Introduction to the Common Information Model
This section introduces the CIM as a model format that is used for power system data and information exchange across applications, platforms, and services. The CIM is used for all power system models in GridAPPS-D, and it is important to have an understanding of the concepts and implementation of CIM for describing power systems using unique mRIDs for each piece of equipment and associated modeling objects.
Introduction
What is the Common Information Model?
The Common Information Model (CIM) is an abstract information model that can be used to model an electrical network and the various equipment used on the network.
CIM is widely used for data exchange of bulk transmission power systems, and is now beginning to find increasing use for distribution modeling and analysis.
By using a common model, utilities, vendors, and researches from both academia and industry can reduce the effort and cost of data integration, and instead focus on developing increased functionality for managing and optimizing the smart grid of the future.
Why is Data Integration Important?
In a typical distribution utility there are hundreds and even in some cases thousands of software solutions and applications that are managed by the IT department. These applications are used and operated independently by the various groups, departments, and organizations within the utility. Whenever a business process requires data from one system or application to be transferred to another system or application, the data needs to be manually extracted from the first database and then converted to the format of the other application’s database.
Two strategies exist for dealing with extreme level of effort needed to manage, update, export, convert, and import data formats between different applications and databases.
Reduce the number of databases by purchasing a large software suite from a single vendor using a single proprietary data format that is internally-integrated and compatible with all the applications needed by utility
Adopt a common data integration platform that allows external integration between multiple software packages using a shared data format
What does CIM Provide?
CIM is an information model, that is an abstract, formal representation of objects, their attributes, the relationships between them, and the operations that can be performed on them. It is NOT a database structure or physical data store. It is a technology-agnostic model for describing the properties of physical power system equipment, power flow data, and messages that can be exchanged between various platforms and applications.
To describe various power system objects, CIM uses Class Diagrams and Sequence Diagrams created using the Unified Modeling Language (UML). It also uses the Resource Description Framework (RDF) to describe classes and attributes in an eXtensible Markup Language (XML) file format. The details of what is covered in each part of the CIM is described in detail below.
Background and Structure of the CIM
UML Class Diagrams
The Unified Modeling Language (UML) provides 13 types of diagrams to define software architecture. One of the is the UML Class Diagram, which visually represents object hierarchies and relationships.
First a review of basic concepts and terminology related to class diagrams:
An object is any thing that we want to describe.
A class represents a specific type of object.
A class hierarchy is a model of the system showing every component as a separate class. The class hierarchy should represent the real-world structure of the system.
A package is a group of classes. Think of folders in a computer file explorer.
Inheritance allows us to define very general “parent classes” and very specific “child classes”.
Attributes are the properties that describe what type of thing the class represents.
Associations are the relationships between various objects and how they are connected to each other.
Class diagrams show all the attributes and associations of various classes in a particular package in a single picture. To read a class diagram, remember that
Lines with an arrowhead indicate class inheritance. For example, in the figure below, ACLineSegment inherits from Conductor, ConductingEquipment, Equipment and then PowerSystemResource. ACLineSegment inherits all attributes and associations from its ancestors (e.g., length), in addition to its own attributes and ancestors.
Lines with a diamond indicate composition. For example, Substations make up a SubGeographicalRegion, which then make up a GeographicRegion.
Lines without a terminating symbol are associations. For example, ACLineSegment has (through inheritance) a BaseVoltage, Location and one or more Terminals.
Italicized names at the top of each class indicate the ancestor (aka superclass), in cases where the ancestor does not appear on the diagram. For example, PowerSystemResource inherits from IdentifiedObject.
A complete set of UML Class Diagrams is provided in the Advanced CIM Modeling section. This section contains class diagrams for all the objects used in GridAPPS-D and tables of properties to help you create and pass your own custom SPARQL queries to the Blazegraph Database.
UML Sequence Diagrams
UML sequence diagrams are used to model the flow of messages, events, and actions between the entities of a system. Time is represented vertically—showing the time sequence of interactions in the system. Displayed horizontally at the top of the diagram are the applications or entities in the system.
CIM uses UML diagrams to represent work flow, operations processes, and other utility use-cases. For the purposes of application development within GridAPPS-D, a detailed understanding of UML sequence diagrams is not required.
Resource Description Framework (RDF)
The Resource Description Framework (RDF) is a method of defining information models that is specified by the World Wide Web Consortium (the W3C). Detailed documentation is available on the W3C website.
RDF focuses on making statements about objects in a subject-predicate-object expression. Each expression is commonly called a “triple” in RDF terminology. The subject is defined by naming a resource, the object denotes traits or attributes associated with the subject, and the predicate expresses the relationship between the subject and the object.
