1. Is an RTU a device or a function?

What exactly is an RTU (Remote Terminal Unit) in this day and age? RTUs started out life as rather humble individuals, merely collecting and collating data for reporting to a master whenever the master was ready for it. These days RTUs seem to be nigh on autonmous beings. And then there are all these nice web enabled RTUs. Is it time to call a halt and lay down definitions for various levels of RTU, say, from dumb RTU right up to self governing fully autonmous rtus, and where do u draw the line before such an RTU turns into a master station? Is the traditional domain of the RTU now seriously under threat from new superior IEDs that seem to be cropping up in ever increasing numbers?

By the letter of the law, an RTU is a Remote TERMINAL Unit, i.e. one which gathers data from local I/O and reports it to a distant master. However, RTUs have become data concentrators, protocol gateways, routers, PLCs, automation application platforms, meters, switch controllers, cap bank controllers, reclosers, and are even beginning to implement some protection functions. Simultaneously, most IEDs are beginning to take on the basic I/O functions of a traditional RTU. There seems to be no need to define a "next generation RTU". The Utility Communications Architecture (UCA) proposes as part of its Generic Object Model for Substation and Feeder Equipment (GOMSFE) that the various functions within a substation be divided into what it calls "building bricks". The generic I/O functions of a basic RTU are just some of those bricks, as are metering functions, protection functions, etc.

Any set of these functions will soon be able to reside in any device in the substation. As that happens, it will cease to matter what any device is actually called. Some devices may be called IEDs, some RTUs, some PLCs, some Relays, some meters, some computers, etc. because their vendors believe that is the main function of the device. The important thing will be whether the subset of functions provided by the device meets a market niche, or whether it is flexible enough to fit several niches simultaneously. (based on an answer from Grant Gilchrist)

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2. What does seamless communication in utility systems mean?

Modern approaches like the IEC 61850 draft standard and UCA are designed in a way that the application information (information models for real world information, e.g. transformer or circuit breaker), the services to access this information (e.g. reporting, control, logging), and the communication stacks (MMS, TCP/IP, Ethernet, ...) are more or less independent of each other.

Seamless means that the information of a device can be accessed independent were the device is installed. To explain the concept of seamless communication, the following cases are discussed:

Case 1 (From Control Center directly to Device)

Location of information Device installed in a substation

Information model Standard circuit breaker model	(IEC 61850-7-4/-3)
Service GetValue mapped to MMS	(IEC 61850-7-2 / 61850-8-1)
Stack TCP/IP	(61850-8-1)

Case 2 (From Control Center through a proxy to Device image)

Location of information Proxy computer in a substation representing device information

Information model Standard circuit breaker model	(IEC 61850-7-4/-3)
Service GetValue mapped to MMS	(IEC 61850-7-2 / 61850-8-1)
Stack TCP/IP	(61850-8-1)

Case 3 (From Control Center directly to Device - different stack)

Location of information Device installed somewere

Information model Standard circuit breaker model	(IEC 61850-7-4/-3)
Service mapped to other Application layer than MMS	(IEC 61850-7-2 / 61850-8-?)
Stack TCP/IP	(61850-8-?)

Discussion:

Seamless in the context of utility applications means:

• The device information is defined ONCE only (in the device itself or in a proxy).
  
• The device information may be located in the end device (information is consistent per definition) or may be located in a proxy server representing any set of devices or systems (consistency of information in proxy and real devices needs good data engineering).
  
• The access to the device information is - from an abstract point of view - the same, independent of the ACSI (IEC 61850-7-2) mapping to a specific application layer, e.g. MMS.
  
• The communication stack may be the same, similar, or different.
  

Seamless does NOT mean: