IEC GOOSE Communication and Its Impact on Substation Performance

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1 IEC GOOSE Communication and Its Impact on Substation Performance Sunil Gupta Department of Electrical & Electronics Engineering, Maharaja Surajmal Institute of Technology, Delhi, India Abstract: IEC standard proposes a very high speed and reliable peer-to-peer communication mechanism over Ethernet LAN called Generic Object Oriented Substation Event (GOOSE). The paper investigates GOOSE message model and identifies the performance advantages associated with it for implementing substation protection schemes. This paper concentrates on major technical features inherited in GOOSE message service mechanism, such as publisher/subscriber communication model, multicasting, and retransmission mechanism for improved transmission reliability. Moreover, the QoS features of switched Ethernet to enhance GOOSE message transmission time performance are explored in depth. GOOSE communication has a huge potential in designing new and improved all-digital distributed substation protection applications. Keywords: Substation automation system, IEC 61850, IEC GOOSE, Switched Ethernet, Communication. I. INTRODUCTION In traditional substation automation systems, point-topoint copper cables connections are utilized for all interfaces between the process level high voltage switchgear and bay level protection & control devices. Thereby, these substations require more time, efforts and huge investments in copper cables with different lengths, and sizes that need to be designed, installed, commissioned, tested, maintained and configured manually. In addition to that, the protective system has limited access to the process level signals and hence requires their own instrument transformers as CTs/PTs outputs cannot be shared with several devices. In IEC Substation Automation Systems (SAS), IEC standard proposes a very high speed and reliable peer-to-peer communication mechanism over Ethernet LAN called Generic Object Oriented Substation Event (GOOSE) [1]. GOOSE has replaced the conventional hardwired interfaces at the station level with fiber optic based standardized Ethernet link for the realtime exchange of substation events among Intelligent Electronic Devices (IEDs) [2]. Thus, GOOSE communication has a huge potential in designing new and improved distributed substation protection applications. Protective functions are performed at the bay level, which involves the Sampled Values (SVs) messages exchange from Merging Units (MUs) to protection & control IEDs II. IEC GOOSE COMMUNICATION and GOOSE messages exchange among substation IEDs. However, GOOSE and SVs messages, over the substation LAN, are mission sensitive that must be highly reliable and possess real time performance characteristic, as per IEC standard, even under network congestion scenarios [3]. Thus process bus communication network is critical for the transmission of GOOSE and SVs in realizing all-digital substation automation applications. GOOSE is an event triggered type message that is sent when certain event like state change occurs; whereas, SV is time triggered and is sent as a continuous stream at a specified sampling rate. Since, the sampling frequency is very high (4000Hz for 50 Hz power system), the transmission of SVs over the digital process bus network results in a large amount of data, and hence occupies a significant portion of the communication channel bandwidth [4-5]. Thus, the transmission of mission sensitive GOOSE messages should not be affected by the presence of huge SVs and other (client/server) network traffic flows over the process bus network. Therefore, it is important to explore and examine the GOOSE message service mechanism as well as the communication technologies/parameters that have a direct impact on GOOSE message performance [6]. The paper investigates GOOSE message model and identifies the performance advantages associated with it for implementing substation protection schemes. This paper concentrates on major technical features inherited in GOOSE message service mechanism, such as publisher/subscriber communication model, multicasting, and retransmission mechanism for improved transmission reliability. Moreover, the QoS features of switched Ethernet to enhance GOOSE message transmission time performance are explored in depth. The rest of the paper is organised as follows: IEC GOOSE communication; its major features and challenges are discussed in Section II. The impact of QoS features to enhance GOOSE communication is described in section III of the paper. Section IV discusses the major impact of GOOSE on IEC substation performance. Finally, concluding remarks are provided in section V of the paper. 26

