J2EE 外文翻译原文

Design and Implementation of Distribution Network SCADA System Based onJ2EE FrameworkZhao Qiang, Chen DanyanDepartment of Computer Science and Technology, North China Electric Power University (NCEPU), Beijing 102206, Chinazhaoqiang01@ABSTRACT: In this paper, the problems exposed by the thirdgeneration of SCADA (Supervisory Control And Data Acquisition) based on the C/S(Client/Server) mode are analyzed. Then, a new distribution network SCADA based on the four-tier architecture of J2EE platform is built. Because of poor information sharing and interoperability of CORBA (Common Object Request Broker Architecture) as the integration bus among application systems of power enterprises, the seamless integration between SCADA heterogeneous data platform which is based on standard IEC (International Electrical Commission) 61970, and other systems is achieved by means of Web services, and such integration is standard and open. The combination of EJB (Enterprise JavaBean) and Hibernate to implement the business logic and make data be persistent separately also makes the SCADA much more scalable and flexible. Because of its flexibility and extensibility, this new SCADA has great benefit to the automation course of electric power systems.KEYWORDS: J2EE framework; SCADA; standard IEC 61970; Web servicesI. I NTRODUCTIONDistribution network SCADA (Supervisory Control and Data Acquisition) includes not only the models and methods of data acquisition and supervisory control but the models and methods of power network topology. The power system models provided by SCADA are consistent, complete and can be used directly [1].With the expansion and market-oriented operation of the power network, the operating and failure modes begin to increase, so does the data needed to be monitored and analyzed. Especially when devices are running down or under emergency conditions, the analysis and decision-making tasks are so pressing and heavy that the operator often can’t accomplish them perfectly [2]. Therefore, they have to depend on the efficient SCADA. The problems of the third generation of SCADA based on the C/S (Client/Server) mode have gradually appeared, such as being short of flexibility, high install cost, being difficult to share data with other systems, etc. Power network SCADA in China has stepped into the fourth generation and its main characters includes standard IEC (International Electrical Commission) 61970 based, object-oriented and continuing to expand the integration between the SCADA system and other systems [3].J2EE platform, as the mainstream enterprise-class platform for distributed applications, is a component-basedand standards-based platform. Developing SCADA with advanced J2EE technologies can resolve a series of problems existing in the traditional SCADA. J2EE platform is the ideal platform to develop the fourth generation of SCADA system.II.T HE DESIGN OF DISTRIBUTION NETWORK SCADABASED ON J2EE PLATFORMAccording to J2EE architecture, this SCADA is designed Figure 1. SCADA system based on J2EE platform.The new SCADA is based on B/S (Browser/Server) and C/S mixed mode: on the one hand, the browser supplies the users with rich graphic display and user interaction functions, including sequence-of-event report, primary electric wiring diagram, etc., on the other hand, independent Java applications, which run directly on the client operating system(or Java Virtual Machine), can make full use of client resources and computing ability, and the result is that it will offer richer functions for the master side, e.g., analyzing the real-time or historical data, monitoring the devices, etc. The monitor screens of the system are realized by SVG (Scalable Vector Graphics) which is recommended by standard IEC 61970. The system reads static map metadata firstly to create SVG screen, and then reads the real-time data according to pre-defined refresh cycle to refresh the screen partially.JSP (JavaServer Pages) and Java Servlet in the Web layer respond to the requests from the client. Having received2009 International Forum on Information Technology and Application 2009 International Forum on Information Technology and Applicationsthose requests, Servlet calls the EJB (Enterprise JavaBean) components in the business layer via factory delegate class, and then returns the processing results to the client. For the purpose of realizing data sharing between SCADA and other power systems, e.g., GIS (Geographic Information System), some business methods are further wrapped and published as Web services according to standard IEC 61970.The business logic is implemented by EJB component. EJB is the core of the whole design and it enables the system to meet the diverse requirements of power enterprises. The detailed business logic in the early SCADA systems is wrapped by EJB component, that is to say Entity Bean is responsible for building dynamic mapping with the database storing historical data [4]. But in this system, such an old structure is discarded. It uses Hibernate framework instead to build dynamic mapping and then