The increasing integration of renewable energy resources such as wind and solar into the electric power grid through power electronic inverters has changed grid dynamics and posed challenges to grid planning, operation, and protection. The growing penetration of renewable energy resources may drive the grid towards weak power grid conditions, under which grid stability issues may affect the operation of inverter-based renewable generators. To more accurately identify the weak grid conditions for guiding grid planning to prevent potential weak grid issues, a method for grid strength assessment is proposed by considering not only the impact the of interaction between interconnected renewable resources, but also the impact of the interactions between shunt capacitors interconnected through the power network. On this basis, we use a real-time digital simulator to explore inverter dynamics under different grid conditions including weak grid conditions. One of the major findings is that there are undesired transient events during the grid restoration process, and the undesired transient events become more significant when increasing the number of solar PV inverters or under weak grid operating conditions. This finding motivates us to study the impact of inverter dynamics on grid protection during the grid restoration period by using a real-time digital simulator. It is found that inverters can act as negative-sequence sources to inject negative-sequence currents into the grid during the grid restoration period and thus can adversely impact the performance of protection schemes based on negative sequence components and potentially cause relay maloperations during the grid restoration period, thus making system protection less secure and reliable.