# On the Use of LoRa Technology for Logic Selectivity in MV Distribution Networks
## 2 Basic Concept of Logic Selectivity  
Logic selectivity (or zone selectivity) is based on the interconnection and the direct exchange of messages between the protections placed along a transmission line in order to identify and isolate only the segment affected by the fault [10; 11; 12]. In particular, a defined number of circuit breakers (CBs) are mounted on the MV network; each CB is controlled by a protection relay (PR) which communicates with the other PRs. According to logic selectivity, each PR that detects the fault sends a blocking signal, namely, Blind, to the upstream protection, inhibiting its intervention [13]. The PR immediately upstream of the fault will not receive any blocking signals from the PR downstream of the fault, because the latter does not detect the overcurrent. Once a defined time has elapsed--referred to as the Maximum Blind Waiting time, \(T_{MBW}\), in the following--the PR commands the tripping of its circuit breaker and selectively isolates the faulty section. If the network topology permits the connection of the MV line to another feeder, the PR that has commanded the CB tripping also must send a Trip signal to the downstream PR and a Close signal to the breaker to establish the interconnection. This way, the powered-off section is minimized, because the line downstream of the fault is supplied by another feeder. For the sake of clarity, Figure 1 shows the operations sequence of an example of an MV transmission network. In this section and in the rest of the paper, the devices involved in this scenario are those described in Table 1.  
The network involves two radial lines, which can be interconnected through the interconnection breaker (IB), controlled by the Tie recloser (TR). On each of the two radial lines, there is a primary HV/MV substation, protected by the SR that controls the substation breaker (SB). The PRs control the CBs along the MV line. The fault management is performed according to the following steps:  
1. _Fault detection_. A fault (for example, a short circuit) in the section between PR2 and PR3 is now considered. After the detection time, SR, PR1, and PR2 recognize the fault. Without a selective protection mechanism, SR, PR1, and PR2 would command the tripping of their breakers, leaving the entire distribution line without power supply. On the contrary, according to the logic selectivity approach, SR, PR1, and PR2 do not immediately control the breaker, but they wait for the duration of the Maximum Blind Waiting time. 2. _Blind transmission_ (Figure 1b). Each PR that senses the short circuit current and detects the faulty condition sends the Blind message to the protection that is immediately upstream. So, PR1 sendsthe Blind message to SR and, at the same time, PR2 sends the Blind message to PR1 to prevent the tripping of the CBs. 3. _Tripping upstream of the fault_ (Figure 1c). The relays that receive the Blind message before the expiration of their Maximum Blind Waiting time (SR and PR1) leave their breakers closed. PR2 does not receive a Blind signal from PR3, which, in fact, does not detect the fault. At the end of its Maximum Blind Waiting time, PR2 commands the trip of its breaker, CB2. 4. _Tripping downstream of the fault_ (Figure 1c). PR2 also sends the Trip message to the downstream relay PR3, which commands the tripping of its breaker CB3 so that the fault is isolated. The line section from the HV/MV station up to CB2 remains powered. 5. _Interconnection closure_ (Figure 1d). PR3 sends the Close message to the boundary TR in order to close the interconnection breaker. Now the section downstream of CB3 is also powered, as it is supplied by the other HV/MV substation.