# Secure and Time-Aware Communication of Wireless Sensors Monitoring Overhead Transmission Lines
## 3 Transmission Line Monitoring Using WSN in Smart Grids
### Smart Grid Architecture  
In general, power grid energy subsystem is divided into power generation, transmission, distribution and consumer site. On the other hand, the power grid communication system is composed of several subsystems, being in fact a network of networks. The core of the monitoring and control of a substation is the Supervisory Control And Data Acquisition (SCADA) system. Remaining communication networks in smart grid systems include, for instance, cellular, microwaves, fiber optics, serial links or wireless local area networks. As in this paper we try to answer questions about time delays generated during measurement data transmission, we consider only the transmission grid. A typical transmission line is presented in Figure 2. Within the transmission grid, WSNs deal with power transmission issues and distribution monitoring. Because sensors handle sensitive and confidential power grid data, which should be securely transmitted to prevent from the theft and to avoid any unauthorized access, secure and reliable protocols need to be implemented. Because the nature of WSN makes the system vulnerable to various attacks, physical and cyber threats, we decide to analyze various WSN deployments (which differ in utilized security mechanisms or routing algorithms) and indicate the architecture, that will be the most suitable for different smart grid applications. An example, 110 kV transmission line, with wireless sensors deployed in miscellaneous places of electric poles (which is the basis of our considerations) is presented in Figure 3. Poles connect transmission lines between two substations. When considering 110 kV transmission lines, the most common distance between two poles ranges between 100 and 300 m. There exist however, transmission lines with the distance between poles equal to 450 m, but these are 400 kV lines, not considered in our case study. In our scenarios, we assumed the distance between two adjacent poles to be equal to 50, 100, 200 and 300 m. There are 43 poles between two substations, and 45 poles in total (including substations). There are 4 sensors and the relay node (also referred to as the sink node) deployed on each pole along the line (Figure 4), which results in 180 deployed sensor devices in total. A group of 4 sensors send the information to the relay on the same pole. After collecting information from all its slave sensors, the sink node on each pole sends the information to the substation. The role of the substation is to transmit the data to the gateway. Figure 2: A typical transmission grid environment. Because the distance between sensors and their relay node is quite small, so a short-range communication technology suffices, we utilized TelosB [15] devices as those located on electric poles and collecting the data (their outdoor range is equal to about 100 m), and Mica2 [16] motes as sink nodes, since their outdoor range is equal to about 300 m (which is quite enough when we consider the distance between two adjacent poles to be equal to 50, 100, 200 and 300 m). Table 2 shows some of the hardware characteristics of the utilized sensor devices. Mica 2 motes, requiring just 58 mm \(\times\) 32 mm of silicon, include data RAM, processing, and communication capabilities. The node combined with a tiny battery is capable of periodically reporting its presence for years to come being a a notable example of a general-sensing-class device. It includes a large interface connector allowing its attachment to an array of sensors. By providing a large number of I/O pins and expansion options, the Mica2 is a perfect sensor node option for any application where size and cost are not absolutely critical. It is, for example, easily connected to motion detectors and door-and-window sensors as the foundation of a building security system. Moreover, the Mica2 is