Converter stations have real and reactive power set points initially determined as part of a main loadflow analysis routinely performed on the AC power system connected to the converter stations. Viable real and/or reactive power set points for the converter stations can be identified by calculating real and/or reactive power set point candidates for the converter stations before the main loadflow analysis is performed again on the AC power system. The power set point candidates are calculated based on information determined as part of a previous iteration of the main loadflow analysis on the AC power system. The power set point candidates which violate an operating constraint imposed on the AC power system are identified, and a region of valid power set points is defined for the converter stations that excludes the power set point candidates which violate an operating constraint imposed on the AC power system.