Ground currents in single-phase transformerless photovoltaic systems

The relative weight of the energy generated by means of renewable sources is constantly increasing. Among all these sources, the photovoltaic (PV) systems present the higher and more stable relative growth. However, the PV system is still too expensive and a significant effort is being done to increase the efficiency and reduce the cost. Concerning the PV inverters, this has lead to the elimination of the low frequency (LF) transformer that has been traditionally included. The LF transformer provides isolation from the grid but reduces the PV inverter efficiency and increases its size and cost. However, the elimination of the transformer might generate strong ground currents, which become now an important design parameter for the PV inverter. The ground currents are a function of the system stray elements. However, there is no simple model and procedure to study the common mode behavior of a PV system, which is required to analyze the ground currents. In this paper, a comprehensible model is proposed which provides a better understanding of the common mode issue in single-phase transformerless PV systems. In addition, a procedure is developed to analyze the global performance, efficiency, grid current quality, and common mode behavior of a PV inverter as a function of its particular structure and modulation technique. Copyright (C) 2007 John Wiley & Sons, Ltd.