This study aims to analyze the impact of integrating a 100MW solar photovoltaic (PV) system into the Port Harcourt 132kV transmission network, specifically to assess its effects on grid performance and stability. The increasing incorporation of renewable energy, especially solar PV, presents operational challenges such as voltage fluctuations, reactive power imbalances, harmonic distortion, and frequency instability. If left unmanaged, these issues can lead to transformer overloading, grid congestion, and increased system losses. To address these challenges, we conducted load flow, voltage stability, and harmonic analyses using the Electrical Transient Analysis Program (ETAP) to model the existing network and evaluate the integration of the 100 MW solar PV systems alongside a battery energy storage system (BESS). The simulation results indicated that prior to integration, critical buses (T1A = 89%, T2A = 89.1%, T3A = 90.8%) and transformers (T1A = 112.8%, T2A = 111.8%, T3A = 91.7%) were operating beyond acceptable limits. After integration, the bus voltages improved to T1A = 96.06%, T2A = 96.11%, and T3A = 97.36%. Additionally, transformer loading decreased to T1A = 71%, T2A = 70%, and T3A = 46.8%, while total network losses significantly reduced from 6086.8 kW + j32740.7 kvar to 1093.172 kW + j9392.581 kvar. These findings demonstrate that the coordinated integration of solar PV and BESS can enhance voltage stability, reduce system losses, and minimize transformer stress. The study recommends supportive policy frameworks to encourage large-scale solar PV integration with energy storage, representing a sustainable approach to improving grid reliability and advancing Nigeria’s transition to renewable energy.