Abstract
Neutral Point Clamped (NPC) converters with n levels are traditionally controlled in such a way that the DC-link capacitors operate at 1/(n − 1) of the total DC-link voltage level. The voltage level across the DC-link capacitors has to be properly regulated by the capacitor unbalance control to contain the harmonic distortion of the converter output voltages. State-of-the-art modulation techniques address the problem of the DC-link voltage regulation for NPC inverters. However, they highly show reduced performance when unbalanced DC-link voltages are considered. In this paper, a novel Space Vector Modulation (SVM) is proposed for NPC converters with an unbalanced DC-link. At every modulation interval, the technique defines the optimal switching pattern by considering the actual unbalanced DC-link conditions. The proposed modulation allows improving the harmonic content of the NPC converter output voltage with respect to a traditional ML-SVM, when the same operating conditions are considered. As an extension, the proposed modulation technique will guarantee the same output voltage quality of a traditional ML-SVM with unbalanced DC-link, while improving the conversion efficiency thanks to a reduction of switching frequency.
Highlights
The widespread adoption of Voltage Source Inverters (VSIs) has pushed research forward in the development of new power converter topologies and modulation techniques, which take into account the non-linear behavior of power semiconductors and components with the aim of improving the performance of the whole power conversion system [1,2,3]
In medium- and high-power applications, Neutral Point Clamped (NPC) inverters [4] are prevailing over conventional two-level
A NPC inverter with n levels takes its name from the typical structure of the DC-link bus, composed by n−1 capacitors forming several Neutral Points (NPs)
Summary
The widespread adoption of Voltage Source Inverters (VSIs) has pushed research forward in the development of new power converter topologies and modulation techniques, which take into account the non-linear behavior of power semiconductors and components with the aim of improving the performance of the whole power conversion system [1,2,3]. Low harmonic content of the output voltages can be guaranteed with a traditional multilevel SVM (ML-SVM) [13] or CBPWM if the capacitors’ voltages are balanced and have a low ripple This is usually achieved increasing the switching frequency causing higher losses, especially in high power density applications with small DC-link capacitors, [2]. To address the distorted output voltage and current due to unbalanced DC-link, several modulation techniques have been developed for 3 levels (3L)-NPC converter based on CBPWM with appropriate zero-sequence component injection or modified SVM [16,17,18,19]. The proposed SVM considers the time-variant voltage unbalance across the DC-link capacitors during the modulation period in order to calculate the duty cycles of the real output voltage space vector [20].
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