Model Predictive Control with Extrapolation Strategy for the Arm Current Commutation Control of Modular Multilevel Converter Operating in Quasi Two-Level Mode

Modular multilevel converters (MMCs) have become an attractive topology for medium-voltage motor drive applications due to their modular construction and voltage scalability. The main drawback of using MMC to drive motors is the large voltage variation of submodule (SM) capacitors when the motor is running at a low speed with a large load torque condition. Operating in the quasi two-level mode of MMCs is an effective solution for such a problem. The arm current commutation control is the most important part for the operation of MMCs in the quasi two-level mode. This paper proposes a new current commutation method. The general idea is to use the delay control method to realize the arm current commutation. Therefore, the key point is to decide when to delay the operation of a submodule at each step during the commutation process. In this paper, the model predictive control (MPC) method with extrapolation technique has been used to determine whether to delay the SM at the present moment. Therefore, the waveforms of the two arms in the same phase may not be completely complementary throughout the whole commutation period. Due to such non-complementary PWMs, the currents between two arms can be exchanged.