# Voltage Problems on Farms with Agricultural Biogas Plants--A Case Study
## 2 Materials and Methods  
The study was conducted at a Polish agricultural biogas plant directly connected to a farm installation. The primary substrate used in this biogas plant was manure from approximately 120 dairy cows. The biogas plant comprises three main facilities: a digester, an engine house, and a digestate tank. The digester had a diameter of 15.5 m and a wall height of 3.7 m, with a total height of 8.7 m, including the biogas tank. During regular operation, the digester held approximately 590 cubic meters of substrate, mixed by two agitators with a rated power of 9 kW. The engine room was constructed as a container housing an internal combustion engine connected to a generator and other technical and control equipment. Biogas purification was achieved using a gas filter and an activated carbon filter. The biogas produced was combusted in two WG1605 combustion engines, each with a power output of 20 kW and an efficiency of 97%. A type 4P/IE2 asynchronous generator, operating at 1500 rpm, was coupled to the engines, which were also equipped with a water-cooling circuit. The digestate, the post-production biogas material, was stored in an open concrete tank. A schematic of the biogas plant is shown in Figure 1. The biogas plant was connected to the farm's main meter switchgear via a 198-meter-long YKY 4 \(\times\) 25 mm\({}^{2}\) cable. The farm's connection power was 40 kW, limited by overcurrent protection C63. The meter switchgear was supplied from the 63 kVA 15/0.4 kV transformer station by a 413 m long overhead line with 50 mm\({}^{2}\) conductors. The overhead transformer station was supplied from the main power point of the electricity system by a 6000 m long overhead line made of steel and aluminium conductors with a cross-section of 50 mm\({}^{2}\). According to information obtained from the Distribution System Operator, the power system had a short-circuit capacity of 102 MVA. The owner of the biogas plant reported the following problems with its operation:  
Due to voltage exceedances at the agricultural biogas plant's connection point, it is impossible to run the second generator. Increasingly frequent cluster malfunction--voltage error displayed. The slurry stayed in the fermenter for an average of two to three weeks but no less than 12 days. Daily, a portion of the digested slurry (regulated by the biogas pressure sensor) was pumped out of the digester into the digestion tank. When the lower biogas pressure threshold value was exceeded, a new slurry was pumped into the digester (from the collection pit) to replenish the digestate. Power quality is defined as a set of parameters describing the characteristics of the power supply process to the user under normal operating conditions, characterising the supply voltage, and determining the continuity of the power supply to the consumer. The following parameters describing power quality were analysed in the study [42; 43]:  
* **Mains frequency** is the number of repetitions in the time waveform of the fundamental component of the supply voltage measured over a specified time interval. The frequency deviation is the difference between a given value and the rated frequency value exhibited during regular power system operation over at least a few seconds. The frequency deviation should, in most cases, not exceed \(+/-\) 1% of the rated grid frequency. * **Voltage deviation (slow voltage variation)** is the difference between the actual and rated mains voltage values. In most cases, the free voltage variation should not exceed \(\pm\) 10% of the rated mains voltage value. * The amplitude of voltage fluctuations is expressed as the ratio of the voltage variation value to the rated voltage. In most cases, this value should not exceed 3%,
- frequency of voltage fluctuation amplitudes or, in the case of periodic fluctuations, the frequency of voltage fluctuations; Short-term flicker index \(\mathrm{P_{st}}\) (index indicating the annoyance of flickering light over a few minutes).