The invention relates to a method (20) of controlling a modular multilevel converter (1) comprising a number of series-connected submodules (21,..., 2n) in at least one first branch (3a_u, 3b_u, 3c_u) and at least one second branch (3a_I, 3b_I, 3c_I). The method (20) is performed in a control device (10) and comprises: detecting (21) a fault condition; and applying (22), in response to the detecting, a pulse blocking procedure for protecting the modular multilevel converter (1), wherein the pulse blocking procedure comprises issuing a pulse blocking signal to at least one subset of all submodules (21,..., 2n).