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Small, light, low cost and high efficiency grid connected photovoltaic (PV) systems can be achieved when the transformer is omitted from the inverter. However, dangerous leakage current will flow between PV array and the grid due to the stray capacitance. The leakage current increases conducted and radiated electromagnetic interference, total harmonic distortion (THD) and losses. Moreover, injection of direct current (DC) to the grid will saturate the distribution transformer along the grid. To solve these issues, different conversion topologies have been proposed. This paper is to model several transformerless grid-connected PV inverter topologies. The operation and common mode performance of these topologies are compared and analyzed.