment can be used for continuous monitoring of the behavior of relaying installed on a power system during the occurrence of either faults or power swing or switching operations. The equipment can be used to explain undesired operations and to assess system performance during correct operation. Analysis of fault records will help in adapting operating and protection practices and in assuring the reliability of a bulk power system. The analysis will also help to isolate problems and incipient failures. In addition, the strategic placement of DFR equipment should provide adequate coverage of the overall system response to any type of system fault or wide-area system disturbance. For this reason, DFR applications and implementation on a bulk power system are mandated by industry standards and regulations.

A review of DFR records for every operation in a system will help to isolate incipient difficulties so that corrections can be provided before a serious problem develops and to provide basic useful information about the performance of the relaying system. A review of all fault records for disturbances on a system can enhance the reliability of a relay system. Systematic analysis of disturbances can play an important role in system blackout avoidance. When they occur during the early stage of analysis, flagging relay and system problems should be addressed before they precipitate into wider-area interruption and system blackouts. This can be accomplished by analyzing correct operations and finding the causes of incorrect operations. In addition, it can provide a better assessment of the validity of relay setting calculations, correct current transformer (CT) and voltage transformer (PT) ratios, and correct breaker operations. It can also enhance the system restoration process by providing fault types and locations and a better measure of power quality.

The proposed NERC Reliability Standard PRC-002-02, “Disturbance Monitoring and Reporting Requirements,” is noted here as a document which ensures that regional reliability organizations establish requirements for the installation of disturbance-monitoring equipment and reporting of disturbance data to facilitate analyses of system events and verification of system models.

1.1 Analysis Function of Power System Disturbances

Analysis of power system disturbances can be summarized on the basis of the following primary functions:

1. The need to view fault data as soon as possible after a fault or disturbance occurs so as to restore the system safely.

2. The need to design the DFR with a reasonable pre-fault time (5 to 10 cycles) to capture incipient initiating conditions (e.g., surge arrester spillover).

3. The need to design the DFR with a long post-fault time, adjustable from 0 to 5 s, to be able to analyze backup protection clearing times (60 cycles or more) and limited power system swings (several seconds) following the occurrence of system disturbances.

4. The need to manipulate the data time base on the DFR record to analyze the effect of faults.

5. The need, finally, to manipulate the DFR data channels and view only those selected.

Ideally, the analysis function should be carried out for all relay operations in a system. The normally cleared events can lead to the discovery of equipment problems and can also be used as a teaching example for power system behavior and phenomena. From the analysis function, monthly disturbance analysis reports can be prepared. In addition, other reports can be generated. The analysis function will focus primarily on providing answers to the following basic questions:

1. What happened?

2. Why did it happen?

3. What is going to be done about it?

In essence, a sequence-of-events report, or time line, needs to be developed. Traditionally, a DFR monitors power system voltages and currents, whereas a sequence-of-events recorder (SER) monitors relay outputs, breaker and disconnect switch positions, alarms, relay targets, and relay communication channels. A DFR can integrate both functions by monitoring events and analog quantities. The following are some of the functions that analysis of DFR records, in conjunction with SER records, can provide:

1. Sequence of operation

2. Fault types

3. Clearing times

4. Reclosing times

5. Relay problems such as:

a. Failure to trip

b. Failure to target

c. Failure to reset

d. Delayed clearing

6. Communication problems such as:

a. False operation of blocking schemes during carrier transmission holes

b. Failure to operate for permissive overreaching transfer trip schemes during signal loss

7. Circuit breaker problems such as:

a. Contact arcing

b. Unequal pole closing

c. Unequal pole opening

d. Re-strike

e. Reignition

8. Fault current and voltage magnitudes to confirm a short-circuit model

9. CT saturation

10. Asymmetrical current caused by dc (direct current) offset

11. Fault locations, currently provided by numerically based distance relaying, can also be provided by DFRs when sufficient analog signals per line are monitored

1.2 Objective of DFR Disturbance Analysis

Data obtained from DFRs and numerical relaying can be used for continuous monitoring of the behavior of the relay system and assist in setting operating margins on critical control and protective apparatus in an electric power system during system disturbance events such as faults, power swings, and switching operations. Analysis of the data can have the dual role of explaining undesired operations and assessing system performance during correct operation.

The primary objective of obtaining and analyzing DFR data is for the purpose of adapting operating and protection practices as well as control strategies to assure the security and dependability of the bulk power protection system. The secondary objective is for the purpose of helping to isolate problems and incipient failures. This requires a review of all DFR data for every operation, to detect and correct incipient troubles before they become a serious problem. The ability should exist for remote interrogation for data analysis and manipulation. Data need to be viewed as soon as possible after a fault or disturbance occurs. The data time base for the DFR record should be manipulated for analysis. The ability should exist to manipulate data channels and view only those of importance. This will ensure that other channels will not obscure vital data.

It is a good idea to analyze all disturbances in a system, but this may require additional personnel who may not be available within the utility's environment. Indeed, it should be realized that the knowledge gained from analyzing mundane operations may prove to be very valuable. Followi...

Table of contents

• Cover
• Title Page
• Copyright
• Dedication
• Preface
• Chapter 1: Power System Disturbance Analysis Function
• Chapter 2: Phenomena Related to System Faults and the Process of Clearing Faults from a Power System
• Chapter 3: Power System Phenomena and Their Impact on Relay System Performance
• Chapter 4: Case Studies Related to Generator System Disturbances
• Chapter 5: Case Studies Related to Transformer System Disturbances
• Chapter 6: Case Studies Related to Overhead Transmission-Line System Disturbances
• Chapter 7: Case Studies Related to Cable Transmission Feeder System Disturbances
• Chapter 8: Case Studies Related to Breaker Failure Protection System Disturbances
• Chapter 9: Problems
• Index