Session Bean realizes the business logic by calling the Hibernate. The business logic encapsulated includes recording sequence-of-even, analyzing failures, etc. Hibernate not only allows the mapping of mapped tables with the domain objects, but also provides data query and recovery mechanisms. Hibernate can replace Entity Bean in the J2EE applications to complete the important task of data persistence [5].The real-time database and the relational database are both adopted. The data from substations are written into the former by the data acquisition module, and then it stores the date needed to record into the latter. The data stored mainly involves telemetry, telesignalling and electrical measurement. III.T HE IMPLEMENTATION OF DISTRIBUTION NETWORKSCADArmation expression and multi-system integrationbased on standard IEC 61970This distribution network SCADA system follows standard IEC 61970 to accomplish the information exchange and interoperation among systems. The integration platform encompasses CIM (Common Information Model), database, import and export tool for CIM/XML (eXtensible Markup Language), CIS (Component Interface Specification) interface implementation and Web services integration bus, as illustrated in Fig. 2.The CIM database mainly used to manage and maintain CIM model data and build bi-directional interface with real-time database. The packages needed to be implemented consist of the core package, topology package, wire package, measuring package and other packages related with CIS service interfaces. The structure of CIM database is built automatically in light of mdl files by conversion tool and the data source comes from SCADA real-time database. The CIM/XML import/export tool will import XML files into CIM database if the whole grid model changes.The CIS interface implementation implements GDA (Generic Data Access) interface and HSDA (High Speed Data Access) interface. The former provides conditional and unconditional queries, writing ability while the latter is used to transfer massive data efficiently [6]. This system offers to read and write data synchronously and asynchronously, subscribe data, etc. Systems like GIS can develop new applications or transfer data flexibly by calling GDA and HSDA implementations on the Web service integration bus. For example, the client can get grid model data by calling GDA methods or get measuring values by calling HSDA methods.The following steps show how to get the whole gird model.1)Get “ResourceID”: Get “ResourceID” for each class by calling method “get_uris”.2)Get record values: Get all record values in the tables by calling method “get_filtered_extent_values”.The system uses the iterator to allow the client to get an arbitrary amount of resources in one time. If the iterator has no resource when the client won’t use it any more, it will be recycled automatically.The traditional integration solutions for heterogeneous system are private and complex, that is to say, different components can’t exchange data directly. In this system, we adopt loosely coupled Web services. It is of great significance in promoting power enterprises to achieve wider application integration.The CIS provided by standard IEC 61970 is identified by IDL (Interface Description Language), which is only for CORBA (Common Object Request Broker Architecture). If want to develop Web services [7], we have to transform IDL CIS into WSDL (Web Service Definition Language) CIS according to conversion rules. Then the CIS Web services server can be built by using the JAX-RPC (Java APIs for XML-Based Remote Procedure Call) server-side model. Here some tools, such as Axis, will be used to mapping WSDL CIS into Java classes.The method using Web services to establish integration bus improves the reusability of the SCADA system and achieves overall information sharing. The third party can visits the SCADA’s run information and grid model in a standard manner.B.Visiting EJB component with design patternsAll the instances created by the factory method pattern usually implement a same interface, where all methods owned by the instances are identified, and these methods always have different implementation in different implementing classes. The present system combines factory method pattern and singleton pattern when creating EJBdelegate factory class. The singleton pattern will provideglobal access point, and the result is that the cost of creatingand destroying Java objects will decrease. The steps for theEJB factory to get EJB remote interface by JNDI (Java Naming and Directory Interface) are as follows.1) Create an EJB factory class: Create an EJB factory class named RemoteFactory, and declare an instance of HashMap for caching EJB remote interface and a private andstatic instance of RemoteFactory which will be returned by asynchronous method. 2) Declaration: Declare the constructor of RemoteFactory to be private. 