Formalization of Functional Requirements for Digital Substation Protection and Automatic Control Systems

Abstract

The series of IEC 61850 standards is currently the most widely used source for description of a digital substation’s equipment and processes; therefore, advanced technologies in the field of designing relay protection and automatic controls (PAC) shall inevitably be developed in compliance with the requirements of these standards. However, the scope of this standard series was originally limited to describing SCADA-controlled facilities and processes, which poses certain restrictions on the standard series capabilities. PAC systems should be designed subject to the IEC 61850 requirements, but at the same time, the IEC 61850 standard does not provide any tools and has no intent for thoroughly describing the structure and operational principle of PAC devices, and only describes the external interface for monitoring the necessary data, which is sufficient from the SCADA system perspective. Such restrictions of the standard series can be clearly seen when it comes to designing centralized PAC systems, PACs with migrating functions and digital substation computer-aided designing and monitoring systems, which pose more stringent (compared to a SCADA) requirements for describing the information model structure and for data exchange between its parts. The fact that the actual algorithmic support of the PAC functions differs from their information models has an impact on designing the functional requirements for logic units (LU).

The functional requirements for LUs cannot be developed unless there is understanding of the logical device (LD) structure that will reflect a certain function; however, IEC 61850 does not specify any logical device structure. As a result, both the structure of logical devices and the functions implemented within one LU may differ essentially from manufacturer to manufacturer, which has a negative impact on the compatibility of the solutions offered in the market. The development of logical devices and units within the research framework revealed a number of problems connected with the specific features of implementing IEC 61850, which result in lack of unambiguity of LU and LD implementations. Thus, the standard does not specify the sources of signals supporting the LU operation, nor does it consider a functional for processing and producing measurements for relay protection as part of the information model. The standard does not describe the functions typical for the domestically adopted PAC designing practice. It should also be noted that the UES Federal Grid Company’s corporate profile specifies them only partially since it inherits the description structure from IEC 61850.

The article suggests a change in the approach to the use of logical units. Currently, the function information model represents only an external interface. This interface makes it possible to obtain, from the midst of algorithmic support, the values of necessary signals with the required semantics, which is functionally useful only for the SCADA. Thus, it turns out that the functional requirements for an LU should define the requirements for the external interface instead of the algorithmic support implementing the PAC function, which to a certain degree makes a nonsense. The article proposes to set up, for each LU, an independent separate algorithmic support, so that the algorithmic support structure would be consistent with that of the information model. In view of this feature, logical devices and units are developed that reflect the PAC functions for facilities of medium and high voltage classes. In addition, data objects are classified into input, output signals and setting parameters, and the extensions of LU information models, should a need to do so arises, are suggested.

The study has revealed multiple applications of similar LUs in different PAC functions, which pose specific requirements for the LU operation. In this connection, it is decided to standardize the entire specific functional within a single LU type with the possibility to choose the necessary operation mode by changing the setting parameters.

The development of the algorithmic support has revealed repeating types of LU algorithmic support structure, which were combined into three categories: measuring, starting-functional, and functional. For each of the LU types, there is a certain set of LU functional blocks.

The designing of PAC functions on the basis of the new approach to the use of LUs has the following advantages: the protection scheme modular structure makes it possible to select the set of protection functions in a wide range at the designing stage; strict definition of interconnections and data exchange between LUs simplifies the understanding of protection operation, and opens the possibility to evaluate the influence of the LU operation mode on the performance of the protection function as a whole; the protection scheme modular structure with a predictable behavior of LUs makes it possible not only to monitor the correct operation of protection using special automatic tools, but also to automatically develop protection schemes. The disadvantage of this approach is the need to develop the PAC algorithmic support from scratch, because the previously developed algorithms do not imply the information model structure.

The process of developing a logical device is shown within the article framework on the example of the medium voltage class overhead power line overcurrent protection with a detailed description of the logic device structure and the purpose of the presented LUs. A configurator software was developed during the study, which makes it possible to configure logical devices based on the library of developed LUs with subsequently making binary files to be uploaded on a hardware platform. The unambiguous description of information exchange between LUs generated the need to extend the SCL syntax, as a result of which a new format of CLD files was produced. The mechanism of applying reference data objects like InRef and BlkRef, that are part of the standard SCL syntax is not applicable because they lack semantic meaning.

The article describes a successful process of testing and validation of the developed algorithms/logic units and logic devices on a hardware platform, as well as the problems revealed during the testing, which are connected with the lack of regulatory requirements for functional testing of logic units and methods for taking into account the impact of various quality labels of data objects.