Switching frequency circulating current analysis in parallel-connected multilevel NPC converters

This paper discusses one of the major issues concerning parallel-connected voltage source converters that is the circulating currents. These currents usually flow between the converters and can be caused either unequal power sharing between the converters or nonsymmetrical PWM control. In the second case they are called switching frequency circulating currents. Detailed investigation on these currents is carried out. Analytical expressions to calculate the maximum values of these currents are obtained for the common case. Knowing the maximum value allow one to estimate semiconductor maximum current more precisely and prevent IGBT or MOSFET overcurrent fail. It is shown that number of the parallel converters has a slight influence on the switching frequency circulating currents maximum value. On the other hand the number of the voltage levels, PWM frequency and sharing reactor inductance proportionally affects this value. Simulation for several different cases is given to verify the obtained equations and prove the correctness of the assumptions made. An experimental verification on the two-level case is presented.