IEC 61850 Logical Nodes and Data Classes in Power System Automation Data Modelling

 

Modern Substation Automation Hierarchical Control 

What is IEC 61850? 

IEC 61850 is a standard for communication networks and systems used in the automation of power generation, transmission, and distribution systems. It was developed by the International Electrotechnical Commission (IEC) to provide a common framework for communication between devices in the substation automation domain. The standard defines a set of protocols, data models, and services that enable interoperability between devices from different vendors, making it easier to integrate and manage complex power systems.

In modern architecture of power system automation, the redundant connections of hard wiring involving RTU, PLC, metering device and instrument transformers can be eliminated with the use of IED (Intelligent Electronic Device). To accomplish this objective, an electrical engineer need to know how to do the data modelling using IEC 61850. 

What is Data Modelling? 

Data modeling is a crucial aspect of IEC 61850, which defines a standard way of representing and exchanging data between devices in a substation automation system. The standard specifies a Common Information Model (CIM) that provides a standardized set of objects, attributes, and services that can be used to represent data and functions within a substation.

The CIM defines a set of object classes, each of which represents a specific type of equipment or function within a substation, such as circuit breakers, transformers, or protection relays. Each object class has a set of attributes that define the properties of the object, such as voltage rating, current rating, or status. The objects and attributes are organized in a hierarchical structure, with higher-level objects representing equipment and lower-level objects representing functions or data points within that equipment. In addition to the object classes and attributes, IEC 61850 defines a set of services that can be used to access and manipulate the data in the substation automation system. These services include read, write, subscribe, and notify, among others. The services enable devices to access and modify the data in a standardized way, making it easier to integrate and manage devices from different vendors.

The use of a common data model enables interoperability between devices from different vendors and simplifies the integration and management of complex substation automation systems. It also enables the use of advanced applications, such as fault location and analysis, load flow analysis, and protection coordination, which require access to data from multiple devices in the substation.

Overall, data modeling is a critical aspect of IEC 61850, providing a standardized way of representing and exchanging data in substation automation systems, enabling interoperability and advanced functionality.

Data modelling in IEC 61850 begins by considering a box that contain the following layers: 

Figure 1. IEC 61850 Data Layers

1. The first layer is the physical device that is connected directly to the network address.  

2. The second layer is the logical device, the IED. Different manufacturers, has different of models of IED that can perform the combined function in substation automation i.e. metering, controlling, monitoring, protection and communication. 

Figure 2. Siemens IED

3. The third layer is the logical which can be considered as the abstract data objects and the main elements of the IEC 61850 object-oriented virtual model. This consists of standardized data and data attributes. IEC 61850 defines the ACSI, which creates objects and services independent of any protocols. This enables a hierarchical class model, in which all class information, services that operate on these classes, and associated parameters, can be accessed from a communication network. 

There are 13 logical nodes available in IEC 61850.

Logical Node Groups	Group Designator
System Logical Nodes	L
Protection functions	P
Protection related functions	R
Supervisory control	C
Generic References	G
Interfacing and Archiving	I
Automatic Control	A
Metering and Measurement	M
Switchgear	X
Instrument Transformer	T
Power Transformer	Y
Further power system equipment	Z
Sensors	S

4. The fourth layer is the data class. In this case, the 13 logical node groups is expanded to 86 data classes. 

Figure 3. Logical Node Groups

Each of the groups are further subdivided into Logical Nodes. There are 86 different types of Logical Nodes defined. Each of these are composed of data that represent some application specific meaning and are intended to provide separate sub-categories of data (poetic license with terminology applied here). For example, The Protection Function group comprises 27 different Logical Nodes, some of these are listed above. To map this to the real world, data from a protective relay with 21 and 51 elements would be mapped to PTOC and PDIS logical nodes respectively

5. The fifth layer is the data. The 86 types of data classes are further expanded to a total of 355 data. 

Figure 4. Data Class

To access a data in a 61850 system is similar to accessing data across a common IT network infrastructure in Windows Explorer wherein the user will browse the network until the data source is located.  

For example, a person is responsible for the HMI wish to animate a CB symbol on a one-line diagram: 

• CB1 is being controlled and monitored by IED1, so they would browse the network until this Logical Device was located.
• They would need enough 61850 nomenclature knowledge to know that the XCBR LN is associated with the status of the CB, then drill down into that “folder”

The picture below shows a sample of data modelling. In this process, the physical device in the left (a simple feeder) which consist of one circuit breaker with CT and VT in the line. Accordingly, the 2 IED's in the right IED 1 and IED 2 can perform as shown. For example, in IED 1, the circuit breaker is modelled thru a logical node data class XCBR while in the same IED, there is also another device which is intended for measurement MMXU. The data classes contained in the IED's above can be combined to perform a certain function based on how the desired outcome (by the designer or programmer).

Figure 5. Virtual Modelling in IEC 61850

The diagram below shows more comprehensive details about the interpretation of the physical world to the virtual world involving logical nodes and data classes under IEC 61850. 

Figure 6. Example on how IEC 61850 Interacts with the Physical World

In conclusion, the Logical Nodes and Data Classes defined in IEC 61850 provide a standardized way of representing and exchanging data in power system automation. The Logical Nodes define the functions and behaviors of devices in the substation, while the Data Classes provide a standardized set of attributes and services for representing data within those devices. Together, they enable interoperability between devices from different vendors, simplifying the integration and management of complex power systems. By adopting these standards, power system operators can improve the efficiency, reliability, and safety of their systems, while also enabling the use of advanced applications and analytics to optimize performance.

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