Calculation of the DC-bus Capacitors of the Back-to-back NPC Converters

Nowadays the most used multilevel topology for variable-speed wind turbines (WTs) is the a back-to back three-level NPC (Neutral Point Clamped) VSCs (voltage source converter). One of the most critical elements in the design of this type of converters is the DC-bus capacitors. The calculus of these depends on the same factors that the same two-levels topology, and, moreover it is function of a low frequency ripple due to the current by NP point. In this work, the value of these capacitors is obtained from the analytical equations of the DC-bus voltage ripple due to the iDC (DC-bus current) and the iNP (NP current). These expressions are verified through simulation and practical results.