# Secure and Time-Aware Communication of Wireless Sensors Monitoring Overhead Transmission Lines
## 1 Introduction
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Phenomena**} & **Time Frame** & \\  & **Seconds (s)** & **Disturbances and/or Actions** \\ \hline \multirow{2}{*}{Wave} & \multirow{2}{*}{\(10^{-7}\) – \(10^{-2}\)} & Switching transients during opening and closing \\  & & power line circuit breakers or caused by lighting \\  & & strike to the transmission line \\ \hline \multirow{2}{*}{Electromagnetic} & \multirow{2}{*}{\(10^{-4}\) – \(10^{0}\)} & Synchronous or sub-synchronous resonances during \\  & & generator or electric motor switching to the grid \\ \hline \multirow{2}{*}{Electromechanical} & \multirow{2}{*}{\(10^{-1}\) – \(10^{2}\)} & Synchronous generator shaft swings caused by \\  & & short-circuits or power balance disturbance \\ \hline \multirow{2}{*}{Thermodynamical} & \multirow{2}{*}{\(10^{1}\) – \(10^{5}\)} & Boiler regulatory actions in thermal power \\  & & plants, secondary frequency control, heating and \\ \cline{1-1}  & & cooling transmission line wires due to weather \\ \cline{1-1}  & & changes or disturbances \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Classification of power system transients and their time scales. Figure 1: The time frames of the basic power system dynamics phenomena together with corresponding WSN monitoring capabilities. along the generator rotor to the turbine until finally the thermodynamics of boiler-turbine-generator slow transients are reached [1]. In today's electric power systems, sensing and monitoring equipment is employed in a limited number of critical assets, such as power transmission lines and substations. Generally, this sensing equipment is not interconnected, and transmits the collected data to a central location using wire communications (such as Ethernet or fiber-optic), where a supervisory control system and dispatchers at the utility headquarters manage grid operations. Wired monitoring systems require expensive communication cables to be installed and regularly maintained. Additionally, the use of wire communication in harsh environments is very often uneconomical or even impossible. Hence, there is an urgent need for cost-effective wireless monitoring and diagnostic systems that can improve system reliability and efficiency by optimizing the management of electric power systems. Recent studies showed [2; 3], that the philosophy of WSNs has a great potential to address the challenges of the existing power grids and fulfill their sensing and communication tasks. The need for uninterrupted electricity supply and the high costs of building new generating and transmitting power facilities are forcing governments and operators to make an efficient use of network infrastructure and increase the regulatory abilities of power grids using new kinds of measurements. Certain and uninterrupted supply of electrical energy became the foundation for the creation of a new Information and Communication Technologies (ICTs) [4] (such as WSN measurement systems). The ability to harvest new information about transmission line parameters implied a rapid development of new algorithms for power systems operation. One way of more efficient usage of grid infrastructure (for instance, overhead transmission lines and increasing its ampacity and transmission flexibility) is the monitoring of weather conditions, mechanical and electrical parameters along lines in order to determine the Dynamic Line Rating (DLR) and allow for controlled overloading during certain time. Additional measurements of the plurality of non-electrical quantities such as temperature, wire strain elongation or sag with the current weather conditions pose an additional information about safe or unsafe operation of transmission lines. Above considerations lead to the conclusion that WSN can provide an important information, which can be used for power system estimation or optimization along with providing an additional decision variable for dispatcher to turn off the line or not.