## 4 Results of an Experimental Study with Power-Line Communication and Selected End-User Equipment
In detail, for particular scenarios of the investigation, the statistics were monitored based on the total number of the connections, the number of failed connections, the number of successful connections, and the duration of failed connections in one hour. The statistics represent 2 h of continuous work of the PLC system. The data were reported using a dedicated ADDAX software solution for residential metering [43]. Following interpretations of the connection statistics may be defined:  
* Increasing the number of attempted connections is the preventive action of the PLC system to keep the transmission successful and can be interpreted as a symptom of deteriorated transmission condition. * Percentage contribution of the number of failed connections is not directly representative quantity indicating the deterioration of the transmission condition due to increasing number of attempted connections. * Increasing the duration of failed connections is the direct symptom of the problems with continuity of the communication (referring to ADDAX recommendation the threshold value for the duration of failed connections indicating communication problem is 180 s in one hour). Case 1 (Supply Voltage Scenario 1, Load Condition 0): Operation of the Tested PRIME PLC System without Loads during Sinusoidal Supply Voltage--Reference Analysis  
As the system itself makes use of the supply voltage (power supplies for meters, meter displays, concentrator, etc. ), it was decided to initially study the impact of voltage distortion without the loads, first of all. This testing stage serves, in a way, as an immunity test of transmission efficiency at a different degree of distortion of the supply voltage, but without the participation of higher frequencies introduced by the loads. At this level of testing, the transmission efficiency statistics available in the master system of the tested PLC are mainly used. With the use of PCR-LA-500 generators, a perfectly sinusoidal waveform of the supply voltage was defined and the first cycle of observation of the operation of the tested PLC system was performed. The results of spectral analysis for the considered case of PRIME PLC system operation in scenario 1 (pure sinusoid supply voltage) without loads are shown in Figure 7. Figure 7a. presents voltage in the point of the equipment connection, which is also the point of energy meter connection with PRIME PLC receiver (denoted in figures as line voltage \"uL\"). The only impact on the primary voltage distortion, i.e., before the PLC communication is switched on, are the currents of the power supplies of the concentrator and the power supplies of the systems and the meter displays. It should be noted that the power supply itself is not an ideal appliance. According to the technical specification, the error of the forced waveform could be up to 0.3%. Figure 7b presents the time-varying nature of the PRIME PLC communication during 20 s of observation. The figure was obtained using a sliding 100 ms window of DFT analysis using grouping in the 200 Hz band in 100 Hz central frequency. It can be visible that the investigated technology uses several packages of broadband signal in the range of 42-89 kHz. In order to identify details of the communication signal and the frequency spectrum of the reference analysis (pure sinusoid supply voltage and no load), the maximum and minimum spectra from the 200 local spectra (20 s of observation with 100 ms window) have been selected. Figure 7c identifies spectra representing voltage, with and without PRIME PLC communication. As shown in Figure 7c, a supply voltage spectrum in investigated cases is characterised by a practically constant value (grey colour in the diagram) not exceeding 60 dB\(\upmu\)V with narrow-band components of higher frequencies. This waveform was obtained under the conditions of supplying the system with a nominal voltage of 400 V, 50 Hz, \(\text{THD}_{\text{U}}\approx\) 0%. This characteristic can be treated as a background for the transmission system (noise).