Network Protection & Automation Guide (NPAG)

In order to provide the modern practicing substation engineer with reference material, AREVA's Network Protection & Automation Guide provides a substantially revised and expanded edition of PRAG incorporating new chapters on all levels of network automation.


The first part of the book deals with the fundamentals, basic technology, fault calculations and the models of power system plant, including the transient response and saturation problems that affect instrument transformers.

The typical data provided on power system plant has been updated and significantly expanded following research that showed its popularity.

The book then provides detailed analysis on the application of protection systems. This includes a new chapter on the protection of a.c. electrified railways. Existing chapters on distance, busbar and generator protection have been completely revised to take account of new developments, including improvements due to numerical protection techniques and the application problems of embedded generation. The chapter on relay testing and commissioning has been completely updated to reflect modern techniques. Finally, new chapters covering the fields of power system measurements, power quality, and substation and distribution automation are found, to reflect the importance of these fields for the modern power system engineer.

The intention is to make NPAG the standard reference work in its' subject area - while still helping the student and young engineer new to the field.

To obtain the NPAG book, by download or CD-ROM, please use the contact form in the link below.


The following is a short description of the content of the individual chapters of NPAG:

Chapter 01 Introduction

The change in protection technology, together with significant changes in Utility, Industrial and Commercial organizations, has resulted in new emphasis on Secondary Systems Engineering.

In addition to the traditional role of protection & control, secondary systems are now required to provide true added value to organizations.

Chapter 02 Fundamentals of Protection Practice

In order to fulfill the requirements of protection with the optimum speed for the many different configurations, operating conditions and construction features of power systems, it has been necessary to develop many types of relay that respond to various functions of the power system quantities.

Chapter 03 Fundamental Theory

In order to apply protection relays, it is usually necessary to know the limiting values of current and voltage, and their relative phase displacement at the relay location, for various types of short circuit and their position in the system. This normally requires some system analysis for faults occurring at various points in the system.

Chapter 04 Fault Calculations

A power system is normally treated as a balanced symmetrical three-phase network. When a fault occurs, the symmetry is normally upset, resulting in unbalanced currents and voltages appearing in the network. The only exception is the three-phase fault, which, because it involves all three phases equally at the same location, is described as a symmetrical fault. By using symmetrical component analysis and replacing the normal system sources by a source at the fault location, it is possible to analyze these fault conditions.

Chapter 05 Equivalent Circuits and Parameters of Power System Plant

Knowledge of the behaviour of the principal electrical system plant items under normal and fault conditions is a prerequisite for the proper application of protection.
This chapter summarises basic synchronous machine, transformer and transmission line theory and gives equivalent circuits and parameters so that a fault study can be successfully completed before the selection and application of the protection systems described in later chapters.

Chapter 06 Current and Voltage Transformers

Whenever the values of voltage or current in a power circuit are too high to permit convenient direct connection of measuring instruments or relays, coupling is made through transformers. Such 'measuring' transformers are required to produce a scaled down replica of the input quantity to the accuracy expected for the particular measurement; this is made possible by the high efficiency of the transformer. The performance of measuring transformers during and following large instantaneous changes in the input quantity is important, in that this quantity may depart from the sinusoidal waveform.

Chapter 07 Relay Technology

The last thirty years have seen enormous changes in relay technology. The electromechanical relay in all of its different forms has been replaced successively by static, digital and numerical relays, each change bringing with it reductions and size and improvements in functionality.

At the same time, reliability levels have been maintained or even improved and availability significantly increased due to techniques not available with older relay types.

This represents a tremendous achievement for all those involved in relay design and manufacture.

Chapter 08 Protection: Signalling and Intertripping

Unit protection schemes, formed by a number of relays located remotely from each other, and some distance protection schemes, require some form of communication between each location in order to achieve a unit protection function. This form of communication is known as protection signalling. Additionally communications facilities are also required when remote operation of a circuit breaker is required as a result of a local event. This form of communications is known as intertripping.

Chapter 09 Overcurrent Protection for Phase and Earth Faults

Protection against excess current was naturally the earliest protection system to evolve. From this basic principle, the graded overcurrent system, a discriminative fault protection, has been developed. This should not be confused with 'overload' protection, which normally makes use of relays that operate in a time related in some degree to the thermal capability of the plant to be protected. Overcurrent protection, on the other hand, is directed entirely to the clearance of faults, although with the settings usually adopted some measure of overload protection may be obtained.

Chapter 10 Unit Protection of Feeders

The graded overcurrent systems described in Chapter 9, though attractively simple in principle, do not meet all the protection requirements of a power system.
Application difficulties are encountered for two reasons: firstly, satisfactory grading cannot always be arranged for a complex network, and secondly, the settings may lead to maximum tripping times at points in the system that are too long to prevent excessive disturbances occurring.

