Voltages balance control in three phase three-level NPC rectifiers

In this work, several strategies to control a three phase three-level Neutral Point Clamped (NPC) rectifiers are presented. This type of converter has two operation modes, as rectifier or as inverter. As rectifier, the dc-link is split in two, being two capacitors placed at the output. The objective of control the dc-link capacitors voltages balance is the goal of this paper. This capacitors voltages difference should be stabilized at zero so that no unbalance appears in the system. Under the assumption of a two-time-scale, the equation of the capacitors voltages balance dynamic is studied and it is shown that it is linear except for an exogenous term, which can be considered as a disturbance. Analyzing this term, it is shown that its expression takes the form of a sinusoidal signal. This fact is used to design the controller by cancellation of the sinusoidal disturbance. Some methods of control using this idea are proposed in this paper and their results are also presented.