Current challenges and future trends in the field of communication architectures for microgrids
Renewable & Sustainable Energy Reviews
The concept of microgrid has emerged as a feasible answer to cope with the increasing number of distributed renewable energy sources which are being introduced into the electrical grid. The microgrid communication network should guarantee a complete and bidirectional connectivity among the microgrid resources, a high reliability and a feasible interoperability. This is in a contrast to the current electrical grid structure which is characterized by the lack of connectivity, being a
... nidirectional system. In this paper a review of the microgrids information and communication technologies (ICT) is shown. In addition, a guideline for the transition from the current communication systems to the future generation of microgrid communications is provided. This paper contains a systematic review of the most suitable communication network topologies, technologies and protocols for smart microgrids. It is concluded that a new generation of peer-to-peer communication systems is required towards a dynamic smart microgrid. Potential future research about communications of the next microgrid generation is also identified. TODAY'S MICROGRID STATUS A Microgrid (MG) (Figure 1 ) is a low voltage distributed network of individual consumers within a building, campus, or community that are interconnected with, at least, one shared distributed generation source (DG). A microgrid consists of a variety of loads, microsources (MS) and energy storages systems (SS), called distributed energy resources (DERs), that acts as a single controllable entity with respect to the main grid , ,  . Microgrid operates mostly connected to the main distribution network but they can be automatically disconnected from the main grid at the point of common coupling (PCC) in case of faults to provide a minimum level of service during a utility grid power outage. They can be reconnected once the fault has disappeared ,  . Microgrids must have their own control to ensure the correct operation and coordination of the different DERs. A Microgrid Controller (MGC) is usually needed to manage the operation within the microgrid, the energy flows and the interconnection with the main grid. In addition, all microgrid devices need to communicate with the MGC. Fig.1. Simplified scheme of a microgrid with a Microgrid Controller Traditionally, this control is carried out by means of a three level hierarchical scheme , : (Figure 2): Distribution Management System or tertiary control (DMS), Microgrid Central Controller (MGGC) or secondary control and load control (LC) or primary control , , . Primary Control: This level of control operates in the time range of milliseconds to minutes, and reacts to the transient dynamics of the DER and the system to respond to any instantaneous deviation in the system's voltage or frequency. This controller acts as local control for each DER unit and utilizes local measurements and responds to short-term events such as islanding detection, sudden real and reactive power mismatches, and power sharing. Secondary Control: This level of control operates in the time range of minutes to hours, and it comprises the discrete dispatch of DER. This level is controlled by the MGCC. This controller is responsible for the optimal coordination and operation of the whole components connected in the same microgrid, assuring the overall maintenance of the grid parameters in both connected and island mode. The secondary control also incorporates control strategies and operations such as intentional islanding, resynchronization, and load shedding. Tertiary Control: This level of control operates in the time range of hours to days, and it involves the communication with the different microgrid central controllers (MGCCs), and management of the MG when it operates on the market. The main entities in this level are the Distribution Network Operator (DNO) and the Market Operator (MO) who are delegates of the main grid. Fig.2. General Hierarchical Architecture of MG control. Communication interfaces for stablishing MG control. Control strategies need communication networks or links between the different levels to achieve an optimal microgrid operation. Communication interfaces must be created to establish a bi-directional communication channels that allows information transfer between the different controllers . (Fig.2, ① and ②) . Generally, data flows between nodes in both directions, i.e., each node are able of receiving and forwarding data over links with other nodes or endpoints. The nodes in microgrids are created by adding information and communication capabilities to the underlying distributed energy resource or component, giving rise to intelligent electronic devices (IEDs)  . In this way, the microgrid controller may communicate with IEDs and other components to provide them data or control commands. For a successful information exchange between nodes within a microgrid system, predefined procedures or protocols for data transmission regulation are needed. A protocol suite consists of a layered architecture where each layer is assigned to a set of functions using one or more protocols  . Data communication networks commonly use multiple levels of protocols based on ISO-OSI (International Standards Organization/ Open Systems Interconnect reference) model  (seen Figure 3) . This allows to convert the information in a form that can be transmitted. Thus, regarding communications, the effectiveness of the control and the communication microgrid infrastructure is linked with the microgrid control scheme and its communication architecture. addition, there are difficulties to manage data in real time of a wide range of devices ,  . As a result, these disadvantages can lead to provide poor services, bottlenecks or underutilization of the network resources. An alternative technology to be used in microgrids is Power Line Carrier (PLC). PLC technologies use the electric power lines as a medium that enable the bidirectional data exchange. It provides a vast coverage and in terms of infrastructure is the most cost-effective technology since the lines already exist. In recent years, microgrids activities have brought a lot of attention to PLC technologies. As an example, the microgrids installed in the University of Seville and NUAA in China, uses PLC as a communication medium for information management and transmitting data  . However, PLC technology has negative effects in the communication channel such as a noisy medium disturbed, distortion, frequency impedance alterations and the risk of signal attenuation  . The increasing introduction of distributed energy resources into the power grid changes the current scenario, because the incorporation of micro-generation allows bidirectional power flows and active consumers, i.e., the end users change their role of passive consumers to active prosumers. Consequently, immediate solutions about microgrid communication architectures that cope with these changes and enable high performance data delivery and real-time monitoring and control are needed, leading to reliable, resilient and sustainable microgrid control systems. TOWARDS AN INTELLIGENT MICROGRID It has been noted that communication system used in today's microgrid has important inefficiencies and it is also localized to support the integrated communications needed for the modern power grid (smart microgrid (SMG)). However, energy systems are increasingly distributed. The integration of DERs into the energy system cause many challenges into the communications field. To incorporate more renewable and alternative energy sources, the communication infrastructure must have the ability to easily handle an increasing amount of data traffic or services requests and must provide a real-time monitoring and control operation of all these nodes. Current serial communications deployed in SCADA systems refer to a set of legacy standards that are still used for low data rate applications and asynchronous bit transfer. Since microgrid operations need timely control actions, a Real-Time Measurement Parameters (RTMP) function is required  . To reach this goal, it´s mandatory to know which bandwidth and which latency ( delay) can tolerate each microgrid application , , i.e., each microgrid function has its own latency and bandwidth requirement depending on the kind of system response it´s dealing with , , . The IEC 61850 and IEEE 1646 standards ,  give specifications for these requirements. The network performance requirements for each microgrid application have been summarized in Table 1. Moreover, the expected communication delay of each kind of microgrid message was specified in , being summarized in Table 2.