These problems led to the concept of 'Unit Protection', whereby sections of the power system are protected individually as a complete unit without reference to other sections. One form of 'Unit Protection' is also known as 'Differential Protection', as the principle is to sense the difference in currents between the incoming and outgoing terminals of the unit being protected.

Other forms can be based on directional comparison, distance teleprotection schemes or phase comparison protection.

Chapter 11 Distance Protection

The problem of combining fast fault clearance with selective tripping of plant is a key aim for the protection of power systems. To meet these requirements, high-speed protection systems for transmission and primary distribution circuits that are suitable for use with the automatic reclosure of circuit breakers are under continuous development and are very widely applied.

Distance protection, in its basic form, is a non-unit system of protection offering considerable economic and technical advantages. Unlike phase and neutral
overcurrent protection, the key advantage of distance protection is that its fault coverage of the protected circuit is virtually independent of source impedance variations.

Chapter 12 Distance Protection Schemes

One of the main disadvantages of a conventional time-stepped distance protection scheme is that the instantaneous Zone 1 protection at each end of the protected line cannot be set to cover the whole of the feeder length that leaves two 'end zones', each being about 20% of the protected feeder length. Faults in these zones are cleared in Zone 1 time by the protection at one end of the line and in Zone 2 time by the protection at the other end of the line.

Unit schemes of protection that compare the conditions at the two ends of the feeder simultaneously positively identify whether the fault is internal or external to the protected section and provide high-speed protection for the whole feeder length. This advantage is balanced by the fact that the unit scheme does not provide the back up protection for adjacent feeders given by a distance scheme.

The most desirable scheme is obviously a combination of the best features of both arrangements, that is, instantaneous tripping over the whole feeder length plus back-up protection to adjacent feeders. This can be achieved by interconnecting the distance protection relays at each end of the protected feeder by a communications channel.

Chapter 13 Protection of Complex Transmission Circuits

For economic reasons, transmission and distribution lines can be much more complicated, maybe having three or more terminals (multi-ended feeder), or with more than one circuit carried on a common structure (parallel feeders). Other possibilities are the use of series capacitors or direct-connected shunt reactors. The protection of such lines is more complicated and requires the basic schemes described in the above chapters to be modified.

Chapter 14 Auto-Reclosing

Faults on overhead lines fall into one of three categories:

  • transient
  • semi-permanent
  • permanent

80-90% of faults on any overhead line network are transient in nature. The remaining 10%-20% of faults are either semi-permanent or permanent.
Transient faults are commonly caused by lightning and temporary contact with foreign objects. The immediate tripping of one or more circuit breakers clears the fault.

Subsequent re-energisation of the line is usually successful. A small tree branch falling on the line could cause a semi-permanent fault. The cause of the fault would not be removed by the immediate tripping of the circuit, but could be burnt away during a time-delayed trip. HV overhead lines in forest areas are prone to this type of fault. Permanent faults, such as broken conductors, and faults on underground cable sections, must be located and repaired before the supply can be restored.

Chapter 15 Busbar Protection

The protection scheme for a power system should cover the whole system against all probable types of fault.

Unrestricted forms of line protection, such as overcurrent and distance systems, meet this requirement, although faults in the busbar zone are cleared only after some time delay. But if unit protection is applied to feeders and plant, the busbars are not inherently protected.

Even if distance protection is applied to all feeders, the busbar will lie in the second zone of all the distance protections, so a bus fault will be cleared relatively slowly, and the resultant duration of the voltage dip imposed on the rest of the system may not be tolerable.

Chapter 16 Transformer and Transformer-Feeder Protection

The considerations for a transformer protection package vary with the application and importance of the transformer. To reduce the effects of thermal stress and electrodynamic forces, it is advisable to ensure that the protection package used minimises the time for disconnection in the event of a fault occurring within the transformer. Small distribution transformers can be protected satisfactorily, from both technical and economic considerations, by the use of fuses or overcurrent relays. This results in time-delayed protection due to downstream co-ordination requirements. However, time-delayed fault clearance is unacceptable on larger power transformers used in distribution, transmission and generator applications,
due to system operation/stability and cost of repair/length of outage considerations.

Chapter 17 Generator and Generator-Transformer Protection

The core of an electric power system is the generation. With the exception of emerging fuel cell and solar-cell technology for power systems, the conversion of the fundamental energy into its electrical equivalent normally requires a 'prime mover' to develop mechanical power as an intermediate stage.
A modern generating unit is a complex system comprising the generator stator winding, associated transformer and unit transformer (if present), the rotor with its field winding and excitation system, and the prime mover with its associated auxiliaries. Faults of many kinds can occur within this system for which diverse forms of electrical and mechanical protection are required. The amount of protection applied will be governed by economic considerations, taking into account the value of the machine, and the value of its output to the plant owner.

