# Secure and Time-Aware Communication of Wireless Sensors Monitoring Overhead Transmission Lines
## 3 Transmission Line Monitoring Using WSN in Smart Grids
### Smart Grid Architecture
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Characteristic** & **TelosB [15]** & **Mica2 [16]** \\ \hline Current Draw (sleep mode) & 1.8 mA & 8 mA \\ Current Draw (active mode) & 5.1 \(\mu\)A & \(<\) 1\(\mu\)A \\ RF power & \(-24\)-0 dBm & \(-20\)-10 dB \\ Receive Sensitivity & \(-90\) dBm (min), \(-94\) dBm (typ) & \(-101\) dBm \\ Outdoor Range & 75–100 m & 305 m \\ Frequency band & 2400–2483.5 MHz & 433 MHz \\ Data Rate & 250 kbps & 38.4 Kbaud \\ Communication Protocol & IEEE 802.15.4/ ZigBee & IEEE 802.15.4/ ZigBee \\ Cost & $182 & $152 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mica2 and TelosB hardware specification. Figure 4: An electric pole with wireless sensors deployed. Figure 3: Default, hop-by-hop WSN architecture deployed on the example transmission grid. capable of receiving messages from nodes attached to high-value assets, including personal computers and laptops, at risk of being stolen. Crossbow's TelosB note bundles all the essentials into a single platform including: USB programming, capability, an IEEE 802.15.4 radio with integrated antenna, a low-power MCU with extended memory and an optional sensor suite. It offers many features, including, among others, IEEE 802.15.4/ZigBee compliant RF transceiver, 2.4 to 2.4835 GHz, a globally compatible ISM band and 250 kbps data rate. When it comes to the measured parameters, in the article we assumed that sensors measure the following quantities: wind speed and direction, atmospheric temperature, pressure and humidity, solar radiation along with conductor current, temperature, tension and inclination. For each pole along the transmission line, each sensor performs 6 measurements (ambient temperature, atmospheric pressure and humidity, solar radiation, wind speed and wind direction are taken into account). Because of the time stamp attached to each measured value, the number of measurements on a pole is doubled, which results in 12 measurement values. There are also 3 current measurements for every phase along with 3 conductor temperature measurements, 4 tension and 4 inclination measurements (for every phase and lighting wire), doubled because of the time stamp. This results in 40 measurement values in total. When we assume that a single measurement can be stored as a single float value (8 bytes), then the size of the packet being transmitted between relay nodes is equal to 320 bytes. The actual cost of the hardware depends on sensor's capabilities. An approximate cost of TelosB device, utilized in our scenarios, is equal to $182, while the relay node can be bought for about $152. According to the analysis given in [2] and the information provided in Table 2, the cost of the deployment of a WSN is indeed lower than the cost of, for instance, cellular links. The values measured by sensors are important for many different smart grid applications. Described parameters are used by wire thermal models of DLR systems for calculating actual value of allowable current or power transfer through the line. Such measurements can be also used by the power system estimation or optimization software. The thermal time constant of the power line conductor ranges from 5 to 15 min and depends on the actual weather parameters. Acquiring data from the whole power line spans in real time determines line operational safety. Let us consider, for instance, the conductor temperature measurement. It can be used for blocking an automatic _reclosing_ action after clearing a power line short circuit. Usually, the first reclosing time ranges from 0.5 to 1.4 s, while the second reclosing time is much longer and ranges from 10 to 20 s. Described reclosing time can be extended if the conductor temperature exceeds maximum allowable value after short circuit. In this particular case, in order to provide the required protection of the relay in the substation, it is essential to know if the measurement data containing conductor's temperature can be delivered within the assumed time period.