Introduction 25.1

Factors influencing the applicationof automation to distribution networks 25.2

Primary distribution system automation 25.3

Secondary distribution networks -urban areas 25.4

Secondary distribution networks -rural areas 25.5

Communications 25.6

Distribution system automationsoftware tools 25.7

2 5 D i s t r i b u t i o n S y s t e mA u t o m a t i o n

25.1 INTRODUCTION

Distribution systems are generally considered to besupply networks operating at 132kV and below, and towhich consumers are normally connected. Within adistribution system, a division into primary and secondarydistribution systems is often made, with primarydistribution systems having voltages above 22kV andsecondary distribution systems voltage below this value.

Automation of distribution systems has existed for manyyears. The extent to which automation has been appliedhas been determined by a combination of technologyand cost. For many years the available technologylimited the application of automation to those parts ofthe distribution system where loss of supply had animpact on large numbers of consumers. Technology wasnot available to handle the large amount ofgeographically dispersed data required for automation ofdistribution systems in rural areas. Even whendevelopments in technology began to overcome theseproblems, the cost of applying the technology was largein relation to the benefits gained. Often, there was nofinancial incentive to apply automation in ruraldistribution systems, and consumers were not entitled tocompensation for loss of supply. As relatively fewconsumers would be affected by a fault on a ruraldistribution system, compared to a similar fault in anurban distribution system, the number of customercomplaints received was not a sufficiently importantfactor to justify investment in network reliability.Interruptions to consumers in rural areas were treated asbeing inevitable.

Recent developments such as privatisation started tofocus attention on the cost to the consumer of a loss insupply. Interruptions in supply began to be reflected incost penalties (directly or indirectly) to the Utility, thusproviding a financial incentive to improve matters. Ruralconsumers gradually became more aware of the disparityin the number of supply interruptions between rural andurban distribution networks. This led, in conjunctionwith an increasing emphasis on Power Quality issues (see

25 D ist r ib ut ion Sys te mA utomat ion

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Chapter 23), to pressure on Utilities to improve thesituation. In addition, the population in rural areasbecame more dependent on electrical equipment, andthus the consequences of a supply outage were moresignificant.

The term automation conjures up the use ofmicroprocessors, maybe linked together over acommunications network and running special purposesoftware to execute a sequence of actions automatically.While such technology is employed and forms part ofdistribution system automation, the term automationmay imply nothing more than the ability to close or opena switch remotely in addition to local (hand) control. Itmay involve nothing more than the addition of an

actuator, and simple on/off remote control facilities.Technology has been applied to reduce the cost of suchdevices, thus improving the economics of theirapplication. Therefore, the field of distribution systemautomation is a very broad one, and the solution appliedto any particular problem will reflect the particularcircumstances of problem and regulatory regime of theUtility concerned.

Figure 25.1 shows typical distribution systems that formthe subject of this chapter, complete with the elementsof the distribution system to which automationtechniques are applied. The remaining sections of thischapter describe the various automation techniquesavailable, together with typical applications.

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110/33kV

110kV

110kV

Large consumers

33kV

RMU

33/11kV 33/11kV

RMU

33/11kV

RMU

33/11kVUrbanDistributionNetwork

RMU RMU

33/11kV

RMU

33/11kV

33kV

Spur lines

Normally open point

Rural distribution network

110/33kV

110/11kV

110kV

Spur lines

Transmission system

Main circuit

networkdistributionUrban

Load Load Load

Load Load Load

Figure 25.1: Elements of a distribution system

25.2 FACTORS INFLUENCING THE APPLICATIONOF AUTOMATION TO DISTRIBUTION NETWORKS

Cost is the main driving factor in the application of anautomation scheme to a distribution network.Regulatory pressure may also influence the decision. Thecost may arise in many different ways. Savings fromimplementing distribution system automation resultfrom reducing:

a. revenue foregone during outages

b. cost of handling customer complaints

c. cost of control/maintenance staff

d. cost of compensation to consumers for outages

Less tangible benefits can also be identified, such asdeferral of system enhancement (i.e. deferral of capitalexpenditure) through better knowledge of networkperformance. The financial advantage to the Utility ofsuch benefits may be more difficult to calculate, butshould be incorporated in any financial comparison for aproposed scheme.

There are inevitably costs incurred through use of anautomation scheme:

a. cost of implementation (capital cost)

b. cost of operation

c. cost of maintenance

and clearly the total costs saved must be in excess of thetotal costs of implementation and use to make a schemeviable.

For many years, automation has been implemented atvoltages above 22kV, simply due to the number ofconsumers inconvenienced by a supply outage and theresulting costs (in whatever form). However, in recentyears, the traditional balance of cost/benefit has beenchanged, due to:

1. increasing dependence by communities/industryon electricity

2. privatisation (in some countries)

3. the spread of electricity supply to ever moreremote areas

4. the cost of training and retaining skilled staff

5. increasing emphasis on Power Quality issues

This change has been in favour of increased automationof the distribution system, including system voltagesdown to LV. Regulatory pressure to improve thereliability and quality of electricity supply to end-usersproduces an outcome that the associated costs are onlyacceptable if technology is applied to automate thesecondary distribution system. Therefore, automation ofthe secondary distribution system has become more

widespread. At the same time, overhead lines in ruralareas suffer many more faults leading to consumersupply loss than urban cable networks. These findingsare not surprising rural distribution networks arecommonly in the form of radial feeders whereas urbannetworks are often in the form of ring or meshednetworks to minimise the chances of supply loss to largegroups of consumers. Similarly, overhead lines arenormally more prone to faults than underground cables.Because the fault incidence on EHV overhead lines issignificantly lower than for those on distributionsystems, it is also arguable that the technical standardsrelating to overhead lines on distribution networks alsorequire review.

Therefore, developments in distribution systemautomation have concentrated largely on applications tothe secondary distribution system.

25.3 PRIMARY DISTRIBUTION SYSTEM AUTOMATION

The primary distribution system is generally accepted ascomprising those elements of the distribution systemoperating at voltages above 22kV. Distribution uses bothcable and overhead lines, and the power levels involvedwill enable either a large group of domestic consumers,or several industrial plants to be served. Very largeindustrial plants may justify their own dedicated feedersfrom the primary distribution substation (Figure 25.2).

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Large consumers

Feeders to secondarydistribution system

distribution systemFeeders to secondary

Bulk transmission network

110kV

50kV

50kV

To Primarydistributionsubstation

110/50kV

Figure 25.2: Primary distribution system

Automation of the primary distribution system is wellestablished, due to the impact of supply loss on the manyand diverse consumers that it serves. In addition, thedistribution system is usually interconnected, so that lossof supply to consumers in the event of a circuit outage isminimised. The circuit breakers and protection systemsused in the system will already be capable of remotecontrol/monitoring. However, status informationon a circuit may be confined to simpleon/off/open/closed/tripped indications, and determinationof the cause of a trip will still require despatch of amaintenance crew to the equipment concerned. Onlyafter the cause of a trip has been determined can faultlocation and rectification take place.

Hence modern network automation techniques can beusefully applied. Application of such techniques bringsthe following advantages:

a. ability to control a much larger area

b. provision of detailed network performanceinformation

c. reduction in space requirements

d. reduction in staffing

25.3.1 Control Area Size

The modern electric power network has tight couplingbetween the var