What is PRP and why does it matter for substation SCADA?

• PRP achieves zero-time failover by sending duplicate data through two separate networks simultaneously. If one network fails, the data already arrived through the other. There is no recovery delay.

• Not all redundancy methods are equal. RSTP, the common alternative, takes hundreds of milliseconds to recover. For IEC 61850 GOOSE protection signals, that window is too long. PRP and HSR eliminate this gap.

• PRP works with standard Ethernet switches and does not require replacing existing network hardware. This makes it practical for both new builds and brownfield upgrades.

• zenon supports PRP for MMS Client and GOOSE communication, enabling redundant communication for the critical data paths in a substation SCADA system.

A communication failure during a grid fault can prevent protection signals from reaching their destination, turning a manageable event into a major incident. High availability in substation SCADA is therefore a safety requirement. It is achieved by combining redundant server architectures with network redundancy protocols that ensure control is maintained when any single component fail.

In the electric power sector, communication network failure is not an inconvenience. It is a safety and reliability risk. The SCADA system must maintain continuous visibility and control of substation equipment, even when a network switch fails, a cable is damaged, or a piece of equipment is taken offline for maintenance.

The communication protocols used in energy environments are designed with reliability in mind. DNP3 was built specifically for remote communication over low-bandwidth or interruption-prone links, using event-driven reporting and integrity polls to ensure data delivery. IEC 60870-5-101/104 provides similar robustness for telecontrol applications. IEC 61850, widely adopted for substation automation, defines high-speed communication services for protection and control. These protocols incorporate mechanisms for detecting and managing communication issues within the data exchange itself.

However, protocol-level reliability alone cannot compensate for a network failure. If the physical or logical network path between devices breaks down, the application data cannot flow, regardless of how robust the protocol is. This is where network redundancy becomes essential. A substation must be able to survive the failure of any single network component, a switch, a cable, or an interface, without interruption to protection or control functions.

To achieve this, utilities deploy redundant architectures: multiple SCADA servers, multiple Ethernet communication interfaces, and network paths that can survive individual component failures. zenon supports this through synchronized primary and standby server operation. The primary and standby servers maintain a redundant connection to all related devices. If the primary server fails, the standby takes over without data loss. zenon also supports a preventive mode called Rated Redundancy, where the system monitors specific metrics and can trigger a controlled switchover to the standby server before a critical state is reached.

Redundant servers solve one part of the availability problem. The other part is the network itself. If the communication path between the SCADA server and the substation devices fails, server redundancy alone is not enough. This is where network redundancy protocols come in.

Substation networks typically use one of three redundancy protocols: RSTP, PRP, or HSR. RSTP reconfigures the network after a failure, recovering in hundreds of milliseconds. PRP and HSR both provide zero-time failover by sending duplicate data frames through separate paths simultaneously, ensuring no data loss during a network fault.

Three redundancy methods are commonly discussed in substation automation:

• RSTP (Rapid Spanning Tree Protocol) creates a logical tree structure across all switches. When a failure is detected, the network reconfigures to use a backup path. Recovery typically occurs within a few hundred milliseconds. For many industrial applications, this is sufficient. For substation protection signals traveling via IEC 61850 GOOSE, it may not be. GOOSE messages must arrive within milliseconds to coordinate protection functions such as tripping and interlocking. RSTP can also be used in combination with link aggregation to improve performance, but it does not achieve the zero-recovery-time behavior expected for the most critical functions.

• PRP (Parallel Redundancy Protocol) takes a different approach. Instead of waiting for a failure and then reconfiguring, PRP prevents the failure from having any effect. It does this by duplicating every data frame and sending it through two completely independent networks simultaneously.

• HSR (High-availability Seamless Redundancy) achieves the same zero-time failover as PRP, but uses a ring topology instead of two parallel networks. Data is sent in both directions around the ring. If the ring breaks at any point, data still reaches its destination from the other direction. HSR requires devices to be connected in a ring configuration and is less flexible than PRP in terms of network design.

A PRP-capable device has two network ports, each connected to a separate network. When data needs to be sent, the device transmits identical frames through both ports at the same time. The receiving device accepts the first frame to arrive and discards the duplicate. If one network fails entirely, the data still arrives through the other. There is no switchover, no reconfiguration delay, and no lost data.

