What are the characteristics of ACSI technology? The mapping method from ACSI to MMS is introduced and explained in detail.

C1618500 is the international standard of substation automation system structure and data communication. Its purpose is to realize the interoperability and information sharing among intelligent electronic devices (IEDs) of different manufacturers in the substation through a standard, and to cancel all kinds of protocol conversion links and conversion devices, so as to make the system debugging more convenient and realize? One world, one technology, one standard? .

Compared with the traditional protocol based on message structure, IEC 6 1850 with object-oriented technology has obvious characteristics and advantages. It puts forward the hierarchical structure of the system, adopts the unified modeling of data objects, separates the mapping method from the specific network, and provides the system configuration management based on SCL, which makes the protocol open enough to meet the requirements of future substation communication development. Therefore, the key to the application of IEC 6 1850 is the realization of ACSI mapping of abstract communication service interface.

1 ACSI overview

Acsi (abstract communication service interface) comes from people's practical experience abstraction of real equipment, and mainly defines various communication services, communication objects and parameters. It is independent of the lower communication system and has nothing to do with the adopted communication protocol and specific implementation method. ACSI mainly sets up various service models, including connection service model, variable access service model, data transmission service model, device control service model, file transmission service model, clock synchronization service model and so on. These service models define communication objects and how to access them, realize the communication between client applications and server applications, and complete real-time data access and retrieval, equipment control, time reporting and recording, and self-description of equipment.

In order to ensure the independence of ACSI and adapt to the changes of future network technology communication development, IEC 6 1850 protocol does not specify the method to realize ACSI, but only provides a special communication service mapping (SCSM) to describe the mapping process, and defines the mapping of ACSI to manufacturing message specification MMS in IEC 6 1850-8- 1. Because different types of SCSM cannot interoperate directly, the standard only stipulates the mapping to MMS at present.

Brief introduction of MMS

MMS (Manufacturing Message Specification), namely ISO/IEC 9506, is an international message specification proposed by ISO TC 184 to realize real-time data exchange and monitoring between intelligent devices in heterogeneous network environment. The service provided by MMS has strong universality, and has been widely used in automobile manufacturing, aviation, chemical industry, electric power and other industrial automation fields.

MMS has the following three advantages: interoperability; Achieve independence; Realize data access in heterogeneous environment. In the past, interoperability provided by communication standards, or network connection, device model, function realization, etc. Too limited or not enough, this situation did not change until the MMS standard appeared. At the same time, MMS has achieved independence, so that users are no longer limited to choosing a fixed equipment provider. As long as they meet MMS standards and can achieve the same functions, they can replace them. This independence is also reflected in the realization of network connection and function. MMS also realizes data access in heterogeneous environment. Most of the previous communication mechanisms only provide a simple byte queue information transmission mechanism in the network, which lacks independence. However, MMS provides more restrictions and structural abstractions on the transmitted information, shields the internal characteristics of the actual equipment, and adopts the BER coding of ASN. 1 in the presentation layer.

3 ACSI mapping implementation model

The key to realize IEC 6 1850 is to realize the mapping from ACSI to MMS or other middleware.

ACSl only provides class models and services, and there is no ACSI PDU. ACSI maps to application layer or CORBA middleware technology through their own specific mapping method SCSM. Taking MMS as an example, the standard stipulates that MMS PDU(MMS protocol data unit) should be constructed by the BER coding method of ASN. 1. For the underlying protocol stack, OSI protocol stack can be designed and implemented by itself or directly based on TCP/IP protocol. SCSM is the concrete realization of ACSI mapping, and describes the process of mapping realization.

Mapping from 4 ACSI to MMS

4. Implementation process of mapping1acsi to MMS

Realizing the mapping from ACSI to MMS is the core of IEC 6 1850. As shown in Figure 2, the design of ACSI is mainly divided into three parts: The first part is the design of various classes of ACSI, the top application layer. Firstly, ACSI services designed 86 logical nodes and public * * * data classes (see Figure 3), imported application-layer data (possibly from databases and XML configuration files), and mapped various logical services, including reporting mechanism, data sets and data sets in IEC 6 1850. The second part is to realize the BER coding of ASN. 1 between the service semantics of the application layer and the data of the application layer in the presentation layer, which is a coding and decoding part. The design of this part is the key, because it involves the interoperability of devices and requires the consistency test of messages. The third part is the transmission form of bit stream message in the network, which is divided into TCP-oriented association mode and UDP connectionless mode between GOOSE message and sampling message.

