Soft Switched Multilevel Unidirectional High Frequency Link DC to AC Converter for Medium Voltage Grid Integration of Solar Photovoltaics
Grounding the frame of photovoltaic (PV) panel is a necessity for the safety of humans. This leads to the formation of large capacitance between PV cells and ground. Hence, to reduce leakage current due to parasitic capacitance between PV cells and ground, the DC output voltage of a photovoltaic panel is normally kept below 1 kV. Conventionally, for medium voltage (3.3 kV-66 kV) AC grid integration of PV panel, the DC output of PV is rst converted to 400V AC and is connected to a 400V collection grid through a line frequency transformer (LFT). This LFT provides isolation and limits circulating current among the PV modules. Another step-up LFT is used to connect 400V AC grid to the medium voltage (MV) transmission grid. These line frequency transformers are bulky and expensive. The line side lters are placed on the low voltage side of the rst LFT and hence, experience high currents leading to higher copper losses. To avoid the limitations of LFTs, power converters with high frequency transformer (HFT) are becoming popular. The HFT is fed from a DC side inverter (DSI) and the output of HFT (which is high frequency AC) is converted to line frequency AC using power electronic converters. This type of converter is known as high frequency link (HFL) DC to AC converter. State-of-the-art HFL DC to AC converters mostly employ a multi-stage power conversion technique where an isolated DC to DC converter is cascaded with an inverter. The stages are controlled independently. The inter-stage voltage sti DC-link is maintained with large electrolytic capacitor. But such an approach requires higher amount of ltering and use of electrolytic capacitor a ects long-term reliability. Moreover, the capacitor voltage needs to be tightly regulated to protect the devices. The grid interfaced inverter is high frequency hard-switched resulting in reduced e ciency. These drawbacks are overcome in a single-stage power conversion approach where the inter-stage lter capacitor is removed and all the power devices are either soft or line frequency switched resulting in reduction in switching loss and improvement in e ciency. In literature, to replace the step-up LFT and to directly integrate the converter to the medium voltage grid, a popular solution is the usage of cascaded multilevel power conversion. Generally, the above discussed multi-stage converter is employed as modules in a cascaded multilevel con guration to produce medium voltage. Moreover, some existing topologies use single-stage converters in a cascaded multilevel con guration to produce medium voltage, but the grid side converters are high frequency switched, leading to higher loss. In this thesis, a new topology is proposed to overcome the drawbacks of existing cascaded multilevel power conversion topologies. In the thesis, a new single-stage high frequency link cascaded multilevel converter topology is proposed for MV grid integration of solar power. A single-stage high frequency link DC to AC converter is used as a module. The DC side of each module is connected to a PV source. The AC sides of multiple such modules are connected in series in a cascaded fashion to interface with the MV AC grid. Proposed modulation of the DC to AC module results in zero voltage switching (ZVS) of the DC side converter and line frequency switching of the AC side converter. ZVS happens for most part of the line cycle. Over a switching cycle, the operation of this module is similar to a phase-shifted full bridge (PSFB) DC to DC converter. In the PSFB converter, during switching transition, the parasitic capacitance of AC side diode bridge along with leakage inductance of HFT forms a resonating circuit. This resonating circuit leads to high voltage stress on the secondary side devices. An active snubber is designed to restrict the voltage overshoot. The operation of PSFB converter, considering all parasitics, is not explored in literature. In this thesis, a detailed analysis of the operation of the PSFB and step-by-step design methodology is given. The hardware is designed and tested with DC input voltage of 400 V, DC output voltage of 1240 V, output power of 1.5 kW and switching frequency of 20 kHz. Experimental results validate the analysis. A method is proposed to observe medium voltage waveforms with the standard low-voltage probe. A method to remotely control the medium voltage converter is developed to ensure safety.