In the online voltage stability monitoring and control， one can find post-fault load flows diverged in many cases. There are three basic requirements on the technology in handling unsolvable power flow problems， i.e. correctly identifying unsolvable power flows caused by power system instabilities， accurately measuring the severity of a unsolvable power flow， and providing sensitivity information used for preventive or emergency controls. The optimal multiplier Newton power flow technique（OM-PF）， the nonlinear programming optimal power flow（OPF） technique and the contingency parameterization continuation power flow technique（CP-CPF） are among effective methods in handling the load flow insolvability problem. The closest critical points in the power injection space can be obtained by OM-PF and OPF methods， and the closest critical point in the contingency parameter space by CP-CPF method. A comparison study on the three methods is done on the IEEE-118 test system. It shows differences between the three methods in terms of insolvability assessment and severity ranking. However， the three methods provide same control sensitivity information.