Delay performance evaluation of the IEC 61850 standard in power transmission substations

IEC 61850 and the coming G5 infrastructure

IEC 61850 is a resilient standard to support a growing demand in different domains. The flexibility of IEC 61850 ensures interoperability between communicating entities of Smart Grid by allowing the same data model to be used on different underlying communication protocols. This research work was focused on evaluating the performance of IEC 61850 in smart transmission power substation.

The End-To-End time delay of the most time critical message ‘GOOSE’ was evaluated for different LAN speed, sampling rate and performance parameters (VLAN & QoS). A combination of literature study and simulation-based evaluation has been conducted to answer the main research question,

Will IEC 61850 meet the time delay requirements if our power transmission substations are upgraded to realize the Smart Grid concept?

The answer from our research work is ‘Yes’. We have evaluated IEC 61850’s performance with high data rates in order to make it close to real-time communication happening in transmission substations.

Development of smart transmission substations based on the IEC 61850 standard is crucial for the future SG infrastructure. We have considered wired Ethernet in our work because it gives higher data rates and bandwidth with low communication delay. It is more secure comparatively to wireless technologies because it doesn’t have air as its transmission medium.

In transmission substations as compared to distribution substations, there are larger equipment and the communication traffic is high, Ethernet is the best suitable technology in all aspects.

The detailed dynamic models of substation automation devices, such as MU and IED were developed using C language of the OPNET tool according to IEC 61850 standard. The modeling of IEC 61850 messages, VLAN and priority tagging based on QoS attribute were also implemented.

Using the OPNET modeler, the ETE delay of GOOSE message for transmission substation were observed which were within the allowable limits. In addition to that, the effects of various parameters such as data rate, sampling frequency, Virtual Local Area Network (VLAN) and Quality of Service (QoS) on delay of GOOSE message was evaluated.

While studying the transmission substation applications over Ethernet, the average GOOSE delay at 100 Mb/s and 1 Gb/s were within the allowable 3-4ms. With LAN speed of 1 Gb/s, average GOOSE delay was much reduced as compared to 100 Mb/s LAN speed. There were spikes of delay along the simulation time which is because of retransmission event and FIFO scheme of Ethernet switch.

The results also showed that the VLAN attribute (IEEE 802.1Q) and QoS attribute (IEEE 802.1p) has to be implemented in the Ethernet for time-critical messages.

For future work, more experiments should be carried out to verify the performance of IEC 61850 for GOOSE when different types of background traffic mix is used. This work was only for GOOSE message, performance evaluation of SV traffic and Client/Server messages should also be evaluated. The impact of GOOSE priority on SV and Client/Server messages can also be studied.

The retransmission event and FIFO scheme that causes the delay spikes can be studied in more detail. Although IEC 61850 suggests to use Ethernet technology in transmission substations, Wireless LANs performance in transmission substations should also be studied.

Thesis Outline

This thesis has four chapters. Chapter 1 includes the brief introduction on power substations, the motivation behind this research and our contribution. It also provides a brief relationship between Smart Grid and IEC 61850 standard.

Chapter 2 provides the salient features of Smart Grid and IEC 61850. It also covers some knowledge about N-1 security criteria and network interdependency between Power and Communication networks. An introduction on OPNET, the software that is used to simulate this research work is also covered.

In Chapter 3, power transmission substation is modeled to corresponding communication network in OPNET. The IEC 61850 messages that we have considered in our communication network are presented. Furthermore, performance parameters that we have considered in our work are discussed in detail. Delay performance evaluation of the IEC 61850 standard for various evaluation parameters are presented with results.

The thesis is summarized and concluded in Chapter 4.

(N-1) Security Criteria and Power/Communication Network Interdependency

In this section, N-1 security criteria in power systems is presented with an example. It also contains a case study in which authors ran experiments to study the cascading failure risk by increasing the network interdependency between Power and Communication networks.

N-1 Security Criteria

The ultimate objective of the transmission system is to deliver electric power reliably and economically from the generators to loads. Power systems must tolerate sudden disruptions caused by equipment failure or weather. A system that is resistant to the outage of any one component is said to be N-1 secure.

N-1 security is fundamental to system operation and must be done in planning any transmission system regardless of its cost.

As an example, PowerWorld Simulator schematic will be used.

In power transmission system shown above, three transmission lines are carrying power from a Generator Substation to a Big Town Substation. The ongoing power and maximum capacity of transmission lines are shown in schematic e.g. the top transmission line is carrying 526 MW and have a total capacity of 725 MW.

Let’s consider a power failure in top transmission line, according to N-1 security criteria, 526 MW will be routed through the other transmission lines that are going to Big Town Substation to meet the power load of 1400 MW as shown below.

As you can see, failure in one transmission line doesn’t affect the system. Power was rerouted through other transmission line as they had capacity to carry the failed line’s power. In our work, we will be considering that our power transmission substation is N-1 secure which can reroute the traffic in case of one outgoing feeder failure.

Power and Communication Network Interdependency

The increased coupling between critical infrastructure like power and communication has important implications for reliability and security of these networks. By increasing the interdependency between Power and Communication Networks, increases the vulnerability in these network structures.