Control and Automation of Power System Substation using IEC61850 Communication

T.S. Sidhu, Fellow, IEEE, Pradeep K Gangadharan, Student Member, IEEE

Abstract-- Automation of control and management of apower system substation is becoming more and more popular.Overcoming the initial hiccups in its acceptance due to highercosts and apprehensions on reliability, more utilities and industries are keen in exploring this option now. In the scenarioof high pressure to improve the efficiency and productivity ofthe power system substation, substation automation is proving to be a cost-effective solution.

With the modern protection relays becoming more powerful,being integrated with more functions, they have the potential to play a key role in implementing control & automation in substations. Finalization of the new communication protocol,IEC61850, for Intelligent Electronic Devices (IEDs), automation and control has become possible, which, when implemented canensure a reliable and efficient power system

This paper discusses the scope of the new communicationprotocol for substation IEDs, IEC61850 and explains theimplementation of few control and automation applications presented in the standard.

Keywords: Substation control and automation, Intelligent Electronic devices, Substation communication, IEC61850, Peer-to-peer communication, GOOSE messages

I. INTRODUCTION

ontrol and automation of power system substation has undergone dramatic changes since the introduction of

powerful micro-processing and digital communication insubstations. Smart, multi-functional and communicativerelays, more popularly called as IEDs (Intelligent Electronic Devices) have replaced traditional panels with dedicated stand-alone relays, meters, control switches, mechanicalstatus indicators and annunciators. IEDs are proving to be a vital link in control and automation of power systems due totheir strategic location and reliability. With increasing effortsto bring the reliability of the other components of the control and automation system, like communication links, switchesalso to the level of the IEDs [4], complete elimination ofhardwired systems is appearing to be a possibility.

Though the primary purpose of most of these IEDs is toprotect the power system equipment against damages duringfaults, their location in the power system has made it an idealdevice to implement control and automation systems as well.With the introduction of microprocessor based

The authors are presently with the University Of Western Ontario, London,Canada

communicable IEDs for protection, including logicalalgorithms and networking multiple relays has becomeeasier. This is found to be a cost-effective solution as compared to providing dedicated devices for control andautomation, which is duplicating the resources in a substation.

Use of communication in substations has been in voguefor more than two decades now. From the time when communication was used only to collect data offline we have come a long way and today we see that much more vital real-time functions are being realized through communication.One major deterrent for the use of communication insubstation automation and control has been the absence of a common communication protocol that was designed for thisapplication. Though many utilities have been using communicable IEDs and interlinking them, it requires huge investments to engineer such systems and maintain them.Experts say that around US$82 billion was spent onapplication integration in 1998, which amounted to 40% of the corporate IT budgets (Forrester 1999)[3]. Added to that,the utilities were ending up with technical bottlenecks ininterlinking devices from various manufacturers, who offered devices with different communication protocols.Inter-operability was a major issue. Also, addition ofmultiple protocol converters and absence of specific standards on the performance requirements of suchcommunication protocols and systems, resulted in the userfinally ending up with a system much different from whatwas originally intended.

IEC started work on developing a common standard forsubstation communication in 1994. At the same time IEEEstarted a similar work on developing a commoncommunication protocol called UCA. In 1997 both IEEE and IEC agreed to work together and develop a commonstandard for substation communication, IEC61850[5]. Thisstandardization process involved leading product/systemmanufacturers and also major utilities. The primary objectiveof the group was to develop a communication protocol for substation communication, which will ensure,

Interoperability: The ability for the IEDs from one orseveral manufacturers to exchange information and use the information for their own functions.Free configuration: The standard shall supportdifferent philosophies and allow free allocation of

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Proceedings of the2005 IEEE Conference on Control ApplicationsToronto, Canada, August 28-31, 2005

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functions.Long term stability: The standard shall be future proof, i.e., it must be able to follow the progress incommunication technology as well as evolving systemrequirements

The IEC 61850 standard is now available and majormanufacturers have started offering IEDs with this new protocol. Implementation of this protocol is expected tosolve many implementation bottlenecks faced till now andalso reduce cost. More and more applications, which were till now realized using hardwired logics will henceforth bepossible using the IEDs employed with this communicationprotocol. This paper introduces the new communicationprotocol IEC61850 and goes on to explain theimplementation of few substation control and automationfunctions using this protocol.

II. IEC61850 STANDARD

A. Description of IEC61850 standardThe standard that defines the new IEC61850 protocol is

divided into 10 parts as shown in figure 1

Fig. 1: IEC61850 standard parts

Part1: This part provides an introduction and overview tothe IEC 61950 standard.

Part2: Contains the glossary of the terminology and definitions used in the context of substation automationsystem in the different parts of the standard.

Part3: Gives the general requirements of thecommunication network with emphasis on their qualityrequirements. It also specifies the environmental operatingconditions to which the communication network devicesshould conform, to ensure reliable operation.

