Active common-mode filter for PV transformerless grid-connected inverters

Abstract

During the last years, it has been possible to witness a steady and progressive increase of energy production from renewable resources. In particular, the greatest increment has been registered for photovoltaic applications, due to the possibility to install low power implants easily integrated in the urban ambient, the so-called domestic photovoltaic.

A photovoltaic system can be islanded, when the energy is extracted from the panels for supplying local loads, as in the case of remote agricultural areas, or grid-connected, where the energy recovered from the panels is directly injected into the mains.

Until now, where there was the possibility, grid-connected system has been considered the easiest and most efficient solution for photovoltaic plants.

Following these considerations, in the last years there has been a remarkable proliferation, in both academic and industrial field, of new solutions for grid-connected inverters that were designed to maximize efficiency and reliability.

Initially, grid-connected inverters were realized employing a line frequency transformer, which, establishing a galvanic insulation between the photovoltaic source and the grid, facilitated the design issues. Nevertheless, because of its bulky dimension, costs, and additional power losses, the use of transformers was progressively abandoned.

Nowadays transformerless inverters are the most efficient grid-connected converters commercialized and some companies arrive to claim values of 98% of efficiency for their products.

Nevertheless, the absence of a galvanic insulation between the photovoltaic source and the grid gives rise to issues, such us ground leakage currents and possible DC current injection into the grid that must be addressed.

The ground leakage current phenomenon was proven to be due to the presence of a parasitic capacitance between the PV cells and the metal structure of the panels, usually grounded for safety reason.

A survey of the actual solutions to avoid the arising of ground leakage current in transformerless single-phase systems was elaborated in this work, and a novel classification for transformerless inverters was proposed as well.

The principal causes of ground leakage current were investigated, and the contribution to the phenomenon of the common-mode voltage generated at the output of the grid-connected inverters during their operation was analyzed. In fact the common-mode voltage at the output of the converters generates currents that flow in the parasitic capacitance throughout the connection to the ground of the neutral wire of the grid at the MV/LV transformer. For this reason the ground leakage current is also known as common-mode current.

A novel approach to cancel the common-mode voltage variations at the output of a transformerless grid-connected converter was proposed. This solution relies on an active common-mode filter connected at the output of the power converter. It is constituted by a common-mode transformer properly supplied by a low-power full-bridge.

The proposed solution is applicable to both stand-alone and grid-connected converters. In particular in this work the active filter was applied to a full-bridge power converter topology driven by the efficient 3-level (unipolar) PWM.

The feasibility of the proposed solution and the capacity to operate with power factor different from one was proven through extensive simulations in Simulink/Plecs environment, and confirmed with experimental results.

On this purpose, a converter prototype was designed and built. It embeds all the components for enabling the connection to the mains in accordance to the Italian legislation CEI 0-21.