•Technical impacts of PV integration into power systems are reviewed.•The PV characteristics causing the impacts, as well as the level and timeframe at which they occur are identified.•PV penetration limits reported in the literature are examined.•The tools and models to analyse the power system impacts are elaborated. As the number of photovoltaic (PV) installations across the world keeps on increasing, their impacts on power systems are becoming more visible and more severe. In this two-part review, the implications of high PV penetration on the stability and reliability of power systems are comprehensively assessed. This paper, the first of the two, reviews the impacts of PV on the power systems’ voltage, frequency, protection, harmonics, rotor angle stability, and flexibility requirement in detail. Factors contributing to those impacts, as well as the level and timeframe at which they occur, are carefully analysed. Subsequently, the limits of PV penetration observed in the literature are reviewed. To allow the readers to verify these impacts and limits, the tools and models typically employed in power system analysis are also elaborated. The second part of the review then completes the investigation by assessing the existing solutions to the PV integration challenges and suggesting the way forward.