Ratan Das, Ph.D., P.Eng.
Electromechanical relay (First gen) shown in block 1, was the only choice till early 1970s. Static or Solid State relay (Second gen), as in block 2, was introduced then. RTU was developed in mid 1970s to interface with static and electromechanical relays initially with SCADA and later with SAS.
Intelligent Electronic Device (IED), as in block 3, was introduced in early 1990s as a Third gen protection and control (P&C) device.
IEDs interfacing with merging unit using IEC 61850-9-2 (Fourth gen P&C device) and connecting with SAS, as in block 4, is primarily the architecture for a digital substation. Digital substation integrates IED with control centers either directly or through SAS.
Evolution of protection, automation and control system and application of microprocessor technology for protective relaying provide good understanding of digital substation. Digital substation also facilitates Microgrid and DER integration, and implementation of FLISR for DA and high impedance fault detection technology.
Next step is the evolution of Centralized Protection and Control (CPC) system within a substation as a Fifth gen protection and control system.