Modeling an IEC61850 Based Substation Automation System

K. Yashwant Department of Electrical Engineering Indian Institute of Technology Madras

Chennai, India yashwant787@gmail.com

K. Shanti Swarup Department of Electrical Engineering Indian Institute of Technology Madras

Chennai, India swarup@ee.iitm.ac.in

Abstract— This paper describes the development of an IEC 61850 based substation automation system (SAS) architecture. IEC 61850 is a standard for the design of electrical SAS. It defines the communication between IEDs in the substation and the related system requirements. The GOOSE messaging, Substation Configuration description Language (SCL) makes IEC 61850 more reliable, and the bottlenecks faced like the one of interoperability have been addressed. In addition to lowering the installation and commissioning costs, a significant reduction in maintenance cost can also be achieved. It minimizes costs of technological obsolescence because of a global acceptance and adoption and future-proof concept of abstract services as well as independence of mapping to protocols. The above architecture is modeled using UML and XML. Lower equipment migration costs, reduced wiring, ease in integration and the ease in adding functionalities to the existing SAS makes IEC 61850 based SAS a widely accepted norm.

Keywords- Substation Automation System; IEC61850; GOOSE; SCL

I. SUBSTATION AUTOMATION SYSTEM (SAS) Substations are very important nodes in a power system

network. Each substation consists of large number of switchgear, which are monitored, controlled and protected by a Substation Automation System (SAS). The advancement in the protection relays, communication technology and automation industry have surely helped SAS gain momentum in becoming more reliable, stable and supporting many more sophisticated functions. Now protection relays have evolved as Intelligent Electronic Devices (IEDs) which have two or more processors and can perform protective functions along with control & measurement functions and can communicate with other IEDs. Advancement in communication technology has helped communication between the IEDs and other SAS equipment become more reliable and faster. Automation industry has helped SAS migrate from old hardwired relay logic to more sophisticated SCADA and Distributed Control systems.

The main system elements in a substation automation system are [1] Intelligent Electronic Devices (IEDs), Bay Module (or Bay Controller), Remote Terminal Unit (RTU), Human Machine Interface (HMI), communications bus or buses, linking the various devices and a link to a remote SCADA system.

A substation control/automation scheme has to support many functionalities and features [1], like, control and monitoring of all substation electrical equipment from a central point, interface to remote SCADA system, control of electrical equipment in a bay locally, monitoring of electrical equipment in a bay locally, status monitoring of automation equipment, system database management, energy management and condition monitoring of substation electrical equipment (switchgear, transformers, relays, etc). Figure 1 shows the architecture of a substation automation system which is based on IEC 61850 standard.

Figure 1: IEC 61850 based Substation Automation System

Substation automation gives the operator perfect control over the substation. Additionally, it improves the quality of electricity transmission and distribution in normal operation and also in disturbance situations and during substation maintenance. Substation Automation system enables on-line

monitoring and control of primary and secondary equipment in a substation.

The main objective of this paper is to understand the reasons for switching over to the IEC 61850 based substation automation system from the conventional systems and to understand some of the concepts which will help us in modeling an IEC 61850 based SAS. The paper is organized as follows. Section II describes the important reasons which are encouraging the utilities to implement the IEC 61850 based SAS. Section III tries to give the overview of the standard. Section IV deals with use of XML and UML in modeling a SAS. Substation configuration description language (SCL) and GOOSE (Generic object oriented substation event) messaging are described in sections V and VI. Conclusions are drawn in Section VII.

II. IEC 61850 BASED SUBSTATION AUTOMATION SYSTEM The advancement in the field of microprocessor-based

multifunctional Intelligent Electronic Devices (IEDs) helped in integrating more functionality into fewer devices which resulted in simpler designs with reduced wiring. IED based protection solutions have been successful because they offer substantial cost savings while fitting very well into pre-existing frameworks of relay application. IEDs’ communication capabilities made information be available at remotely places which resulted into fewer visits to the substation [1].