Part4: Pertains to the system and project management

with respect to, engineering process and its supporting tools,life cycle of IEDs and overall system, and quality assurance.

Part5: This part defines the performance requirement of different functions being implemented using communication.All known functions are included. This part is the basis on which the architecture of the communication network and the applications that can be implemented for a given networkare to be decided.

Part6: Specifies a file format for describingcommunication related IED configurations and IED parameters, communication system configurations,switchyard (function) structures, and the relations betweenthem. The main purpose of this format is to exchange IEDcapability descriptions and SA system descriptions, betweenIED engineering tools and the system engineering tool(s) ofdifferent manufacturers in a compatible way. The definedlanguage is called Substation Configuration descriptionLanguage (SCL). The configuration language is based on theExtensible Markup Language (XML) version 1.0.

Part 7-1: The purpose of this part of the IEC 61850 standard is to provide “ from a conceptual point of view “assistance to understand the basic modeling concepts and description methods.

Part 7-2: Applies to the ACSI (Abstract CommunicationService Interface) communication in substations and feeder applications. The ACSI provides.a) Abstract interface describing communications between

a client and a remote server. b) Abstract interface for fast and reliable system-wide

event distribution between an application in one deviceand many remote applications in different devices and for transmission of sampled measured values.

Part 7-3: Specifies common attribute types and commondata classes related to substation applications. This standardis applicable to the description of device models andfunctions of substations and feeder equipment.

Part 7-4: This part specifies the information model ofdevices and functions related to substation applications. Inparticular, it specifies the compatible logical node names anddata names for communication between IEDs. This includesthe relationship between Logical Nodes and Data. The names defined in this document are used to build thehierarchical object references applied for communicatingwith IEDs in substations and on distribution feeders. The naming conventions of IEC 61850-7-2 are applied in thispart.

Part 8-1: This part specifies a method of exchangingtime-critical and non-time-critical data through local-area networks by mapping ACSI to MMS and ISO/IEC 8802-3 frames.

Part 9-1: This part of IEC 61850 specifies the mappingsfor the communication between bay and process level and it

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specifies a mapping on a serial unidirectional multi-droppoint to point link in accordance with IEC 60044-8.

Part 9-2: Defines the Specific Communication Service Mapping (SCSM) for the transmission of sampled valuesaccording to the abstract specification in IEC 61850-7-2. The mapping is that of the abstract model on a mixed stackusing direct access to an ISO/IEC 8802-3 link for the transmission of the samples in combination with IEC 61850-8-1.

Part 10: To specify the procedure for conformancetesting of products implemented with this communicationprotocol.

B. Approach of IEC61850To meet the basic requirements of the standardization

process, that is interoperability and to be future proof, theIEC61850 standard is built over a standard OSI 7 layermodel. The data services and applications related to thepower system substation are built above the 7th layer (application) of the OSI model. This ensures that thesubstation communication can evolve with the evolution of communication technology using its strength. Figure 2 below shows this approach to the standardization process.

Fig. 2: IEC61850 approach to standardization

The data models are divided into logical groups calleddevices, nodes, classes and data.

Each functional element is defined as a logical node. Aphysical device (IED) can house multiple logical nodes in it.Each logical node is a collection of standard data classes.The possible values that can be assigned to the data classes are called as data. Figure 3 pictorially represents the physicaldevice, logical nodes, data classes and data.

Fig. 3: Organization of Logical device, logical nodes, data classes and data

Every control or automation function (infact anyfunction) can be broken down to a collection of differentlogical nodes. These logical nodes can be housed in a singleIED or distributed among multiple IEDs. All the different logical nodes of a specific application are interconnectedusing logical connections. These logical connections can be over a single or multiple physical connections.

Fig. 4: Building functions from multiple logical nodes [1]

Figure 4 illustrates how functions are realized usinglogical nodes and logical connections. In this example twofunction, F1 and F2 are shown. Function F1 is split into 5logical nodes (LN1 to LN5). Function F2 is split into three logical nodes LN3, LN5 and LN6. These logical nodes are housed in three different physical devices (IEDs), PD1, PD2and PD3. The logical node LN0 is the node carrying theidentification of the physical device. The logical connectionsbetween the logical nodes are marked as LC and the physicalconnections between the physical devices are PC.

