Neutral point (NP) clamped (NPC) inverters produce low-frequency NP voltage oscillations when modulated with nearest three-vector (NTV) methods for certain loading conditions. Non-NTV modulations can address these oscillations at the expense of increased switching frequency, output voltage distortion and complexity. This study presents a new generalised algorithm to implement hybrid modulation strategies which can also eliminate NP voltage oscillations using a combination of the aforementioned methods. The proposed algorithm avoids abrupt changeover between the two modulations and ensures minimum contribution of non-NTV operation and hence its drawbacks. Moreover, the algorithm is computationally efficient compared with its counterparts, since it only uses redistributed NTV duty ratios to generate NTV and non-NTV modulations instead of using two separate complex algorithms which result in increased computational burden. A non-NTV method called selected three-vector (STV) modulation is chosen to perform the detailed analysis of above proposed algorithm for hybrid NTV-STV modulation. Additionally, a simplified STV (SSTV) is derived to develop a new hybrid NTV-SSTV modulation strategy which further reduces losses and complexity compared with the hybrid NTV-STV modulation, without affecting the performance of NPC. The proposed algorithm and the derived hybrid modulation strategy are validated using simulation and experimental results.
In this paper, a Diode Clamped Multilevel Topology operating in Rectifier mode is presented. A simple voltage control strategy is implemented in order to control the DC-link voltage at the output side of the rectifier. A Level shifted Carrier based Sine PWM or SVM techniques can be incorporated to generate switching signals to the devices of Five-level configuration. For Diode Clamped Converter if the number of levels increased beyond three, charge balance among the capacitors is a major issue. A simple charge balance circuit is analysed for 5-level converter and a new voltage balance control scheme is proposed for maintaining equal capacitor voltages. MATLAB based simulation results are presented for the proposed scheme along with the conventional scheme for comparison. The converter also works as High Power Factor Rectifier and draws sinusoidal supply currents with unity power factor. Index Terms-Multilevel-Converters, Voltage balancing, Diode Clamped Converter (DCC), Space Vector Modulation (SVM).
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