In terms of SCADA integration, the first of such systems were not very successful especially, in cases where the end-user got locked into a solution offered by a single vendor. As a result, the IEDs from other vendors could not be integrated easily, which invariably led to interoperability issues on the SCADA side. Owners of such systems also faced long-term support and maintenance issues. Hence, more customizable solutions were needed.

Early SCADA systems integrated information from generation stations and substations through remote terminal units (RTUs) to provide operators with system-wide knowledge. To gather system information these RTUs and SCADA systems typically used additional transducers and contacts that were separate from the protection systems. The information updates for these systems were not available at fast speeds. The information was available at the order of several seconds to minutes. On the other hand it was quite evident how IEDs were performing more system automation and control functions. Almost all the information required by system operators (and more) was available from these IEDs once they are networked together. Thus, the use of these substation IED networks reduced or eliminated additional transducers, input and output contacts, and even RTUs. More importantly they were also able to provide data at much faster speeds.

Hence to accommodate these new and increasingly popular IED network functions, substation communications infrastructure also experienced dramatic changes. Substation integration systems now were based on IED networks that were built using EIA-232 point-to-point and EIA-485 multi-drop communications ports within the IEDs. These ports communicated at a speed equal to or less than 38.4 kilobits per second (Kbps). The information exchanges were carried out

using register/address-based protocols such as DNP3, Modbus [2]. But as the data from substations was huge in size, it was clear that the networks now should be capable of carrying this huge amount of data comfortably. The need for instantaneous availability of information required much higher communication speeds. Hence, substation IED network communications now started migrating to Ethernet based communication networks and used protocols like Internet Protocol (IP), Transmission control protocol (TCP) and File transfer protocol (FTP) for information and file exchanges.

The above reasons led the Electric Power Research Institute (EPRI) in the US to work on a Utility Communications Architecture (UCA) in the early 1990s to develop a communications architecture that would facilitate the design of systems for protection, control, monitoring, and diagnostics in the substation. The goal was to produce industry consensus regarding substation integrated control, protection, and data acquisition, and to further reduce the amount of engineering and wiring required. This work developed to produce UCA2 which showed that true interoperability was possible. With many years of competing protocols and integration challenges, UCA2 was taken forward by International Electrotechnical Commission’s (IEC) Technical Committee 57 (TC 57: Power system control and associated communications) to produce the standard IEC61850 that revolutionized substation automation [3].

The IEC 61850 has been developed by representatives from utilities, suppliers and communication experts over a number of years who were focused on the development of a standard in which devices from all vendors could be connected together to share data, services, and functions. The key targets of the standard were to ensure interoperability of products from different suppliers, easy expansion of existing systems, and future-proof protocols & data structures which could be applied across a wide range of interconnection technologies, even as they developed further [4].

III. IEC 61850 – THE STANDARD IN A NUTSHELL IEC 61850: Communication networks and systems in

substations.

IEC 61850 is a standard for the design of electrical substation automation. It defines the communication between IEDs in the substation and the related system requirements. It supports all substation automation functions and their engineering. The standard defines a consistent methodology for interconnecting IEDs in substations using Ethernet LAN based technologies, a set of communication services, and applications built on a standardized set of plant and equipment models (logical nodes). IEC 61850 also supports the free allocation of functions to IEDs, and therefore supports different approaches in function integration, function distribution and substation automation architecture [5].

The standard contains an object-oriented data model that groups all data according to the common user functions in objects called Logical Nodes (LN). All related data attributes are contained and defined in these Logical Nodes. Access to all the data is provided in a standardized way by the services of the standard, which are defined to fulfill the performance

requirements. The data model and services of the standard are mapped to a mainstream communication stack consisting of Manufacturing Message Specification (MMS), TCP/IP and Ethernet with priority tagging.

Part 6 of the standard specifies a file format for describing communication related IED configurations and IED parameters, communication system configurations, switchyard (function) structures, and the relations between them. This part describes the system and configuration parameter exchange file format based on XML containing primary system schematic (single line) description, communication connection description and IED capabilities. To do so Substation Configuration description Language (SCL) is used. SCL is based on the Extensible Markup Language (XML) version 1.0.

