Three-Phase NPC-LDN Hybrid Transformerless Multilevel Inverter

The proposed inverter topology is evolved from multiple level-doubling-network (LDN) based state-of-the-art topology, where power frequency isolation-transformer is used at the ac output. The proposed work removes the transformer by replacing the main bridges by three-phase TNPC (T-type neutral point clamped) converter. The major issue of this hybrid topology is capacitor balancing of NPC with LDN operation. To solve this issue two different methods are proposed in this article. In these methods, an additional circuitry is added for capacitor balancing. The worst-case power handled by this additional converter is negligible compared to power handling capability of the entire converter. Hence, a full-capacity transformer is replaced by a nonisolated dc–dc converter of fractional rating. Both manufacturing cost and operating cost (losses) are substantially reduced. The proposed topology does not require any isolated auxiliary power supply as well as only one dc source is used for entire topology. Operation of the proposed topology is verified with PV fed grid-connected single stage operation and restricted the leakage current within limit. To validate the concept simulation is carried out in MATLAB/Simulink. A laboratory prototype is developed for experimental verification of the concept. Hardware results matched well with the simulation.

I. Introduction

Multilevel inverters (MLIs) [1], [2], [3] depict significant advantages over two-level voltage source inverters, notably for medium voltage high power applications. It offers high efficiency, power quality, output power capacity, and low electromagnetic interference. Cascaded H-bridge MLI (CHBMLI), flying capacitors, and diode-clamped MLI are well-known MLI topologies used in industry. The key limitations of these MLIs include shifting of NPC neutral point voltages, complicated control techniques to balance capacitor voltages of FC, and the high number of switches and dc sources in CHB.