# Secure and Time-Aware Communication of Wireless Sensors Monitoring Overhead Transmission Lines
## 1 Introduction  
A smart grid is an initiative to promote and transform the traditional power systems to modern and automated power grids. An electrical power system is a set of many elements such as transmission lines, transformers and generators connected together into a larger system, that can generate, transmit and distribute electric power. Different kinds of electrical elements imply a large variety of dynamic actions or responses to disturbances. Some power system disruptions can affect single element, others can affect larger fragments. Some failures can spread and affect the system as a whole. As each dynamic effect reflects certain unique feature of power system dynamics, some of them can be grouped according to their cause, consequence, time frame, physical character or the place in the system that they occur. Based on the physical character of the disturbance, different power system dynamics can be divided into four groups, defined as: wave, electromagnetic, electromechanical and thermodynamic [1]. This classification also corresponds to the time frame involved (Figure 1 and Table 1). The fastest transients are related to the wave effects or surges in high voltage transmission lines and correspondto the propagation of electromagnetic waves caused by lightning strikes or switching operations. The time frame of these dynamics ranges from microseconds (\(\mu\)s) to milliseconds (ms). Much slower are the electromagnetic dynamics, that take place in the machine windings following a disturbance, such as a short-circuit, operation of the protection devices like the distance or overcurrent protection as well as the interaction between the electrical machines and the power system. Their time frame ranges from ms to seconds (s). Due to the oscillation of the rotating masses of the generators and motors that occurs usually after disturbance like a short-circuit or switching off large amount of generation, the electromechanical dynamics are even much slower. Electromechanical transients are also caused by operation of the protection system such as underfrequency or undervoltage protection. The time frame of these dynamics is from around a second to several seconds. The slowest dynamics are the thermodynamic transients, related to the active power generation control in power plants (boiler-turbine-generator control) or transmission line wires temperature variations due to varying weather conditions and line current flow. (The line current flow is a consequence of weather conditions or disturbances like long-term line overloading because of high ambient temperatures, short-term line overloading due to power swings in the system or very short-term high overcurrent due to short-circuit.) A careful inspection of Figure 1 shows the classification of power system dynamics with respect to the time frame. As it is evident from Figure 1, they are closely related to where the dynamics occur within the system. For instance, moving from left to right along the time scale in Figure 1 corresponds to moving through the power system from the electrical circuits (that contains elements such as resistance (or resistor), inductance (inductive coil or solenoid) and capacitance (or capacitor)) of the transmission network, through the generator armature windings to the field and damper winding, then