IEC 61850 - A BRAND NEW WORLD#3
The foundation of the IEC 61850 is the concept of virtualization, i.e. providing a virtual representation of the behavior of real primary or secondary substation devices.
As mentioned earlier, the virtualization covers only the relevant and communications visible components of the model. The figure below shows the use of this process to model an overcurrent stage of a protection relay from any vendor as an IEC 61850 logical node.
Virtualization of protection relays
The modeling approach in the standard uses the principles of functional decomposition and UML notation. It is used to understand the logical relationships between components of a distributed function and is presented in terms of the model hierarchy that describes the functions, sub-functions and functional interfaces.
The data flow is used to understand the communication interfaces that must support the exchange of information between the distributed functional components for different applications. The information modeling on the other hand is used to define the abstract syntax and semantics of the information exchanged. It is presented in terms of the data object hierarchy that includes data object classes, types and attributes.
A very important differentiating factor of IEC 61850 compared to other communication protocols is that everything in this model has a name. This allows the definition of standard device models that support self-description and use of meta-data to be used for development of different engineering tools.
The models of multifunctional protection IEDs that include both protection and non-protection functions such as control, measurements, monitoring and recording are discussed. Two basic modeling approaches are possible:
Single Logical Device based model
Multiple Logical Devices based model
The modeling of a complex multifunctional protection IED such as a modern distance relay is possible only when there is good understanding of the problem domain. At the same time we should keep in mind that the models apply only to the communications visible aspects of the IED.
The functions in relatively simple IED, such as a low-end distribution feeder or transmission line protection relays, are fairly easy to understand and group together in order to build the object model. That is not the case for the more complex devices like a distance protection. The distance protection function has different components that need to be taken into consideration in the model. Complex to represent are also advanced transmission line protection schemes that typically exist in distance relays, as well as distributed functions based on high-speed peer-to-peer communications between multiple IEDs.
IEC 61850 defines not only the object models of IEDs and functions in a substation automation system, but also the communications between the components of the system and the different system requirements. It is very important to understand that the fact that one can model a function in a device or substation automation system does not mean that the standard attempts to standardize the functions. This is especially true for the distance elements. There are so many different algorithms and characteristics, as well as preferences and opinions, that this will be an extremely difficult task. Instead, the model represents the communications visible attributes and behavior of the device. This is one of the main reasons that there is a difference in the modeling requirements between IEC 61850 configuration applications and analysis or testing tools.
It is important to also remember that the changing technology introduces new methods for interface between the instrument transformers or sensors in the substation and the distance or other protection relays. They need to be able to interface with conventional and non-conventional sensors in order to allow the implementation of the system in different substation environments.
A simplified diagram with the communications architecture of an IEC 61850 Process Bus based substation automation system is shown below.
Simplified IEC 61850 based communications architecture
The Merging Unit (MU) multicasts sets of measured sampled values to multiple IEDs in the substation over the substation local area network. In some cases it is called the “process bus”. Status information for breakers and switches is available through an input/output unit (I/O). In some cases the merging unit and the input/output unit can be combined in a single device.
The receiving devices then process the data, make decisions and take action based on their functionality. The action of protection and control devices in this case will be to operate their relay outputs or to send a high-speed peer-to-peer communications message to other IEDs in order to trip a breaker or initiate some other control function, such as breaker failure protection, reclosing, etc..
The modeling of complex multifunctional IEDs from different vendors that are also part of distributed functions requires the definition of basic elements that can function by themselves or communicate with each other. These communications can be between the elements within the same physical device or in the case of distributed functions (such as substation protection schemes) between multiple devices over the substation local area network. The basic functional elements defined in IEC 61850 are the Logical Nodes.
A Logical Node is “the smallest part of a function that exchanges data”. It is an object that is defined by its data and methods. When instantiated, it becomes a Logical Node Object. Multiple instances of different logical nodes become components of different protection, control, monitoring and other functions in a substation automation system. They are used to represents individual zones or steps in a protection function.
A multifunctional protection IED has a complex functional hierarchy that needs to be modeled according to the definitions of the IEC 61850 model. It has two main groups of functions - protection and non-protection. The protection functions can be further divided into main protection functions, backup protection functions and protection related functions.
Each device sub-function then can be split in functional elements. Functional elements can be defined as the smallest functional unit that can exist by itself and also can exchange signals or information with other elements within a device or a system.
The modeling of complex protection devices depends not only on their functionality, but also on the configuration of the substation where they are installed. The model will be different if the transmission line is connected to a bus with a single breaker compared to the case of a breaker-and-a-half or ring bus.
The modeling of multifunctional devices needs to reflect the functional and modeling hierarchy described earlier.
Distance protection relay - simplified object model