Connecting IEDs using PRP
Cisco Connected Grid Switch CGS-2520 supports the IEC 61439-2 Clause 4 Parallel Redundancy Protocol (PRP) that allows data to overcome any single network failure without affecting the data transmission. Intelligent Electronic Devices (IEDs) that implement PRP over Ethernet can connect to the CGS-2520 to achieve IED link redundancy and “hitless” failover for critical IEC 61850 GOOSE messages.
IED vendors can validate interoperability with Cisco Connected Grid Switches for PRP link redundancy and build customer confidence in the joint solution with the “Cisco Compatible” certification and logo.
Figure 1 PRP Topology with IEDs
In legacy substation implementations (without IEC 61850 protocols), the IED relay equipment is often interconnected with fixed cabling. This is accomplished by connecting discrete copper wires between a relay’s output contact terminals and another IED’s input contact terminals. This allows for a relay to have logic implemented that signals another relay by opening/closing its relay contacts when certain conditions are qualified. The transition is detected at the far end IED, which can respond by executing a predetermined action.
For critical protection and control functions such as “trip” messages or “breaker failure”, the entire transition needs to complete in one-quarter to one-half of a 50 Hz or 60 Hz power cycle (60 Hz cycle = 16 ms), or 4–8 ms. As next-generation substation automation is considered an Ethernet/IP based replacement for the legacy wired approach, the requirements have been identified in IEC 61850 to meet or exceed the traditional copper wiring approach.
IEC 61850 GOOSE is the favored approach to replace the copper wiring for critical intra-substation IED-to-IED communication. To meet or improve on the legacy approach, Ethernet transport must provide a path between IEDs with latency that is 4 ms or less. For GOOSE to be adopted as a protection and control replacement for time-proven copper wiring, there is also an industry expectation that the failover from a failed Ethernet link/device also occur in 4 ms or less.
PRP Implementations may vary from vendor to vendor. This solution example uses IEDs that support PRP by having two NICs per IED for transmission of GOOSE traffic. These interfaces are connected to separate parallel substation networks. See Figure 1. In a minimal configuration, each of the rings may be collapsed to a single CGS-2520.
The IED replicates and sends the messages on both it’s interfaces and also gives them a sequence number, which then traverse the parallel networks to arrive at the receiving IED on two separate NICS. The receiving IED selects the first message that it receives and the second copy of the message. In addition to the parallel paths, the sending IED sends each packet out multiple times as part of the GOOSE protocol. In the event of a failure in the network attached to one of the IED interfaces, the receiving IED still receives one copy of the message and thus works in a hitless failover manner. This approach guarantees that one of the packet streams arrives at the destination in less than the required 4 ms.
The suggested solution validation topology consists of two Cisco CGS-2520 rings running Resilient Ethernet Protocol (REP) for convergence and IEDs with dual NICs and PRP capability. One interface of these IEDs is connected to each of the rings. Each of these CGS-2520 rings is completely separate and operates independently of failures in the other ring. PRP has currently not been validated with a Flexlink or Spanning Tree topology; however, in general PRP is not dependent on a particular LAN topology for its operation.
IED 1 and IED 2 under test are capable of implementing PRP on their dual NICs. These IEDs are configured to exchange GOOSE messages as rapidly as the IEDs can send them, and the time when each GOOSE message is sent and received by the IED is recorded in the logs of the relays. The testing consists of shutting down or performing a cable pull for IED interfaces on both ends sequentially. After this, the logs from the two relays are compiled together and no break in connectivity on the application layer (data transfer end-to-end) is verified in the logs.
Additional tests may consist of network outage testing (i.e. breaking the ring) or testing at high packet rates. More information on suggested test plan, equipment configurations, etc. is available to Cisco Developer Network for Connected Energy (CDN/CE) members.