3) Create a lookup method: Create a lookup method whose return type is EJBObject. In this method, lookup the object of EJBObject in the above HashMap cache pool firstly.If it does exist, then return it. Otherwise, lookup all the homeinterfaces’ parent interface by JNDI in the context, and then a new object of EJBObject which will be put into the existing HashMap later will be created by the interface.After the factory class being created, the client just needsto call the lookup method of it and transfer concreteparameters. Such a universal method can avoid a largenumber of repeat lookups and calls for the method “create”from the client.C. The integration of EJB and HibernateAs the object-relational mapping solution of Java,Hibernate provides a convenient framework for the mapping from object-oriented domain model to the traditional relational database. On the merits of Hibernate, the system will integrate EJB and Hibernate. Hibernate provides the operation to the database and it will be deployed to the Weblogic application server, while Session Bean is responsible for calling Hibernate. The steps of the integration are as follows.1) Deploy Hibernate: Deploy Hibernate to the WeblogicApplication Server. After the deployment, then Weblogic will generate a JNDI for Hibernate. 2) Create a StartUp class: In order to ensure the pre-creating of Hibernate’s SessionFactory, the system needs to create a StartUp class which implements the Weblogic T3StartUpDef interface. This class will begin to run in thebackground when the application server starts. The coremethod is as follows.public String startup(String name, Hashtable args) throwsException {Configuration con = newConfiguration().configure("/hibernate.cfg.xml");SessionFactory sf = con.buildSessionFactory();}3) Call Hibernate: Session Bean can get the Hibernate SessionFactory through the above-mentioned Hibernate JNDI. After getting the instance, Session Bean can operate the database through Hibernate.Context ctx = new InitialContext(); SessionFactory sf = (SessionFactory) ctx.lookup("hibernate/session_factory"); D. Real-time Java TechnologyBecause of Java’s inherent uncertainty such as thread management, garbage collection, class loading, etc., the standard Java applications that run on general JVM (JavaVirtual Machine) can only meet soft real-time demand in a few hundred milliseconds.For a real-time system, predictable execution is critical.In the SCADA systems, when the fault accidents happen, SCADA system should give reaction timely, but the conventional Java thread uses the time slice rotation scheduling mechanism. The mechanism doesn’t provide any priority protections to threads.By using fixed priority scheduling strictly, real-time thread scheduling of RTSJ (Real Time Specification for Java) is perceptible. The class HighResolutionTime and Clock provide high resolution clocks for real-time systems and canbe accurate to nanosecond [8]. The mechanisms that RTSJprovides can meet the needs of the real-time SCADA system.To provide real-time functions, the SCADA systemrequires the underlying operating system to support real-time,for example, Solaris 10 has a real-time scheduler. Also theprogram needs to import the javax.realtime package beforewe use any functions provided by RTSJ implementation. Follow following steps can create and start a real-time thread. 1) Initialize parameters: Initialize related parameters of the constructor of real-time thread. A typical constructor is like this: RealtimeThread(SchedulingParameters scheduling, ReleaseParameters release, MemoryParameters memory, MemoryArea area, ProcessingGroupParameters group, ng.Runnable logic). 2) Instantiating the real-time thread: Instantiating a real-time thread object using above constructor. 3) Start the thread: Call the “start” method of the real-time thread.Generally in the system, the common RealtimeThread will be used in the mission that can tolerate delays caused by the garbage collector. High-priority NoHeapRealtimeThreadis rarely interrupted or occupied, so they will be used in the most demanding tasks which can’t tolerate delays. IV. C ONCLUSION This paper applies the four-layer J2EE architecture to the development of the distribution network SCADA system. After analyzing the architecture of existing SCADA systems, the new system gets a lot of improvements by using many new software technologies that sprang up in recent years. With introduction of the EJB component, SVG, real-time Java technology, standard IEC 61970 and Web services, practices have proved that the new distribution network SCADA system has greater flexibility and scalability than the old system. It will play an important role in thedistribution network automation systems.R EFERENCES[1]Chen Tang, Zhao Zukang, and Chen Xingying, Distribution NetworkSystems and Their Automation Technologies. Beijing: China Electric Power Press, 2004.[2]Du Junhong, Le Quanming, and Tenghuan, “Design of powerdispatching automation systems based on J2EE and CORBA,”Automation of Electric Power Systems. Nanjing, vol. 28, pp. 78-81, March 2004.[3]Ma Zhengwu, “Summarize of SCADA&RTU in the course ofrealizing the enterprise automation,” Program-mable Controller & Factory Automation. 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