Digital Protection for Power Systems
Buy e-book PDF
Digital protection is based on the use of computers in power line relaying. Since the late 1960s, digital devices and techniques have been applied to almost all new protection schemes. Today the technology is moving towards standardised hardware platforms; at the software level, however, there remains a huge variety in approaches and protection algorithms. This book gives a fairly detailed understanding of the principles and techniques underlying the application of digital technology and algorithms to protection. It avoids going into detail of specific products: up-to-date information on these is available from the manufacturers. Instead it aims to give the reader a thorough understanding of the generic problems of digital protection. The text covers the mathematical basis of numerical techniques and relay algorithms, the basic elements of digital protection and the fundamentals underlying the commonest algorithmic forms, particularly as applied to line protection. It deals with the fundamentals of travelling-wave techniques and their application to transmission lines, and with digital differential protection of transformers and lines.
Inspec keywords: power transformer protection; power system protection; power system relaying; power transmission lines
Other keywords: digital protection; transmission lines; power transformers protection; travelling-wave techniques; power system protection; relay algorithms; digital differential protection; line protection; power line relaying
Subjects: Power system protection; Transformers and reactors; Power transmission lines and cables
- Book DOI: 10.1049/PBPO015E
- Chapter DOI: 10.1049/PBPO015E
- ISBN: 9780863413032
- e-ISBN: 9781849194310
- Page count: 216
- Format: PDF
-
Front Matter
- + Show details - Hide details
-
p.
(1)
-
1 Introduction
- + Show details - Hide details
-
p.
1
–6
(6)
This chapter discusses digital-based relaying. The power system equipment in a substation could be protected using digital computers, and since that time, research in digital protection has attracted many investigators. Research activity has covered virtually every protection technique, and many novel algorithms and associated hardware implementations have emerged.
-
2 Mathematical background to protection algorithms
- + Show details - Hide details
-
p.
7
–38
(32)
To understand the principles underlying digital protection technology, it is necessary to review briefly the mathematical basis of numerical algorithms. The topics covered in this Chapter therefore include finite differences, numerical differentiation, curve fitting and smoothing, Fourier analysis, Walsh analysis, and the relationship between Fourier and Walsh coefficients. It is not the intention that the material presented should be highly rigorous in the mathematical sense, but rather that it should give a working knowledge of the numerical techniques used and thus provide a basis for the work on specific protection algorithms that is presented in later Chapters.
-
3 Basic elements of digital protection
- + Show details - Hide details
-
p.
39
–59
(21)
In this Chapter the basic principles underlying the conversion of analogue signals into equivalent digital forms are explained. We also explain the essentially common features of various digital relaying schemes, other detailed aspects being discussed in later chapters.
-
4 Sinusoidal-wave-based algorithms
- + Show details - Hide details
-
p.
60
–71
(12)
The algorithms covered in this Chapter assume that the post-fault current and voltage waveforms are sinusoidal. This assumption is not, of course, generally valid, particularly when EHV or UHV network applications are involved. However, in practice, the signals processed are often prefiltered and, in lower voltage distribution systems in particular, the waveforms often very quickly attain a nominally sinusoidal form.
-
5 Fourier analysis and Walsh function based techniques
- + Show details - Hide details
-
p.
72
–92
(21)
In this Chapter we present digital relaying algorithms which are based on Fourier and Walsh analyses. From the point of view of this discussion, Fourier analysis includes Fourier series and Fourier transform-based methods, while Walsh analysis includes Walsh series only.
-
6 Least squares based methods
- + Show details - Hide details
-
p.
93
–102
(10)
In this Chapter we discuss techniques used to fit faulted current and voltage waveforms, each to a sinusoidal waveform containing a fundamental component, a decaying/constant DC component and/or harmonics. These techniques use the least squares (LSQ) method to minimise the fitting error, and all have the common goal of extracting the fundamental components of voltage and current waveforms, to calculate the impedance to the fault or the comparison of current-based signals in digital differential protection.
-
7 Differential equation based techniques
- + Show details - Hide details
-
p.
103
–114
(12)
In this Chapter no special assumption are made with regard to the content of faulted current and voltage waveforms. The fundamental approach, which is common to all algorithms covered in this Chapter, is based on the fact that all protected equipment can be normally represented by differential equations of either first or second order. The methods are described by reference to transmission-line protection, since it is in this application that they are mainly used. However, the methods can easily be extended to other items of plant. For the purposes of this Chapter we shall assume the line length is such that shunt capacitance can either be neglected or can be lumped into a single 'equivalent' value.
-
8 Fundamentals of travelling-wave based protection
- + Show details - Hide details
-
p.
115
–147
(33)
In this Chapter, transmission lines are treated as distributed circuits, in order to explain how travelling-wave phenomena are propagated and detected. The basic principles of travelling-wave schemes, the formation of relaying signals, Bergeron's equation and discriminant functions using single-phase line models are also explained. These principles are then extended to three-phase lines by decomposing the line into three equivalent single-phase lines using the modal decomposition approach.
-
9 Travelling-wave protective schemes
- + Show details - Hide details
-
p.
148
–170
(23)
This Chapter is designed to extend knowledge about travelling-wave techniques by presenting the underlying principles of some specific implementations. Information on further development of specific products is available from manufacturers' literature.
-
10 Digital differential protection of transformers
- + Show details - Hide details
-
p.
171
–189
(19)
This Chapter gives a brief general review of the principles of transformer differential protection. This is followed by an explanation of the application of digital techniques and the algorithms that have been developed specifically for application to transformer protection. The algorithms covered include finite duration impulse (FIR) filters, least-squares curve fitting, the digital Fourier algorithm and the flux-restrained current differential algorithm. Finally, the basic hardware arrangement for implementing digital techniques to the protection of transformers is described. It is, however, important to note that closely similar techniques can be applied to the protection of generators, although, in this case, the transformation ratio of currents is the same on each side of the protected zone.
-
11 Digital line differential protection
- + Show details - Hide details
-
p.
190
–199
(10)
As power systems grow both in size and complexity, it becomes common to use long and heavily loaded two-terminal lines as well as multi-terminal and tapped lines. This in turn has created difficult problems for their protection.
-
Back Matter
- + Show details - Hide details
-
p.
200
(1)