# On the Use of LoRa Technology for Logic Selectivity in MV Distribution Networks
## 4 Enabling Technologies for Logic Selectivity  
As stated in the previous section, in order to make the logic selectivity work, the **Blind** message has to be received before the Maximum Blind Waiting time expires. It has to be considered that, from the fault occurrence to the fault clearing, the protection requires some delays to perform its operations, and these have to be taken into account. In particular, when a fault occurs, the protection requires a time interval to detect the fault, typically equal to (i) 27 ms for unidirectional faults and (ii) 57 ms for directional faults [22; 23]. Furthermore, an MV circuit breaker takes about 60 ms to open the circuit [24]. As an example, the timing of the operations sequence of the protections PR1 and PR2 for the considered fault is reported in Figure 2. A maximum allowed fault clearing time of 500 ms is hypothesized. Figure 2: Timing of the operations of PR1 and PR2 if a fault downstream of PR2 occurs. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & & \multicolumn{3}{c}{**SF**} \\ \hline
**BW** & **7** & **8** & **9** & **10** & **11** & **12** \\ \hline
150 & 37.12 & 64 & 128 & 256 & 430.08 & 860.16 \\
250 & 18.56 & 32 & 64 & 128 & 215.04 & 430.08 \\
500 & 9.28 & 16 & 32 & 64 & 107.52 & 215.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Values of \(T_{packet}\) (total transmission time of the LoRa packet), expressed in ms, versus BW and SF, with CR = 1, implicit header (IH = 1), CR = 1, cyclic redundancy check (CRC) = 1, payload of 10 bytes. Both the protections recognize the fault after a delay of \(T_{D}\), representing the detection time. In the figure, the fault detection is highlighted by the rising edge of the signal FAULT DETECTED. As soon as the fault is detected, PR2 sends the Blind message to PR1; this event is displayed by a pulse of the signal TX BLIND. The protection PR1 waits for the Blind message, but it has to guarantee that the circuit is opened before that the maximum allowed time for fault clearing \(T_{FC}\) of 500 ms expires. Considering that from the tripping command to the effective tripping of the circuit breaker a time delay \(T_{TRIP}\) is required, PR1 waits for the Blind message for a Maximum Blind Waiting time \(T_{MBW}\) equal to \(T_{FC}\)-\(T_{D}\)-\(T_{TRIP}\). In the hypothesis that the fault is unidirectional, \(T_{D}=27\) ms, so each protection awaits the blind message for about \(T_{MBW}=500-27-60=413\) ms. In the example in Figure 2, PR1 receives the Blind message (event shown with the pulse of the signal RX BLIND) after a delay time of less than \(T_{MBW}\). So, PR1 inhibits the operation of its circuit breaker, which remains closed, as can be noted by the level of the signal CB1 STATUS remaining low. Similarly, PR2 waits for a Blind message from PR3; when the \(T_{MBW}\) of 413 ms expires and the Blind message has not been received, PR2 commands its circuit breaker to trip. In the figure, this event is shown by the level of the signal CB2 STATUS becoming high. Therefore, logic selectivity requires adherence to the precise constraints of communication times. This involves a series of issues concerning the design of a suitable network infrastructure that is able to guarantee fast and reliable communications and that must connect electrical devices to each other that may be a few kilometers away. Various enabling technologies [25], both wired and wireless, have been taken into consideration by researchers and operators in the sector [26]. Inside the primary substation, the devices are connected in a local network through an optical fiber which ensures message exchange with a latency below 1 ms [23; 27]. Outside the primary substation, the optical fiber is not available everywhere because of the costs related to building the infrastructure. In some cases, in the presence of overhead power distribution lines, some pilot projects have foreseen the installation of the optical fiber under the guard wire.