Ratan Das, Ph.D., P.Eng.
Electromechanical relay (First generation) shown in block 1, was only available for protection till early 1970s at which point Static or Solid State relay (Second generation), as in block 2, was introduced. RTU was developed in mid 1970s to interface with static and electromechanical relays initially for SCADA, & later for integration with Substation Automation System (SAS).
Intelligent Electronic Device (IED) shown in block 3, was introduced in early 1990s as a Third generation protection and control (P&C) device. They started showing up in mid 1980s as microprocessor-based relay.
IED interfacing with merging unit using IEC 61850-9-2 (Fourth generation P&C device) and connecting with SAS, as in block 4, is primarily the architecture for a digital substation. Digital substation also integrates IED with control centers either directly or through SAS and improve asset management with condition-based maintenance.
To understand digital substation better, you can read evolution of protection, automation and control system and application of microprocessor technology for protective relaying.
Next step is the evolution of Centralized Protection and Control (CPC) system within a substation as a Fifth generation protection and control system.