2 Fig. 1. IEC Substation Concept Fig. 1 illustrates the basic concept of the GOOSE communication. IEC defines a Generic Substation Event (GSE) service based on the concept of an autonomous decentralization method which supports multicasting [7]. Two classes of messages are defined in IEC : GSSE (Generic Substation State Event) - is an older message type which provides the capability to convey only binary event state change information. GOOSE (Generic Object Oriented Substation Event) - is more flexible that supports the exchange of a wide range of possible common data (binary, analog and integer) organized by a Data-Set. Thus, the major difference between the GOOSE service and the GSSE service is the kind of information to be exchanged. The GOOSE service provides a flexible means to specify which information is to be exchanged; while the GSSE service only supports a simple list of binary status information. The detailed description regarding the abstract data classes and services of the GOOSE control model are presented in the standard [7]. The common components that facilitate the IEC GOOSE class object are shown in Fig. 2. Fig. 2. GOOSE Control Block Model (Gocb) Model A. Publisher-Subscriber Model in GOOSE Communication IEC GOOSE message exchange in the communication network is based on publisher/subscriber mechanism. The publisher/subscriber communication model, as shown in Fig. 3, is meant for the fast and reliable transmission of event data to multiple receivers [8-9]. The publisher writes the value in the local buffer at the sending side; the receiver reads the values from a local buffer at the receiving side as shown in Figure 1. Specific Communication Service Mapping (SCSM) services of the communication system are responsible to update the local buffers of the subscribers automatically. Values received in the reception buffer at the subscriber side are forwarded to the relevant applications. The GOOSE message communication is based on the Multicast Application Association Class (MCAA). Multicasting is an efficient way of data communication which enables simultaneous delivery of the same event information from one source (publisher) to one or many devices (subscribers) in its peer group. 27

3 Fig. 3. IEC Communication Models [2] Thus, the multicast nature of GOOSE allows the publisher IED to send a single copy of the data such as analogue value or state change to several subscribed IEDs at the same time. Thereby, it reduces the traffic load over the network and hence achieves high network performance. However, the transmission reliability is reduced as this publisher/subscriber model is connectionless in contrast to connection oriented client/server communication model. The GOOSE message transmission reliability is enhanced by retransmission mechanism as described in next subsection, and hence, they need not to be acknowledged. Further, publisher/subscriber communication mechanism usually addresses Quality of Service (QoS) features of switched Ethernet for the guaranteed delivery of real-time messages over the Substation Communication Network (SCN) even in network congestion scenarios. However, the transmission time depends on the communication network situations such as, data rate, network traffic load, and also on the IED s local information processing abilities. B. Retransmission Mechanism of GOOSE To ensure that the GOOSE message is received by the subscribed IEDs in a specified time interval, as demanded by application, IEC [1-2] specifies a retransmission scheme that repeatedly broadcasts GOOSE message over the station LAN. After initiation of any status change, GOOSE messages are published repeatedly. At the time of configuration, parameter called maximum time is set in each GOOSE message to wait between message publication, and the name of the data set to include in the message. A GOOSE message is published each time on the network when either the maximum time expires or any data set value changes. After a change in the data set elements i.e. some event 28 takes place, GOOSE messages are sent repeatedly with incremental periods, to make sure that all subscriber IEDs will receive them across non deterministic Ethernet. Thus, GOOSE has a mechanism to ensure that each message reaches to its destination without loss, and any communication failure is reported to substation operator immediately for some necessary corrective actions. However, depending on the choice of final stable retransmission time, it may not be sufficient to guarantee the reliability of time-critical tasks [10]. C. Multicast Protection Messages Over Process Bus Network The standard defines different types of communication services like client/server, GOOSE, SVs depending on their performance requirements. The message transmission time depends on the type of the message and the application performance class. IEC specifies a strict time performance requirements for the delivery of these IEC messages, including the time critical GOOSE and SVs [5]. GOOSE (Type 1 A Trip messages) and SVs (Type 4 raw data messages ) for P2 and P3 applications must have a total transmission time below 3 ms. Process bus communication network is critical for the transmission of mission critical messages i.e., GOOSE and SVs in all-digital substation automation applications. This transmission time involved the time taken to transfer the message from the publishers (MUs/NCITs at process level) to the subscribers (protection and control (P&C) IEDs at bay level). As discussed, this must meet the acceptable maximum delay performance requirements under any network operating conditions, i.e., the transmission of these mission sensitive messages should not be affected by the presence of other (client/server) network traffic flows over the process bus network. Thus, the GOOSE and SVs messages have high priority, reliable, and safe transmission needs, and the benefits will not be realized if the performance of process bus network is inadequate [11]. To support this, IEC follows a communication approach in which the abstract data models and services are mapped to real communication protocols such as MMS, TCP/IP, and Ethernet [12-14]. Since, GOOSE and SVs have real transmission needs. To reduce the additional overhead caused by TCP/IP layers, GOOSE and SVs messages are directly mapped on the Ethernet link layer of ISO/OSI based seven layers communication stack. However, this elimination of TCP/IP layers reduces the reliability of packet transmission. Therefore, to enhance the transmission reliability of GOOSE, the same

