By Robert Uluski and Grant Gilchrist, EnerNex Corporation
Most automation by electric utilities has been applied in the substation and at the enterprise level. Relatively few utilities have deployed distribution automation-the automation of devices outside the substation fence (i.e., out on the feeders themselves). This is due primarily to the potentially large expense of implementing DA, the lack of economic justification for such expenditures, and the unique and difficult technical challenges of implementing DA on a widespread basis.
But, this situation is changing.
Widescale deployment of automation on distribution feeders is becoming a reality in the electric power business. Drivers include:
- increased customer expectations in terms of power quality and reliability;
- growing number of regulatory incentives, both positive and negative;
- increased performance and affordability of DA communications choices; and,
- increased variety and capability of automation devices and software.
An efficient, reliable and secure communication infrastructure is vital for a successful DA implementation. Unfortunately, there is no single “cookbook” communication solution or model for success that can be applied to all utilities. Each utility’s unique characteristics-geography, distribution feeder electrical capabilities and constraints, customer density, human and financial resources, customer demographics and preferences, and a host of other characteristics-will determine the requirements for the communication system. Some of the more important factors in this decision are described in Table 1 (pg. 22).
In addition to these factors, a DA communications system must address today’s needs while providing the ability to add future functionality.
A wide array of technology and configuration alternatives exist for DA, including both wireless and wired technologies configured in a variety of network architectures. It is important to remember that most networks will be a hybrid of several of these technologies.
Fixed Radio Solutions
Fixed radio solutions include pagers, licensed or unlicensed radios, cellular or satellite.
Pagers can receive alert signals or short messages from feeder devices or transmit simple on/off commands. The network costs of leasing from a third-party paging provider are usually very inexpensive; often less than $5 per month per site, and sometimes just $1 per month per site. The hardware costs for telemetry paging generally range from $75 to $100. For a large number of sites (possibly hundreds) and low data volume (less than 100 bytes per month), it may be possible to negotiate rates well below $1 to $3 per site for two-way paging.
The actual, raw data rate for paging technology is quite slow (2,400 bps) and the latency for one-way and two-way paging may vary from 20 seconds to several minutes. However, this is acceptable for some DA applications, such as capacitor switching.
Licensed radio can be used with a variety of methods and configurations in the Very High Frequency (VHF) and Ultra High Frequency (UHF) spectrums. Each frequency spectrum (band) has its strengths and weaknesses. The VHF band offers non-line-of-sight propagation characteristics and is less affected by atmospheric noise and physical obstructions (buildings, trees, etc.) than higher frequencies. Its weakness is that it provides lower available bandwidth (9.6 kbps or less). However, this is not a major problem for most DA systems. (See Figure 1, previous page.)
Licensed radio is well suited for implementing automatic sectionalizing, if suitable licenses can be obtained by the utility. WiMAX, a new licensed radio technology operating from 2-11Ghz and 10-66GHz, promises to provide WiFi-like service over areas ranging in the tens of kilometers.
Spread spectrum radio technology has been used since World War II because it was largely immune to enemy interference and jamming. These qualities are still desirable for networks that require high reliability, security and availability. Advances over the last several years have made it a dependable and proven communications technology.
Spread-spectrum radios operate in the 900 MHz, 2.4 GHz, and 5.8 GHz unlicensed frequency ranges. All users within wireless range of each other share the entire band, with each using only a small portion of frequency band in a random fashion. The chance of more than one user being on the same channel at the same time and causing interference is very low, even in densely populated areas.
The bandwidth offered by spread-spectrum product lines is generally around 100 to 250 kbps-more than enough for DA applications. Communication signals can be obstructed by foliage (especially pine needles) on a direct path from the source. Some spread-spectrum products offer the flexibility for mesh-networking (peer-to-peer architecture), which can be used to communicate via alternate routing around a partially obstructed direct path (see Figure 2).
Recently, unlicensed radio has been the most popular choice for handling DA communications due to its combination of low cost, high overall reliability and availability, advanced security features, and excellent data throughput capabilities. In addition, eliminating the application process required for licensed frequencies reduces the startup time for the communication system. Coverage can be a problem in some terrains, but most utilities have been able to mitigate this problem through the use of repeaters and mesh networks.
Large infrastructure investments by cellular carriers and the economies of scale provided by millions of subscribers have made cellular technology an attractive option for DA. One drawback for utilities is the possible lack of certain service options and even basic coverage in some rural markets. To mitigate this, it may be possible to connect to a digital cellular system over greater than normal distances by using radios with a higher power rating and better antenna than a typical handset.
A modem or module is needed to use cellular data services-these generally cost around $200, which is a reasonable price for a DA node. Ongoing monthly charges can be inexpensive, too. Digital cellular data service can be obtained in some areas for as low as $7 to $11 per month, making digital cellular one of the most economical approaches for DA communications, although it can mean “locking in” to one particular vendor.
