## 1 Introduction  
The substation automation system (SAS) plays a vital role in the optimal and reliable operation of modern power systems. The SAS is used for automating the control systems of substations and developing the remote monitoring and control mechanisms in energy industry [1]. Furthermore, it defines protocols for critical communications among Intelligent Electronic Devices (IEDs), which should be fast, reliable, and secure enough to address the power system operation, control, optimization, and protection issues. These all bring a new level of challenges to the power system control and protection. Generally, communication protocols and data model for data exchange among the IEDs are defined through substation automation standards. Although a variety of standards, protocols, and technologies have been developed in this regard, many of them are vendor dependent and they do not properly address the interoperability issues in complex systems, and also do not support high speed communication technologies (e.g., Ethernet). Among all these standards, IEC 60870, MODBUS, and DNP3 are still being used in industry [2]. The IEC 61850 standard is the newest one which tries to address the above-mentioned drawbacks through new data models and protocols [3]. The IEC TC57 developed the IEC 60870 for basic remote-control communications between remote units and master stations and defined the profiles for different vendors to address the compatibility issues [4]. On the basis of the open systems interconnection (OSI) model [5], which categorizes the communication network into seven layers, this standard mainly acts on the data link layer and uses RS-232, RS-485, or fiber optic interfaces at the physical layer for point to point network topology communications. Nowadays, the MODBUS protocol is used widely in industrial applications because it supports different network technologies such as serial communications, optical/radio networks, RS-232, RS-485, and RS-422. The MODBUS was basically developed for response/answer manner and acts at different layers of the OSI model [6]. The MODBUS is optimal protocol for serial communications and originally was developed for master-slave communication manner which means that it inherently is not suitable for Ethernet communications. Although the MODBUS plus covered the internetworking remote communications over the internet by adopting the TCP/IP protocol, this protocol still suffers from some disadvantages. For example, the sequence of events can be missed due to a lack of time stamping action. The distributed network protocol (DNP3) is usually used within the SCADA systems and defines the rules for interconnecting the automation systems [7]. Considering the OSI model, the DNP3 is a Layer-2 protocol with related addressing mechanism, link control, multiplexing, etc. Although the DNP3 is highly used in power industry applications, its main drawback occurs in the interconnection with data units and substation transport events, where the DNP3 protocol data packets lose their logical context. In response to the above-mentioned standard limits, the IEC 61850 standard was developed to implement the modern networking technologies, data model, and communication protocols [8; 9; 10]. This is a comprehensive standard with advanced protocols for device integration, data encapsulation, and network services, and therefore it provides a highly flexible design environment for each project and considers the communication technologies, object definition, and requirements. For example, there are three defined data models in this standard for different applications, i.e., GOOSE, sampled measured values (SMV), and manufacturing message specification (MMS). The GOOSE stands for the generic object-oriented substation event and is used for critical, time-sensitive, and multicast applications such as tripping/blocking signals within the wall of a substation. This type of communication occurs over the data layer (Layer 2) of the OSI model and must be completed in less than 4 milliseconds (ms).