24-Substation Control and Automation

Introduction 24.1

Topology and functionality 24.2

Hardware implementation 24.3

Communication protocols 24.4

Substation automation functionality 24.5

System configuration and testing 24.6

Examples of substation automation 24.7

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24.1 INTRODUCTION

The sometimes complex interlocking and sequencecontrol requirements that are to be found in asubstation of any significant size lend themselvesnaturally to the application of automation. Theserequirements can be readily expressed in mathematicallogic (truth tables, boolean algebra, etc.) and this branchof mathematics is well-suited to the application ofcomputers and associated software. Hence, computershave been applied to the control of electrical networksfor many years, and examples of them being applied tosubstation control/automation were in use in the early1970’s. The first applications were naturally in the bulkpower transmission field, as a natural extension of atrend to centralised control rooms for such systems. Thelarge capital investment in such systems and theconsequences of major system disruption made the costof such schemes justifiable. In the last ten years or so,continuing cost pressures on Utilities and advances incomputing power and software have led to theapplication of computers to substation control/automation on a much wider basis.

This Chapter outlines the current technology andprovides examples of modern practice in the field.

24.2 TOPOLOGY AND FUNCTIONALITY

The topology of a substation control system is thearchitecture of the computer system used. Thefunctionality of such a system is the complete set offunctions that can be implemented in the control system– but note that a particular substation may only utilisea subset of the functionality possible.

All computer control systems utilise one of two basictopologies:

a. centralised

b. distributed

and the basic concepts of each are illustrated in Figure

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24.1. Early examples of substation automation used thecentralised concept, due to limitations in technology,both of processor power and communication techniques.Latest examples use a distributed architecture, in that anumber of Intelligent Electronic Devices (IED’s) – such asmicroprocessor based relays – may be linked via amultidrop serial link to a local processor. The localprocessor may control one or more bays in a substation.All of the local processors are, in turn, connected to aHuman Machine Interface (or HMI), and possibly also toa local or remote SCADA system for overall networkmonitoring/control.

24.2.1 System Elements

The main system elements in a substation control systemare:

a. IED’s, implementing a specific function orfunctions on a circuit or busbar in a substation.The most common example of an IED is amicroprocessor based protection relay, but it couldalso be a microprocessor based measurementdevice, interface unit to older relays or control, etc.

b. Bay Module (or controller). This device willnormally contain all of the software required forthe control and interlocking of a single bay (feeder,etc.) in the substation, and sufficient I/O tointerface to all of the required devices required formeasurement/protection/control of the bay. The

I/O may include digital and analogue I/O (forinterfacing to discrete devices such as CB close/tripcircuits, isolator motors, non-microprocessor basedprotection relays) and communications links (serialor parallel as required) to IED’s

c. Human Machine Interface (HMI). This is theprincipal user interface and would normally takethe form of a computer. The familiar desktop PC iscommonly used, but specialised computers are alsopossible, while normally unmanned substationsmay dispense with a permanently installed HMIand rely on operations/maintenance staff bringinga portable computer equipped with the appropriatesoftware with them when attendance is required.It is usual to also provide one or more printerslinked to the HMI in order to provide hard-copyrecords of various kinds (Sequence of Eventsrecorder, alarm list, etc.)

d. A communications bus or busses, linking thevarious devices. In a new substation, all of theelements of the automation system will normallyuse the same bus, or at most two busses, to obtain cost-effectiveness. Where a substationautomation system is being retrofitted to anexisting substation, it may be necessary to useexisting communications busses to communicatewith some existing devices. This can lead to amultiplicity of communications busses within theautomation system

e. A link to a remote SCADA system. This may beprovided by a dedicated interface unit, be part ofthe HMI computer or part of an IED. It perhapsmay not be provided at all – though since one ofthe benefits of substation automation is thecapability of remote control/ monitoring, thiswould be highly unusual. It may only occur duringa staged development of an automation scheme ata time when the bay operations are beingautomated but the substation is still manned, priorto implementing remote control capability

24.2.3 System Requirements

A substation control/automation scheme will normallybe required to possess the following features:

a. control of all substation electrical equipment froma central point

b. monitoring of all substation electrical equipmentfrom a central point

c. interface to remote SCADA system

d. control of electrical equipment in a bay locally

e. monitoring of electrical equipment in a bay locally

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Outstation

(b) Distributed topology

Control Centre

(a) Centralised topology

Outstations

Outstation

Outstation

Outstation

Outstation

Outstation

Controlcentre

Controlcentre

Controlcentre

Figure 24.1: Basic substation automationsystem topologies

Chap24 exe 20/06/02 15:15 Page 424

f. status monitoring of all connected substationautomation equipment

g. system database management

h. energy management

i. condition monitoring of substation electricalequipment (switchgear, transformers, relays, IED’s)

The system may be required to be fault-tolerant,implying that redundancy in devices and communicationpaths is provided. The extent of fault-tolerance providedwill depend on the size and criticality of the substationto the operator, and the normal manning status(manned/ unmanned). Many of the functions may beexecuted from a remote location (e.g. a System ControlCentre) in addition to the substation itself.

Certain of the above functions will be required even inthe most elementary application. However, the selectionof the complete set of functions required for a particularapplication is essentially the responsibility of the end-user (Utility, etc.). Due to a modular, ‘building block’approach to software design, it is relatively easy to addfunctionality at a later stage. This often occurs throughchanging operators’ needs and/or electrical networkdevelopment. Compatibility of the underlying databaseof network data must be addressed to ensure thathistorical data can still be accessed.

24.3 HARDWARE IMPLEMENTATION

To form a substation control system, the variouselements described above must be assembled into someform of topology. Three major hardware topologies canbe identified as being commonly used, as follows:

24.3.1 HMI-based Topology

This takes the form of Figure 24.2. The software toimplement the control/automation functions resides inthe HMI computer and this has direct links to IED’s usingone or more communications protocols. The link to aremote SCADA system is normally also provided in theHMI computer, though a separate interface unit may beprovided to offload some of the processor requirementsfrom the HMI computer, especially if a proprietarycommunications protocol to the SCADA system is used.