C. Data flow among the logical nodes Depending on the application, the logical data flow

among the different logical nodes and their performancerequirement may vary. The data flow can be broadlyclassified into one of the following types;

Polling – When a logical node from the client requeststhe logical node from the server for data transfer atperiodic intervals is called polling. A typical examplewould be when a master station polls for metering

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values at periodic intervals from an IED. The advantage of this method is that the traffic on the network is fixedand also multiple clients can get the data. However thistechnique has the limitation of not being able to cater totime critical applications and also events occurring between two consecutive polling can be lost.Unbuffered reporting – In this technique a logicalnode housed in the server sends data to one or multiplelogical nodes. The data that is being transmitted is not buffered in the server. Thus in the event of momentarycommunication interruption all events occurring at that time is lost. However this technique can be used fortime critical applications. Buffered reporting – This is similar to the above case with the exception that the server buffers data for a limited time. Thus the chances of missing events duringmomentary interruption in communication are remote.Log – This type of data transfer occurs when the servers send data on the occurrence of events and the clientsstore them in a sequential order. Peer to peer data value publishing – This is a bi-directional data transfer that occurs between logical nodes. The initiation of the data transfer depends on the application that could be triggered on satisfying any pre-defined condition. Exchange of generic object orientedsubstation events (GOOSE), or transfer of raw sampledata from instrument transformer to IEDs are examplesof this type of data transfer.

Fig. 5: Interface between IEC61850 object model and OSI stack

To meet the service and performance requirements of thedifferent types of data transfer, different data models aremapped to the communication services as shown in figure 5.Time critical data like GOOSE messages and raw sampledata are sent directly to the data link layer. To increasereliability, the data transmission is repeated. Also for somespecific applications like protection tripping, VLAN servicesof data link and physical layers are used.

III. COMMUNICATION IN POWER SYSTEM SUBSTATION

Communication in power systems is used in manyapplications. These applications can be broadly classifiedinto two, based on geographical spread as:

Inter substation: Applications involving exchange ofdata between substations over a wide area network(WAN) is classified under this categoryIntra substation: Applications involving exchange ofdata within a substation over a local area network(LAN) is classified under this category

Based on the time criticality of the data transfer, the IEC 61850 classifies the messages into 5 types

Type 1: Fast messages - This type of messagetypically contains a simple binary code containingdata or command. The receiving IED will act immediately on receipt of these messages. Examplesof this type of message are, trip, close, start, block,etc. The total transmission time of these messagescan be anywhere between 3 to 100 ms depending on the application.Type 2: Medium speed messages - In this type of message, time at which the message originated isimportant but the transmission times are relativelyless critical. These messages are time tagged usingthe IEDs local clock. Example for this type of message is the rms value of measured signals.Typically the total transmission time in this case shallbe less than 100ms.Type 3: Low speed messages - Complex messageswhich are time tagged and which require atransmission time of less than 500ms are classifiedunder this type. Data like event recording, slow speed auto control functions are examples of this type.Type 4: Raw data - Sampled raw data from theinstrument transformers (CTs and PTs) are classifiedunder this type. The data in this case is a continuousstream of digitized data.Type 5: File transfer functions - This type ofmessage is used to transfer large files of data forrecording and information purpose. Setting files,disturbance record files are examples of this type of messages.Type 6: Time synchronization messages - This typeof message is used to synchronize the internal clocksof different IEDs that are networked together.

Most of the control and automation functions are implemented within a substation using fast, medium speed orlow speed messages. In some applications inter substationcommunication may also be used. The feasibility of applyingany control or automation function using communication and IED depends primarily on the service capabilities of the

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network available. The following section explains theimplementation of control and automation applications usingIEC61850.

IV. APPLICATIONS USING IEC 61850 The formulation of a common standard for

communication in power systems has created a potential toimplement many automation and control functions usingIEDs. Some of the new algorithms proposed [2], [6] can alsobe adapted to work in this scenario. Implementation of twoexamples presented in the IEC61850 standard [1] isexplained here.

The two applications discussed in this paper are;Automatic voltage regulationPoint-on-wave switching control

These control applications are built around the transformer in the substation. Figure 6 shows a powertransformer along with the breaker, CT, PT breaker andOLTC. This transformer is protected by a single IED withmultiple protection and control functions. Layout of thelogical nodes, physical device and interconnections for a transformer bay is shown in Figure6.

Fig. 6: Transformer bay IED with logical nodes

A. Voltage regulationThe target is to maintain the voltage at the LV bus at the

rated value. This is an implementation of an automaticcontrol task. The position of the OLTC (on-load tapchanger) is calculated based on the voltage at the load bus. To make it more adaptive the present amount of load canalso be taken into consideration.

Fig. 7: Logical nodes interconnection for voltage regulation application [1]

The logical nodes involved in achieving this function are shown in figure 7. The node “IHMI” is physically present inthe master PC of the operator. This node is assigned in thisapplication to provide the status information to the operator.The logical nodes PTOV (over-voltage protection), PTUV(under-voltage protection), PIOC (over-current protection),ATCC (automatic tap change control), MMXU (measuringprocess) are present inside the IED. The logical nodes TVTR1 and TVTR2 (voltage transformers), TCTR (currenttransformer), XCBR (circuit breaker), YLTC (tap changer)are also housed in the IED. However it is possible to have had these nodes external to the main IED when theconcerned primary equipment has communication capability.The connection between the logical nodes is implementedusing logical connections.

