COMMUNICATION TECHNOLOGY PLC COMMUNICATION – THE WEAKEST LINK? There is no doubt that power lines are part of critical infrastructure. Power engineering is now in its most dramatic development phase. Unfortunately, progress in technologies and political decisions tend to complicate development. After the EU strategic plan 20-20-20 is realised, we are likely to face the paradox of a reliable supply of power still not being ensured. Fortunately, however, even empty slogans such as ‘smart grid’ bring rational considerations and decisions to the area of stability and safety of power supply. The way to control end devices, including residential e-meters, is very clear ... despite reservations PLC technology will prevail. It is the only technology with reliability that equals that of the power supply. Shannon schematics of communication systems reveal the structure of both modem and surroundings that influence total efficiency of transferred information. A real communication system is obviously significantly more complicated; however, this simplified model is sufficient to identify the key effects (See Figure 1). The theoretical calculation for the highest possible transmission capacity C 0 (Shannon–Hartley theorem) looks as follows: C 0 = B log 2 (1+ ) [bit/s; Hz; W; W] Figure 2: Idealised distribution network. Elements L 1,2,3 and S 1,2,3 have character of lines and they could be described by spare schematics – see Fig. 3 Elements Z G (source impedance), and Z L , and Z 1,2,3 (load impedance) describe both the source and appliance part of the power network (impedance characteristics defines that elements have frequency dependency). where B is equal to bandwidth; S is equal to signal power over bandwidth and N is equal to noise power over bandwidth. Based on this, it is clear that transmission capacity does not depend on how we wish it to look, but depends on the characteristics of the transmission channel. In the case of PLC technology, that channel means power lines. The distribution can be generalised on serial-parallel structures with relatively high attenuation (because of significant parallel capacity and serial inductance). Parallel ordering of single consumption places with highly dynamic connection and disconnection brings strong and very dynamic change of impedance. Unfortunately, there is always the customer’s consumption at the end. Customers can through their behaviour influence the distribution network and therefore the communication channel (see Figure 2). The spare schematic characteristics shown in Figure 3 highlight the real value of power The communication channel is always the weakest link in the transmission chain. It would seem that based on the frailties of this channel, it would be more convenient to leave this channel ... the exact opposite is, however, true Figure 1: Shannon schematics of a communication system, highlighting negative effects. METERING INTERNATIONAL ISSUE - 5 | 2014 33