The development of future energy systems in accordance with the smart grid (SG) paradigm requires a radical change in the management of the electricity distribution network, which needs to become intelligent and adaptive. Smart distribution networks (SDN) have systems in place to control a combination of distributed energy resources (DERs). Distribution system operators (DSOs) have the possibility of managing electricity flows using a flexible network topology [1
]. The transition towards SDN involves software, automation, and controls to ensure that the power distribution network, not only remains within its operating limits (e.g., node voltages and branch currents within acceptable limits), but is also operated in an optimal way. In the SDN context, therefore, Information and Communication Technologies (ICT) are not a simple add-on to the electrical system, but their availability and efficiency are essential for operating the entire power distribution system. In fact, the electric system is managed and controlled through ICT network, which allows a bidirectional exchange of large amounts of data, creating a keen interdependence between electric system and ICT system. In the ICT system, the communication between the SDN components is characterized by non-idealities such as latencies and packet losses that may reflect upon the power system operation; furthermore, components such as antennas, routers, modems, etc. are subjected to faults and malfunctioning that may cause system reliability reduction or service interruption [3
In this context, this article aims at providing—by means of a co-simulation-based assessment method—an evaluation of the performance of LTE as communication technology for smart grid application, considering a highly time-critical application like fault location, isolation and system reconfiguration (FLISR).
1.2. Literature Review on Simulation of Communication Systems for Smart Grids
With recent enhancements in wireless solutions, which guarantee a reliable low-cost communication, a strong interest is upon the possibility of exploiting last generation communication systems for supporting the transition of distribution network towards a Smart Grid scenario. However, the best option for communication technology solution to fit SG applications is still not clear, even though LTE technology is considered one of the most promising. LTE, with its widespread distribution, broad coverage, high throughput, device-to-device (D2D) capability, despite not being originally designed for smart grid applications, represents a valuable candidate for usage in a SG communication system [4
]. A comprehensive analysis of an LTE-based smart grid operation analysis with a co-simulation approach is still missing in literature. In [6
], the communication challenges when choosing a technology supporting distribution automation applications was investigated with the communication software OPNET. LTE performances were analyzed in terms of coverage, delay and reliability with variable real-world deployment constraints, but the impact on distribution network was only analyzed in terms of requirements, and no interrelation between communication network and distribution network was analyzed in a joint way. A similar approach was adopted in Reference [7
], where OPNET simulated using LTE for transmitting Phase Measurement Unit (PMU) packets in a fault monitoring system. Performances in terms of latencies, channel utilization, and response with variable load were examined. An analogous methodology was applied in [8
], where LTE was analyzed in an OPNET environment to investigate the impact of SG communication on public shared LTE networks. Finally, in [9
], LTE latencies were theoretically investigated based on requirement documents released by the National Institute of Standards and Technology (NIST), and the traffic distribution of smart grid distribution automation considering a smart grid application reserved bandwidth.
All the mentioned publications miss catching the cyber-physical behavior of smart grid, where electric and communication systems are strictly interdependent. A simulation platform where both domains are jointly simulated is fundamental in order to correctly analyze the smart grid behavior providing test platforms for smart grid applications that can be used for engineering smart grids from use case design to field deployment [10
Smart grid simulators may be classified according to their modeling capabilities of power and communication systems. Three alternative approaches have been proposed in literature to tackle this kind of studies: Co-simulation, comprehensive simulation, and hardware-in-the-loop.
