# Voltage Problems on Farms with Agricultural Biogas Plants--A Case Study
## 3 Results and Discussion  
### Analysis of the Supply Network Parameters of the Agricultural Biogas Plant under Study  
The recorded grid frequency values are shown in Figure 2. Although frequent changes in frequency were noted, the maximum deviation from the rated value was slight, not exceeding 0.18%. However, a particular recurring pattern is worth noting: the lowest recorded frequency values occurred in the afternoon during peak electricity network load and increased as the load decreased. The waveforms of the variation in voltage levels and the percentage deviation of the voltage from the rated voltage recorded during the tests are shown in Figures 3 and 4. The analysis of the recorded voltage waveforms reveals a cyclic pattern of changes similar to those observed in frequency. Voltage fluctuates with changes in the energy system's load, decreasing as the load increases. Increases in voltage deviation values accompany these voltage decreases. The voltage varied from approximately 200 V to nearly 260 V. Such significant fluctuations can cause interference with electronic equipment. The recorded values of the long-term flicker annoyance factor (Plt) are included in Figure 5.  
Figure 4: Variation in voltage deviation values recorded at the connection point of an agricultural biogas plant. Figure 3: Voltage variation recorded at the connection point of an agricultural biogas plant. Figure 2: Frequency value variation recorded at the connection point of an agricultural biogas plant. The values of the long-term flicker annoyance index also vary cyclically. The lowest \(\mathrm{P_{lt}}\) index occurs at night, and the highest in the afternoon. It is usually related to the presence of so-called unstable loads (loads that consume electricity that are significant in value and variable in time) in the power system. The voltage asymmetry factor also varies cyclically (Figure 6). The recorded waveform usually reaches a maximum in the afternoon, while the minimum occurs at night. The recorded \(\mathrm{THD_{U}}\) values for each phase are shown in Figure 7.  
The values of the total voltage distortion coefficient recorded at the biogas plant also vary in the diurnal system, mainly due to the cyclic operation of equipment drawing current distorted from the sinusoidal waveform from the grid. The highest values were recorded mainly in the afternoon. Figure 5: Variation in the long-term flicker nuisance factor (\(\mathrm{P_{lt}}\)) recorded at the connection point of an agricultural biogas plant. Figure 6: Variation in the voltage asymmetry factor (\(\mathrm{\alpha_{U}}\)) recorded at the connection point of the agricultural biogas plant. The results of the analysis of the active power levels generated by the agricultural biogas plant are summarised in Figure 8.  
Figure 8 and information from the owner indicate that only one turbogenerator is operational at the agricultural biogas plant. The data also show long periods when the biogas plant is not generating electricity (the power is negative). The times when the agitator is running are visible on the waveform, with the recorded power dropping close to zero when the generator is running or around \(-17\) kW when no energy production occurs. Therefore, it can be inferred that one 20 kW generator is necessary to meet the peak needs of the agricultural biogas plant. Without the agitator running, the biogas plant consumes about 1 kW (for control systems, monitoring, etc.). The inability to switch on the second generator is due to the mains voltage exceeding the permissible value of 253 V. Even with one generator running, voltage exceedances were  
Figure 8: The recorded course of variation in the three-phase active power value generated at an agricultural biogas plant. Figure 5.0 The course of variation in the total voltage distortion coefficient (\(\rm{THD_{U}}\)) was recorded at the connection point of the agricultural biogas plant. recorded (Figure 3), causing the generator unit to disconnect from the grid to protect other electrical equipment sensitive to voltage spikes.