Part 7 of the standard, which talks about the Basic communication structure for substation and feeder equipment, is divided into 4 parts. Part 7-1 provides the “conceptual point of view” to understand the basic modeling concepts and description methods. Part 7-2 gives the description of the Abstract communication service interface (ACSI), specification of the abstract communication services and the model of the device database structure. It also provides an abstract interface for fast and reliable system-wide event distribution between an application in one device and many remote applications in different devices and for transmission of sampled measured values (SMV). Part 7-3 specifies common attribute types and common data classes related to substation applications. Part 7-4 gives the definitions of logical node classes and data classes (logical node classes are composed of data classes). It also specifies the compatible logical node names and data names for communication between IEDs. In short this part specifies the information model of devices and functions related to substation applications.

IEC 61850 includes high-speed peer-to-peer communications using GOOSE messages (Generic Object Oriented Substation Event) and Sampled Analog Values (SAV) [9]. To implement these mechanisms, the hard wiring between the relays and between the control station and the breakers has been replaced by communication infrastructure. Figure 2 shows the important concepts of the IEC61850 standard.

Figure2: Important concepts of the IEC 61850 standard

IV. MODELING AN SAS USING UML AND XML Modeling of an IEC 61850 based Substation Automation

System is based on object oriented approach towards modeling. Various components in a substation automation system like primary and secondary equipment in a substation, IEDs, communication infrastructure are modeled using this approach.

Modeling of these components is done using object oriented methodology because of the fact that there exists numerous interconnections (can be physical connections or can also be in the form of information exchange channels) between the above mentioned components. Hierarchical functionalities among these equipment to provide protection and control functions add to the necessity of an object oriented approach to the modeling of such a SAS.

Modeling an IEC 61850 based SAS requires modeling of two types: one, the modeling of various interconnections between the components and hierarchical functionalities, and two, the modeling of the huge amount of data that is generated from various components such as (intelligent sensors, transducers, IEDs, event recorders, etc.). The first type of modeling employs Unified Modeling Language (UML), and the second type employs Extensible Markup Language (XML) for proper encoding of the data generated. Both UML and XML are specifications (set of rules) which will help us model the system in a standardized format.

UML is a modeling language that is widely used in the field of object oriented development of systems. UML is defined and maintained by Object management group (OMG). The latest formal release of UML is its version 2.3 (i.e. UML 2.3) which was released in May 2010. The UML models are represented using UML diagrams. These UML diagrams are broadly used to represent three different models of the system: Static view (structure diagrams), Dynamic view(behavior diagrams) and Interaction diagrams. These three models (views) are further divided into many other models like class diagrams, composite structure diagrams, sequence diagrams, activity diagrams, state machine diagrams, communication diagrams, timing diagrams etc. These different types of diagrams are used to represent data structures, device and operator interactions or any other automation and protection related processes. Various tools like Rational Rose, Umbrello, and ArgoUML are used to model the systems using UML. IEC 61850 standard in its various parts uses UML to represent in a standardized graphical way the complex models of multi-functional substation IEDs and their interface with the primary substation equipment and the communications network[11]. Figure 3 shows UML diagram of a SCL schema.

Figure3: UML diagram overview of a SCL schema [6]

The Substation configuration description language (SCL), which is used to model the IED and communication system, is

based on World Wide Web Consortium (W3C)'s XML schema. XML stands for eXtensible Markup Language. XML is a markup language which as mentioned before has been developed by W3C. It is an open format which got extended from SGML (Standard Generalized Markup Language) was started in 1996. XML is a markup language much like HTML. XML was mainly designed to carry data, not to display data (unlike HTML). The main strength of XML lies in the fact that the XML tags are not predefined. The user can define his own tags according the situation and the requirement. This makes XML more adaptive and flexible as we can define our own tags for our applications. In Part 6 of the IEC 61850 standard, the SCL is defined which is based on UML and XML.