The subject, or resource, in an RDF model is expressed as a Uniform Resource Identifier (URI). URIs are similar to the Uniform Resource Locators (URLs) used as web addresses but are more general because they are not limited to accessible data on the web. The predicate and object are also technically URIs and so also are just identifiers. The subject-predicate-object triplets takes the form of expressing syntactical constructs like “a substation has a name”.
RDF Schema (RDFS) files describe the classes, attributes, and relationships of an information model and typically use an .rdfs file format. RDF instance files describe object instances and typically use an .xml extension. RDF incremental files describe changes to a set of object instances as described by an instance file, and typically use an .xml extension.
CIM uses RDF instance files to define power system models with unique master resource identifier (mRID) issued by a model authority. The mRID is globally unique within an exchange context. Global uniqeness is easily achived by using a UUID for the mRID. It is strongly recommended to do this. For CIM XML data files in RDF syntax, the mRID is mapped to rdf:ID or rdf:about attributes that identify CIM object elements.
Key Concepts & Terminology from RDF
URI References – CIM and GridAPPS-D use two URI references to identify properties and resources. These identify the RDF format and the CIM classes used.
<http://www.w3.org/1999/02/22-rdf-syntax-ns#><http://iec.ch/TC57/CIM100#>
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Summary of CIM XML Classes
This section provides a brief look at the classes of equipment modeled in CIM XML and used in GridAPPS-D.
Details of each package, the class diagram, and attributes of each class are provided in the relevant sections of the reference guide to this lesson.
Names, Nodes, Terminals
The Core package provides very high level information of the distribution feeder model
IdentifiedObject
The Core package contains a class called IdentifiedObject. This class is very abstract and only contains attributes used to reference the object either by a user or in software. The attributes of IdentifiedObject include *mRID*, which is the master resource identifier that should be a globally 3-18 unique identifier of objects; the mRID does not have to be human-readable. This identifier is generally intended to be used by software systems.
The attributes name, description, aliasName, and pathName are intended for providing identifiers that are human-readable. It is common for names of objects within a utility to not be unique due to historical naming conventions, the results of mergers and acquisitions, and the inability of other software systems to manage uniqueness. For these reasons, there are no constraints on these names requiring them to be unique.
PowerSystemResource
The PowerSystemResource class inherits from IdentifiedObject and provides another relatively abstract class used in the CIM. The PowerSystemResource class supports an association to a Company class. This relationship identifies the company that operates the resource.
ConnectivityNode
The ConnectivityNode class has a relationship to the Terminal class. Each ConductingEquipment object has Terminals, which are then connected to ConnectivityNodes. The terminals can be thought of as being closely related to the conducting equipment, and the connectivity nodes are the glue that defines what equipment is connected to what other equipment.
CIM also includes the TopologicalNode class, which is used to convert breaker-switch oriented power system models to bus-branch models. This object is not used in GridAPPS-D, which does not feature transmission substation configurations (e.g. breaker-and-a-half, main-and-transfer-bus, ring-bus, etc.) that require topological processing of breaker and switch positions to determine network topology and line connectivity.
Power System Equipment
CIM XML provides a number of classes for defining physical power system equipment, including lines, switches, transformers, regulators, capacitors, and reactors.
Equipment and ConductingEquipment
The ConductingEquipment class inherits from an Equipment class which inherits from PowerSystemResource. This is the parent class for most of the physical equipment that are used to model the power system.
Conductor and ACLineSegment
Directly inheriting from ConductingEquipment is the Conductor class. This class specifies the length of the conductor.
Each segment of a distribution line is defined in a CIM model as an ACLineSegment. This class contains the electrical attributes commonly associated with a line needed for steady state analysis, including the positive-sequence and zero-sequence resistance, reactance, conductance, and susceptance.
More details are available in the `LineModel class diagram <>`__ and `list of attributes <>`__
PowerTransformer, TransformerWindings, and TapChanger
These three classes specify the portions of a step-down transformer and regulator.
The PowerTransformer class inherits from Equipment (not ConductingEquipment) and has associations to the TransformerWinding class.
The majority of the electrical characteristics associated with the transformer are actually associated with the TransformerWinding objects.
An association from the TransformerWinding class to the TapChanger class is used when the transformer has a tap changer. The TapChanger class has as attributes for things like the tap steps and nominal setting. The TapChanger class inherits from the PowerSystemResource class instead of the Equipment class, so it has few inherited attributes and associations.
References
Portions of this tutorial have reproduced verbatim text and information from the EPRI report An Introduction to the CIM for Integrating Distribution Applications and System and the CIM Ontology Diagrams
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