4 GOOSE message is repeated several times according to the IEC standard; whereas, the same SV is not repeated as they are time triggered and transmitted at sampling frequency rate (80 samples per cycle for protection applications); which reduces the transmission reliability of SVs over the process bus network [6]. Since, the time critical messages are directly mapped to data link layer 2, all IEDs and MUs should support Ethernet data link layer-2 multicasting capability. This can be achieved through Ethernet MAC (Media Access Control) source and destination addresses, which are defined in the Ethernet packet frame. Multicasting enables simultaneous delivery of the same information from the publisher to the several subscribed devices in its peer group, i.e., the same message need not be repeated several times to different locations. Thus, multicasting nature of GOOSE and SVs brings a significant reduction in the network traffic. III. SWITCHED ETHERNET TECHNOLOGY FEATURES Multicast GOOSE and especially the time-triggered Sampled Values (SVs) messages (transmitted at 4000Hz for 50 Hz power system) increase network traffic on the SCN. Therefore, real time performance of process bus communication network is vital for the transmission of SV data from the conventional or NCITs to the bay level IEDs, and for the transmission of trip commands to the circuit breaker IEDs at process level by high speed GOOSE messages according to IEC To reduce the network traffic burden of multicast messages, network traffic management is critical in a process bus environment. Thus, to improve security and the performance of protection over the substation LAN, GOOSE messages are configured to use some of the advanced QoS features of the switched Ethernet technology like priority tagging (IEEE 802.1P), VLAN (IEEE 802.1Q), RSTP, and multicast addresses filtering of the Ethernet frames. Full duplex and collision free environment allow devices connected to the switch to simultaneously send and receive data without suffering collision. A 4 byte extension to the standard Ethernet frame header is defined by IEEE 802.1q standard that includes 12 bit field for VLAN identifier and a 3 bit priority tagging field for traffic classification. VLAN and multicast filtering allows the prioritization and segregation of network traffic, and also reduce the processing workload of IEDs as they have to process only those multicast frames they actually needed. The 3 bit user priority field allows total eight classes of services. This field can be set between 0 (i.e. lowest priority) and 7 (i.e. highest priority) to set the 29 priority for different types of data flow in a network. Time critical protection and control data such as GOOSE are set with highest priority whereas client/server based file transfer applications i.e. measuring and recording event data are set with low priority [15-22]. Thus, VLAN, priority tagging, RSTP and full duplex are some of the key enabling technologies that allow the efficient use of available network bandwidth and minimize the several delays thereby; increase the availability of GOOSE based applications. Although these features are applied for the GOOSE and SVs transmission, but they do not ensure the deterministic communication delays and packet loss on the network during worst case conditions [6], [23]. IV. MAJOR IMPACT OF GOOSE COMMUNICATION ON SUBSTATION PERFORMANCE IEC standard supports peer-to-peer communication services like GOOSE and SVs for real time data transfer instead of master-slave model for communication in legacy protocols. The GOOSE communication offers various benefits over conventional hardwired analog circuits in implementing substation control and protection applications. GOOSE replaces a complex network of multiple hardwired copper connections with simpler and lighter few fiber optic based Ethernet communication network at the station level for inter-ied communication. Trip, blocking, status or any logical signals can be transferred through the communication network using GOOSE. It brings reduction in time, cost and efforts for designing control and protection applications in substations such as interlocking, reclosing, load shedding and breaker failure protection etc. over conventional hardwired schemes. Fast signaling response and multicasting nature of GOOSE improves the reliability, accuracy and performance of control and protection functions of SAS by satisfying the real time performance requirement of these applications for which the transfer time should not exceed 3ms. In the same way, GOOSE service can also be used for testing the performance of IEC based protective relays and the power system applications. In GOOSE based system, several virtual inputs /outputs are available through software configuration which enables new and improved functionalities to be added in substations in near future, at lower installation, commissioning, maintenance and testing cost as compared to the costly and manually configured complicated hardwired schemes.

