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NETWORK / 25 / JULY/AUGUST 2017 HSR (HigH-availability SeamleSS Redundancy pRotocol) Like PRP, HSR applies the principle of parallel operation to a ring interconnecting IEDs with two links. HSR relies on ring topology, sending data not over two networks but in both directions of the single ring. Unlike PRP, however, HSR does not allow single and double network attachment devices (non-redundant and redundant devices) to be mixed on the same LAN. Because of that, laptops and worksta- tions must be connected to the network via a dedicated redundancy device called a redundant box (RedBox). The principles of HSR are: l One Ethernet network ring connects each doubly attached node with HSR (DANH). l A DANH source sends a frame simultaneously on both ports and blocks the sent messages if received. l In normal operation, all DANHs receive frames from both ring connections and instantaneously forward them. l A DANH destination receives both frames in normal operation. It uses the first and discards the duplicate. l If a link fails, a DANH destination operates with the frames from the other healthy path. l A singly attached destina- tion receives only one frame via the redundant box (RedBox) it is connected to. In other words, if an IED fails, its protection function and the communication redun- dancy are lost, but the other IEDs continue to communicate, protect, and control the electri- cal equipment through a single communication network. Because HSR forwards all messages from all IEDs, the communication bandwidth is proportional to the number of IEDs. Each device needs to send not only its own mes- sages but also pass along all messages coming from all other IEDs. Therefore, HSR is limited to about 16 communi- cating devices. HSR, like PRP, is also plug and play. And because it em- ploys single ring topology, addi- tional switches are not needed. That makes deployment less expensive than PRP. PRP and HSR solutions are based on IEC 62439-3. danH: doubly attached node HSR The operating principles of PRP are: l Two completely separated Ethernet networks (LANs) are operated in parallel. l Each doubly attached node with PRP (DANP) has an interface to each LAN. l A DANP source sends a frame simultaneously on both LANs. l A DANP destination re- ceives both frames (in nor- mal operation) and discards the duplicate. l A singly attached desti- nation receives only one frame. If a LAN fails, a DANP destina- tion operates with the frames from the other LAN. When applied, those principles amount to this: if an IED fails, only its protection function is lost. The other IEDs continue to communicate over the redundant LAN to protect and control equipment. And if a switch fails, communica- tion redundancy is lost, but not the protection and control functions. That's because messages sent on the healthy network are not disturbed. Further, PRP allows single and double network attach- ment devices (non-redundant and redundant devices) to be mixed on the same LAN, thus allowing laptops and worksta- tions to be connected to the network with standard Ether- net adapters (non-redundant devices). And because PRP uses a double star topology, the communication bandwidth on each star made with a single link is not different than for a non-redundant network. PRP can thereby accommodate more IEDs than HSR (high availability seamless redun- dancy protocol). The PRP protocol is also plug and play. In other words, no special network engineering is required. However, additional switches are required to build the double star network infra- structure, making PRP deploy- ment costs higher than HSR. to gain your cpd accreditation for reading this article, simply go to http://cpd.networks. online/test1 and answer the questions below correctly. We will then email you your cpd certificate. QueStionS l How do devices talk to each other under IEC 61850? a) Over an Ethernet connection b) Over a local area network connection c) Via copper cables l What is the benefit of deploy- ing a client-server communi- cation exchange? a) Faster communication b) It avoids any central point of failure c) It saves money l Do devices have to have been made by the same manufac- turer to communicate under IEC 61850? a) Yes b) No l What is the benefit of using GOOSE messaging for I/O? a) It's quicker than other messaging b) It's less likely to fail c) Adding new logic variables and virtual inputs and outputs can be simplified, requiring the engineer to only modify the device configurations l What is the benefit of PRP? a) It's cheaper b) It prevents data loss in case of failure c) It's easier to implement l What is the advantage of PRP over HSR? a) PRP allows single and double network attach- ment devices to be mixed on the same LAN b) PRP offers a more resil- ient connection c) PRP is cheaper danH ied danH ied danH ied