In this paper, we propose decentralized controllers for the design of the Automatic Generation Control (AGC) layer in power networks equipped with Flexible AC Transmission Systems (FACTS) devices. We focus on the capability, provided by FACTS, of redirecting power flows by controlling physical parameters of tie-lines. Control design is decentralized as the procedure for synthesizing a controller for a generation area uses information from neighboring areas only. Moreover, our method is termed plug-and-play because, if a generation area is plugged in or out, at most neighboring areas must update their controllers, leaving the rest of the network unaffected. Performance brought about by the proposed controllers is discussed on a benchmark example.