Direct Current Control for Grid-Connected Diode-Clamped Inverters

The accurate control of the line current, robust and dynamic behavior even under distorted grids and other system faults are the most important requirements for grid-connected inverters. Applying direct current control to multilevel inverters combines the advantages of both and is an enabling step towards improved inverter performance. Direct current controllers suffer from high complexity at increased level count, which is resolved by using geometrical principles and simple analytical calculations for switching vector selection within the space vector diagram. The simplified, novel parametric controller concept is scalable to inverters with arbitrary level count, which does not require any switching tables and the new fully symmetric setup guarantees equal switching frequencies among the three phases. The hypotheses are further confirmed on a real hardware test setup on the example of a three-level NPC inverter hardware and a Xilinx development platform. Experimental results prove the expected behavior of the direct current control under various conditions, which shows a general approach to balance the dc-link capacitors of diode-clamped inverters within the theoretical limits and demonstrates the benefits of using field programmable gate arrays as a controller platform.