V. USE OF SCL IN MODELING AN IEC61850 BASED SAS The XML based substation configuration description

language is defined in IEC 61850-6 [7]. The exchange of IED configuration data between different vendors is achieved by using SCL which is defined in IEC 61850 defined. Using this each vendor can use his private tools to export data into a standard format and can be integrated into a common product using standard tools as well. Interoperability, one of the major advantages of IEC61850 can be attributed to the use of SCL. SCL makes possible, in a standardized way, the functional and communication capability of devices, their configuration, a concrete communication of a substation automation system, and the allocation of devices to the substation primary equipment.

Figure4: Conceptual Modeling approach of an IEC 61850 based SAS [7]

The configuration of the entire substation and its IEDs is described in configuration description files. The IED capability description file (.icd) describes the capabilities and (optional) the preconfigured data model of the IED. IED Configuration

Tool and the libraries of the IEDs are used to generate .icd file. The System Specification Tool and the corresponding libraries are used to generate the system specification description file (.ssd) which describes the single line diagram of the substation with the associated logical nodes. Now, the .icd files and .ssd file are used in conjunction with System Configuration Tool to produce the substation configuration description file (.scd) which describes the complete substation configuration. Finally the .scd file is fed to the IED Configuration Tool to generate the configured IED description file (.cid) describing a instantiated IED with all configuration parameters relevant for that IED. This file includes the device specific configuration data. The use of the .cid file to configure the IED is optional All these configuration files are in XML format [8]. Figure 4 shows the conceptual view of the modeling of an IEC 61850 based substation automation system.

VI. GOOSE MESSAGING One of the very important features of the IEC 61850

standard includes two real-time, peer-to-peer communications methods which have been to particularly useful to protection engineers: Generic Substation Event (GSE) messaging and Sampled Values (SV) messaging. The two types of GSE messages, Generic Object Oriented Substation Event (GOOSE) and Generic Substation State Event (GSSE), can coexist but are not compatible. GSSE is also known as UCA GOOSE which is an older, binary-only message type (for binary event status data), but all new systems use the more flexible GOOSE, which conveys binary, analog and integer data.

GOOSE and GSSE messages use multicast services that allow simultaneous delivery of the same substation event message to multiple IEDs. The GOOSE message can serve several different applications that each have different performance requirements. IEC 61850 classifies application types based on how fast the messages are required to be transmitted among networked IEDs. The standard also specifies the performance of each type of application, documented as time duration of message transmission. For example: Type 1A corresponds to “Fast messages (Trip)” which have the required transmission time of 10ms for “P1” performance class and 3ms for “P2/P3” performance class. Likewise, the other types of messages are: Fast messages (other), Medium Speed, Low Speed, Raw data, File transfer and Time synchronization [7]. To ensure highest level of reliability, GOOSE messages are repeated as long as the state persists. To maximize dependability and security, GOOSE message has a “hold time“ parameter, which defines the time for which the message will live and afterwards will expire unless the same status message is repeated or a new message is received prior to the expiration of the hold time. The repeat time for the initial GOOSE message is short and the repeat times and hold times can be increased for the subsequent messages until the maximum is reached. Normally, all devices sending GOOSE messages shall continue to send the message with a long cycle time, even if no status/value change has occurred. This ensures that devices which have been activated recently will know the current status values of their peer devices. GOOSE message definition and the parameters in a

GOOSE message have been clearly standardized in IEC 61850 standard. The standard also gives the overview of the classes and services of the GOOSE model, also specifies a retransmission scheme to achieve a highly dependable level of message delivery.

VII. CONCLUSION The advantages of using IEC 61850 over the legacy

protocols help in achieving the Substation Automation System with utmost performance at considerably lower costs. The communication system for the SAS which helps IEDs and other equipment in the substation has been standardized and helps in configuring the SAS with much ease. The GOOSE messaging, SCL adds to the advantages of IEC 61850 being more reliable, and the previous bottlenecks faced like the one of interoperability have been addressed quite remarkably. In addition to lowering the installation and commissioning costs, a significant reduction in maintenance cost can also be achieved. These and many other advantages will make the acceptance, design and implementation of IEC 61850 based SAS continue at a much higher pace.

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