For this topology, a powerful HMI computer is clearlyrequired if large numbers of IED’s are to beaccommodated. In practice, costs usually dictate the useof a standard PC, and hence there will be limitations onsubstation size that it can be applied to because of aresulting limit to the number of IED’s that can beconnected. The other important issue is one of reliabilityand availability – there is only one computer that cancontrol the substation and therefore only local manual

control will be possible if the computer fails for anyreason. Such a topology is therefore only suited to smallMV substations where the consequences of computerfailure (requiring a visit from a repair crew to remedy)are acceptable. Bay Modules are not used, the softwarefor control and interlocking of each substation bay runsas part of the HMI computer software.

24.3.2 RTU-based Topology

This topology is an enhancement of the HMI topologyand is shown in Figure 24.3. A microprocessor-basedRTU is used to host the automation software, freeing theHMI computer for operator interface duties only. TheHMI computer can therefore be less powerful and usuallytakes the form of a standard PC, or for not-normally-manned substations, visiting personnel can use aportable PC.

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Master clock(GPS, radio)

Telecontrol orbus interface

Legacy bus

I/O, devicesCT, VT

The RTU, telecontrol interface and the bus interface could be: • separate equipment • integrated into the same computer

IED's

RTU

HMI

SCADAinterface

Internetor PSTN

Bus interface

Figure 24.3: RTU-based topology

Bus interface

Telecontrol orbus interfaceStation bus

Legacy bus

I/O, devicesCT, VT

The HMI, telecontrol interface, and the bus interface could be: • separate equipment • integrated into the same PC

IED's

IED'sComputer

Remote HMI

HMI

Master clock(GPS, radio)SCADA

interface

Internetor PSTN

Bus interface

Figure 24.2: HMI-based hardware topology

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The RTU is purpose designed and can house one or morepowerful microprocessors. A greater number of I/Opoints can be accommodated than in the HMI topology,while the possibility exists of hosting a wider variety ofcommunication protocols for IED’s and the remoteSCADA connection. Bay Modules are not required, theassociated software for interlocking and controlsequences is part of the RTU software.

24.3.3 Decentralised Topology

This topology is illustrated in Figure 24.4. In it, each bayof the substation is controlled by a Bay Module, whichhouses the control and interlocking software, interfacesto the various IED’s required as part of the control andprotection for the bay, and an interface to the HMI. It ispossible to use an HMI computer to take local control ofan individual bay for commissioning/testing and faultfinding purposes. The amount of data from the varioussubstation I/O points dictates that a separate SCADAinterface unit is provided (often called an RTU orGateway), while it is possible to have more than one HMIcomputer, the primary one being dedicated to operationsand others for engineering use. Optionally, a remote HMIcomputer may be made available via a separate link. Itis always desirable in such schemes to separate the real-time operations function from engineering tasks, whichdo not have the same time-critical importance.

The connection between the various Bay Modules andthe HMI computer is of some interest. Simplest is thestar arrangement of Figure 24.5(a). This is the least-costsolution but suffers from two disadvantages. Firstly, abreak in the link will result in loss of remote control ofthe bay affected; only local control via a local HMIcomputer connected to the bay is then possible.Secondly, the number of communication ports availableon the HMI computer will limit the number of BayModules.

Of course, it is possible to overcome the first problem byduplicating links and running the links in physicallyseparate routes. However, this makes the I/O portproblem worse, while additional design effort is requiredin ensuring cable route diversity.

An alternative is to connect the Bay Modules, HMIcomputer and SCADA gateway in a ring, as shown inFigure 24.5(b). By using a communication architecturesuch as found in a LAN network, each device is able totalk to any other device on the ring without any messageconflicts. A single break in the ring does not result inloss of any facilities. The detection of ring breakage andre-configuration required can be made automatically.Thus, the availability and fault tolerance of the networkis improved. Multiple rings emanating from the HMIcomputer can be used if the number of devices exceedsthe limit for a single ring. It can be easier to install on astep-by-step basis for retrofit applications, but of course,all these advantages have a downside. The cost of sucha topology is higher than that of the other solutions, sothis topology is reserved for situations where the highestreliability and availability is required - i.e. HV and EHVtransmission substations.

Redundancy can also be provided at the individual devicelevel. Relays and other IED’s may be duplicated, thoughthis would not be usual unless required for other reasons(e.g. EHV transmission lines may be required to haveduplicate main protections – this is not strictly speaking

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(a) Star connection of bay modules

HMIcomputer

(b) Ring connection of Bay Modules

HMIcomputer

BayModules

BayModules

BayModules

BayModules

BayModules

BayModules

BayModules

BayModules

Figure 24.5: Methods of hardwareinterconnection

Master clock(GPS, radio)

Telecontrol orbus interface

Legacybus

I/O, devicesCT, VT

The Bay Module and businterface could be: • separate equipments • integrated into the Bay Module

IED's

Bay Module Computer

Station busComputer

Remote HMI

HMI

SCADAinterface

Internetor PSTN

Bus interface

Figure 24.4: Decentralised topology

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duplication of individual devices - which would requireeach individual main protection to have two identicalrelays voting on a ‘1 out of 2’ basis). It is usual to havemore than one operators’ HMI, either for operationalreasons or for fault-tolerance. The system computer maybe duplicated on a ‘hot-standby’ or ‘dual-redundant’basis, or tasks may be normally shared between two ormore system computers with each of them having thecapability of taking over the functions of one of theothers in the event of a failure.

