Guest Editorial

In the last decade, renewable energy, in particular wind and solar energy, has experienced one of the most substantial growths of any power generation source. At present, in many areas across the world, wind and solar energy are providing substantial proportions of the total electrical demand, rising to over 50% in certain regions. However, many renewable energy sources, e.g. modern variable speed wind turbines and photovoltaic systems, are significantly
different from conventional thermal and hydropower generating technologies, since they synchronize to the electricity grid via power electronics converters, and so are not directly responsive to system frequency transients. As a result, there are significant challenges for maintaining a reliable and secure power system, particularly in areas with high penetration of wind/solar energy. On
the other hand, flexible control strategies can give modern wind turbines and solar generation units, as well as large scale wind/solar farms, the ability to provide active power support to the grid during frequency transients over a wide timeframe. This Special Issue brings together papers focused on the recent
advances and breakthroughs in the technology of active power control (APC) from wind and solar (including photovoltaic and solar thermal) energy generation systems, ranging from individual wind turbines and solar generation units to large wind/solar farms. 135 manuscripts were submitted to this Special Issue, and underwent a formal review process, after which only 24 papers
were accepted for publication. The accepted papers are broadly classified into four themes: (I) inertia and primary frequency control of wind turbines; (II) grid integration of photovoltaic generations; (III) power system operation and stability with renewable energy generations; and (IV) Wind power predictions and other issues. A brief discussion of each paper and the authors' contributions are presented below.