# Smart Grid Architecture for Rural Distribution Networks: Application to a Spanish Pilot Network
## 2 Typical Rural Distribution Networks
### General Features of Rural Distribution Networks  
Focusing on the rural systems, the majority of rural distribution networks comprise long overhead Medium Voltage (MV) lines, which are operated between 1 kV and 36 kV [5; 23] with bare cables [4]. From these MV lines, overhead Low Voltage (LV) lines feed customers up to few hundred meters away by crossing valleys, mountains and forests. Furthermore, due to the orography, these networks are difficult to access, which is aggravated by adverse meteorological conditions. Both line types can basically be considered resistive. Historically, overhead lines with bare cables were erected because they are the most cost effective way to provide power supply to rural and remote areas [4]. This type of electrical line is critical because the majority of failures suffered in rural distribution systems occur precisely in overhead bare lines during severe weather conditions. In particular, according to [4], about 62% of all middle voltage network faults are caused by nature; with adverse weather conditions, this figure increases up to 92%. The rural distribution circuits are sized according to their mechanical factors rather than according to voltage drops or maintaining the continuity of supply to a number of large consumers [5; 23]. Accordingly, Distributed System Operators (DSOs) have been replacing the LV overhead bare lines with overhead or underground three- or four-wire cables, which are more reliable but more expensive. However, the majority of MV lines are still overhead bare conductors. The typical topology of rural systems is radial [5; 8; 23] and usually connects dispersed consumers across a wide territory. This yields important variations in voltage levels between the feeder and different consumption points [4]. In contrast to the urban distribution networks, the capacity of rural lines is limited by the important voltage drops and mechanical requirements, not by the thermal rating of the conductor [5; 33; 34]. In addition, the rural distribution networks lack strong interconnections with transmission systems, thus converting rural networks into weak systems [33]. The configuration of rural systems cannot be easily changed because there are few switches and disconnectors, in addition to being manual and in remote locations [4; 35]. The electrical protections are also weak because they are sacrificial protections like fuses [4; 35]. In contrast to urban, industrial and commercial mesh networks, the rural ones usually have only an upstream protection device that disconnects the whole system in case of any fault. This greatly reduces the network resilience and increases the number of hours without supply to the consumers. Therefore, it is recommended to improve the resilience and performance of the system by sectionalising the network with recloser or automatic recloser circuit breakers, including back-up distribution substations, and by deploying Distributed Generation (DG) and fault indicators [4]. Rural distribution grids are more vulnerable than other grids. The aged infrastructure of a rural network combines with the other usual challenges in this kind of location, where we can find lower quality indices (number or time of interruptions), difficulty of access after electrical contingencies, voltage variations, grid congestion, tree pruning and scattered consumption. All of this requires updated solutions and cutting edge technologies for facing them. Traditionally, electrical utilities have solved these issues by building new secondary substations, lines, and other electrical infrastructures. Even in specific cases, capacitors are deployed in order to compensate reactive power flows and reduce voltage drops. In addition, the new DGs have exacerbated the issues mentioned above, demanding more static and non-flexible reinforcement of the network. In addition, rural grids, unlike urban ones, usually have no communication infrastructures.