Wind and solar energy play an important role in the de -carbonization of electricity generation. However, high shares of these Variable Renewable Energies (VREs) challenge the power system considerably due to their temporal fluctuations and geographical dispersion. This paper systematically reviews and analyzes transmission grid extensions as an integration measure for VREs. Effects of grid extensions for fundamental properties of the power system as a function of the penetration and mix of wind coupled with solar energy were revealed in the study. The paper also provides an overview of the system implication of wind and solar PV energy and investigates a way to partly overcome transmission grid extensions.