Supressão de correntes circulantes em conversores multiníveis modulares: comparação entre os controladores direto em coordenadas dq e indireto baseado em energia

Abstract

This work presents a comparative study between two control methods to compensate the circulating currents in modular multilevel converters (MMC): a direct method, in dq coordinates, and an indirect method, also know as energy based control (EBC). Despite their simple implementation, direct suppression controllers have limitations, such as the absence of control over the upper and lower arm voltages of the converter. Related to this issue, the dynamics of the DC side of the MMC are prone to poorly damped oscillations or stability problems when the second harmonic components of the arm currents are mitigated using the direct method. On the other hand, the energy-based controller allows complete control of the internal dynamics of the converter, attenuating direct current oscillations during transients. The study of this type of control is also relevant for expanding the application field of MMC. Initially, the fundamentals of the converter are presented, including its modular structure, current flow through the converter, and modulation and equalization strategies used to regulate submodule capacitors voltages. Subsequently, the modeling of the converter connected to the grid is performed. With the model equations in hand, the control loops for grid currents and circulating current are outlined. Among the direct methods, the circulating current suppression controller (CCSC) in the synchronous reference frame is employed. For the indirect method, energy-based control in the natural coordinates system is adopted. The chosen structure, in this case, is based on the literature review. Details regarding the design of the controllers are provided for both methods. Modifications to the EBC structure are also presented to enable its application in an unbalanced grid condition. Digital simulation results obtained with a modular multilevel converter with 50 submodules per converter arm, modeled in the PSCAD/EMTDC program, are used to validate the theoretical analyses and compare the performance of the two control techniques.