Substation protection and control architectures went through a number of changes over the last 100 years. They evolved from electro-mechanical and solid state relays with primitive communications infrastructure, based on hard-wired copper wiring and use of analog telephone lines, to integrated protection and control systems utilizing Ethernet networks and based on advanced protocols such as IEC 61850. The current integrated substation automation systems based on IEC 61850 architecture are using two level communication networks – process bus, that connects merging units (MU) and intelligent integrated devices (IEDs) and station bus, that connects IEDs and control SCADA system. The process bus carries digitized primary equipment data in the form of sampled values (SV) and GOOSE messages whereas station bus carries mostly control data. In future Centralized Substation Protection and Control (CPC) systems, IEDs as standalone devices will disappear and their functions will be integrated into the centralized computing platforms (and to a lesser extent into the MUs). This will lead to converged network architecture with process and control buses physically connected but logically separated. The computing platform will provide protection, control, management and monitoring functions. MUs will have a minimum intelligence and may integrate only primary equipment protection functions. This centralized protection and control architecture will require a reliable and secure communications infrastructure. In principle, this communications infrastructure connects MUs with computing platforms and as a result must support all types of protection and control functions. These networks must handle processing of real time mission-critical events in a few milliseconds without data loss while supporting SCADA/control traffic at the same time. Robust network redundancy protocols are the key in building resilient networks. The latest IEC 62439-3 HSR/PRP  protocols are considered one of the best standardized options available for substation CPC communications architecture as they provide true zero-frame loss communication. There are also emerging technologies like Time Sensitive Networks (TSN) and Software Defined Networks (SDN) etc. that will further improve Ethernet networks performance by making them deterministic and easy to manage and configure.
Centralized protection and control (CPC) – resilience – redundancy – communications – infrastructure – converged networks – protection and control (P&C)
The Smart Grid communications infrastructure is converging to Ethernet/IP based networks; the same applies to future substation CPC systems. Despite this clear trend, there is still a mix of many legacy type communication technologies used in today’s substations: different types of serial interfaces and time-division multiplexing (such as DDS, G.703, IEEE C37.94 etc.) used in majority of protective relays with Sonnet/SDH in the backbone. The mission critical nature of P&C systems imposes very specific requirements on network performance. A number of protection schemes require lossless communication with minimum latency and packet delay variations (packets jitter). Lossless communication is also extremely important in minimizing overall latency and preventing retransmits; for example, digitized sensor data carried in the form of Sampled Values (SV) is not retransmitted – the loss of a few samples can prevent signal restoration and impact P&C functions. The existing properties of Ethernet, as a best effort network, require enhancements to address the mission critical nature of P&C systems – until then, the circuit-switched networks will still co-exist in the substation environment. Advancements in developing Deterministic Ethernet (DE), adoption of lossless redundant schemes like HSR/PRP and other improvements will solve these performance issues thereby making Ethernet the network for future CPC systems. The CPC system is defined as a “system comprised of a high-performance computing platform capable of providing protection, control, monitoring, communication and asset management functions by collecting the data those functions require using high-speed, time synchronized measurements within a substation” .
At its core, the future CPC architecture will consist of three major blocks – Intelligent Merging units (IMUs) that carry time-synchronized data from primary equipment, resilient communications infrastructure and CPC Computing (CCPC) devices . The conceptual diagram can be presented as follows: