In order to accurately analyze the performances of cascaded H-bridge with star connection, cascaded H-bridge with delta connection and modular multilevel converter in handling the unbalanced load under the asymmetrical grid fault condition, firstly, this paper builds mathematical models of these three multilevel converters based on the principle of power equilibrium. Then it gives contrastive analysis. The final analysis conclusion is that the STATCOM based on cascaded H-bridge with star connection converter is not suitable for compensating unbalanced load, while the STATCOM based on cascaded H-bridge with delta connection converter can work well to compensate unbalanced load when grid is normal. But the capability of compensating unbalanced load is greatly suppressed when asymmetrical grid fault happens. The the STATCOM based on modular multilevel converter works well to compensate reactive power and negative-sequence current even under serious asymmetrical grid fault condition. Finally, Matlab/simulink simulations verify the correctness and effectiveness of theoretical analysis.