The total I/O count in a major substation will becomelarge and it must be ensured that the computer hardwareand communication links have sufficient performance toensure prompt processing of incoming data. Overload inthis area can lead to one or more of the following:

a. undue delay in updating the system statusdiagrams/events log/alarm log in response to anincident

b. corruption of system database, so that theinformation presented to the operator is not anaccurate representation of the state of the actualelectrical system

c. system lockup

As I/O at the bay level, both digital and analogue willtypically be handled by intelligent relays or specialisedIED’s, it is therefore important to ensure that thesedevices have sufficient I/O capacity. If additional IED’shave to be provided solely for ensuring adequate I/Ocapacity, cost and space requirements will increase.There will also be an increase in the number ofcommunication links required.

A practical specification for system response times isgiven in Table 24.1. Table 24.2 gives a typicalspecification for the maximum I/O capacities of asubstation automation system.

A significant problem to be overcome in theimplementation of communication links is the possibilityof electromagnetic interference. The low voltage levelsthat are used on most types of communication link maybe prone to interference as a result. Careful design ofthe interfaces between the devices used and thecommunication bus, involving the use of opto-couplersand protocol converters, is required to minimise the risk.Care over the arrangement of the communication cablesis also required. It may also help to use a communicationprotocol that incorporates a means of errordetection/correction. While it may not be possible tocorrect all errors, detection offers the opportunity torequest re-transmission of the message, and also forstatistics to be gathered on error rates on various partsof the system. An unusually high error rate on a part ofthe communication system can be flagged tomaintenance crews for investigation.

24.4 COMMUNICATION METHODS

Digital communication between items of hardware isdivided into three elements:

a. the protocol, consisting of the hardware, such asconnectors, connector pin functions, and signallevels

b. the format, consisting of the control of the flow ofdata

c. the language, or how the information in the dataflow is organised

Each of these areas is covered so that an appreciation ofthe complexities of digital communications isunderstood.

24.4.1 Communication Protocols and Formats

Anyone trying to connect up the various elements of aHi-Fi system if they have purchased them from differentmanufacturers will be aware of the number of differentprotocols in use. The situation is the same in theindustrial field. Manufacturers of devices are oftentempted to utilise a proprietary protocol, for no betterreason sometimes than to encourage the sole use of theirdevices. Users, of course, have the opposite interest;they would like every manufacturer to use the sameprotocol so that they have the widest choice. In practice,protocols have evolved over time, and some protocols aremore appropriate to some communication requirementsthan others. The protocol used is also linked to theformat used, since the number of conductors requiredmay depend on the format used.

There are two basic formats in use for datacommunications:

a. serialb. parallel

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Signal Type Response Time to/from HMI

Digital Input 1s

Analogue Input 1s

Digital Output 0.75s

Disturbance Record File 3s

Table 24.1: Practical system response times fora substation automation scheme

I/O Type Capacity

Digital Input 8196

Digital Output 2048

Analogue Input 2048

Analogue Output 512

Table 24.2: Typical I/O capacities for a substationautomation system

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Serial format involves sending the data one bit at a timealong the communication channel. Parallel formatinvolves sending several bits simultaneously. Clearly,parallel communication requires more wires than serialcommunication (a disadvantage) but can transmit agiven amount of data faster. In practice, parallelcommunication is limited to communication over a fewmetres, and hence the majority of communications useserial format. There are a number of popular serialcommunication protocols in common use in thesubstation automation field.

24.4.1.1 RS232C Protocol

The RS232C protocol allows for full duplexcommunications between two devices. The basicspecification is given in Table 24.3. The hardwarespecification can vary – nine conductors are the minimumrequired for a full implementation, while a 25 pinconnector is commonly encountered. If flow control ofdata is not required, only three signals are required (datatransmit/receive and ground). Being limited tocommunication between two devices, this protocol is notuseful in substation automation schemes. However, it isdescribed, because it is regularly encountered in remotecommunication applications, such as those between asmall substation and a control centre using modems totransfer the data over a telephone line.

24.4.1.2 RS485 Protocol

This protocol is detailed in Table 24.4, and is much moreuseful for substation automation schemes. This isbecause, many devices can be attached to one datachannel, the maximum distance over whichcommunications can take place is quite large, and themaximum bit rate is quite high. It only requires a simpletwisted pair connection, with all devices ‘daisy-chained’on the link, as shown in Figure 24.6.

Thus devices can be located throughout a substationwithout causing communications problems and significantamounts of data can be transmitted rapidly. The maindrawback is that it is a half-duplex system, so thatcommunications use a kind of question and answertechnique known as ‘polling’. The equipment that needsthe data (e.g. a substation computer or bay controller) mustask each device in turn for the data requested and thenwait for the response prior to moving on to the next device.

Where devices connected to the communicationschannel may need to flag alarm conditions, this dictatescontinual polling of all devices connected to thecommunications channel. If more than 31 devices needto be connected, more than one RS485 communicationslink can be provided.

24.4.1.3 IEC 60870-5 Protocols

The two commonly used protocols are IEC 60870-5-101and IEC 60870-5-103.

IEC 60870-5-101 is used for communications betweendevices over long distances. A typical application wouldbe communications between a substation and a CentralControl Room (CCR). A bit serial communicationtechnique is used, and transmission speeds of up to64kbit/s are possible, depending on the transmissionprotocol selected from those specified in the standard.Modems can be used, and hence there is no practicallimitation of the distance between devices.

