modelling and simulation of digital substation automation ... · 978-1-5386-6159-8/18/$31.00 ©2018...

978-1-5386-6159-8/18/$31.00 ©2018 IEEE

Modelling and Simulation of Digital Substation Automation for Inter-Substation Line Protection

Anuj M Nichani, Student and K. Shanti Swarup, Senior Member, IEEE

Abstract— Power systems have grown to become complex

energy networks, they consist mainly of substations connected by

transmission lines. These lines are key components of the

networks and need to be protected to ensure the integrity and

reliability of the system. The control and protection of equipment

within a substation has evolved from manual to hardwire

automated to Supervisory Control And Data Acquisition

(SCADA) enabled to Intelligent Electronic Device (IED) enabled

and finally now digital automation. The present standard of

substation automation globally accepted is the International

Electrotechnical Commission 61850 Standard. While this

standard has guidelines and rules for the mechanisms within a

substation, it does not fully address the use of digital

communication for Transmission line protection. This paper

shall explore the possibility of transmission line protection using

digital substation automation concepts.

Keywords— Digital Substation; Line Protection; Riverbed; IEC 61850; Digital Automation;

I. INTRODUCTION

The modern day substation relies on microcontroller based Intelligent Electronic Devices (IEDs) to perform the functions previous performed by mechanical relays. While this has made the protection and control (P&C) functions highly automated and reliable it brings with it several issues, mainly related to the communications between the Instrument Transformers (ITs), the IEDs and the Circuit Breakers (CBs).

Firstly, each IED must be physically connected to every IT and every CB with copper cables to receive and transmit signals. Secondly, the protocols used for such communication are inevitably proprietary protocols. This makes the utility highly dependent on the original equipment manufacturers (OEMs) and also makes updating/replacing faulty units troublesome because of compatibility issues.

Author A.M.Nichani is a student with the department of Civil Engineering IIT Madras, Chennai, India. Author K.S.Swarup is a Professor, Department of Electrical Engineering, IIT Madras, Chennai, India.

The International Electrotechnical Commission (IEC) formulated the 61850 standard to solve exactly these issues. The standard has first, introduced the concept of the Process Bus (PB) and eliminated the need for expensive and redundant copper wiring between individual elements. It has also standardized not only the communication protocols, but also the data formats and data handling systems which each vendor must build into the IEDs, hence solving the interoperability and compatibility issues. Substations which adhere to this standard are termed as Digital Substations. A detail description of the standard has been taken up in [1].

Figure 1. Comparison of conventional and Digital Substation [2]

When it comes to the protection of transmission lines, there exist two options: The distance protection scheme, and the differential protection scheme. While both have their pros and cons, in the present day the application of differential protection to transmission lines is not preferred due to latency in communication and synchronization issues, and the distance protection scheme is widely used. This paper’s objective is to first model the data communications of both schemes on Riverbed Modeler and study the improvements that can be made to each of these schemes by extending the IEC 61850 paradigm to elements outside the boundary of the substation.

II. PROTECTION SCHEMES

The protection of equipment within a digital substation is implemented as follows:

1) The values of currents and voltages are read by Non-Conventional Instrument Transformers (NCITs), their output is of a standard digitized format called Sampled Values (SVs), and these SVs are published onto the process bus (PB). In case of conventional Instrumentation Transformers (ITs), a

Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India

merging unit (MU) is used to convert their output values to the standard SV form and publish them on the PB.

2) All the IEDs which require these SVs will read them off the PB. The IEDs are programed to monitor the values and are enabled to make decisions based on them.

3) If the SVs indicate a fault, the IED responsible will send out a Generic Object Oriented Substation Event (GOOSE) message over the PB to the concerned CB to initiate a trip.

4) Once a CB receives a GOOSE message to trip, the contacts open and the fault is isolated. It then reports the same as a GOOSE message over the PB. From the above sequence of events, it is obvious that SVs and GOOSE messages are time critical. Hence under the IEC 61850 standard these messages are sent directly over the physical layer of the substation local area network (LAN), on high priority, using a publisher subscriber mechanism. While this is the fastest way to deliver these messages over LANs. When we apply these concepts to transmission line protection, the SVs and GOOSE messages must be transmitted over Wide Area Networks (WANs), direct delivery in such cases is not possible, and we must transmit these messages using the User Datagram Protocol (UDP) over the internet protocol (IP) for WANs. These guidelines have been set in the latest revision of the IEC 61850 standards under part 9. It is to be noted that when SVs and GOOSE messages are sent over a WAN they are referred to as Routable- SVs (R-SVs) and Routable- GOOSE (R-GOOSE).

Figure 2. Message – Protocol layer mapping

The two protection schemes of transmission lines are adopted to digital automation as follows:

A. Differential Protection

In reference [3], the authors have highlighted the advantages of line differential protection and then explored the application of IEC 61850 standards to differential line protection. The authors have described the basic principle involved in differential line protection using the IEC 61850 paradigm. In reference [4] these concepts are furthered, the authors have identified the logic nodes to be used. They have also designed the data communication required.

