Parallel Operation of Modular Single-Phase Transformerless Grid-Tied PV Inverters With Common DC Bus and AC Bus

In order to enhance the efficiency and reliability of dc-module-type photovoltaic (PV) generation systems, the single-phase transformerless full-bridge grid-tied inverters with a common dc bus and ac bus are utilized as the dc-ac stage. The high-frequency circulating current issue of parallel-operated single-phase grid-tied inverters with different topologies and modulation strategies are investigated. The conditions of high-frequency circulating currents elimination without additional hardware costs are revealed and presented. The analysis indicates that there are no high-frequency circulating currents between parallel-operated inverters with bipolar pulsewidth modulation (PWM). The parallel-operated inverters with unipolar double frequency PWM have high-frequency circulating currents, but the effects of the high-frequency circuiting currents on the fundamental components of split-filter inductor currents can be mitigated. The parallel-operated inverters with a unipolar PWM also have high-frequency circulating currents, but their effects on the fundamental components of split-filter inductor currents cannot be reduced. The single-phase transformerless full-bridge inverters with low-leakage currents, such as H5 and highly efficient and reliable inverter concept, meet the elimination conditions of the high-frequency circulating currents. Hence, these inverters are free of high-frequency circulating currents. A prototype of dc module-type grid-tied PV generation system is built up, and experimental tests with different topologies and PWM methods verify the analysis.