1 Ethernet Redundancy in Smart Grid Substations

Before digital and smart grid substations, traditional substations relied on analog systems and manual operations to monitor and control electricity distribution. Due to the rising need for a system that manages power distributions with greater efficiency, flexibilty, and reliablity, the industry transitioned to digital grid infrastructure. Digital grids use advanced sensors and networks to gather and process data. This industry update enabled real-time monitoring and automated control, unlike that of the previously used traditional grids. Smart grids are a type of digital grid that takes it a step further by having advanced analytics, management, and decision-making capabilities allowing for the optimization of grid performance across a wider scale.

A smart grid substation is a key component of the electricity-grid infrastructure, located everywhere from the high-voltage power generation facilities throughout the distribution network to the low-voltage feeders serving residences and businesses. Substations are a primary factor in transforming voltage levels for transmission and performing important functions such as switching, monitoring, and protecting sub-systems in order to maintain grid efficiency and reliability. To meet these important functions, fast, fault tolerant communication that helps operators achieve cost efficient and reliable operations are necessary. Substation networks require communications that have built-in redundancy and time synchronization with no down time due to a single fault in the network. If a failure does occur, the network must recover within a given short time. Redundancies create more than one path between the source and destination to reroute traffic at the time of failure.

Figure 1-1 Typical Electric Smart Grid

Operators need to continually monitor the health of networks and take action to maintain the operation with efficiency. This need leads to the requirement for reliable and low-latency communications between the control center of the operator and high-value nodes such as substations. Due to the different requirements of station and process bus networks, different protocols need to be implemented according to their specific performance characteristics.

Within a substation, there exists three main different levels of communication: the station level, bay level, and the process level. The 'station bus' handles communication between the station level and the bay level and the 'process bus' handles communications between the bay level and the process level. Each bus within the three main levels carry Ethernet networks that connect various Intelligent Electronic Devices (IEDs) - such as protection relays, bay controllers, etc. - and smart control/SCADA systems. The station bus network is mainly used to carry event-driven Ethernet messages for supervising the system and it interconnects the whole substation and provides connectivity between the central management and individual IEDs at the bay level. The station bus typically carries Generic Object Oriented Substation Event (GOOSE) traffic and TCP/UDP traffic. Traffic in the station bus can tolerate frame losses.

The process bus also carries GOOSE messages, but is also mainly used to carry measurement traffic, in the form of sampled values (SV) traffic. These are small Ethernet frames carrying measurement values, sent by merging units for example - a merging unit is a device within the substation that measures current and voltage signals from instrument transformers. SV traffic does not tolerate frame loss, therefore, media redundancy must work without interruption.

The process bus is redundantly connected to the station bus through, for example, the High-availability Seamless Redundancy protocol (HSR) or Parallel Redundancy Protocol (PRP). The process bus traffic is sent to the station bus for analysis and for additional monitoring.

Figure 1-2 Process Bus vs Station Bus

The need for high performance, high reliability, and predictable Ethernet networks drove the motivation for the IEC 62439 standard with the goal to create low cost, easy to maintain, and interoperable common network infrastructure with built-in redundancy.