IEC60870-5-103 specifies a communication protocolbetween a master station and protection devices (e.g.protection relays). The standard is based on, and is asuperset of, the German VDEW communication protocol.Either fibre optic transmission or an RS485 link can beused, and transmission speeds are either 9600kbit/s or19200kbit/s. Maximum transmission distance is 1000musing fibre-optic transmission . Communication is on a‘master/slave’ basis, in which the master stationcontinually polls the slaves (relays) to determine if anyinformation is ready to be sent by the slaves. While somemessages are defined by the standard, these are of

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Max. number of transmitters 1

Max. number of receivers 1

Connection type 25 core shielded

Mode of operation DC coupling

Maximum distance of transmission 15m

Maximum data rate 20kbit/s

Transmitter voltage 5V min, 15V max

Receiver sensitivity 3V

Driver slew rate 30V/µsec

Table 24.3: RS232C specification

Max. number of transmitters 32

Max. number of receivers 32

Connection type Shielded Twisted Pair

Mode of operation Differential

Maximum distance of transmission 1200m

Maximum data rate 10Mbit/s

Transmitter voltage 1.5V min

Receiver sensitivity 300mV

Table 24.4: RS485 specification

IED IED

IED IED IED IED

Terminatingresistor

Masterstation

Figure 24.6: ‘Daisy-chain’ connectionof RS485 devices

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limited functionality. In addition, the standard allows theuse of manufacturer-specific ‘private’ messages. Thesepermit much greater functionality, but at the same timehinder interoperability of equipment from differentmanufacturers because there is no need for the format ofsuch messages to be made public. This is arguably thegreatest drawback of the standard, since extensive use of‘private’ messages by manufacturers of devices essentiallyturns the standard into several proprietary ones.

24.4.2 Network Protocols

So far, the protocols described are useful forimplementing communications over a relativelyrestricted geographical area. A substation automationscheme may extend over a very wide area, and hencesuitable protocols are needed for this situation. Themost common protocols in use conform to the ISO 7-layer model of a network. This model is internationallyrecognised as the standard for the requirements forcommunications between data processing systems.

24.4.2.1 ISO 7-layer model

The ISO 7-layer model is shown in Figure 24.7. Itrepresents a communications system as a number oflayers, each layer having a specific function. Thisapproach ensures modularity, and hence assists inensuring that products from different vendors thatcomply with the standard will work together. Thefunctions of each layer are best described by making ananalogy with a telephone call, as given in Table 24.5.

There are a number of network protocols that arecompliant with the OSI model, such as TCP/IP, Modbus,DNP. This does not mean that the devices using differentprotocols are interchangeable, or even that devices usingthe same protocol are interchangeable.

The same data item may be stored at different addresseswithin different devices, and hence re-programming ofthe client that receives the information is necessarywhen one device is replaced by a different one, even ifthe functionality is unchanged. It can easily be seen howa substation equipped with a variety of devices fromdifferent manufacturers and maybe using differentprotocols for communication makes the problem ofapplying an automation system very difficult andexpensive. The major cost in such cases is developing thesoftware translation routines for protocol conversion andbuilding of the required database specifying where eachitem of data to be acquired is held.

24.4.2.2 Utility Communications Architecture protocol

A recent protocol, the Utility CommunicationsArchitecture v2.0 (UCA v2.0), seeks to overcome thesehandicaps by adopting an object-oriented approach tothe data held in a measurement/control device, plus aninternationally recognised protocol (ISO 9506) in theapplication layer. Data objects and services availablewithin a device follow a specified naming system. Theclient can extract a description of the data objects thata device can supply, and services that it can perform, sothat it is easier to program the client. Scaling factorsand units for data items are built into theself-description, so that the effort requiredduring commissioning is reduced. Devices are notinterchangeable, in the sense that a device from one

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Table 24.5: OSI 7-layer model – Telephone call analogy

Telephone Call Analogy

Conversion of voice into electrical signals. Defines typeof connector, no. of pins, signal levels, etc. Optical fibresand wires that make up the physical telephone network

Message transmission, error control and conferencing facilities.Words not clearly received are requested to be re-transmitted,

using agreed procedures. For conferencing, defines how controlpasses from one person to the next.

Call routing, by specifying the method of allocating telephonenumbers and provision of dialling facilities. Includes operator

facilities for routing to extensions. If the message is from severalsheets of paper, ensures that all sheets have been received and are

in the correct order.

Monitors transmission quality and implements procedures if qualityis unaceptable - e.g. requests both parties to hang up and oneto re-dial. Also provides a mechanism to ensure that the correct

persons are communicating, and searches for them(e.g. uses telephone directory) if not.

Provides facilities for automatically making calls at pre-defined times,and ensures that the correct persons are present when the callis made. A session may be interrupted and re-established later,

using the same or a different network/transport connection.As calls are half-duplex, provides flow control procedures -

e.g one person says 'over' to invite the other to speak.

Removes language difficulties by ensuring that the same languageis spoken by both parties, or provides translation facilities.

Also provides encryption facilities for confidential calls.

Specifies the format in which a message will be sent when usedin a specific application- e.g. if the application is to convey

information about meetings attended by a person, will definethe format used for the place, time, and purpose of the meeting.

OSI Layer

Physical

Data Link

Network

Transport

Session

Presentation

Application

Application

Presentation

Session

Transport

Network

Data Link

Physical

Selects appropriate service for application

Provides code conversion, data reformatting

Co-ordinates interaction between endapplication processes

Provides for end-to-end data integrity andquality of service

Switches and routes information

Transfers unit of information to other end ofphysical link

Transmits bit stream to medium

Figure 24.7: OSI 7-layer interconnectionmodel

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manufacturer cannot be removed and replaced by adevice of similar functionality from another vendor.Rather, this protocol ensures interoperability; that is theability for devices from different suppliers and ofdifferent functionality to communicate successfully witheach other. The transport protocol has been separatedfrom the application protocol, so problems with registeraddresses, etc. no longer exist. All that has to beaddressed is the transport protocols used, and clients willnormally be able to communicate with devices using oneof a number of common transport protocols. Thisstandard has an IEC equivalent, IEC 61850. To beginwith, IEC 61850 covers only the field of substationautomation, but will gradually be extended to cover thesame fields as UCA v2.0. Manufacturers are increasinglymoving away from protocols with a proprietary elementin them to UCA v2.0/IEC 61850. It is likely that within ashort time, most protection and control devices will useone or other of these standards for communications.One important reason guiding this change is that thesestandards permit the use of the XML language forexchange of data between databases. As theinformation stored in an automation system or controlcentre comprises a series of databases, informationexchange is therefore facilitated.

