# An Improved Mixed AC/DC Power Flow Algorithm in Hybrid AC/DC Grids with MT-HVDC Systems
## 4 Results and Discussions
### Case 6: AC/DC Power Flow Considering Converter Station Limits and Converter Outage  
In the sixth case, the impact of the converter station limits on the AC/DC PF solution is investigated. When an active power set-point of a converter station equipped with the \(P\) controller is outside of the P-Q capability chart (P-Q capability chart shows the possible operation points.), the active power order should be reduced to comply with the predefined limit. Similarly, when a reactive power set-point of a converter station equipped with the \(Q\) controller is outside of the P-Q capability chart, the reactive power order should be reduced to comply with the predefined limit, subject to not reaching the active power limit. In addition, when a converter station equipped with the \(V_{DC}\)-droop controller reaches its limit, the converter station should be set to a constant \(P\) injection equal to the maximum active power limit of the converter station. In addition, when a converter station equipped with the \(V\)-droop controller reaches its limit, the converter station should be set to a constant \(Q\) injection based on the predefined limit. For both converter stations equipped with the \(Q\) controller and \(V_{DC}\)-droop controller, the priority is given to active power over reactive power, when enforcing the limits. It should be noted that however, all the DC-slack buses are disregarded from the analysis, they are rechecked at the end of AC/DC PF calculations.  
According to the above explanations, it is assumed that the converter station #1 reaches the reactive power limit and the converter station control is changed from constant \(V\)-mode to constant \(Q\)-mode. Tables 22-25 illustrate the results of PF calculations in all both AC and DC buses and branches after enforcing the converter station's current and voltage limits. Meanwhile, the algorithmis converged in 0.60 s and in three iterations. After three iterations, the reactive power set-point of the converter station #1 is changed from \(-\)40 MVAR to \(-\)36.25 MVAR.  
To demonstrate the impact of changing the set-point of the converter station, the active power set-point of the converter station #1 is set to \(-\)130 MW so that simultaneously both active and reactive power violate their limits. Tables 14-15 show the results of PF calculations both in all AC and DC buses and in branches after changing the active power set-point of the converter station #1 to \(-\)130 MW. It should be noted that the algorithm is converged in 0.71 s and in three iterations.  
After three iterations, the active and reactive power set-points of the converter station #1 are changed from \(-\)130 MW to \(-\)121.21 MW, and \(-\)40 MVAR to \(-\)8.96 MVAR, respectively. In this case, the total active power generated by the generation units is increased (tangible changes are related to the one connected to bus 2). The overall power losses on the AC lines are decreased. As the converter station #1 is connected to the grid and its set-points are reached their maximums limits, the active power losses (correspond to \(Zl^{2}\)) are increased. Therefore, the PF on the DC lines is increased.