Abstract

Grid-tied photovoltaic inverters must fulfill several requirements, including high efficiency and reduced cost and complexity of the overall system. Hence, transformerless operation is advantageous in order to achieve the prior requirements. Meanwhile, such operation results in several demerits, such as the dc current component injection into the grid. This component should be effectively mitigated in order to avoid some impacts, such as the saturation of the transformers in the distribution network. On the other hand, limiting this component up to few milliamperes is a challenging issue due to the various measurement errors. Accordingly, different blocking and measurement techniques have been proposed and studied to overcome this issue, where some demerits are seen behind each technique such as the implementation complexity, the common-mode voltage problems, and the high filter requirements. Moreover, none of them measures the dc component directly, but predicts its value using different approaches. Hence, this letter proposes a new technique to measure this dc current component with high accuracy using a coupled inductor combined with a small-range Hall effect current sensor in order to achieve the lowest possible cost with the highest possible accuracy. The proposed technique is introduced, analyzed, and tested experimentally to verify its principle of operation. Also experimental measurement of the dc current component using a 5-kVA transformerless grid-tied voltage-source inverter is introduced with and without the proposed technique in order to validate its operation.

V. CONCLUSION

This paper has proposed a coupled-inductor-based dc current component measurement technique for transformerless grid-tied inverters. The merits behind this method are low cost, simplicity, and high accuracy. Meanwhile, additional current sensors are used, but these sensors has a small range. Unlike all the conventional measurement techniques, the proposed one is able to measure the dc component itself, i.e. it does not predict its value from the measured dc offset between the inverter outputs. The proposed measurement technique is verified experimentally and tested in a 5 kV A three-phase three-wire transformerless grid-tied voltage source inverter, in which the dc component has been effectively reduced from 60 mA to 2 mA.