XML-based Modeling of Communication Protocols for the Power System

Transcription

1 XML-based Modeling of Communication Protocols for the Power System ZHIMIN HE, JIA WANG Engineering Training Center Shanghai University of Engineering Science No.333, Longteng Road, Songjiang District, Shanghai CHINA Abstract: The XML technology currently widely used in network communication is also able to play an important role in the automation system of the power Based on some specifications and standards of the current power system, the study conducted self-description of the information and data modeling of substation communication protocols with XML and its object-oriented technology, and discussed basic principles and methods of the modeling of communication protocols for the power system mainly with the case study of Standards. It is also the basis of heterogeneous data exchange and transmission with XML in the power Key-Words: XML; Communication Protocols; Modeling; Heterogeneous Data; Power System; Information Exchange; CIM 1 Introduction The openness of the current power automation system is limited due to the widespread use of proprietary databases provided by the supplier. With the development in the power industry, the automation system will always be faced with many new requirements, so it becomes more and more urgent to achieve the interconnection between the automation and information However, most proprietary database systems of power automation systems (such as EMS, etc.) are designed and developed according to the special functions and requirements of those provided by suppliers. In order to achieve correct and accurate interaction among the applications provided by different suppliers, it is necessary for these applications to have a shared and concerted understanding for the exchanged information. In the meantime, the information semantics in the power system should be defined as a uniformed data model so as to prevent integration engineering at some stage from becoming a wide area of isolated automation Island. Relevant international organizations have developed many relevant standards and protocols so as to meet the demand of the application of the power IEC has worked out a number of international standards on communication networks and system configuration for the power system, such as IEC 61870[2], the transmission communication protocol, that provide a uniform platform for information interaction in the power Based on these existing specifications and standards, the study discussed the basic principles and methods of describing with XML the data structure of communication protocols for the power In the development of previous data acquisition system of the power system, protocols were explained with themselves as objects. When a new protocol is required to be added to the system, the whole system needs to be changed and new program codes added, thereby causing a heavy workload. After the semantic information model of IEC series of transmission protocols was analyzed, objects in the protocols were obtained layer by layer with the XML technology so as to achieve the reuse of objects and self-definition of standard protocols in this study. With XML, the specific protocols in different fields are able to be described by layer by layer and the system can be smoothly transferred into the new protocol standard without any changes. 2 XML-based Information Exchange Model of the Power System Common Information Model (CIM) is, according to the definition of Object Management Group, a standard object of operation and application of the systems used for production, transmission, distribution, marketing and retail in power, water and gas industries. In the field of power, CIM defines, with its standard for description of power objects and their relationships, the standard object model of the power ISSN: ISBN:

2 industry used for development and integration of power engineering, planning, management, operation, business and other applications. CIM has greatly improved the openness and extensibility of the power automation However, as a large and complex system, it is impossible for all kinds of applications of the power system to be completely integrated onto the same database platform. Instead, data exchange between application systems are required to achieve the sharing and integration of data and functions. Extensible Markup Language (XML) is a markup language for marking electronic documents with structural information. The definition of exchanging data objects is standardized with CIM, while XML provides an effective means for this. The core issue of data exchange is the standardization of information description, which mainly solves the understandability of the information. Due to its ability to describe data and the consistency in format of data description, XML becomes a powerful tool for data exchange, especially suitable for data exchange among heterogeneous systems. With XML technology, all types of information in the power system, textual or binary, can be marked so that XML can be used as a communication medium among heterogeneous systems. Moreover, the extensibility and customization of XML has made it sufficient to express all types of data, which can be either received and processed by each automation system or transferred among different databases. Therefore, the issue of the uniform interface[1] among different automation systems in the power system can be better solved with the XML technology. There are no fixed labels and elements that can be applied to all users in all fields, because XML is a meta-markup language, but it allows developers and writers to define elements based on their requirements. X in XML represents extensibility, that is, XML documents can be extended so as to meet a variety of needs. The data and information described by XML documents are not only clear and readable, but also more precise in data search and positioning through extensibility. Since XML emphasizes the description of data themselves and the organization and storage structure of data contents, different users can extract relevant data out of their own needs for different purposes. Any tool that can read text files can read XML documents since they are text. Therefore, the data described by XML can be kept for a long time without having to worry about the problem of recognition. XML s characteristic of self-description allows the separation of messages from the application, and their modification or expansion will also not affect the current message processing. Therefore, it is generally accepted to encode messages with XML. For an enterprise-level power application system, data exchange can be achieved with external application systems or other power companies based on the information interaction model encoded by XML. Fig.1 is a schematic diagram of the structure of an XML-based for the power automation Topology Power Flow Calculation Network Modeling XML XML XML XML XML XML Database Middleware MIS Application EMS Information Release Other system Fig.1 The System Structure of XML-based CIM model for Power Automation System It can be seen from figure 1 that the realization of data exchange among different modules of the system through the XML interface is mainly because each system is described with the standardized XML modeling. 3 XML based Communication Protocols Description 3.1 Brief Introduction of Functions of IEC Series of Protocols In the substation automation communication system, there are a series of provisions in the sending and receiving processes of the communication to ensure that the two sides of communication can transmit data in a correct, effective and reliable way so as to regulate the two sides to work in a correct and coordinated way. These provisions are called data transmission control protocols, or in brief, communication protocols. At present, many international organizations and authorities are actively carrying out the work of standardizing substation automation [3]. For substation automation users and manufacturers, only with the standardization of transmission protocols and transmission networks, can the interchangeability and interoperability of the equipment in the substation automation system be achieved and can the drawbacks of inability to interconnect and connect equipment from different manufacturers be overcome. As the first international standard on communication networks ISSN: ISBN:

