Large electrical transmission networks are susceptible to undergo very large blackouts due to cascading failures, with a very large associated economical cost. In this work we propose segmenting large power grids using controllable lines, such as high-voltage direct-current lines, to reduce the risk of blackouts. The method consists in modifying the power flowing through the lines interconnecting different zones during cascading failures in order to minimize the load shed. As a result, the segmented grids have a substantially lower risk of blackouts than the original network, with reductions up to 60% in some cases. The control method is shown to be specially efficient in reducing blackouts affecting more than one zone. •Controlling the power flowing through interconnections in power grids during a cascading failure can reduce the size and risk of blackouts.•The control consists in choosing the impedance of the lines interconnecting different zones in such a way that minimizes the load shed in each dispatch.•Contrary to other control methods, the one proposed in this work reduces the size of blackouts without increasing their frequency.•In terms of risk reduction, segmenting large power grids in zones of similar size works better.