## 6 Deployment Guidelines for NB PLC System Deployment
### PLC Signal Injection in SSs
#### 6.1.1 Multi-Transformer Scenarios  
In a typical SS there is usually more than one LV panel, associated either to one or several distribution MV to LV transformers. Multi-transformer environments are typical of dense urban areas, where the density of customers is very high, and the evolution of the area has made the LV infrastructure grow. The PLC connectivity of LV panels connected in multi-transformer environments is not generally known, and LV PLC connection among these elements is unclear.  
LV panel injection in multi-transformer environments has been addressed in [145]. The lack of previous references dealing with LV signal injection may be attributed to the fact that traditional AMR systems have been proprietary solutions from specific system providers. The details of the solutions used for these multi-transformer situations probably remain the know-how of the parties involved in the deployments.  
Reference [145] assumes that the connectivity among the various transformers in a SS cannot be determined from the existing electricity company database information, and it states that according to the tests performed in a group of representative substations a certain correlation exists between the connection of neutral wires of the transformers and the likelihood of having a variable quality PLC communications channel among the LV panels connected. However, as certain cases were found where the PLC communications channel established among the LV bus-bars, was not good enough for network performance, even if neutral connectivity among the transformers of the SS was modified to improve the PLC connectivity, and that only via an on-site test this physical connectivity at PLC frequencies could be checked, reference [145] proposed a PLC injection mechanism that has been proven optimal in such grids [100]. This mechanism is based in the connection of the so-called Auxiliary Nodes, which manage to extend Base Node (subnetwork controller) access to the different parts of the LV grid connected to other LV panels.  
Reference [145] leaves several aspects open. One of these aspects is the connectivity of the Base Nodes. Ethernet and coaxial cables are presented as examples of this connectivity, but are not compared. Depending on the various implementation scenarios, Ethernet or coaxial cable connectivity offer different characteristics that need to be assessed based on specific equipment's possibilities to take the most convenient implementation decision. Some other connectivity options, such as wireless, could also be explored to obtain the expected results.  
The second aspect that deserves attention is based on the figures of merit used to select Auxiliary Node against the multiple Base Nodes approach. Reference [145] selects the strategy based on the metric of the success rate, which is appropriate for conservative Smart Metering scenarios. However, reference [145] also states that latency results are slightly better in scenarios where the multi Base Node approach is used. This result needs to be combined with the time to retrieve all measurements, which is also reduced with the multi Base Node approach. Thus it may result that the multi Base Node approach could be suggested for Smart Metering deployments where the speed to perform tasks (e.g., meter readings collection) is the most important figure of merit. The only drawback of this strategy is that PRIME (the technology used in [145]) does not allow today the coordination of Base Nodes, so that overall PRIME collisions are minimized. However, these collisions could de-facto be small in multi-transformer environments where multiple LV panels do present a low PLC communications feasibility, as collisions are by default not present in isolated panels. As a consequence, it could be desirable to explore the possibility of developing intelligent devices able to dynamically select either Base Node or Switch Node capability that depending on the objective and the specificities of the scenario could apply the best resulting strategy.