الفهرس 
صفحة العنوانOnly 14 pages are availabe for public view
 |  | from | 213 |  |  |
| | | from | 213 | | |
Abstract
Renewable energy sources are considered a crucial energy transformation to reduce carbon emissions, and for the upcoming years it is expected that solar PhotoVoltaic (PV) will become the second-prominent generation source after wind energy.
Usually, photovoltaic inverters are connected to the low voltage or medium voltage grid. Thus, some technical requirements apply to the grid-connected PV inverters, and international standards bounding the Total Harmonic Distortion (THD) of the grid injected currents and power factor should be met.
Bypass diodes are used to mitigate the partial shading impacts. The global maximum power point is difficult to track during partial shading, due to multiple local Maximum Power Points (MPP) generated during partial shading, and the MPP location changes. Consequently, there is a fraction of photovoltaic energy that is lost.
This thesis proposes a novel solution without using passive components or bypass diodes and/or changing the interconnection of photovoltaic modules. A novel multi-terminal Current Source Inverter (CSI) based grid-connected PV system is proposed. This multiple-terminal single-stage converter allows to a number of N PV panels to be connected directly to the three-phase grid without the need for extra additional boosting converters, and provides independent maximum power point tracking for each PV, thus mitigating the impact of partial shading. Additionally, the reactive power is controlled to inject/consume reactive power into the grid during the day. A grid filter is designed to meet the technical requirements of grid-connected PV inverters, to guarantee less than 5%THD in the grid current and controllable power factor at the point of connection.
A non-linear Direct Power Predictive Controller (DPPC) is proposed, based on the finite control set of the predictive model controller method. It is a flexible approach allowing fast and accurate dynamic response, where the constraints and non-linearities can be included in the control. The DPPC proposed in this thesis is designed to control the generated power of each PV independently and the grid reactive power at minimum sampling time to improve the performance of DPPC.
An experimental prototype and the simulations in MATLAB/Simulink software environment are used to validate the proposed system for the two-terminal and four terminal CSI (4T-CSI) based PV system, as case studies of multi-terminal CSI based PV systems.
The simulation and experimental obtained results of two-terminal CSI based PV system during uniform irradiation levels and injection of reactive power into the grid during the day, show the high performance of DPPC to track the MPPT and in the control of the reactive power, respectively. The measured value of the grid currents THD is 4.1 % at the maximum PV power, thus less than the 5% set as the maximum by international standards. However, the generated power of PV during partial shading is reduced. Also, in case of converter failure no power is injected in the grid, although the PV is operational.
For the 4T-CSI based PV system, the results are carried out controlling the current or the power at different operation scenarios. The obtained results show the ability of the multi-terminal CSI to solve the problem of partial shading by providing the independent PV power control. Thus, the loss of power represents only the impact percentage of shaded PV and does not affect all the connected unshaded PVs, as in the two-terminal CSI. Also, in case of partial converter failure, 4T-CSI is still able to inject power into the grid even if one of the converter’s leg fails. The simulated and experimental values show that the current THD is kept below 5% and the power factor is guaranteed to be unitary as well.
Keywords: Multi-Terminal Matrix Converter, Photovoltaic System, Photovoltaic Shading, Predictive Controller, Direct Power Controller