Co-simulation usually involves the integration of two or more simulators to capture cyber physical interdependency of a process or system. By co-simulating conventional power system simulation with communication and automation systems, the impact and dependencies of communication on the system can be investigated [11
]. In co-simulation, each system is analyzed by its own dedicated simulator, and all simulators are executed simultaneously by appropriately designed run time interfaces (RTI) and coordinated simulation management. Various solutions for realizing a co-simulation tool, that differ in the targeted field of researching smart grids, and consequently in architectural choices, e.g., software components, time synchronization strategy, and scalability, can be found in the literature. Among them, for instance, EPOCHS is recognized to be one of the first co-simulation tools for power systems [13
]. It was developed integrating three different commercial software: PSCAD/EMTDC and GE Power Systems Load Flow Software (PSLF) simulating the power grid, and ns-2 simulating the telecommunication network. PSCAD/EMTDC is dedicated to simulate electromagnetic transients, whilst PSLFs simulates the electrical system for long-term scenarios. Another important pioneer platform for co-simulation is GECO [14
]. It exploits the event-driven method for synchronizing the simulation of the power system (with PSLF) and the communication network (modeled with ns-2). In this tool, each iteration of the numerical solution of the power flow is an event. All events are integrated in the event scheduler of ns-2, allowing a perfectly integrated simulation and minimization of synchronization errors. If compared to time step synchronization, event driven synchronization permits reducing simulation time and simulating large power systems with reduced computational burden. An alternative approach is comprehensive simulation, that combines power system and communication network simulation in one environment. In this case, the main concept is to bring together both system models and solving routines which leads either to integrate power systems simulation techniques into a communication network simulator or vice versa. A comprehensive simulation approach has been adopted for instance in Reference [15
], where the authors presented a modular simulation environment based on OMNeT++, exploiting existing models for the communication network but purposely developing extra models for the electrical network. Finally, co-simulation could be realized with hardware in the loop (HIL) with software simulators and hardware components integrated in a real test bed, often used for testing control and protection systems in power systems [16
]. HIL approach allows a perfect correspondence with a real system but with higher investment costs. A detailed state-of-the-art review of appropriate tools for simulating both domains of power system and ICT processes in the evolution of smart grids was presented in Reference [17
The authors of Reference [18
] proposed a classification of different fields of application of the co-simulation/HIL approach for smart grid analysis. Three macro-areas were identified:
wide area monitoring and control (WAMC);
optimization and control in distribution networks;
integration of distributed generation.
In these fields of application, co-simulation approach allows emphasizing several critical aspects related to the interaction between the electrical system and ICT for smart grid operation, in particular:
impact of latencies on correct operation of the electrical system [19
use of artificial intelligence in the management of smart grid [21
effect of cyber attack on smart grid management algorithms [23
1.3. Contributions of This Paper
The objective of this paper is to demonstrate that LTE may provide appropriate performance for supporting data communication required to perform fault location, extinction, and a subsequent network reconfiguration in smart power distribution networks. For this reason, the co-simulation tool adopted has been purposely developed to simulate the highly time-critical smart grid application of fault management and network reconfiguration and permits reproducing and evaluating the behavior of the public mobile telecommunication system 4G LTE as communication technology for smart grid applications. In particular, this study focuses on the impact of LTE performances on network operation during fault management and reconfiguration. The architecture for co-simulation proposed in this paper coordinates two software packages, i.e., OMNeT++ for the ICT system and DIgSILENT PowerFactory with a Python script for the power system. A MATLAB Graphical User Interface (GUI) which allows the user to personalize the input data and to interact with the simulation as shown in Figure 1
The co-simulator uses electromagnetic transient analysis capabilities of PowerFactory and the wide choice of libraries for communication systems analysis that are offered in the OMNeT++ open source environment. Specifically, in this paper, a system-level simulator for LTE and LTE-Advanced networks (SimuLTE) was used [27
The Python script coordinates both dynamic simulations and allows data exchange between software packages through dedicated interfaces. PowerFactory provides a Python API that allows accessing software functionalities, element parameters, simulation results, etc.
The integration of OMNeT++ in the co-simulation framework was obtained with a TCP socket connection programmed in C++. The Python script is the Run Time Interface (RTI) of the co-simulation tool. For each time step Δt, the script calls PowerFactory for solving the differential algebraic equations that describe the electric network analyzed during the time interval, and contemporarily calls OMNeT++ that executes the simulation during the subsequent time step. The scheduler is the heart of the simulation in the OMNeT++ environment, as its purpose is to handle the event list and run the scheduled event for the next instance. A customized scheduler was purposely developed in RTI to properly coordinate the OMNeT++ simulation. In the proposed application, when a short circuit condition is detected in the electric network, a new event is scheduled in OMNeT++ simulating the communication among the distributed devices involved in FLISR.