On examination of a differential line protection scheme, the flow of data is as follows:

Figure 3. Data flow scheme

With reference to Fig. 3., MU12 should report its SVs to P&C1 over LAN and P&C2 over WAN. Similarly MU21 should report its SVs to P&C2 over LAN and P&C1 over WAN. P&C1 and P&C2 will be programmed to compare these different values and if a fault is detected; P&C1 must trip CB12 by sending a GOOSE over the LAN of station 1, while P&C2 must trip CB21 by sending a GOOSE over the LAN of station 2. Briefly, for differential line protection the LANs of the substation will carry SVs as well as GOOSE messages, whereas the WAN between the stations need only carry R-SVs. It is therefore necessary that the end to end delay of R-SVs over the WAN be simulated. This will be taken up in the following sections.

B. Distance Protection

Distance protection in its present form is quite robust and is hence widely used for the protection of transmission lines. In reference [5], the authors have highlighted how selectivity is an issue intrinsic to distance protection, especially when faults occur at the extremes of zones. The authors have identified the logic node configuration as well as the data communication required for implementing line distance protection within the IEC 61850 paradigm. The authors however halt the discussion at this point and do not comment on the practical applications. In reference [6], the author proposes the formation of an Inter – Substation Process Bus (ISPB) for improved line distance protection. This ISPB is basically a process bus shared across the substations of a line over a WAN.

Figure 4. Proposed ISPB [6]


The IEDs at each point constantly update their state to the IEDs of the neighboring zones via GOOSE messaging over the ISPB. In line distance protection, the LANs of the stations would carry GOOSE as well as SVs. The WAN however would only need to carry GOOSE messages. We will simulate the data communication over an ISPB in the sections ahead and comment on the practicality of such distance protection schemes.

III. MODELING

The modeling is carried out on Riverbed Modeler (formerly OPNET modeler). This modeler is a powerful communications network simulation tool. Although it has a primary focus on computer communications, the tool is flexible and allows us to model custom communication applications such as GOOSE messages and SVs. Our aim will be to first model the communications networks required for the different line protection schemes. Next, model the custom applications which will run on the network. Then finally run simulations and capture the time delay in delivering the time critical messages (GOOSE and SV) over the WANs.

Figure 5. Workflow of modelling

A. Topology Modeling

Elements of the substation are represented by elements in the modeling environment based on the type of traffic they must generate, handle and receive. IEDs (P&C,MU and CB) have been represented by Ethernet workstations. The PB is represented by an Ethernet switch. LANs of substations are represented by 100Mbps Ethernet links. The WAN link is represented by an appropriate optic connection, each station is connected to the WAN by an appropriate router. The topology modeling of both the schemes are as follows:

Figure 6. Differential protection Topology

Figure 7. Distance Protection Topology

B. Application Modeling

Application modeling is necessary to accurately estimate the type and volume of traffic on the network. Applications are formed of several tasks, each task is further broken down to phases. Each phase is made up of requests which in turn are a frame of packets that are sent out by an element. For the definition of a custom application such as R-SV, we must identify, from the bottom up, the nature of packets, phases and tasks that make up the R-SV application. We then need to appropriately configure these details along with the parameters such as inter request time, inter packet time, etc. In our case, with respect to R-SV, the packet size is configured to be 128 bytes. As R-SVs for protection are generated at 4 kHz, the inter packet time is adjust appropriately. Next, as SVs are peer to peer messaging, we must configure the originating node and terminating node. SVs are messages which do not receive any confirmation, hence this unidirectional nature is also configured in the request – response pattern. Finally, as SVs are time critical they are assigned a high priority (Reserved (7)) as directed by the IEC standard. Further, the complication of configuring phases and tasks is simplified as the SV application has only one job. The configuring of SVs and R-SVs is identical in all respects except one, SVs will be configured for direct delivery whereas R-SVs will be configured for delivery over UDP/IP.


Figure 8. SV Task Modelling

GOOSE and R-GOOSE applications are modeled in the same fashion. The packet size for GOOSE is set as 256 bytes, also the frequency of GOOSE messages is much less and this is configured appropriately.

C. Profiling

Before the deployment of applications they must be profiled. This is simply a grouping of all the applications a particular element will use. For example, the P&C element will use both GOOSE and R-GOOSE applications. By creating this profile and assigning it to the appropriate nodes all the applications bundled under it get assigned to that element.

D. Application Deployment

Once the applications are profiled they must be deployed. This is an important step in the modelling as the logical connections between nodes and the flow of traffic is defined in this step. In the earlier step of configuring tasks, we define abstract nodes as the source and destination for the traffic generated by a particular task. In application deployment, the first step is to link all the physical nodes of the network to their respective abstract definitions. This is done by using the application deployment wizard.