When the bus voltage goes below (or above) a thresholdset in the ATCC logical node, it decides to increase (ordecrease) the tap. This decision also depends on the status of the protection elements (they should not have operated) and the power flow. It also calculates the number of taps to beraised (or lowered). This information is then given to theYLTC through the logical connection. The YLTC node inturn gives pulses to the OLTC control motor to change thetap position. The YLTC returns the present position of thetap to the ATCC node. For implementing this application a low speed data transfer is sufficient (500ms).

B. Point on wave switching controlTo minimize the stress on the breaker while closing it, the

breaker poles have to be closed when the voltage across itscontacts are minimum. This is done by measuring thevoltages at either side of the breaker and issuing closecommands at the appropriate time to the circuit breaker.Similarly, to reduce the stress when opening a circuitbreaker, the current signal is monitored and the point on thecurrent waveform that will give the minimum stress iscalculated. The logical nodes used to implement the point-on-wave switching is shown in figure 8.

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Fig. 8: Logical nodes interconnection for point-on-wave switching function [1]

Figure 9 shows the sequence of steps involved inexecuting the point-on-wave switching function. The processis started by a command from the IHMI node to perform apoint-on-wave switching (either opening or closing). Thefirst action is to select the appropriate signals namelyTVTR1 & 2 for closing and TCTR for opening. Then onreceipt of the confirmation of the signal selection, the execute command is sent from the IHMI node to the CPOWnode. The CPOW node then calculates the point-on-wave onthe voltage or current waveform, which will result inminimum stress. This point is then converted to anappropriate switching instance. The CPOW node thenreleases the close command to the XCBR node at the right instant. Depending on whether the breaker is equipped withthree pole or single pole operation, a single or threeindividual commands are given. The XCBR node thenperforms the switching and reports back the status. For thisapplication the requirement of the communication network isdifferent for different links. The logical link between theXCBR and CPOW nodes should have ability to perform fastmessage transfer. The other communication links can have medium or low message transfer capabilities.

Fig.9: Sequence of execution of the point-on-wave switching application [1]

V. PHYSICAL MEDIUM AND ISSUES

The data transfer capability of a communication networkdepends on the amount of traffic, bandwidth of the link, typeof switching device used (hub, bridge, switch, router, etc),the type of configuration (star, multi-drop, etc) and the typeof physical connection (twisted pair copper, optic fibre, etc).Reference [4] discusses this in detail.

VI. CONCLUSION

Combined protection, monitoring and control devices and LAN based integrated substation automation systems are setto become more and more popular. Modern communicationtechnologies including the internet are used for remotemonitoring, setting, control and retrieval of load and faultdata. Higher performance at lower cost has resulted in a fast acceptance of the new technology. With the finalization of a common communication standard for substation IEDs, mostof the bottlenecks faced till now have been overcome.

In future the trend of system integration will continue at amuch higher pace, given that IED based substation controland automation will be a cost effective solution with theavailability of a common communication protocolIEC61850. In addition to lowering the installation and commissioning costs, a significant reduction in maintenancecost can also be achieved.

VII. REFERENCES

[1] IEC61850, “Communication networks and systems in substation”,Parts 1 to 9.

[2] IEEE PSRC working group H5 report to the CommunicationsSubcommittee, “Application of peer-to-peer communications forprotective relaying”

[3] Karlheinz Schwarz, “Standard IEC 61850 for substation automation and other power system applications”, Power Systems and Communications Infrastructures for the future, Beijing, September2002

[4] Marzio P. Pozzuoli, “Ethernet in substation automation applications – issues and requirements “

[5] Klaus Peter Brand, “IEC61850 tutorial”, CIGRE September 2003.[6] E. Demeter, T.S. Sidhu, S.O. Faried, “An Open System Approach to

Power System Protection and Control Integration”, accepted forpublishing in the IEEE Trans. Power Delivery.

VIII. BIOGRAPHIES

Tarlochan S. Sidhu (SM’94-F’04) is Chair of the Department of Electricaland Computer Engineering and Professor and Hydro One Chair in PowerSystems Engineering at the University of Western Ontario, London, ON,Canada. He is a Fellow of the IEEE, Fellow of the Institution of ElectricalEngineers of the U.K., Fellow of the Institution of Engineers, India, a Professional Engineer registered in the Province of Ontario and a CharteredEngineer in the U.K.

Pradeep Kumar Gangadharan (S’04) is currently pursuing his PhD degree at the University of Western Ontario, London, Canada. From 1995 to 2003, he worked in various capacities at the Energy Automation andInformation business of ALSTOM (presently AREVA).

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