Level-phase-shifted pulse-width modulation for cascaded H-bridges

Cascaded H-bridges (CHB) is a multilevel inverter topology with a modular structure, and one of its main benefits is the low distortion of the output power. These features make CHB inverter an excellent fit for medium voltage high power applications. The pulse-width modulation (PWM) implementation of CHB has several approaches. With increasing number of output voltage levels, centralized modulators, such as space vector modulation and selective harmonic elimination PWM, demand excessive amount of data transfer between the H-bridge cells and the main modulator. Carrier-based modulators, such as phase-shifted PWM (PS-PWM) and level-shifted PWM (LS-PWM), can be run as distributed modulators, making the implementation more flexible for a high number of cascaded H-bridge cells. However, both of these classical PWM methods have their own flaws, with either impaired output voltage waveform with PS-PWM, or asymmetric loading for the H-bridge cells with LS-PWM. This paper proposes a new carrier-based PWM method, which combines the favorable features of PS-PWM and LS-PWM. The proposed level-phase-shifted modulator is simulated with seven-level and nine-level CHB inverters to demonstrate the operation.

I. Introduction

Cascaded H-bridges (CHB) inverter topology, illustrated in Fig. 1, was first introduced to variable speed drives application in [1]. Since, the application range has vastly increased with the applications like static synchronous compensator (STATCOM) and solid state transformers [2], [3], [4], [5]. The key driving factors for CHB inverters in medium voltage applications are its modular structure and the ability to produce excellent output power quality. One key factor is that the modular H-bridge power electronic building blocks (PEBB) can be implemented with low-voltage semiconductor switches, which make it more appealing for multilevel inverter applications with higher nominal voltage [2].