Impact of IEC 61850 Edition 2 on the Object Modeling of Motor Protection IEDs

Author: Alexander Apostolov, USA

The article concentrates on the impact of Edition 2 of IEC 61850 on the object modeling of motor protection Intelligent Electronic Devices (IEDs).
The first part of the article analyzes the functional hierarchy of motor protection IEDs. It then describes the challenges in modeling their functionality based on the IEC 61850 Edition 1 standard. The reason is the relatively flat hierarchy of the model - server containing Logical Devices, which then contain Logical Nodes. This hierarchy did not match the multilayer hierarchy of the existing motor protection IEDs.
The second part of the article describes the changes in the modeling hierarchy in IEC 61850 Edition 2 with the main focus on the nesting of Logical Devices. The complex model of a motor protection IED is presented with multilayered logical devices.
The inheritance principles of the modeling are described considering specifically the Mode and Behavior of Logical Devices and Logical Nodes in the IED hierarchy.
The last sections of the article analyze the benefits of the modeling of motor protection IEDs using the principles defined in IEC 61850 Edition 2. Improving the efficiency of testing as well as maintaining the availability of protection and control functions, especially during their maintenance testing will result in improved reliability of the distribution power system.

Functional Hierarchy
The modeling of a complex multifunctional protection IED such as a modern motor protection device is possible only when there is good understanding of the problem domain and the modeling principles. 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 low-end motor 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 an advanced motor protection IED. Figure 1 shows an abstract multifunctional IED containing several functions (Fi). Each function contains a multi-layer hierarchy of sub-functions (SFi). At the bottom of the modeling hierarchy are the function elements (FEi) – the building blocks of the object model.

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 steps in protection functions.
A multifunctional motor 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. The main protection functions in an advanced motor IED are:

  • Differential protection
  • Overcurrent protection
  • Thermal overload protection
  • Overvoltage protection
  • Undervoltage protection
  • Arc protection
  • Power protection

Non-protection functions are of several categories:

  • Measurements
  • Control
  • Condition monitoring and diagnosis
  • Recording

Figure 2 shows a simplified functional model of a motor protection IED.
 Each of the above described functions can be divided in sub-functions that represent groupings of related functional elements.

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