Figure 9. Profile and Tier Deployment

For example, under the tasks of R-SV, we defined “MU” as the source of traffic and “P&C” as the destination, here we now link all the MU nodes of the network to the abstract “MU” source and all the P&Cs to the abstract “P&C” destinations. This creates a logical connection between all the MUs and all the P&Cs. In practice the traffic flows from a particular MU to particular P&Cs, this must be configured individually on each node by changing the Traffic Destination settings. Once this has been done, the exact flow of traffic is accurately modeled and can be verified by checking the logical connections.

Figure 10. Logical connections for R-GOOSE messages for

Line Distacne Protection

IV. SIMULATION AND RESULTS

Finally simulations can be run on the network. The time delay for end to end delivery of messages are plotted in Figures 11 – 14. And Summarized in the table below:

Message Average Delay Time (ms) GOOSE (LAN) 1.3

R-GOOSE (ISPB – Adjacent) 3.7 R-SV (WAN) 3.2


Figure 11 GOOSE LAN Delay

Figure 12 ISPB R-GOOSE Delay

Figure 13 R-SV Delay

Figure 14 R-SV & R-GOOSE delay times

The following observations are made from the simulations:

A. Differential Scheme

The transmission delay of R-SVs is 3.2ms, and the subsequent delay of GOOSE message is 1.3ms. This is obtained under normal working. A different scenario where the direct link between two substations is failed was simulated. Even under such a scenario the WAN delay time was not affected.

B. Distance Scheme

The results for the simulation of the ISPB show that when a R-GOOSE message is sent to a neighboring station the delay is 3.3ms. Further, if the R-GOOSE has to be sent to any further station, the delay time increases greatly, this is because of the routing delay the message incurs at every router along the ISPB.

V. CONCLUSION

The time delays obtained for LAN messages are in line with previously simulated and tested results within the field. This indicates that the models of the applications developed on Riverbed are reliable. The application of digital communication to line differential protection makes it feasible for use as well as improves the reliability of the system. The application of digital communication applied to distance protection will decrease backup protection times. However, the application of the ISPB for messaging across substations which are not adjacent cannot be used for time critical data.

VI. RECOMMENDATIONS FOR FUTURE WORKS

For future work, the simulated results for WANs may be verified with a setup. This would require hardware compatible of wrapping GOOSE and SVs with UDP/IP, along with a dedicated WAN. Secondly, while transmitting GOOSE and SVs over WANs there arises a need of securing the communication and network, various cyber security processes can be employed to achieve this. These will however affect the performance of the protection applications in several ways including adding to the ETE delay of messaging. It would be fruitful to study these effects and impacts of cyber security.

REFERENCES [1] R. P. Gupta, “Substation Automation Using IEC61850 Standard”, Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008 [2] Steven A. Kunsman, Going digital: A look at the modern substation, (online: https://www.windpowerengineering.com/business-news-projects/going-digital-look-modern-substation/) [3] Yiqing Liu, H. Gao, Weicong Gao, Naiyong Li and Mingjiang Xiang, "A design scheme of line current differential protection based on IEC61850," 2011 IEEE Power Engineering and Automation Conference, Wuhan, 2011, pp. 520-523. [4] I. Ali, S. M. S. Hussain, A. Tak and T. S. Ustun, "Communication Modeling for Differential Protection in IEC-61850-Based Substations," in IEEE Transactions on Industry Applications, vol. 54, no. 1, pp. 135-142, Jan.-Feb. 2018.


[5] B. Falahati, Z. Darabi and M. Vakilian, "Implementing distance line protection schemes among IEC61850-enabled substations," 2014 IEEE PES T&D Conference and Exposition, Chicago, IL, USA, 2014, pp. 1-5.

[6] J. Wu, D. Dostanov and M. Redfern, "Fault passage protection based on IDMT relaying with IEC61850-90 inter-substation communications," 12th IET International Conference on Developments in Power System Protection (DPSP 2014), Copenhagen, 2014, pp. 1-6. [7] Application modelling user guide, Simon Fraser University [8] A. Guzmán, J. Mooney, G. Benmouyal, N. Fischer and B. Kasztenny, "Transmission line protection system for increasing power system requirements," 2010 Modern Electric Power Systems, Wroclaw, 2010, pp. 1-11. [9] A. Schmitt and J. De La Ree, "Implementation of a purely digital substation system using the IEC 61850 standard," 2016 IEEE PES 13th International Conference on Transmission & Distribution Construction, Operation & Live-Line Maintenance (ESMO), Columbus, OH, 2016, pp. 1-5.

[10] T. S. Sidhu and Y. Yin, "Modelling and Simulation for Performance Evaluation of IEC61850-Based Substation Communication Systems," in IEEE Transactions on Power Delivery, vol. 22, no. 3, pp. 1482-1489, July 2007.


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