4.2 Implementation method of mapping from ACSI to MMS

As the most important part in the application layer of MAP (Manufacturing Automation Standard), MMS standard hides the specific internal characteristics of equipment by introducing the concept of VMD (Virtual Manufacturing Equipment), and sets up a series of types of data to represent the functions of actual equipment. At the same time, a series of MMS services are defined to operate these data, and the actual equipment work is manipulated by accessing VMD model. The VMD concept of MMS introduces the idea of object-oriented design into process control system for the first time.

MMS has no specific implementation method for all kinds of services specified by it, which ensures the openness of implementation. As shown in Figure 4, the data object model of IEC 6 1850 is mapped to the VMD of MMS, the logical nodes MMXU and PBPR of IED are mapped to the domain of VMD, and the logical node Volts is mapped to the named variable, so that the attribute value of the voltage range in the logical node MMXU can be obtained by accessing the variable MXU$Volts$rang.

MMS adopts the basic coding rule (BER) of ASN. 1. There are four tag types defined in ASN. 1, namely universal, application, context-sensitive and private. When transmitting the code, TLV(TAG Length Value) is adopted, that is, the tag, the length value and the value are transmitted at the same time. Both parties can encode and decode according to TLV mode when receiving and parsing, and encoding and decoding is an opposite process. The adoption of ASN. 1 code enables both parties to understand the meaning of ASN. 1 code and the MMS service it represents in a heterogeneous environment, regardless of how the two parties implement and transmit ASN. 1 code.

The concrete implementation of association-oriented MMS communication service is shown in Figure 5. This is a process of receiving information. First, the interception function notifies the main function through the pipeline command when it receives the communication data stream. The main function calls the management ACSE function of MMS to confirm the legality of the connection. After the authorization is confirmed, the read function is called to read the bit stream, and then the bit stream is decoded by the MMSASN. 1 function, and the connection is handled at the same time, managing the VMD domain, variables and types of MMS. After decoding, the data is processed according to the application of a specific application layer.

The adoption of ACSI in IEC 6 1850 is beneficial for users to understand the functions of each module of the equipment by directly viewing the configuration file or obtaining the logical nodes and data in it. So the abstract modeling of different manufacturers may be different. At the same time, the current IEC 6 1850 standard lacks China's participation, and many definitions of data and logical nodes need to be expanded according to China's own situation. Try not to expand logical nodes in expansion, and limit the length of expansion plus ex to 7 digits when expanding data.

4.3 Comparison of mapping ACSI to MMS and XML

IEC 6 1850 standard only stipulates the mapping to MMS for the time being, but at the same time points out other possibilities, such as XML. As a heterogeneous processing technology, XML can be combined with HTTP service. Six performances.

A small number of ACSI services can also be mapped to HTTP/HTML/XML. As can be seen from the figure, mapping from ACSI to MMS can achieve more services than mapping from HTTP to XML, such as InfoRepor and Journal, while only the most basic GET and POST in HTTP can achieve reading and writing services. The biggest difference between them is that mapping to MMS is a BER code that transmits ASN. 1, while mapping to XML or HTML is a file that transmits text format. Therefore, according to specific applications, when only basic services are needed, functions are not complicated and real-time requirements are not high, the way of mapping ACSI to XML can be adopted.

5 conclusion

Aiming at the purpose of equipment development in the specific application of IEC 6 1850, this paper studies the mapping implementation of ACSI and draws the following conclusions:

(1) By analyzing the technical characteristics of ACSI, the model of ACSI mapping realization is constructed, which lays the foundation for ACSI mapping realization;

(2)IEC 6 1850 standardizes the mapping from ACSI to MMS, studies and designs the detailed flow of mapping from ACSI to MMS, and puts forward three parts of ACSI implementation, which provides a framework for the implementation of ACSI mapping;

(3) The specific mapping method from ACSI to MMS is designed, including logical node mapping, data model mapping and the design of main functions of MMS communication server, which provides a technical scheme for developing IEDs conforming to IEC 6 1850.

(4) Comparing the mapping from ACSI to MMS and to XML, we can adopt the mapping from ACSI to XML when the real-time performance is not high and the function is not complicated.