Satellite communication is an excellent “gap filler” technology for hard-to-reach areas. New service offerings have the following characteristics:
Satellite technology is not recommended for widespread use in DA applications. This is primarily due to the high cost of installing the required facilities and the high ongoing monthly expenses. Latency is potentially another problem, but is tolerable for most DA applications.
Power Line Communications
In powerline communications, the high-voltage primary distribution wires act as the communications medium. System-wide implementation of power line communication facilities requires a major financial investment and commitment that usually cannot be justified solely for DA purposes. However, many utilities have already deployed power line communication systems for handling automatic metering, and DA systems can make use of this existing infrastructure.
With conventional power line carrier (PLC), a signal is injected into the primary lines via an interface at the distribution substation. The signal travels from the main office to the substation on leased telephone line, point-to-point radio link, or other communications channel. The signal then passes through the PLC interface, and propagates down the primary wire into the DA end devices.
A disadvantage of PLC is that line capacitors attenuate the signals, resulting in some portions of the feeder that have no signal coverage. Special techniques, such as capacitor isolators, may be needed to eliminate this problem. Another problem is that PLC signals may be disrupted when a line fault occurs and during power line outages, making PLC unsuitable for automatic sectionalizing.
Communication via PLC is relatively slow. Data throughput ranges from 100 bps to 2 bits/hour, which may be too slow for some DA applications.
An emerging technology that utilizes the power line as its communication media is broadband over power line (BPL). Unlike conventional power line carrier, the signal is injected at locations out on the distribution feeders themselves (not in the substation). The BPL signal is typically delivered to the feeder locations via optical fiber.
The strength of the injected signal is sufficient to reach customer locations up to a mile or so away without isolating the line capacitor banks. Because distribution feeders are usually much longer than a mile or two, several signal injection points are needed to reach customers across the feeder.
Adding DA applications to an existing BPL system that includes other utility applications (like automatic metering) would be technically feasible and cost-effective. However; implementing a BPL system that is dedicated to DA would not be cost-effective at this time.
DA Communications Protocols
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 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:
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 to ensure that a master station can build an accurate historical events log. It permits devices to give data different priorities and to report data spontaneously without a request from the master station. This is extremely useful for DA applications in which nothing may change for days on end but then must be acted on immediately when, for instance, a fault occurs. The protocol is very efficient, reliable and effective for DA purposes.
Because it is an International Standard, IEC 60870-5-101 is widely used throughout Europe, the Middle East, and Asia as well as elsewhere in the world. However, it is rarely encountered in North and South America because of the emergence of DNP3.
The Distributed Network Protocol (DNP3) is the most popular electric utility protocol in the Americas and is the national standard for water utilities in both Australia and the United Kingdom. It was developed by Westronic Inc. when that organization became impatient with the speed of progress of the IEC 60870-5-101 standard. Like Modbus, DNP3 is now managed by an independent organization, the DNP Users Group (www.dnp.org).
DNP3 was based on the IEC 60870-5 family of standards and implements almost all the same functions as IEC 60870-5-101, but the two are not interoperable. For some applications, DNP3 can be much more efficient because it permits multiple types of data to be transmitted in the same message, which “101” does not. It is arguably more reliable in noisy environments. It has a few more advanced features, such as “data sets” and limited self-description. Other than that, they are very similar.
Another advantage that both DNP3 and IEC 60870-5-101 have over Modbus is a standard mechanism for securely determining that the sender’s identity is correct and that the message has not been tampered with. This common authentication mechanism, known as IEC 62351-5, is in development.
Not all protocols are interchangeable between substation and distribution automation. Modbus, IEC 60870-5-101, and DNP3 were all developed as low-bandwidth serial protocols for DA use and were later expanded to operate over TCP/IP and Ethernet for use in substation automation. They are all effective in this application.
However, the substation automation world is now abuzz regarding IEC 61850, which is arguably the next step in utility communications standards. IEC 61850 moves away from the “data points” concept and gives all data standardized names which can be read by human beings and machines alike, reducing configuration time and errors by as much as 75 percent. It also includes a set of high-speed peer-to-peer protocols for use in protection tripping and waveform transfer in real-time over the LAN. IEC 61850 may soon be used between two substations or between substations and control centers.
Most of IEC 61850 is not practical for use in distribution feeder automation in the near future, however. The bandwidth required will not be available until more high-speed wide area networks are used out to the pole-top.
The Future of DA Communications
As you can see, there is no one “best” solution for all utilities. However, some trends can be expected.
The biggest challenge, as always, remains the business case for DA. However, many utilities are beginning to realize that DA, when based on a well-planned communication network, can be the foundation of a truly intelligent grid: one that does not just react to emergencies, but predicts and prevents them.
Bob Uluski is a senior consultant for EnerNex Corporation. He has assisted a number of electric utilities with distribution automation projects that involved planning, business case development, implementation and training.
Grant Gilchrist is a consulting engineer for EnerNex Corporation. A recognized expert in data communications for the electric power industry, he has worked extensively in utility communication architecture development and standards.