Testing of IEC 61850 Sampled Value based Devices and Systems

Author:Alexander Apostolov, USA

The successful integration of these devices

  • The first component of the test system is the configuration tool that takes advantage of the standard substation or IED configuration files defined in Part 6 of IEC 61850
  • The second component is a Simulation Tool that generates the current and voltage waveforms. The specifics of each simulated test condition are determined by the complete, as well as the configured functionality of the tested device
  • The third component is the Test Evaluation Tool that includes the monitoring functions used to evaluate the performance of the tested elements of a distributed sampled analog value based system. It receives, compares and evaluates the multicast by the merging unit sampled analog values with the original current and voltage waveforms applied to the merging unit. The test system may also retrieve and compare the waveform records from the tested device to the original waveforms from the simulation tool
  • The fourth component is the Reporting Tool generating the test reports based on a user defined format and the outputs from the simulation and evaluation tools

Distributed Functions Based on Sampled Analog Values
Sampled analog values transmitted over the substation local area network are one of the key differences between the typical communication protocols and IEC 61850. They allow the elimination of copper wiring between the substation instrument transformers and the protection, control, monitoring and recording devices. As a result the user can achieve significant savings in wiring, while at the same time improve the safety of the substation environment and the functionality of the substation automation system, as well as to minimize the commissioning and maintenance costs. According to IEC 61850 all communications in the substation are based on logical interfaces. The logical interface of interest in the case of distributed sampled analog values applications is IF4: CT and VT instantaneous data exchange (especially samples) between process and bay level.

With the continuous improvements in Ethernet technology and the reduced cost of switched local area networks, multicast become the preferred choice for sampled measured values applications.
IEC 61850 defines a number of different groups of logical nodes that represent functional elements of primary or secondary substation devices. It also describes functions as tasks, which are performed by the substation automation system, and exchange data with other functions. The functions are performed by IEDs (physical devices) and may be split in parts residing in different IEDs but communicating with each other (distributed function) and with parts of other functions. In the context of this standard, the decomposition of functions or their granularity is ruled by the communication behavior only. Therefore, all functions considered in the standard, can be modeled by logical nodes that exchange data. A function is considered distributed when performed by two or more logical nodes located in different physical devices – the typical case in systems using IEC 61850 sampled values.

The above is another key characteristic of IEC 61850 that determines the functional requirements for the testing of multifunctional devices or distributed applications using sampled analog values.

Analog Signal Processing for Protection and Control
The analog input function in IEC 61850 is modeled by multiple instances of two logical nodes from the Instrument Transformers group T - TCTR and TVTR. Both use one instance per phase. These three or four instances of TCTR(n) and TVTR(n) may be allocated to different physical devices mounted within the instrument transformer per phase.

The currents and voltages from TCTR and TVTR accordingly are delivered as sampled values to the protection, measuring or recording functional elements.
In the case of a multifunctional IED all the logical nodes are located within the same physical device and the interaction between them is performed over the internal digital data bus of the device. This is not the case in a distributed function using the sampled measured values communications defined in IEC 61850. Even that the application will be modeled using the same logical nodes, they are not anymore located within the same physical device – i.e. we have a case of distributed function according to the IEC 61850 definitions and this has a significant impact on the testing.

The processing of the secondary currents and voltages represented by the TCTR and TVTR is performed in a new type of device – the Merging Unit. It is defined as an interface unit that accepts multiple analog CT/VT inputs and produces multiple time-synchronized serial unidirectional multi-drop digital point-to-point outputs.
The merging unit is synchronized using 1 PPS (Pulse per second) signal from a GPS receiver. As can be seen from the Figure 4, there is a time delay D = D1 + D2 introduced within the device. If this time delay is not compensated, it will be seen as a phase shift that will affect all functions using the sampled analog values. It is clear that such phase shift will have an impact on some protection functions. That is why it is very important that the merging unit is properly tested in order to evaluate its performance.

Distributed applications based on sampled values include protection, control, measuring, monitoring, and recording. They are different functionally but will have similarities in the requirements for functional testing.

BeijingSifang June 2016