The increasing penetration of electricity generation from renewable energy is posing new technical challenges to the operation of power systems. Some of them are related to the different electrical behavior of the generators when compared with traditional directly coupled synchronous generators. This work is focused on the case of renewable generators connected to the grid through full power electronic converters and the improvement of its response to voltage sags. It is proposed a system based on the use of an electronic on load tap changer to increase the generator contribution to the short circuit current during grid voltage depressions, to cooperate in the post-fault voltage recovery of the power system. It is also presented a specific control system on the converter, based on a nonlinear current source, to secure the necessary fast response. An experimental validation of the developed system has been carried out by building and testing a laboratory prototype. The results of the more relevant tests are also presented.