  The utilization of renewable energy sources (RESs) has become significant
throughout the world especially over the last two decades. Although high-level
RESs penetration reduces negative environmental impact compared to conventional
fossil fuel based energy generation, control issues become more complex as well
as total inertia to the system is significantly decreased due to removal of
conventional synchronous generators. Some other technical issues, high
uncertainties, low fault ride through capability, high fault current, low
generation reserve, and low power quality, arise due to RESs integration.
Renewable energy like solar and wind are highly uncertain due to intermittent
nature of wind and sunlight. Cutting edge technologies including different
control strategies, optimization techniques, energy storage devices, and fault
current limiters are employed to handle those issues. This paper summarizes
several challenges in the integration process of high-level RESs to the
existing grid. The respective solutions to each challenge are also discussed. A
comprehensive list of challenges and opportunities, for both wind and solar
energy integration cases, are well documented. Also, the future recommendations
are provided to solve the several problems of renewable integration which could
be key research areas for the industry personnel and researchers.
