Factor Coverage
Data Throughput
Latency
Description Capability of the system to cover the required service territory. Amount of information that can be transported in a given time, typically in bits per second (bps). The delay from when an event occurs in the field to the time the dispatcher is informed about the event. One-way or two-way.
Some technologies require line-of-sight, or are affected by physical obstructions like mountains, buildings, hills, etc. Some may not be available in rural areas, e.g. cellular. 9,600 bps to each remote device is generally acceptable for today’s DA applications. May sometimes only be transmitting a single control command or switch position.
Instantaneous response is typically not required for DA. Some applications normally require data to be sent in 4 to 8 seconds, while others can tolerate delays of 30 seconds or longer.
Direction
Availability and Reliability
The ability of the system to perform its job at any time.
Security
The ability to prevent unauthorized use of the system or its data.
Cost
The cost of installing, operating and maintaining the system.
Some DA applications, like auto-sectionalizing, require two-way traffic, while others, like capacitor bank control, only need to transmit a command in one direction. Must be resistant to electrical noise, environmental effects, and be available during power outages. Some technologies, like power line carrier, are disrupted just when DA applications need to communicate most, e.g. when a fault occurs. DA systems are a challenge to secure because they typically have little or no physical security (i.e., no fences, no gates) and rely on wireless technologies, which are more vulnerable to eavesdropping. DA equipment locations are small and numerous. Therefore, the communication cost per location is a key factor in making the business case.
by the Modbus-IDA organization (www. modbus.org).
Like most utility protocols, Modbus is based around the idea of data points: small pieces of information that are either digital (on/off) or analog (variable) and represent either inputs (read-only monitored data) or outputs (write-only switches, controls, etc.). Each protocol has its own terminology for this “SCADA data model.” Modbus calls them “registers” and “coils,” for instance.
It’s possible to find a Modbus protocol implementation for almost any device you need, especially sensors. However, Modbus has some drawbacks for DA applications:
• Modbus has no standard method for the representation of time stamps, no concept of a historical events buffer, and no method for synchronizing time to millisecond accuracy. This makes it difficult to build a sequence of events log for tracking faults and restoration activities.
• The contents of registers may vary widely. For instance, it is common practice in Modbus implementations to “pack” different types of data (e.g. digitals, BCD, timestamps) into Modbus registers. This requires specialized software at the master station to convert from one format to another.
system that is dedicated to DA would not be cost-effective at this time.
Deciding on a physical communications technology for DA is only the first part of designing a complete communications system. Regardless of whether devices in a DA system use licensed radio, cellular, or PLC, they must speak a common language over that network.
Fortunately, deciding on the protocol is not nearly the daunting task it used to be. There was a time when each type of remote device and every master station spoke its own unique, proprietary protocol.
While this problem has not completely disappeared, two factors have reduced it to bearable status. The first factor was the early emergence of data concentrator devices which convert one protocol to another. The second factor was the emergence of a few standardized, open protocols which belong to no particular vendor. A few of the most common open protocols used for DA are discussed here.
Modbus is an extremely popular
protocol that was originally developed
by the programmable logic controller
vendor Modicon and is now managed
Some DA applications, like capacitor bank control, are not affected by these problems, and Modbus continues to be widely used in both substation and distribution automation.
IEC 60870-5 is the official standard developed by the International Electrotechnical Commission (IEC) for “telecontrol,” i.e. DA purposes.
Like Modbus, IEC 60870-5-101 is based on the idea of data points. However, it specifies time synchronization, confirmation messages, and many other measures
References:
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