5 Monitoring the status of hardwired connections in traditional hardware based schemes is a crucial and difficult task. Whereas, GOOSE enabled communicationbased schemes enhances the supervision of time critical protection and control messages along with virtual connections, for any abnormalities, in the system. If case of any failure of communication in any device or in communication path, it can be rectified automatically or reported to substation operator for necessary action. Thereby, it supports the self healing capability in SAS. Thus, the inherent features in GOOSE communication service mechanism provides an opportunity to design new and innovative distributed protection and control applications in IEC substations. 30 V. CONCLUSION A very high speed peer-to-peer GOOSE messages information exchange among substation IEDs are included in IEC that are particularly meant for developing fast and reliable control and protection applications. This paper has presented an important extract of the major features possessed by this IEC GOOSE communication and investigated its impact on the design, operation and performance of substation automation systems. GOOSE message structure, GOOSE message service mechanism and the performance advantages associated with GOOSE message communication have been examined in detail. GOOSE offers several benefits such as reduction in overall losses, savings in overall life-cycle cost, new and improved functionality in designing innovative all-digital distributed substation protection applications. VI. REFERENCES [1] IEC : Communication Networks and Systems in Substations, Part 7-4: Basic Communication structure for substations and feeder equipment- Compatible logical node classes and data classes. Available: [2] Communication Networks and Systems for Power Utility Automation-Part 8-1: Specific Communication Service Mapping(SCSM) Mappings to MMS (ISO and ISO ) and to ISO/IEC , IEC ed2.0,2011. [3] Communication Networks and Systems for Power Utility Automation-Part 9-2: Specific Communication Service Mapping(SCSM) Sampled Values Over ISO/IEC , IEC ed2.0, [4] IEC LE: Implementation guideline for digital interface to instrument transformers using IEC , UCA International Users Group. [5] IEC : Communication Requirements for Functions and Device Models, IEC INTERNATIONAL STANDARD, July [6] Kanabar, Mitalkumar G., and Tarlochan S. Sidhu. "Performance of IEC Process Bus and Corrective Measure for Digital Relaying", IEEE Transactions on Power Delivery, [7] IEC : Communication Networks and Systems in Substations, Part 7-2: Basic Communication structure for substations and feeder equipment-abstract Communication Service Interface (ACSI). Available: [8] M. Yalla, M. Adamiak, A. Apostolov, J. Beatty, S. Borlase, J. Bright, J. Burger, S. Dickson, G. Gresco, W. Hartman, J. Hohn, D. Holstein, A. Kazemi, G. Michael, C. Sufana, J. Tengdin, M. Thompsonand, E. Udren, Application of peer-to-peer communication for protective relaying, IEEE Transactions on Power Delivery, vol. 17, no. 2, pp , April [9] D. Reckerd and J. Vico, Application of peer-to-peer communication for protection and control at seward distribution substation, 58th Annual Conference for Protective Relay Engineers, 5-7 April 2005, pp [10] D. Hou and D. Dolezilek, IEC What it can and cannot offer to traditional protection scheems, proceedings of the 35th Annual Western protective relay conference, Spokane, WA, October [11] Ingram, David, Pascal Schaub, Richard Taylor, and Duncan Campbell. "Performance Analysis of IEC Sampled Value Process Bus Networks", IEEE Transactions on Industrial Informatics, [12] IEC 61850: Communications Networks and Systems in Substations, , Available [Online]: [13] T.S. Sidhu and P.K. Gangadharan, Control and automation of power system substation using IEC communication, Proceedings IEEE Conference on Control Applications, Toronto, Canada, August 2005, pp [14] R.E. Mackiewicz, Overview of IEC and benefits, IEEE PES Transmission and Distribution Conference and Exhibition, May 2006, pp [15] T. Skeie, S. Johannessen and C. Brunner, ETHERNET in substation automation, IEEE Control Systems Magazine, vol. 22, no. 3, pp , June 2002.

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