24.4.3 Languages

A communications language is the interpretation of thedata contained in a message. The communicationslanguage normally forms part of the overallcommunications protocol. Obviously, it is necessary forboth transmitter and recipient of the message to use thesame language. While a number of communicationsstandards attempt to specify the language used, there isoften flexibility provided, leading to manufacturer-specific implementations. A popular work-around is fora number of organisations to agree common standardsand set up a certifying body to check for complianceagainst these standards. Thus, equipment that compliesbecomes to large degree, interoperable. However, thelatest trend, as exemplified by the UCA v2.0/IEC 61850

protocol, is to define the language very precisely at ahigh level, and require such details to be included as apart of each message so that the recipient can interpretthe message without the need for any translatorsoftware.

24.5 SUBSTATION AUTOMATION FUNCTIONALITY

The hardware implementation provides the physicalmeans to implement the functionality of the substationautomation scheme. The software provided in thevarious devices is used to implement the functionalityrequired. The software may be quite simple or extremelycomplex – Table 24.6 illustrates the functionality thatmay be provided in a large scheme.

The description of the electrical network and thecharacteristics of the various devices associated with thenetwork are held within the computer as a database orset of databases. Within each database, data isorganised into tables, usually on a ‘per device’ basis thatreflects the important characteristics of the device andits interrelationship with other devices on the network.Electrical system configuration changes requiremodification of the database using an appropriatesoftware tool supplied by the automation system vendor.The tool is normally a high level, user-friendly interface,so that modifications to the one-line can be drawndirectly on-screen, with ‘pick-and–place’ facilities forrelays, IED’s, etc. This work would normally be doneoffline on the Engineers’ workstation, if available, or as abackground task on the control computer if not. Carefuland extensive checking of the data is required, bothbefore and after entry into the database, to ensure thatno errors have been made. Full testing on the newconfiguration using a simulator is recommended prior touse of the new database on the main control computerto ensure that there is minimal possibility of errors.

The software is written as a set of well-proven, standardmodules, so there is little or no need for new modules tobe written and tested for a particular substation. Therequired data for the calculations performed by the

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Functional area Functionality

Interlocking CB's Isolators ContactorsTripping sequences CB failure Intertripping Simultaneous tripsSwitching sequences Automatic transformer changeover Automatic busbar changeover Restoration of supply following fault Network re-configurationLoad management Load shedding Load restoration Generator despatchTransformer supervision OLTC control Load managementEnergy monitoring Import/export control Energy management Power factor controlSwitchgear monitoring AIS monitoring GIS monitoringEquipment status Relay status CB status Isolator statusParameter setting Relays Transformers Switching sequences IED configuration

Access control One-line views System views Event loggingHMI functionality Trend curves Harmonic analysis Remote access Disturbance analysis

Interface to SCADA Alarm processing 512

Table 24.6: Typical substation automation functionality

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software is held in the network database. This meansthat adding functionality later is not difficult, so long asthe database design has considered this possibility.There may be problems if the electrical systemconfiguration is altered or additional functionality addedin reading historical data prior to the change. Training ofoperations personnel will inevitably be required inoperation of the system, configuration management andautomation system maintenance. Automation systemsuppliers will be able to provide configurationmanagement and system maintenance services undercontract if required, often with defined cost schedulesand response times so financial management of theautomation scheme once installed is well-defined.

The issuing of commands to switching devices in thesystem has to be carefully structured, in order to preventcommands that would cause a hazard from being issued.A hierarchical structure is commonly used as shown inFigure 24.8, beginning with the requirement for anoperator wishing to issue a command to switchingdevices to log-in to the system using a password.

Different levels of authority, allowing for restrictions onthe type and/or location of switching commands capableof being issued by a particular operator may beimplemented at this stage. The next level in thehierarchy is to structure the issuing of commands on an‘issue/confirm/execute’ basis (Figure 24.9), so that theoperator is given an opportunity to check that thecommand entered is correct prior to execution.

The final level in the hierarchy is implemented insoftware at the bay level and is actioned after theoperator confirms that the switching action is to be

executed. At this stage, prior to execution, the operationis checked against:

a. devices locked out (i.e. prevented from operation)

b. interlocking of devices/switching sequences

to ensure that the command issued is safe to carry out.The action is cancelled and operator informed if it is notsafe to proceed, otherwise the action is carried out andthe operator informed when it is complete.

In a number of systems, some routine switchingoperations (e.g. transfer of a feeder from one busbar tothe other in a double-bus substation) are automated insoftware. The operator need only request the ‘bus-transfer’ action to be carried out on a particular feeder,and the software is able to work out the correctswitching sequence required. This minimises thepossibility of operator error, but at the expense of someextra complexity in the software and more extensivechecking at the factory test stage. However, sincesoftware is modular in nature, substation electricaltopology is restricted to a small number ofconfigurations and such sequences are very common, thesoftware development is essentially a one-off activity forany particular substation control system. Thedevelopment cost can be spread over the sale of anumber of such systems, and hence the cost to anyindividual user is small compared with the potentialbenefits.

24.5.1 Future Developments

The functionality of a substation automation system isstill evolving, with new applications being steadilyadded. Expansion of the functionality of such systems isproceeding in many areas, but two main areas currentlyare attracting significant interest. These are conditionmonitoring and web-access.

Condition monitoring packages are already implementedin automation systems for switchgear, while stand-alonepackages are available for transformers (Chapter 16).Under development are similar packages for generators,CT’s, VT’s, and batteries. It can be expected that all ofthese facilities will be offered as part of a comprehensivecondition monitoring package in substation automationschemes in the near future. The advantage for the useris that the condition monitoring package can then forma component of the Asset Management policy, in orderto determine the schedule for maintenance andreplacement, plus the acquisition of statistics on failurerates. These can then be used in conjunction withmanufacturers to enhance the design to improveavailability.

