# Application of a DC Distribution System in Korea: A Case Study of the LVDC Project
## 4 Construction of Actual DC Distribution Line
### Operation Results  
The DC distribution was applied to the actual gird line to find problems and to improve the performance. Also, by establishing operating data for the pilot lines, design, and operation standards for DC distribution lines were created. The performance of the AC/DC converter applied to the actual grid line was verified. The average daily data from 14 February 2017 to 30 July 2017 are summarized. Approximately 86,000 data points were collected per day, and the average was calculated. The voltage was very stable, with a fluctuation of 5% over the 6 months. Except for the 23 February 2017 the efficiency remained near 98%. On 23 February 2017 because the load usage was zero from 06:37:54 to 10:09:14, the daily average efficiency was low. The data showed that the performance of the AC/DC converter system was satisfactory. As shown in the Section 3.1 \"Fault detection,\" the IMD was installed on the output side of the AC/DC Converter as the IMD installation location does not impact its performance. In particular, this paper tested the feasibility of the outdoor operation of IMD by installing it in an outdoor environment, which is rarely done. The insulation resistance value (R\({}_{\text{INS}}\)) of the energized-line was measured upon the installation of the IMD, following the operation manual shown in Figure 4. The measured value was 487 k\(\Omega\) so R\({}_{\text{INS}}\) is 750 (line voltage) \(\times\) 100 \(\Omega\)\(\times\) 1.5 = 112.5 k\(\Omega\) or more. Therefore, the alarm set value is set as follows. (i) Alarm 1--pre-warning R_Alarm1 (k\(\Omega\)) = line voltage (V) \(\times\) 100 (\(\Omega\)) \(\times\) 1.5 = 750 \(\times\) 100 \(\times\) 1.5 = 112.5 (k\(\Omega\))  
(ii) Alarm 2--warning R_Alarm2 (k\(\Omega\)) = line voltage (V) \(\times\) 50 (\(\Omega\)) \(\times\) 1.5 = 750 \(\times\) 50 \(\times\) 1.5 = 56.25 (k\(\Omega\))  
Next, Figure 11 is a measure of the IMD resistance value obtained from the actual DC distribution system. The average daily data from 14 February 2017 to 30 July 2017 are summarized. As can be seen from the waveform, the insulation resistance value is maintained at approximately 486 k\(\Omega\) during the measurement period, indicating alarms and faults did not occur. It was confirmed that the insulation resistance value was monitored in real time. Figure 11: Measurement of insulation resistance. Figure 10: Actual DC distribution line configuration. The actual DC distribution system was completed in October of 2016 and continues to operate normally. The waveforms below in Figure 12 shows the operation data of the AC/DC Converter (input/output voltage, power) from 22 February 2017. As the load usage fluctuation is constant, a representative day's waveform was selected to be analyzed. The load usage ranged between 3 kW and 4 kW, with a sudden maximum peak at 9 kW. Although the load was not large, the protection operation of the power conversion device occurred intermittently in a sudden change of load condition that was not periodic. The representative waveform at the time of the protection operation is shown in Figure 13. It is evident that the operation of the converter was interrupted at the moment when the protection operation was initiated. To understand this deeply, the current data at the time of the incident was acquired and analyzed with increased sampling time. As shown in the waveform in Figure 13, 121 A of output current triggered the protection system. This exceeds the rated capacity of the converter and the threshold current of triggering protection system (Maximum 60 A) that indicates the need for more analysis. A field survey confirmed that frequent water pump motor operations were found at farmhouses. The motor inrush current is commonly known to be 500%-700% of the rated current. However, in this incident, the inrush current reached 10 times the rated current. The DC/AC inverter system improvement plan is proposed as follows.