3 and systems of the substation automation system, IEC61870 conducts uniform modeling of data objects in the substation automation system, uses object-oriented technology and the abstract communication service interface independent of the network structure, and supports TCP/IP communication protocols for the automation It is an open, future-oriented and a new generation of communication protocols of the substation automation XML is used to achieve the conversion of communication protocols, in which the most important task is to model the communication frames. In the IEC61870 series of standards[4], describes the formats of the transmission frame which are mainly classified as FT1.1, FT1.2, FT2 and FT3. The frame formats of FT1.2 described later can be further divided into those of variable frame length, fixed frame length and single control byte, of which the specific formats are shown in fig.2 68H DataLength (L) DataLength (L) 68H ControlDomain (C) AddressDomain (A) 10H ControlDomain (C) AddressDomain (A) Link User Data Area CheckSum (CS) EndCharacter 16H E5H A2H Link User Data Area CheckSum (CS) EndCharacter 16H Fig.2 The specific formats divided into variable frame length, fixed frame length and single control byte formats 3.2 Protocol-based XML-Schema When the protocols are described with XML, XML-Schema is first to be developed in the field of protocols application[5], which defines semantics and syntax used in protocol description with XML, embodying the characteristics of XML customization. In the XML-Schema, it is necessary to define elements, tags, attributes and entities used in describing the protocols, reflect the logical relationship between the protocol data described and ensure that XML documents are readable and searchable. With the format of FT1.2 variable frame length in IEC as the example, XML-Schema document Protocols.xml of the link layer is described as follows: (1) <? xml version = 1.0 encoding = UTF-8?> (2) <Schema xmlns = urn:schemamicrosoft-com:xml-data xmlns:dt = urn:schema-microsoft-com:datatyp es > (3) <ElementType name = PRM (4) <ElementType name = FCB (5) <ElementType name = FCV (6) <ElementType name = ACD (7) <ElementType name = DFC (8) <ElementType name = FunctionCode (9) <ElementType name = (10) <ElementType name = Length dt:type = int /> (11) <ElementType name = AddressDomain (12) <ElementType name = CheckSum (13) <ElementType name = EndCharacter (14) <ElementType name = ControlDomain dt:type = eltonly /> (15) <element type = PRM > (16) <group order = seq minoccurs = 0 maxoccurs = 1 /> (17) <element type = FCB /> (18) <element type = FCV /> (19) </group> (20) <group order = seq minoccurs = 0 maxoccurs = 1 /> (21) <element type = ACD /> (22) <element type = DFC /> (23) </group> (24) <element type = FunctionCode /> (25) </ElementType> (26) <ElementType name = LPCI content = eltonly > (27) <element type = minoccurs = 1 maxoccurs = * /> (28) <element type = Length minoccurs = 1 maxoccurs = * /> (29) <element type = ControlDomain /> (30) <element type = AddressDomain minoccurs = 0 maxoccurs = 1 /> (31) <element type = CheckSum /> ISSN: ISBN:

