# An Improved Mixed AC/DC Power Flow Algorithm in Hybrid AC/DC Grids with MT-HVDC Systems
## 4 Results and Discussions
### Case 7: AC/DC Power Flow for the Large-Scale Hybrid AC/DC Grids with MT-HVDC Systems
It is also noticed that compared to the other DC lines, the changes of the PF on the lines 5-6, 5-7, and 5-8 are considerable and the changes of the PF on the DC lines 6-7 and 6-8 are almost zero. To minimize the power losses, in the next case, it is considered that the converters are equipped with the droop controllers. Table 16 illustrates the detailed information of the droop settings for each converter station. Tables 17-17 illustrate the results of mixed AC/DC PF considering droop control settings provided in Table 16 (Case 7-3). Analyzing the obtained results shows that the changes of the PF on the AC lines 1-39, 2-3, and 17-27 are almost negligible but the total power losses on the AC lines are decreased. The DC lines 1-4 and 2-3 have the most power losses. In addition, no power is transferred on lines 6-7 and 6-8. Therefore, they can be considered as reserve lines for planning purposes. It is worth mentioning that compared to Case 7-2, the total power losses on the DC lines are decreased. To analyze the impact of MT-HVDC systems on the PF analysis, AC lines 3-4, 3-18, 4-14, and 14-15 as the main interconnected corridors between each of area are disconnected. Tables 17 and 17 show the results of AC PF without DC grids in the case of disconnecting the AC lines 3-4, 3-18, 4-14, and 14-15 (Case 7-4). The disconnection of the mentioned lines is caused that some of the lines reach their maximum transfer power capabilities. The changes of the PF on the AC lines 4-5, 5-6, 6-11, and 10-11 are drastically changed. However, the changes of the PF on the AC lines 10-13 and 13-14 are deceased. The rest of the lines have either no or very slight changes on the PF. To improve the reliability of the hybrid AC/DC grids, the MT-HVDC systems are connected to compensate for the disconnections of the AC lines 3-4, 3-18, 4-14, and 14-15. Tables 17-17 show the obtained results of the mixed AC/DC PF calculations when MT-HVDC systems are connected to the IEEE 39-bus test system considering the disconnections of the AC lines 3-4, 3-18, 4-14, and 14-15 (Case 7-5). Based on the obtained results, the significant changes on the PF are related to the AC lines 1-2, 1-39, 10-13, 13-14, and 17-27 (increase) and 2-25, 5-6, 6-11, 10-11, 15-16, 17-18, and 26-27 (decrease). The total power losses on the AC lines are closed to the previous case. In addition, DC lines 5-6, 5-7, and 5-8 are the DC lines with high power losses. In order to minimize the power losses, it is assumed that the converter stations are equipped with the droop controllers as Table 16. Tables 17-17 show the obtained results of the mixed AC/DC PF calculations when MT-HVDC systems are connected to the IEEE 39-bus test system considering the droop parameters and disconnections of the AC lines 3-4, 3-18, 4-14, and 14-15 (Case 7-6). The obtained results show that compared to the previous case, only the PF of the AC lines 4-5, 5-6, 15-16, 17-18, and 26-27 are increased and the total power losses both on the AC and DC lines are decreased.