PRP does not require special switches. The two networks can use standard Ethernet switches and can be structured identically or differently. Devices with a single network port can be connected through a Redundancy Box (RedBox) that provides access to both networks. This makes PRP practical to implement in both new builds and brownfield upgrades.

It is possible to meet the requirements of IEC 61850 using a combination of RSTP and link aggregation at a lower cost. However, when running critical functions on a SCADA system, it is widely recommended to use PRP or HSR to achieve the network failover times needed. PRP, with its use of two separate, independent networks, provides operational benefits for maintenance and is widely preferred for solutions that require high availability

IEC 61850 defines high-speed communication services for substation automation, including GOOSE messaging that must arrive within milliseconds. To meet these requirements, the standard references PRP and HSR as redundancy protocols. PRP's ability to use standard Ethernet switches and support independent network designs makes it a widely adopted choice for substation SCADA.

The IEC 61850 standard establishes communication methods for intelligent electronic devices (IEDs) in electrical substations. It defines services including MMS for client/server communication, GOOSE for high-speed protection messaging, and Sampled Values for transmitting digitized measurements. These protocols run on high-speed switched Ethernet to meet the response time requirements of protection systems.

The standard references PRP and HSR as the redundancy protocols to enhance communication resilience. This is driven by the need for recovery times that are effectively zero for critical protection functions. IEC 61850 GOOSE messages must reach their destination within milliseconds. A network recovery time measured in hundreds of milliseconds, as with RSTP, means protection signals may be lost during that window. PRP and HSR eliminate the recovery window entirely.

For substation SCADA applications, PRP's ability to work with standard Ethernet switches, its support for independent network design, and its flexibility in integrating existing single-port devices make it a widely adopted choice. It allows one network to be worked on for maintenance while the other continues to carry all traffic without interruption.

zenon Software Platform supports high-availability SCADA by combining PRP network redundancy with server redundancy, communication monitoring, and alternative data source switching. This integrated approach ensures critical communication paths remain operational during network failures, while automated monitoring and switchover capabilities help prevent disruptions before they occur.

zenon supports PRP across its Ethernet-based communication services, such as IEC 61850 including MMS Client for data acquisition, MMS Server for data provision and GOOSE for protection signaling. This means critical communication paths between the SCADA system and substation devices continue to operate even if a network component fails. The operator sees no change in system behavior. The failure can be investigated and repaired while the substation remains fully operational.

Beyond network-level redundancy, zenon provides additional capabilities for maintaining availability:

• Server redundancy. zenon supports synchronized primary and standby server operation with hot standby redundancy. If the primary server fails, the standby takes over without data loss. Rated Redundancy mode continuously monitors system health and can trigger a preventive switchover before a critical state is reached.

• Communication monitoring. zenon protocol drivers provide statistics on connection health and data quality. When communication degrades, the system can alert maintenance staff, trigger alarms, or automatically switch to alternative data sources. This allows utilities to investigate and resolve communication issues during normal operation.

• Alternative data sources. zenon includes a native function to feed datapoints from multiple value sources. If a primary data source becomes unreliable, the system can automatically switch to an alternative source according to a configurable algorithm, helping to maintain continuity of operation.

• Compliance and security. zenon's IEC 61850 client driver is Edition 2.0 certified by TÜV SÜD, confirming reliable operation with current protocol functions. For cybersecurity, zenon supports TLS encryption according to IEC 62351-3. Security mechanisms such as encryption and authentication also contribute to communication reliability by verifying data integrity and the authenticity of communicating entities.

The Simple Network Management Protocol (SNMP) provides visibility into the health of network devices such as switches, routers, and firewalls. It enables operators to monitor whether devices are functioning correctly, which ports are in use, and whether errors have occurred.

zenon includes an SNMP driver and can serve as an SNMP manager, collecting data from network devices and displaying it as variables within the SCADA system. This data can be shown in process graphics, evaluated in reports, or stored in archives. zenon can also trigger alarms based on SNMP data and intervene automatically if configured to do so.

Additionally, zenon can act as an SNMP agent, sending operational state information to a higher-level SNMP manager through the zenon Process Gateway. This allows a superordinated system to monitor the operational state of zenon itself, for example, when zenon is used as a control system in an unmanned substation.

SNMP complements PRP and other redundancy measures by providing the monitoring layer that detects problems before they cause failures. While PRP ensures the network survives a failure, SNMP helps operations teams identify degraded components and address them proactively.