4 (32) <element type = EndCharacter /> (33) </ElementType> (34) </Schema> Where (1) is an XML type declaration statement, indicating that it is an XML document encoded by UTF-8 and in accordance with the specifications of version 1.0. (2) is a Schema declaration statement, including Schema namespace declaration. Two namespaces are used above, of which xmlns = urn:schema-microsoft-com:xml-data specifies that the document is an XML Schema document and xmlns:dt = urn:schema-microsoft-com:datatypes defines the data type used in the document. (3)~(13) are element definition statements. A total of 11 elements are defined according to the format of FT1.2 variable frame length in Protocol IEC : PRM (start packet bit), FCB (frame count bit), FCV (frame count valid bit), ACD (asked to visit bit), DFC (data flow control bit), FunctionCode (function code), (start character), Length (length), AddressDomain (address domain), CheckSum (frame checksum) and EndCharacter (end character), data types of which are int. (14)~(25) define the sub-element ControlDomain (control domain) in the Schema and specify its sub-elements. (26)~(33) define the third-level element LPCI (Link Protocol Control Information) in the Schema and specify its sub-elements. (34) is a closing tag statement, indicating the end of Shcema description. 3.3 XML-based Protocol Description The customization of protocol-based XML-Schema provides a set of syntax and semantics to describe protocols with XML, which can be used to describe the protocols in a layered way to accomplish the transition from protocol-oriented to layer oriented in the data acquisition system and make the protocols configurable. The XML description of one function frame in Protocol IEC for substation automation system is shown below also in the FT1.2 frame format. The XML-Schema customization of protocols in the Protocols.xml forms the semantics and syntax for protocol description with XML, which are described with XML as follows: <? xml version = 1.0 encoding = UTF-8?> <Protocols xmlns = x-schema:protocols.xml > <LPCI> <>104</> <Length>15</Length> <ControlDomain> <PRM>0</PRM> <ACD>1</ACD> <DFC>0</DFC> <FunctionCode>8</FunctionCode> </ControlDomain> <AddressDomain>a</AddressDomain> <CheckSum>b</CheckSum> <EndCharacter>22</EndCharacter> </LPCI> <DUI> <DUT> <TYP>6</TYP> <VSQ>129</VSQ> </DUT> </DUI> <InfObj> <IOD> <FUN>255</FUN> <INF>0</INF> </IOD> <InfEle> <Time> <Year>c</Year> <Month>d</Month> <Day>e</Day> <DayOfWeek>f</DayOfWe ek> <Hour>g</Hour> <Minute>h</Minute> <Second>i</Second> <MilliSec>j</MilliSec> <Time> <InfEle> <InfObj> <Asdu> </Protocols> In this XML-based description of Protocol 103, the element ACD is 1, indicating that the interval unit is required to transmit level-one user data to the control system; the element DFC is 0, indicating that new data can be accepted; the element AddressDomain is a; the frame check and CheckSum are b; in the element Time, j is millisecond, i is second, h is minute, g is hour, f is week, e is day, d is month, and c is year. Fig.3 shows the program s flow chart of protocol components in the direction of monitoring. Protocol-based XML-Schema developed through the above principles and methods and XML-based protocol description can solve the problem of data heterogeneity in the power system caused by the adoption of different communication protocols. ISSN: ISBN:

5 Matching at Link layer Form XML documents described at link layer Frame data flow Matching at Application layer Form XML documents described at application layer Future, Proceedings of the IEEE, Vol.93, No.11, 2005, pp [4] Lin Wang, Celia Li, Helen Cheung, etc., PRAC: A Novel Security Access Model For Power Distribution System Computer Networks, Proceedings of IEEE PES General Meeting 2007, Vol.1, 2007, pp [5] Alexander Apostolov, XML Applications to UCA 2.0/IEC Object Modeling, IEEE/PES Summer Meeting, July, 2002, pp Assemble XML documents of XML-based protocol description Fig.3 The Flow Chart of Protocol Components in the Monitoring Direction 4 Conclusion Although the issue of the interaction of heterogeneous data in the power system has attracted more attention in recent years and has been discussed in some relevant research, some drawbacks still exist in the current methods, especially their inability to meet the requirements of network-based information exchange. The emerging network technology XML provides a common data exchange format, making it possible the smooth interaction on the network of heterogeneous data among the power automation systems. Based on the characteristics of data modeling in the power system, the study discussed the basic principles and methods of describing and establishing with XML substation communication protocols of the common data structure in the power However, there are still some challenges for XML to be widely used in the power system, which mainly includes interoperability based on XML and the potential and serious network security issues. Objectively speaking, these challenges are not to be easily overcome, and some of them are not purely technical ones. So it is also an issue of research as to how to tackle these challenges. References: [1] Sun Jing, Li Xiaoming, Zhang Lianmei, Method of the exchange of the data among heterogeneous databases in heterogeneous power information system, Automation of Electric System, Vol.21, No.2, 2003, pp [2] IEC (Committee Draft), Communication Networks and Systems in Substation[S], 1999 [3] Felix F. Wu, Khosrow Moslehi, Anjannn Bose, Power System Control Centers: Past, Present and ISSN: ISBN: