| Abstract |
The paper describes, through a theoretical analysis, the phenomena causing in an NPC inverter the
neutral point voltage to deviate from the ideal value, due to unbalance between the voltages of the two capacitor
banks of the D.C. Bus. The authors give the equations that permit to calculate such deviation in different
operating conditions, for both the dipolar and the unipolar modulation methods. By this analysis arises that the
neutral point voltage is influenced by non perfect circuit symmetry and modulation method. In effect the neutral
point voltage shows a deviation due to the unbalance between the capacitor banks of the DC link. Besides, during
the unipolar modulation it appears a third harmonic ripple that is not present during the dipolar modulation. The
authors deal with the problem of the compensation of the neutral point deviation and propose a new solution
based on a linear method.
A closed loop system compensates the neutral point voltage deviation and a particular procedure based on an
open loop structure control reduces the third harmonic ripple.
These procedures have been implemented and tested on 9MVA NPC Inverter with active front end as reported in
this paper. |
