## 5 Deployment of PLC for Smart Metering and Smart Grid Systems
### Grid Description  
The electricity system is composed of four main elements: generation plants, transmission lines, substations and the distribution system. Electricity grids are different in the different regions of the world [125]. Grids are also different depending on the specific country we consider, and even its components are different depending on the age of the infrastructure. The Smart Grid finds different instantiations depending on the specific need and segment of the network it is referred to. PLC, for the purposes of Smart Grid in distributed grids, is a telecommunications access technology and consequently it is used in the access segment of the electricity grid. This refers to the so-called MV and LV segments of the grid. MV and LV segments of the grid are the ones closer to the PoS (see Figure 1). The MV segment [87] is the part of the grid that transitions from the HV grid to the so-called SSs (sometimes called transformer centers, as transformers from MV to LV are found in these premises). MV, depending on the cable configuration, may be classified as overhead or underground; this classification is not only important in terms of the cost and difficulty when building the grid, but it is relevant for the applicability and performance of PLC communications. Impedance of the power lines [126] is different in both types of networks, propagation is usually favored in overhead scenarios, access to the cables is of a different nature (signal couplers need to be installed differently), and more important, architecture is different as overhead power lines present a bus topology with mechanical switches in parts of the grid, whereas underground power lines configure a point to point network (where the \"points\" are the SSs) which is very attractive for the control of the telecommunications network built on top. LV segment [89] is the part of the grid in direct contact with the PoS. The LV segment presents two main characteristic: the first one is its capillarity and heterogeneity, as it needs to reach everywhere with the minimum investment; the diversity of configurations and the number of different components installed over the years, are some of the factors to be considered to understand the diversity of this scenario; and the second one is the susceptibility to the customer premises, where loads (customers and their appliances) are connected and disconnected due to the need to provide the universal electricity access, and that have different electricity consumption patterns, and connect and disconnect a variety of different devices at different moments. This circumstance implies that impedances change in different parts of the grid, and that noises appear and disappear during the day and the season without any clear and traceable origin. As a consequence this variability of the physical transmission media leads to different telecommunication subnetworks (\"subnetworks\" are each one of the PLC networks associated to each specific SS) which may be considered unique in their characteristics and performance. Notwithstanding the description above, it is often hard to generalize on the nature and structure of electricity grid, as these aspects are usually a consequence of the historical evolution of socio-economic conditions in the different regions. However, there are some factors that need to be addressed as they are important to understand how and if PLC is to be used for communications:  
* MV segment. Voltage levels need to be considered in the design of MV PLC solutions. The absolute value of the voltage level is a critical factor to design the couplers that need to transition the telecommunication signals from the transmitters to the receivers through the MV grid. Figure 1: Electricity distribution grid. * LV segment. Distances, amount of PoS, and its density in LV grid. These three parameters are presented together, as they are not independent variables (see [100]).