A HYBRID UP-PWM SCHEME FOR HERIC INVERTER TO IMPROVE POWER QUALITY AND EFFICIENCY

Abstract

The Highly Efficient and Reliable Inverter Concept (HERIC) inverter is a cost-effective topology, which has low leakage currents and a relatively high efficiency. Thus, it is very suitable for transformerless PV systems. However, with the reported modulation methods, it is difficult to simultaneously maintain the high efficiency, good power quality, and reactive power injection of the HERIC inverter. In this project, a hybrid UniPolar Pulse Width Modulation (UP-PWM) scheme is thus proposed to achieve those performances. The hybrid scheme adopts the conventional UP-PWM in the case of generating the positive power. When generating the negative power, a modulation scheme, which only requires the operation of freewheeling switches, is specifically proposed. Additionally, in the region of the output voltage and current zero-crossing points (ZCP), an UP-PWM with modified dead time is introduced. In order to validate the effectiveness of the proposed scheme, simulations and experiments are performed on a 4-kW HERIC inverter system with a 20-kHz switching frequency. The results demonstrate that the proposed hybrid UP-PWM method achieves a better performance in terms of reactive power injection than the conventional UP-PWM scheme, and a higher efficiency than the UP-PWM with dead time. In addition, the proposed UP-PWM scheme also enables a better power quality.

Existing System

In the existing system, bipolar pulse width modulation (BP-PWM) technique is used.

Proposed System

This project proposes a hybrid UP-PWM scheme for the HERIC inverter. The main contribution of this project can be summarized into two parts. Firstly, the proposed scheme combines three modulation methods, i.e., a conventional UP-PWM, an UP-PWM with the modified dead time, and a modulation that only requires the operation of AC bypass power switches. Therefore, the proposed hybrid UP-PWM method can achieve a better performance in terms of reactive power injection than the conventional UP-PWM. Additionally, it also achieves a higher efficiency than the UP-PWM with dead time. Secondly, with the modified dead time strategy, the proposed UP-PWM scheme can reduce the current ZCD in the region of the grid voltage and current ZCPs, and thus, an improved power quality is ensured.

Architecture

Hardware schematic of the single-phase single-stage grid-connected HERIC inverter system