There has already been discussion on the variouscommunication techniques available. Use of the Internet

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Operator/authorised

person

Select user List of availablefunctions

Seniorauthorised

person

Engineer

Systemengineer

Administrator Password

Password

Password

Password

Password

Figure 24.8: Hierarchical command structure

Interlocking

Deviceselect

List ofavailableactions

Actionselect

Actionconfirm

Actionexecute

Cancel

Figure 24.9: Device selection/operation

Chap24 exe 20/06/02 15:15 Page 431

communication techniques for communications to/froma substation offers a cheap, well-proven, widelyaccessible route for this function. It also enables accessto the data from a broader community, which may beuseful in some circumstances. However, great emphasismust be placed on the use of secure Internetcommunications techniques, such as those used in thefinancial sphere, as the opportunity for unauthorisedmalicious access leading to major incidents or loss ofconfidential data is much greater. As cost is the maindriver, it can be expected that automation systems usingsuch communications techniques will appear in thefuture, using secure communications techniques, andthat users will have to become more aware of the threatsinvolved in order to apply suitable countermeasures.

24.6 SYSTEM CONFIGURATION AND TESTING

These tasks, along with project management, are the mosttime consuming tasks in the process of realising a control andmonitoring system for an electrical network. The strategiesavailable for dealing with these problems vary betweenmanufacturers, but typical approaches are as follows.

24.6.1 System Configuration

Software tools exist that assist in configuring a modernsubstation or network automation system. The extent towhich the task is automated will vary, but all require as aminimum the details of the network to be controlled,extending to the individual device level (circuit breaker,isolator, disconnector, etc.). Where communication to anexisting SCADA system is required, data on the logicaladdresses expected by the SCADA system and devicescontrolled remotely from the SCADA system will also bepart of the data input. Use can also be made of existingdatabases that cover pre-defined network configurations –for example the interlocking equations for a substation bay.

Software tools will check the data for consistency, priorto creation of:

a. the required equipment that forms the automationscheme, together with the requiredinterconnections

b. the databases for each individual device

The data will be divided into domains, according to theuse made of the data:

a. process – CB/isolator position, interlockingequation, values of current/voltage

b. system – number of bay computers, hardwareconfiguration of each bay computer, automatedsequences

c. graphical – the links between each mimic display

and the data to be displayed

d. operator – security access levels, alarm texts, etc.

e. external constraints – data addresses for externaldatabase access

Once all the data has been defined, the configuratortools can define the hardware configuration to providethe required functions at least cost, and the datarequired for implementation of the automation scheme.

24.6.2 System Testing

The degree of testing to be carried will be defined by thecustomer and encapsulated in a specification for systemtesting. It is normal for testing of the completefunctionality of the scheme to be required prior todespatch from the manufacturer. The larger and morecomplex the automation scheme, the more important forall parties that such testing is carried out. It is acceptedwisdom that the earlier problems are discovered, thecheaper and quicker it is to fix them. Remediation ofproblems on-site during commissioning is the mostexpensive and time-consuming activity. Manual testingof a network automation scheme is only practical forsmall networks, due to the cost of testing. Simulationtools are necessary for all other automation schemes.These tools fall into two categories:

a. simulator tools that re-create the network to becontrolled by the automation system.

b. test management tools

24.6.2.1 Simulator tools

Simulator tools are dedicated to the network beingtested. They will normally be provided with a simulationlanguage that the test team can use to play scenarios,and hence determine how the automation system willreact to various stimuli.

Process simulator tools may be hardware and/or softwarebased and emulate the response of the various devices tobe controlled (CB’s/isolators/VT’s/protection relays, etc).They must be capable of closely following the dynamicresponse of such devices under multiple and cascade faultconditions. Specific tools and libraries are developed asrequired, including the use of complex software such asEMTP for simulation of the response to impulse-typephenomena and the dynamic response of protectionalgorithms. They may simulate the response of equipmentwithin the control span of the automation equipment, orthat of equipment outside of the span of control, in orderthat the response of the automation system can be tested.

Communications simulator tools are used both to loadthe internal communications network within the

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automation system to ensure that all devices arecommunicating correctly and that performance of theoverall automation system is within specification duringperiods of high communications traffic. These simulatorsare standardised and a single simulator may be able toemulate several items of equipment. Externalcommunications simulators test the communications withan external system, such as a remote control centre. Thesewill normally be customer-specific, but some standardsimulators may be possible if a standard communicationsprotocol such as IEC 60870-5-101 is used.

24.6.3 Test Strategy

The strategy adopted for the testing of the automationsystem must naturally satisfy client requirements, andgenerally follow one of two approaches:

a. a single test is carried out when all equipment forthe scheme has been assembled,

b. incremental tests are carried out as the automationsystem is built up, with simulator used to representmissing equipment.

The former solution is quickest and cheapest, but cangive rise to problems where it is not easy to locateproblems down to the device level. It is therefore usedprincipally when an upgrade to an existing system isbeing carried out.

It is usual for all of the functionality to be tested,including that specified for normal conditions andspecified levels of degradation within the automationsystem. This leads to a large number of tests beingrequired. Over 500 separate tests may be required for anautomation system of average size in order todemonstrate compliance with the specification.

24.6.4 Management of System Tests

The large number of tests required to demonstrate thecompliance of an automation system with specificationmakes manual techniques for management of the testscumbersome and time consuming. The end result isincreased cost and timescale. Moreover, each test mayresult in a large amount of data to be analysed. Theresults of the analysis need to be presented in an easilyunderstood form and stored for some time. If changesare made to software for any reason over the lifetime ofthe equipment, the different versions must be stored,together with a record of what the changes betweenversions were, and why they were made. Themanagement of this becomes very complex, andsoftware tools are normally used to address the issues oftest schedules, test result presentation, software versioncontrol, and configuration management.

