Network Protection & Automation Guide

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Network Protection & Automation Guide

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  • Network Protection & Automation Guide

  • 1 I n t r o d u c t i o n

  • Relay hardware is becoming even more standardised, to the point atwhich versions of a relay may differ only by the software they contain.

    This accurate prediction in the preface to the Third Edition of the ProtectiveRelay Application Guide (PRAG), 1987, has been followed by the rapiddevelopment of integrated protection and control devices. The change intechnology, together with significant changes in Utility, Industrial andCommercial organisations, has resulted in new emphasis on Secondary SystemsEngineering.

    In addition to the traditional role of protection & control, secondary systemsare now required to provide true added value to organisations.

    When utilised to its maximum, not only can the integration of protection &control functionality deliver the required reduction in life-time cost of capital,but the advanced features available (Quality of Supply, disturbance recordingand plant monitoring) enable system and plant performance to be improved,increasing system availability.

    The evolution of all secondary connected devices to form digital controlsystems continues to greatly increase access to all information available withinthe substation, resulting in new methodologies for asset management.

    In order to provide the modern practising substation engineer with referencematerial, the Network Protection & Automation Guide provides a substantiallyrevised and expanded edition of PRAG incorporating new chapters on all levelsof 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 affectinstrument transformers.

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

    The book then provides detailed analysis on the application of protectionsystems. This includes a new Chapter on the protection of a.c. electrifiedrailways. Existing chapters on distance, busbar and generator protection havebeen completely revised to take account of new developments, includingimprovements due to numerical protection techniques and the applicationproblems of embedded generation. The Chapter on relay testing andcommissioning 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 reflectthe 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. We trustthat you find this book invaluable and assure you that any comments will becarefully noted ready for the next edition.

    1 Int roduct ion

    N e t w o r k P r o t e c t i o n & A u t o m a t i o n G u i d e 3

  • Introduction 2.1

    Protection equipment 2.2

    Zones of protection 2.3

    Reliability 2.4

    Selectivity 2.5

    Stability 2.6

    Speed 2.7

    Sensitivity 2.8

    Primary and back-up protection 2.9

    Relay output devices 2.10

    Relay tripping circuits 2.11

    Trip circuit supervision 2.12

    2 F u n d a m e n t a l so f P r o t e c t i o n P r a c t i c e

  • N e t w o r k P r o t e c t i o n & A u t o m a t i o n G u i d e 5


    The purpose of an electrical power system is to generateand supply electrical energy to consumers. The systemshould be designed and managed to deliver this energyto the utilisation points with both reliability andeconomy. Severe disruption to the normal routine ofmodern society is likely if power outages are frequent orprolonged, placing an increasing emphasis on reliabilityand security of supply. As the requirements of reliabilityand economy are largely opposed, power system designis inevitably a compromise.

    A power system comprises many diverse items ofequipment. Figure 2.2 shows a hypothetical powersystem; this and Figure 2.1 illustrates the diversity ofequipment that is found.

    2 Fu n d a m e n ta l so f P ro te c t i o n P ra c t i c e

    Figure 2.1: Power station

  • N e t w o r k P r o t e c t i o n & A u t o m a t i o n G u i d e





    ls o




    n P





    e 2.Figure 2.2: Example power system

    Figure 2.1: Example power system

    R1 R2

    G1 G2

    T1 T2T

    R3 R4

    G3 G4

    T10 T11


    T16 T17


    T12 T13

    R5 R6

    G5 G6

    T7 T8T




    T5T T6 T3T T4T


    L3 L4











    Hydro power station




    CCGT power station


    Steam power station



    33kV D' 110kV




    Grid380kV F'

  • N e t w o r k P r o t e c t i o n & A u t o m a t i o n G u i d e 7

    2 F











    Many items of equipment are very expensive, and so thecomplete power system represents a very large capitalinvestment. To maximise the return on this outlay, thesystem must be utilised as much as possible within theapplicable constraints of security and reliability ofsupply. More fundamental, however, is that the powersystem should operate in a safe manner at all times. Nomatter how well designed, faults will always occur on apower system, and these faults may represent a risk tolife and/or property. Figure 2.3 shows the onset of a faulton an overhead line. The destructive power of a fault arccarrying a high current is very great; it can burn throughcopper conductors or weld together core laminations ina transformer or machine in a very short time sometens or hundreds of milliseconds. Even away from thefault arc itself, heavy fault currents can cause damage toplant if they continue for more than a few seconds. Theprovision of adequate protection to detect anddisconnect elements of the power system in the event offault is therefore an integral part of power systemdesign. Only by so doing can the objectives of the powersystem be met and the investment protected. Figure 2.4provides an illustration of the consequences of failure toprovide appropriate protection.

    This is the measure of the importance of protectionsystems as applied in power system practice and of theresponsibility vested in the Protection Engineer.


    The definitions that follow are generally used in relationto power system protection:

    a. Protection System: a complete arrangement ofprotection equipment and other devices required toachieve a specified function based on a protectionprincipal (IEC 60255-20)

    b. Protection Equipment: a collection of protectiondevices (relays, fuses, etc.). Excluded are devicessuch as CTs, CBs, Contactors, etc.

    c. Protection Scheme: a collection of protectionequipment providing a defined function andincluding all equipment required to make thescheme work (i.e. relays, CTs, CBs, batteries, etc.)

    In order to fulfil the requirements of protection with theoptimum speed for the many different configurations,operating conditions and construction features of powersystems, it has been necessary to develop many types ofrelay that respond to various functions of the powersystem quantities. For example, observation simply ofthe magnitude of the fault current suffices in some casesbut measurement of power or impedance may benecessary in others. Relays frequently measure complexfunctions of the system quantities, which are only readilyexpressible by mathematical or graphical means.

    Relays may be classified according to the technologyused:

    a. electromechanical

    b. static

    c. digital

    d. numerical

    The different types have somewhat different capabilities,due to the limitations of the technology used. They aredescribed in more detail in Chapter 7.

    Figure 2.3: Onset of an overhead line fault

    Figure 2.4: Possible consequence of inadequate protection

  • N e t w o r k P r o t e c t i o n & A u t o m a t i o n G u i d e

    In many cases, it is not feasible to protect against allhazards with a relay that responds to a single powersystem quantity. An arrangement using severalquantities may be required. In this case, either severalrelays, each responding to a single quantity, or, morecommonly, a single relay containing several elements,each responding independently to a different quantitymay be used.

    The terminology used in describing protection systemsand relays is given in Appendix 1. Different symbols fordescribing relay functions in diagrams of protectionschemes are used, the two most common methods (IECand IEEE/ANSI) are provided in Appendix 2.


    To limit the extent of the power system that isdisconnected when a fault occurs, protection is arrangedin zones. The principle is shown in Figure 2.5. Ideally, thezones of protection should overlap, so that no part of thepower system is left unprotected. This is shown in Figure2.6(a), the circuit breaker being included in both zones.

    Figure 2.52.6

    For practical physical and economic reasons, this ideal isnot always achieved, accommodation for currenttransformers being in some cases available only on oneside of the circuit b