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