DC/DC converters for grid integration of large-scale solar photovoltaic systems

The global trend towards larger size ground mounted solar photovoltaic (PV) power plants is set to continue, with the development of several projects in the 200MW range and higher. This installation trend will challenge the current PV plant architectures by requiring power converters with a higher power rating and a higher voltage level at the point of common coupling (PCC), which can lead to higher ratio transformers or more transformation stages to be used for the connection of the solar farm with the electricity grid.Two possible solutions are proposed in this thesis to minimize the number of transformer stages and/or the transformer turns ratio of a grid-connected PV plant without changing the standard configuration of the system.The first solution is a multistring PV system architecture based on a high-voltage-gain DC/DC converter. By introducing a high-voltage-gain DC/DC converter, the PV system can be connected to a medium voltage grid through a single transformer stage and the turns ratio of transformer can be reduced, thus resulting in reduced cost and increased efficiency of the PV system. A 1MW section of a PV plant has been modeled and simulated using MATLAB/Simulink and PLECS Blockset. The simulation results of three different case studies are presented to evaluate the performance of the proposed PV system configuration.The second solution is a PV system based on a cascaded H-bridge (CHB) multilevel converter topology. Despite the fact that the CHB converter topology can deal with the aforementioned challenges, it faces the issue of leakage current that flows through the solar panel parasitic capacitance to ground which could damage the PV panels and pose safety problems. This thesis proposes a CHB topology with multiphase isolated DC/DC converter for a large-scale PV system which eliminates the leakage current issue. At the same time, the multiphase structure of the DC/DC converter helps to increase the power rating of the converter and to reduce the PV voltage and current ripples. A 0.54 MW rated seven-level CHB converter using multiphase isolated DC/DC converters has been modeled and simulated using MATLAB/Simulink and PLECS Blockset. Simulation results of different case studies are presented to evaluate the performance of the proposed PV system configuration.The proposed PV system based on a high-voltage-gain DC/DC converter has achieved satisfactory performance for boosting the voltage, thus the PV system is connected to a medium voltage grid through a single transformer with low turns ratio. Moreover the interleaved configuration of the high-voltage-gain DC/DC converter helps to increase the voltage gain and power rating of the converter.The medium voltage grid connection with a single transformer stage also has been achieved in the proposed PV system based on a CHB topology with multiphase isolated DC/DC converter. Moreover, the use of a multiphase isolate DC/DC converter has completely removed the leakage current issue and has resulted in better maximum power point tracking (MPPT) efficiency than the single-phase converter case