الفهرس 
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Abstract
A fundamental prerequisite for raising industry and public awareness of sustainable and renewable energy sources is photovoltaic power plants. When compared to other renewable energy sources, solar power plants are expanding quickly, and their source is distinguished by its infiniteness, inexpensive production, naturalness, availability, sustainability, and friendliness.
Despite the fact that solar energy, in particular, is a very promising and advantageous source, some safety precautions and protection requirements should be taken into account for the installed Photovoltaic (PV) power plant, whether it is grid-connected or standalone, as both are vulnerable to various faults and problems, including Temporary Overvoltages (TOVs) and transient overvoltages. Although the installation of PV power plants is placed in very large areas to receive as much as possible from the incident solar irradiation and prevent the shading effects that can reduce its performance, the likelihood of lightning strikes over these plants is high. Lightning strikes are thought to be the most common cause of transient overvoltages. This leads the electrical or electronic equipment exposed to lightning strikes nearby. Solar grid-connected plants are vulnerable to those direct and indirect lightning strokes due to their expanded surface in the wide area.
This thesis is aimed to analyze and mitigate the transient overvoltages resulting from the direct and indirect lightning strokes.
Firstly, a steady state study for a 1 MW solar PV grid-connected power plant was done. Lightning strikes were applied at different positions in the grid to test its effect on the PV farm’s components. Also, the effect of the lightning characteristics was studied. Different mitigation techniques are applied such as, Metal Oxide Surge Arrester (MOSA) with a suitable rating, High Voltage Filter (HVF), and parallel combination between them. The modeling and analysis were implemented using PSCAD/EMTDC software. Secondly, the same system with connecting a down conductor to the PV array frame impacted with lightning stroke for simulation the Indirect Lightning Stroke (ILS) through the PV grid connected power plant which cause Back Flow Current (BFC) through the grid due to ILS effect were discussed. Common grounding between the DC and AC side of the grid was implemented with discussing the effect of BFCs on the peak voltage measured at different positions at the grid. Different mitigation techniques were proposed.
The results showed that when the direct lightning stroke is strike nearby the grid’s inverter position, it negatively harmed the PV farm where the voltages is increased by about a 16.1 %. Also, when the tail time of the lightning stroke increased from 50 µs to 300 µs, it caused an increase in the peak voltage value at the striking position by 71.7%. The results showed that, the parallel combination of MOSA and HVF represents the most efficient mitigation technique, where it can reduces the induced overvoltages by about 96.3% comparing with using MOSA only and 95% comparing with using HVF. For ILS, when the grounding grid resistance, Rg, of the system under study reduced from 5Ω to 1Ω, the peak voltage was reduced by about 91.3% at the striking position. Also, at Rg equals to 1Ω, the peak measured voltage is still high about 1.6 kV. So, the four models of EGLAs are suggested which their alleviation ability to the BFC differs from each other according to the most absorbent type for the discharge voltage. These mitigation techniques improve the performance of the PV power systems.