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.
This paper uses a novel approach for the control of three-level neutral-point-clamped (NPC) rectifiers in order to tackle the capacitor voltage balance problem. A distinctive feature of the new control approach is that it is based on a model which is written in terms of the duty ratios for each phase at each level. Hence, the system model presents nine duty cycle variables. Despite the fact that this formulation is different from the usual ones, it is shown that the control problem of currents and dc-link voltage can be formulated in a similar way to conventional methods. Furthermore, the control of the capacitor voltage balance can be expressed by means of equations that are decoupled from the currents and dc-link voltage dynamics, which results in a specific controller for the voltage balancing that does not affect the previous dynamics. A key point of the proposed approach is that part of the modulation stage is implicit in the formulation. Two particular controllers are compared in this paper. The first one fulfills the different control objectives at the expense of a large number of commutations. This problem is overcome in a new proposed controller, which presents similar performance and a satisfactory number of commutations. Experimental results are performed showing the effectiveness compared with a modified virtual space vector modulation with capacitor voltage balance capabilities.
This paper analyzes the stability of the well-known three-phase two-level power converter. Focusing on the rectifier operating mode, the dynamics of the system, when the instantaneous power and dc-link voltage controllers are included, are described by a set of complex equations that results in a nonlinear autonomous singularly perturbed system. Hence, the closed-loop system can be studied under the assumption of separate time scales. The analysis proposed in this work follows a novel three-time-scale approach, where the fast time scale corresponds with the instantaneous power dynamics, the mid-range time scale is related to the dc-link voltage dynamics, and the slow time scale is associated with the dc-link voltage regulator dynamics. In this way, the analysis leads to the decomposition of the closed-loop system into three simpler subsystems: fast, medium, and slow subsystems. These subsystems approximate the closed-loop system behavior over the three different time scales. Finally, since the equilibrium point of each subsystem is exponentially stable and some other conditions are satisfied, it is shown that the equilibrium point of the closed-loop system also presents exponential stability. Experimental results for a synchronous three-phase power rectifier prototype are included to corroborate the analysis carried out.
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