# Voltage Problems on Farms with Agricultural Biogas Plants--A Case Study
## 3 Results and Discussion
### Analysis of the Feeding System of the Biogas Plant under Study
Analysis of the Main Supply Parameters of the Studied Agricultural Biogas Plant after Changing the Supply Cable  
The value standardised in Polish regulations [42; 43] is the 95% quantile. It is defined in the standard [42] as the highest value obtained from 95% of the recording time during a week. A summary of the recorded supply voltage frequency values is shown in Table 1.  
Figure 16: Recording of the course of the variation in the three-phase active power value generated in an agricultural biogas plant. Figure 15: The waveform of the total voltage distortion coefficient (THDU) variation was recorded after changing the power cable. A summary of the measured voltage levels in each phase of the system supplying the agricultural biogas plant is shown in Figure 9:0 Several relationships emerge from analysing these values. Noticeable voltage asymmetry exists in the individual phases. The lowest voltage occurred in the third phase, likely due to the asymmetrical connection of single-phase loads to the grid. However, in each phase, the voltage recorded after changing the power cable was lower than that of the primary circuit, translating into voltage deviation values. After the power cable change, although momentary exceedances of the permissible voltage deviation values were registered, the regulatory normal quantile of 95% slightly exceeded 9%. The operation of the biogas plant causes an increase in voltage at its point of connection, which, as seen from the measurements, is kept close to the upper limit allowed by regulations. Switching on the digester mixer reduces generation by almost half, resulting in a drop in voltage values. Turning it off causes the voltage to rise again; this cycle repeats every 30 minutes. This cyclic, step-by-step change in voltage is not beneficial for both the generated electricity's quality and consumer equipment's durability. It is mainly noticeable as a change in the intensity of the lighting installed on the farm and can lead to malfunctioning of voltage-sensitive equipment, potentially causing deactivation or even damage. Table 2 provides the answer to the malfunction of the milking apparatus--its shutdown. Before the power cable was replaced, the lowest recorded voltage value was only 197 V, below the usually accepted voltage limit of 200 V for milking clusters. Consequently, the clusters reported a voltage error and went into emergency mode. After replacing the power cable, the lowest recorded voltage value was 208 V, higher than the minimum value allowed by regulations. As a result, there were no more emergency shutdowns of the consumers during the test period. A summary of the results of the statistical analysis of the values of the voltage asymmetry coefficient in the power supply system of the biogas plant under study is presented in Figure 9:1 As was shown during the analysis of the recorded waveforms of variation in the \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Before the Changes**} & \multicolumn{2}{c}{**After Changes**} \\ \cline{2-5}
**Parameter** & **Frequency** & **Deviation from Rated Value** & **Frequency** & **Deviation from Rated Value** \\  & **[Hz]** & **[\%]** & **[Hz]** & **[\%]** \\ \hline Average value & 49.995 & \(-\)0.010 & 49.996 & \(-\)0.008 \\ Minimum value & 49.910 & \(-\)0.180 & 49.900 & \(-\)0.200 \\ Maximum value & 50.080 & 0.160 & 50.070 & 0.140 \\ Quantile 95 & 49.970 & 0.040 & 49.970 & 0.040 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Results of statistical frequency analysis.