Chapter 18 Industrial and Commercial Power System Protection.

The protection and control of industrial power supply systems must be given careful attention. Many of the techniques that have been evolved for EHV power systems may be applied to lower voltage systems also, but typically on a reduced scale. However, industrial systems have many special problems that have warranted individual attention and the   development of specific solutions.

Chapter 19 A.C. Motor Protection

There are a wide range of a.c. motors and motor characteristics in existence, because of the numerous duties for which they are used. All motors need protection, but fortunately, the more fundamental problems affecting the choice of protection are independent of the type of motor and the type of load to which it is connected.

There are some important differences between the protection of induction motors and synchronous motors, and these are fully dealt with in the appropriate section.
Motor characteristics must be carefully considered when applying protection; while this may be regarded as stating the obvious, it is emphasised because it applies more to motors than to other items of power system plant.

Chapter 20 Protection of A.C. Electrified Railways

In the case of electrified railways, there is a high probability that sustained electrical faults of any type (high resistance, remote breaker/protection failure etc.) may be associated with overhead wire damage or a faulty traction unit. Fallen live wires caused by mechanical damage or accident represent a greater safety hazard with railways, due to the higher probability of people being close by (railway personnel working on the track, or passengers). Traction unit faults are a fire hazard and a safety risk to passengers, especially in tunnels. For these reasons, there will be a bias towards dependability of back-up protection at the expense of security. The consequences of an occasional unwanted trip are far more acceptable (the control centre simply recloses the tripped CB, some trains are delayed while the control centre ensures it is safe to reclose) than the consequences of a failure to trip for a fallen wire or a traction unit fault.

Chapter 21 Relay Testing and Commissioning

The testing of protection equipment schemes presents a number of problems. This is because the main function of protection equipment is solely concerned with operation under system fault conditions, and cannot readily be tested under normal system operating conditions. This situation is aggravated by the increasing complexity of protection schemes and use of relays containing software.

The testing of protection equipment may be divided into four stages:

  • type tests
  • routine factory production tests
  • commissioning tests
  • periodic maintenance tests

Chapter 22 Power System Measurements

The accurate measurement of the voltage, current or other parameter of a power system is a prerequisite to any form of control, ranging from automatic closed-loop control to the recording of data for statistical purposes.
Measurement of these parameters can be accomplished in a variety of ways, including the use of direct-reading instruments as well as electrical measuring transducers.

Chapter 23 Power Quality

Over the last thirty years or so, the amount of equipment containing electronics has increased dramatically. Such equipment can both cause and be affected by electromagnetic disturbances. A disturbance that affects a process control computer in a large industrial complex could easily result in shutdown of the process.
The lost production and product loss/recycling during start-up represents a large cost to the business.

Similarly, a protection relay affected by a disturbance through conduction or radiation from nearby conductors could trip a feeder or substation, causing loss of supply to a large number of consumers. At the other end of the scale, a domestic user of a PC has to re-boot the PC due to a transient voltage dip, causing annoyance to that and other similarly affected users. Therefore, transporters and users of electrical energy have become much more interested in the nature and frequency of disturbances in the power supply. The topic has become known by the title of Power Quality.

Chapter 24 Substation Control and Automation

The sometimes complex interlocking and sequence control requirements that are to be found in a substation of any significant size lend themselves naturally to the application of automation. These requirements can be readily expressed in mathematical logic (truth tables, Boolean algebra, etc.) and this branch of mathematics is well-suited to the application of computers and associated software. Hence, computers have been applied to the control of electrical networks for many years, and examples of them being applied to substation control/automation were in use in the early 1970's. The first applications were naturally in the bulk power transmission field, as a natural extension of a trend to centralised control rooms for such systems. The large capital investment in such systems and the consequences of major system disruption made the cost of such schemes justifiable. In the last ten years or so, continuing cost pressures on Utilities and advances in computing power and software have led to the application of computers to substation control/ automation on a much wider basis.

Chapter 25 Distribution System Automation

Automation of distribution systems has existed for many years. The extent to which automation has been applied has been determined by a combination of technology
and cost. For many years the available technology limited the application of automation to those parts of the distribution system where loss of supply had an
impact on large numbers of consumers.

Recent developments such as privatisation started to focus attention on the cost to the consumer of a loss in supply. Interruptions in supply began to be reflected in
cost penalties (directly or indirectly) to the Utility, thus providing a financial incentive to improve matters.

Power. Flexible. Easergy.
BeijingSifang June 2016