This paper investigates the integration of a 5 MW photovoltaic (PV) distributed generation (DG) system into the 66 kV Tragan sub-transmission network in southern Libya, a weak grid with poor voltage performance and high reactive power demand. The PV system is sized using PV*SOL, while load flow and contingency analyses are carried out using NEPLAN. The results show that PV-DG significantly improves network performance. Real power losses are reduced by 30.6% (from 3.501 MW to 2.429 MW), while the minimum bus voltage increases from 90.4% to 97.8%, eliminating all five voltage violations. Under N-1 contingency conditions, the minimum voltage during transformer outage improves from 87.2% to 94.2%, satisfying operational limits. The analysis also shows that PV alone cannot fully support the evening peak, and a 3 MW/6 MWh battery is recommended. Furthermore, an economic assessment based on actual generation cost gives a payback period of 6.5 years, and a negative carbon abatement cost (–$149/tCO₂). Environmentally, annual CO₂ emissions are reduced by 4,813 tons. Overall, the study confirms that PV-DG is a practical and effective solution for improving weak desert networks such as Tragan, although battery storage, transformer upgrades, and proper protection coordination are needed to achieve full system reliability.