Method for operating a modular multilevel converter comprising a plurality of submodules (1, 2, 3, 4, 5, 6), each of which has at least two electronic switching elements, an electrical energy store, two submodule connections, a bypass switch for bridging its submodule (1, 2, 3, 4, 5, 6) in the modular multilevel converter, and a communication element for communication with a communication device of the modular multilevel converter, the method comprising the following steps: (a) ascertaining that the plurality of submodules (1, 2, 3, 4, 5, 6) have a submodule (1, 2, 3, 4, 5, 6) which is defective such that the communication element in the defective submodule (1, 2, 3, 4, 5, 6) does not communicate with the communication device, characterized by (b) determining whether a present arm current resulting from an operating point of the modular multilevel converter is below a predetermined threshold value, (c) generating or amplifying an internal circulating current in the modular multilevel converter with the defective submodule (1, 2, 3, 4, 5, 6) if the arm current resulting from the operating point of the modular multilevel converter is below the predetermined threshold value, the internal circulating current flowing via the electrical energy store of the defective submodule (1, 2, 3, 4, 5, 6), the defective submodule (1, 2, 3, 4, 5, 6) being blocked before generating or amplifying the internal circulating current in the modular multilevel converter, as a result of which the electrical energy store in the defective submodule (1, 2, 3, 4, 5, 6) is charged by means of the internal circulating current, and the bypass switch of the blocked defective submodule (1, 2, 3, 4, 5, 6) being closed in order to bridge the blocked defective submodule (1, 2, 3, 4, 5, 6) in the modular multilevel converter if the electrical energy store of the blocked defective submodule (1, 2, 3, 4, 5, 6) has reached a predetermined charging by means of the internal circulating current, and (d) monitoring the closing of the bypass switch not only by the relevant submodule itself, but also by another submodule, in particular by a neighboring submodule, each submodule (1, 2, 3, 4, 5, 6) of the plurality of submodules (1, 2, 3, 4, 5, 6) being connected to a bypass switch of another submodule (1, 2, 3, 4, 5, 6) of the plurality of submodules (1, 2, 3, 4, 5, 6) in terms of communication technology by means of a respective communication element in order in each case to monitor the state of a bypass switch of each submodule (1, 2, 3, 4, 5, 6) of the plurality of submodules (1, 2, 3, 4, 5, 6) by means of another submodule (1, 2, 3, 4, 5, 6) in each case, generating or amplifying the circulating current in the modular multilevel converter with the defective submodule (1, 2, 3, 4, 5, 6) being effected if furthermore the communication element of a submodule (1, 2, 3, 4, 5, 6) which monitors the defective submodule (1, 2, 3, 4, 5, 6) and which is connected to the bypass switch of the defective submodule (1, 2, 3, 4, 5, 6) in terms of communication technology communicates to the communication device the fact that the bypass switch of the defective submodule (1, 2, 3, 4, 5, 6) is open. (a) ascertaining that the plurality of submodules (1, 2, 3, 4, 5, 6) have a submodule (1, 2, 3, 4, 5, 6) which is defective such that the communication element in the defective submodule (1, 2, 3, 4, 5, 6) does not communicate with the communication device, characterized by (b) determining whether a present arm current resulting from an operating point of the modular multilevel converter is below a predetermined threshold value, (c) generating or amplifying an internal circulating current in the modular multilevel converter with the defective submodule (1, 2, 3, 4, 5, 6) if the arm current resulting from the operating point of the modular multilevel converter is below the predetermined threshold value, the internal circulating current flowing via the electrical energy store of the defective submodule (1, 2, 3, 4, 5, 6), the defective submodule (1, 2, 3, 4, 5, 6) being blocked before generating or amplifying the internal circulating current in the modular multilevel converter, as a result of which the electrical energy store in the defective submodule (1, 2, 3, 4, 5, 6) is charged by means of the internal circulating current, and the bypass switch of the blocked defective submodule (1, 2, 3, 4, 5, 6) being closed in order to bridge the blocked defective submodule (1, 2, 3, 4, 5, 6) in the modular multilevel converter if the electrical energy store of the blocked defective submodule (1, 2, 3, 4, 5, 6) has reached a predetermined charging by means of the internal circulating current, and (d) monitoring the closing of the bypass switch not only by the relevant submodule itself, but also by another submodule, in particular by a neighboring submodule, each submodule (1, 2, 3, 4, 5, 6) of the plurality of submodules (1, 2, 3, 4, 5, 6) being connected to a bypass switch of another submodule (1, 2, 3, 4, 5, 6) of the plurality of submodules (1, 2, 3, 4, 5, 6) in terms of communication technology by means of a respective communication element in order in each case to monitor the state of a bypass switch of each submodule (1, 2, 3, 4, 5, 6) of the plurality of submodules (1, 2, 3, 4, 5, 6) by means of another submodule (1, 2, 3, 4, 5, 6) in each case,