Abstract: The possibility of eliminating the step-up transformer is one of the most attractive features of cascade multilevel converters.However without a transformer,the converter could inject significant amount of direct currents（dc） to the grid.In high voltage converter applications,where current transformers（CT） are often used as a feedback element,the DC injection problem can be more crucial than inverters using Hall-effect current sensor,as CT can’t sample direct currents.This paper deals with this DC-injection problem first by introducing CT model into a cascade static synchronous compensator（STATCOM） model,and then analyzing the mechanism of how CT can cause large DC-injection.A DC component suppression method was proposed.This method can eliminate the DC component of output currents even when direct current can’t be sampled.This method has been verified on a 25-level cascade multilevel STATCOM prototype.