The US Federal Energy Regulators Commission (FERC) has published recently an updated Smart Grid Policy (Docket No. PL09-4-000, Issued July 16, 2009).
Excerpt from the summary of the report: "This Policy Statement provides guidance regarding the development of a smart grid for the nation’s electric transmission system, focusing on the development of key standards to achieve interoperability and functionality of smart grid systems and devices. In response to the need for urgent action on potential challenges to the bulk-power system, in this Policy Statement the Commission provides additional guidance on standards to help to realize a smart grid. ...".
The essential term used is "Interoperability" (Interoperability is described as exchanging meaningful information between two or more systems and achieving an agreed expectation for the response to the information exchange while maintaining reliability, accuracy, and security; according to GridWise). The term "Interoperability" is used 89 times throughout the policy statement.
One of the crucial standards that supports interoperability in power system automation, protection and control is the standard IEC 61850 ... also referred to in the policy statement: "The Commission stated that IEC Standards 61970 and 61968 (together, Common Information Model), along with IEC 61850 (Communications Networks and Systems in Substations), could provide a basis for addressing this issue."
Interoperability is impacted by many aspects (standard definition, implementation, subsetting, resources available in a device, ...). Two IEDs that are fully compliant may not be able to talk together, because of resource restrictions. A Server IED may support 3 TCP connections. A fourth client that wants to retrieve some information from that server cannot even open a TCP connection - due to the resource restriction. From an application point of view the two devices cannot interoperate.
There is a crucial difference in the use of TCP for general web applications and IEC 61850 (and other close to real-time applications). Usually a client opens a TCP connection posts a request, gets some responses, and closes the TCP connection. Web browsers, in their simplest mode of operation, would just connect to download a page and then disconnect. This simple transactions use very little resources. The resources are free after each transaction. But connecting and disconnecting repeatedly to the same server does carry an overhead and slows the communication down.
To allow close to real-time information exchange over TCP requires to keep the connection open all time. This has the drawback that the server needs to reserve resources for each client - independent if there is little or high traffic. For that reason IEDs in power systems (often with very limited resources) have a limited number of clients that can communicate with them at the same time. Once resources are consumed, there is no interoperation with one additional client possible at all.
This kind of limited resources in automation devices makes interoperability a challenge. If two devices do not operate: please do not start to blame it to the standard or to the implementation ... A system integrator needs to know many details on limitations. Be aware: Everything is limited!
The discussed challenge is independent of the upper layer protocols like DNP3, IEC 60870-5, IEC 60870-6, IEC 61850, ... it is typical for all protocols in the automation domain that use TCP.
Click HERE for the complete FERC Smart Grid Policy.
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