The control of personnel working in the system test areais also of importance, to ensure tests are unbiased. Tomeet this objective, test team personnel are normallyindependent of those of the design team. If incrementaltesting is used, it is sound practice that the finalintegration test team is also independent of the testteam(s) that carried out the incremental tests.

24.7 EXAMPLES OF SUBSTATION AUTOMATION

A significant advantage to an asset-owner of using asubstation automation system is the space-savings thatresult. Space costs money, and hence minimisation ofspace enables new substations to occupy a smallerphysical space. Alternatively, expansion of an existingsubstation can be undertaken making use of currentlyspare bays, but where there is a problem in tightly packedrelay rooms in accommodating the extra equipment.

A common need is to update an existing substation,presently based on electromechanical or electronicrelays, with modern devices. Figure 24.10 illustrates howthe transition to use of a substation automation systemmay be managed – of course, there are other possibilitiesdepending on the priority assigned by the asset-owner.

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Controlroom

RTU Sequence of eventsWall mimic

Marshalling cabinets

(a): Current situation3 cubicles/bay

Cubicles

.......... ..........

Protection 1

Protection 2

Auxiliaryrelaying

Auxiliaryrelaying

Protection 2

Protection 1

Controlroom

Sequence of events

Wall mimic


3 cubicles/bay

(b): Step 1: RTU Renovation (HW obsolescence & new SCADA protocol)

New RTU

Cubicles

.......... ..........

Protection 1

Protection 2

Auxiliaryrelaying

Auxiliaryrelaying

Protection 2

Protection 1

Figure 24.10: Upgrade path for an existing substation

Chap24 exe 20/06/02 15:18 Page 433

Examples of automation systems on order or installed aregiven in the following sections.

24.7.1 Industrial Network Automation Project

A large industrial network was significantly expandeddue to the addition of extra processing facilities. As partof the expansion, a new substation automation system

was installed, using an ALSTOM PSCN3020 substationautomation system. The simplified 33/11kV one-linediagram is shown in Figure 24.11. Total generationcapacity amounts to over 170MW. Not shown on thediagram is an extensive LV network and a number of3.3kV switchboards feeding motors.

The system has two features that make it unusual froma control point of view. Firstly, the generation within thesystem is distributed, and this results in the possibility ofseveral island networks being created in the event of amajor electrical incident, each of which are to be runindependently until such time as paralleling of theislands becomes possible. Secondly, the grid system isweak, so that import has to be limited to a maximum of40MW, even under transient disturbances such as thesimultaneous loss of two generators, each of over 30MWcapacity.

As a result of these requirements, the standard softwarewas enhanced to allow simultaneous control of up to 3autonomous islands within the overall network, eachisland having the full range of control facilities includingcircuit/device switching, active/reactive power control ofgenerators, voltage and frequency control of each islandand load shedding. Due to the restrictions on gridimport, a fast load shedding algorithm was developed, asstudies indicated that conventional under-frequency

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Figure 24.10 (cont): Upgrade path for an existingsubstation

Controlroom


New RTU

3 cubicles/bay

Cubicles

.......... ..........

Substation controlHMINew SOE

(c): Step 2: SOE Renovation and wall mimic change

Protection 1

Protection 2

Auxiliaryrelaying

Auxiliaryrelaying

Protection 2

Protection 1

Controlroom


New RTU

2 cubicles/bay

Cubicles

.......... ..........

Substation controlHMINew SOE

Protection 1

Protection 2

Auxiliaryrelaying

Protection 2

Protection 1

Bay computer

(d): Step 3: Progressive decentralisation and protection integration

Controlroom

2 cubicles/bay

Cubicles

.......... ..........

Substation controlHMI

Protection 2

Protection 1

Bay computer

Protection 2

Protection 1

Bay computer

(e): Step 4: Full decentralisation

132kV network

11kV

11kV

11kV

33kV

Figure 24.11: HV Single-line diagram: industrialsystem substation automation example

Chap24 exe 20/06/02 15:18 Page 434

load shedding did not provide the required performance.The fast load shedding scheme involves continuouscalculation of the amount of load to be shed in the eventof loss of one or more generators and/or the gridconnection, and determination of which loads should beshed, based on operator-set priorities and actual powerconsumption. In the event of generation loss, load thatis at least equal to the amount of lost generation isdisconnected immediately, after which a conventionalunder-frequency/grid import load shedding strategy isinvoked to cater for any further generation/loadimbalance occurring.

The substation automation configuration is shown inFigure 24.12, while a sample operator display capturedduring system testing is shown in Figure 24.13.

24.7.2 Utility Substation Automation Project

This project concerns a 345/138/20kV substation. Thesubstation consists of two 345kV lines, 2 x 345/138kVtransformers and 2 x 345/20kV/20kV transformers. Eachof the 345kV and 138kV busbars is of conventionaldouble-bus configuration, with bus couplers connecting

the main and reserve busbars. Each 345kV bus is splitinto 4 sections, with bus section CB’s linking thesections. Similarly, the 138kV busbars are split into 3sections. The 20kV busbar is also of double busconfiguration. An ALSTOM PSCN3020 substationautomation system has been installed to provide localand remote control and monitoring of the switchgear atall voltage levels. For the 138kV and 20kV busbars,monitoring is provided by MiCOM M301 MeasurementCentres, communicating with BM9100 or BM9200 BayModules using K-Bus proprietary communications link.Control is exercised directly from the Bay Modules.Protection relays are generally from ALSTOM’s K-seriesand EPAC range, also communicating with the BayModulus using K-Bus. However, line differential andtransformer differential relays are from anothermanufacturer, and communicate with the same BayModules using the IEC 60870-5-103 protocol, thusillustrating the use of Bay Modules to provide more thanone communications protocol. For the 345kV busbars,existing electromechanical-type relays were in use, andmonitoring of these is by use of contacts on the relayswired back to the Bay Modules.

