Modulation Technique for Single-Phase Transformerless Photovoltaic Inverters with Reactive Power Cap

Abstract

This paper underpins the principles for generating reactive power in single-phase transformerless photovoltaic (PV) inverters. Two mainstream and widely adopted PV inverters are explored, i.e., H5 and HERIC. With conventional modulation techniques, reactive power cannot be realized in H5 and HERIC due to the absence of freewheeling path in negative power region. Based on the study, modulation techniques are proposed to provide bidirectional current path during freewheeling period. With proposed modulation technique, reactive power control is achieved in H5 and HERIC inverters, without any modification on the converter structures.

Existing System

Pulse-Width Modulation Method.

Proposed System

In this paper, the general principles of reactive power generation for single-phase transformerless PV inverter are underpinned. In order to generate reactive power, a new current path is required in order to achieve zero-voltage state during negative power region. Based on the analysis, modulation techniques are proposed which provides bidirectional current path during freewheeling period. As a result, reactive power control is realized in H5 and HERIC inverters, without any modification on the converter structures. Furthermore, the common-mode behavior of which is not compromised. The CMV is maintained at constant which helps to suppress the leakage current. The overall performances were verified via simulation and experimental investigation. The results prove that with the proposed modulation method, H5 and HERIC inverters are suitable for transformerless and reactive power applications. The similar principles can be extended other single-phase transformerless PV inverter topologies. In short, the proposed working principles extend the conventional inverters applications to next-generation PV systems which require reactive power capability.

Architecture

H5 inverter Structure

HERIC inverter Structure