Principal Investigator: Dr. Adel Nasiri
Many studies have been presented on various inverter topologies and control algorithms to obtain high efficient, compact and cost effective inverter designs for grid- connected PV systems. However, these studies are mainly focus on low voltage grid systems. The proposed inverters are usually used with a low frequency transformer (LFT) in order to connect to medium voltage grid. The LFT increases the size, volume and cost and decrease the efficiency of the system. The proposed project will develop a novel inverter architecture for solar PV systems, which enables direct connection of the inverter to the Medium Voltage (MV) grid without any low frequency transformer. The proposed system employs an H-bridge inverter, high frequency transformer (HFT) and full-bridge diode rectifier. The DC-DC converter stage is capable of reaching MV voltage with appropriate transformer turn ratio. In the proposed system the DC-DC converter stage is controlled to generate rectified sine wave voltage and current at the secondary side DC bus. Thus, the inverter employed at the secondary side operates at line frequency and only inverts the rectified sinewave voltage and current to AC. This design and operation at line frequency reduces the grid side transients and improves efficiency. Replacing the LFT with the HFT provides significant improvements in terms of size, volume, cost and efficiency. In addition, by employing LLC resonant converter, which can achieve zero voltage switching (ZVS) and zero current switching (ZCS), in DC-DC converter stage total switching losses can be decreased and the efficiency of the system is increased. The proposed single-phase resonant converter cells operating in buck-boost mode also enables much wider Maximum Power Point Tracking (MPPT) range. In addition, the proposed system is able to provide Volt-VAR support for the electric distribution systems.
In summary, the proposed system provides an unprecedented level of efficiency improvement by using (i) resonant PV side converter operating at zero current switching minimizing switching loss, (ii) using high frequency transformer, reducing the loss significantly compared with low frequency transformer, and (iii) using low frequency unfolding circuitry to practically eliminating the switching loss. The conventional 60Hz transformer creates a continuous power loss in the system even if the solar PV system does not generate any power. The proposed system additionally improves the efficiency by de-energizing the high frequency transformer when there is no power generation.