IEC 61850-9-2 Based Process Bus

Authors: Tarlochan S. Sidhu, Mitalkumar G. Kanabar, and Mohammad R. Dadash Zadeh, Department of Electrical and Computer Engineering, University of Western Ontario, Canada

The merging unit (MU) is a key element of the IEC 61850 process bus. It gathers information, such as phase voltages and currents from instrument transformers, and status information from transducers using proprietary links.

All these analog values are converted to digital, and merged into a standard data packet format. This data packet is sent to corresponding bay level   protection and control IEDs using standardized Ethernet based communication links. As Figure 1 shows, MU has a time synchronization source (GPS clock), which provides a time stamp on each data packet. Time synchronization of each packet is required for protection and control IEDs to estimate accurate phasor.

Benefits from IEC 61850-9 Process Bus are:

  • Interoperability:  All devices connected to IEC 61850-9 process bus are able to share any piece of information with any IED
  • Simple connection: Point -to-point copper wire connectors are reduced to just few connections of communication links
  • Overall cost reduction: Deployment of IEC 61850-9 process bus reduces the installation and labor cost, even though material cost may be higher

Challenges with the performance of IEC 61850-9-2 process bus network: According to IEC 61850, the acceptable maximum communication delay for the highest class is as few as 3 msec. This has to be achieved independent from the traffic flows on the process bus communication network. The time critical messages on process bus are of two types:

  • Raw data message or sampled values (SV)

Process bus time critical messages are directly mapped to the Ethernet link layer. That means, these time critical messages are communicated with no transmission reliability due to elimination of TCP/IP layer (TCP acknowledges the receipt of the packet, and re-transmit if packet is lost). Therefore, to enhance the transmission reliability of GOOSE, IEC 61850-8-1 has proposed repetition of the same GOOSE messages multiple times. However, IEC 61850-9-2 does not suggest same message repetition for SV packets, which is justified because SV packets are continuously injected from several MUs at the rate of 80 samples per cycle. Repetition of the same SV packet several times would increase the network load tremendously. This means, there is no assurance or reliability measures for the SV packet communication over the process bus.

All the time critical messages will have priority tagging along with VLAN (IEEE 802.1Q) to enhance the performance over the process bus. However, IEEE 802.1Q may not be able to improve the performance especially in the worst case scenarios. For example, if higher priority packets arrive during the transmission of the lower priority packets with large size, higher priority packets will have  to wait until the transmission of the lower priority packets ends. This worst case scenario also depends upon the size of the packet and traffic on the network. That is why implementation of QoS using IEEE 802.1Q in Ethernet switch networks does not provide deterministic ETE  (End-To-End) delay on the network during worst case conditions. Detailed  analysis and performance evaluation of SV packets over the IEC 61850-9-2 based process bus has already  demonstrated the SV packet loss and delay in certain worst case scenarios.

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