Communication from the Master Station to the BayModules is by a dual-redundant fibre-optic ring (EFI.P).Time synchronisation uses a GPS interface to the MasterStation. Remote control/monitoring facilities areprovided, both from a Remote Control Room and aremote Network Control Centre. The latter uses theDNP3.0 protocol, so that the complete scheme uses 4different communication protocols.

Figure 24.14 illustrates the system architecture, whileFigures 24.15/16 show part of the 345kV and 138kVbusbars respectively.

24.7.3 Substation Control for an Electrified Railway

A high-speed (auto-transformer fed) railway has a routelength of 500km. A total of 8 traction supply substationsand 41 auto-transformer substations are required toprovide traction power and auxiliary supplies to the railline.

All of the forty-nine substations are interconnected bymeans of an Ethernet OPC fibre-optic network, formingthe communications spine of the system. Eachsubstation has a proprietary EFI.P fibre-optic ring(3.5Mbit/s) that interconnects the Bay Modules with thecommunications spine and local operator workstations.The ring is composed of dual fibre-optic cables in a singlesheath, thus providing two communications channels.Figure 24.17 illustrates the network involved.

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Figure 24.13: Sample operator display: industrialsystem substation automation example

Figure 24.12: System architecture: industrial systemsubstation automation example

Chap24 exe 20/06/02 15:19 Page 435

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Local control roomModem

K-bus

K-bus

K-bus

K-bus

K-series relays

K-series relays

K-series relays

MiCOMM301

MiCOMM301

MiCOMM301

K-series relaysMiCOMM301

K-bus

K-bus

K-series relays

K-series relaysMiCOMM301

MiCOMM301

K-bus

MiCOMM301

K-bus

MiCOMM301

K-bus

MiCOMM301

K-bus

MiCOMM301

EPACrelays

EPACrelays

K-bus

MiCOMM301

EPACrelays

Modem

Remote control room

Networkcontrolcentre

es: Cambuci 1 & 22 X BM9100

345/138kV Earthing transformers2 X BM9200

345kV: Bus section 11 X BM9100

345kV: Bus coupler2 X BM9100

20kV Busbar2 X BM9100

138kV Bus section 21 X BM9100

138kV Lines:Ipiranga 1 & 22 X BM9100

345kV: Bus coupler1 X BM9100

345kV: Reactor 11 X BM9100



345kV Line: Cabo Norte 1 & 2

K-bus

IEC 60870-5-103

MiCOM M301

K-bus

K-bus

K-series relays

K-seriesrelays

MiCOMM301

MiCOMM301

Relay PQ741

EFI.P Dual redundantFibre optic ring

345/138kV Transformers:Lado de Alta4 X BM9100

345/138kV Transformers: Lado de Baixa2 X BM9100

345/138kV Transformers

138kV Lines: Wilson 1 & 22 X BM9100

345/20/20kV Transformers2 X BM9100

Gateway

GPS

EOP-1EOP-2

Hot stand by

Data acquisition

HUB

Figure 24.14: System architecture: Utility substation automation project

Chap24 exe 20/06/02 15:19 Page 436

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Section 4B

138kV Busbar

138kVSection 3B

Section B

Earthing transformer 2

138kV LineWilson 2

345/88/138kVTransformer 3

Section ASection 3ASection 4A

138kV Line:Ipiranga 2(future)

345/88/138kV(future)

Transformer 2

138kV Line: Ipiranga 1(future)

138kV Line:Mariana 2(future)

138kV Line:Brigadeiro 2

(future)

Figure 24.16: Single line diagram: Utility substation 138kV busbar (part)

Line: Cabo Norte 1

Section 2C

Section 1C

2B

Transformer 2345/20/20kV

2A

Transformer 1345/88/138kV

Reactor 1

345/20/20kV

1B

Transformer 1

1A

345KV

Earthing transformers

Section D

(Future)

Section 2D

Section 1DSection C

345kV Busbar

Line: Cabo Norte 2

Figure 24.15: Single line diagram: Utility substation 345kV busbar (part)

Chap24 exe 20/06/02 15:19 Page 437

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The substation automation scheme used is the ALSTOMPSCN3020. Traction substations have an incomingsupply at either 225kV or 400kV, transformed down to± 27.5kV for traction and lower voltages for auxiliarysupplies. Redundancy in control and supervision isprovided through the operator at each substation beingable to view and control those substations immediatelyadjacent as well. There is an overall Control Centre tomonitor the complete system, using a Gateway on theEthernet spine. Approximately 500 Bay Modules areused, providing control and measurement facilities andalso acting as interfaces to the protection relays.

The significant aspect of this application is the distanceover which the automation scheme is applied using astandard substation automation scheme. The overall

length of 500km is large for a substation automationscheme and illustrates the geographical span nowpossible. Figure 24.18 shows the topology of thesubstation automation equipment at a tractionsubstation, while Figures 24.19-21 show the differentlevels of detail available to a substation controller via theHMI. Operator functions include control and monitoringof the substations, remote setting of all relays andautomatic retrieval of disturbance recordings from relaysfor remote analysis. Data is refreshed at approximately1 second intervals. A notable automation feature is theautomatic reconfiguration of the power distributionnetwork during faults or other outages to maintaincontinuity of traction power supplies.

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Figure 24.18: Configuration of a traction substation

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Figure 24.19: Overview of traction power supplies

Figure 24.20: Autotransformer one-line diagram

Chap24 exe 20/06/02 15:19 Page 440

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Figure 24.21: Incoming supplies at a traction substation

Chap24 exe 20/06/02 15:19 Page 441

Documents

24-Substation Control and Automation