Communication media for AMR - which technology is the most appropriate?
In this article we examine PLC, satellite and telephone communications. Future issues will consider radio, fibre-optic and cellular options.
UNB power line carrier
Metering via power line carrier (PLC) has a long and somewhat chequered history. High frequency carriers have been used on high voltage power transmission lines quite successfully for relay control and voice messages. High frequencies propagate very nicely on transmission lines, because the lines are long, simple and carefully controlled.
Distribution lines, however, can create real challenges for carrier systems. High frequency waves love to bounce off any change in line impedance like branches and taps, transitions between underground and overhead, capacitor banks, even the loads themselves. All those waves bouncing around tend to reinforce in some places and cancel in others. That makes high frequency carrier systems very difficult to predict or control on the ever-changing power lines.
To solve the problem, distribution line carrier systems evolved toward lower frequencies. They dropped from the 100 kilohertz range down to the 10 kilohertz range. Ripple carrier systems using frequencies in the 100 Hertz range had wonderful propagation, and were used successfully for peak load management for many years. This all made propagation problems more manageable, but they didn’t go away.
One persistent problem is that as frequencies go lower, the size and cost of the transmitters get larger. As a result ripple technology has usually been limited to one way messaging, where a single large transmitter is installed at a substation and low cost receivers are located downline. That ’s good for load management but not for reading meters .
Sequential waveform distortion is a variation of ripple – a carrier that doesn’t think in terms of frequencies at all. It works in the time domain, transmitting messages down line from a substation by changing the shape of the voltage sine wave. A transceiver located downline can respond to the message by creating large current spikes which can be detected upstream at the substation. In this way a meter can be polled by the central equipment.
NEW CONCEPT WITH DISTINCT ADVANTAGES
Ultra narrow bandwidth (UNB) power line carrier technology is a relatively new concept that has distinct advantages over other carrier systems. Signals in UNB systems have the long distance and reliability virtues of low frequency signals used in ripple carrier systems, but UNB transmitters are inexpensive and can be built small enough to fit easily inside the average kWh meter.
These meter reading devices don’t have to be polled, because each one is transmitting all the time. Each unit transmits on its own private frequency (rather like radio and television stations) and thousands of these transmitters can be sending signals up the power line simultaneously.
All those continuous signals travelling up the line from each customer can provide an important advantage to a power distribution company. They help maintain the system by providing rich fields of data waiting to be mined. If a customer loses power, the signal for that meter will disappear. Any subtle variations in the power system will cause changes in the received signal. Alert personnel use that information to locate bad grounds , arcs , power outages , outage blinks ,tampering and so on. Because they know which phase the signal travels on, engineers can keep the phases balanced and system records up to date.
Further advantages stem from the fact that meter readings are reported daily. This helps engineers and technicians watch trends and usage profiles. Among other things, utilities can use the information to resolve high bill complaints. It eliminates the need for special reads when customers move. When these readings include peak and time-of-peak information, engineers can use it to size transformers, reducing line loss and transformer failure.
THE SECRET IS TO GO SLOW
UNB technology has all these advantages because it goes slowly. In communication theory, getting your message across above the noise is the key challenge. When you transmit data very slowly, it occupies a narrow bandwidth which can pierce through noise like an arrow.
Recent advances in technology allow this principle to be taken to the extreme. For example, one popular implementation of UNB carrier sends data at the incredibly slow rate of .0005 baud. This is fast enough to send a reading every day, yet slow enough to allow even a tiny transmitter to send signals hundreds of kilometers up a power line, through megawatts of noisy power.
The low bandwidth also allows the carrier to operate at very low frequencies, which travel everywhere the power goes, through transformers and capacitors. This helps to make a complete working system simple, low cost and easy to install. Because distance is not a consideration, such a system is especially attractive in rural environments. The real magic in a UNB system is in the receiver. Located up line from the meters, usually at a substation, the receiver monitors the current on the lines. It contains a computing power-house called a digital signal processor (DSP) – a specialised computer that emulates physical processes. In one system , the DSP emulates 3000 FM radios simultaneously, each receiving a signal from a different meter. Without the magic of DSPs, the 3000 receivers would be completely impractical.
WHAT IS BANDWIDTH?
To a communications engineer, band-width is like real estate. Some of it is considered valuable; some is considered wasteland. If you want to use a portion, you have to wrestle with Mother Nature to keep it clean and suitable for your purpose. Then you have to compete with other people who also want to use it.
A UNB signal is so tiny that it can use bits of real estate that previously were considered worthless. It can be unobtrusive even in crowded neighbourhoods where space is valuable. Its small size also makes it more immune to the ravages of nature, and tends to make it lower cost. Using a physical analogy, if an AM radio broadcast signal is one mile wide, then an FM radio signal is about 20 miles wide. A standard television channel is 600 miles wide – and a UNB signal is the width of a human hair. It takes that much less power to transmit because it is that much more immune to noise.
UNB METERING POISED TO EXPAND
UNB sprang from the concept that every disadvantage has compensating advantages. When everybody wants to communicate faster, see what you can gain from going slower. The advantages of UNB metering systems become clear when they are evaluated for reliability and efficiency in certain applications. Rural utilities have been some of the first to implement UNB systems because of their economical long distance capabilities, but the technology offers benefits for many industries using PLC. Meanwhile newly focused research and development is discovering a bright future for this technology.
Low - earth orbit (LEO) satellites offer monitoring information from remote assets, positioning and tracking of commercial assets and personal e-mail communications via the Internet to a broad range of end users, including customers in utilities, oil and gas, transportation , heavy equipment, marine and defence industries.
The introduction of LEO satellites has made remote global network links both possible and affordable. Unlike geostationary earth orbit (GEO) satellites, LEO systems require less power to transmit or receive messages. Lower power requirements translate into lower some systems have implemented proven and readily available VHF technology in the deployment of their networks.
Some new primary applications that are of particular interest to the global utility industry include residential utility metering and national account metering.
Secondary applications include sub-station monitoring, outage notification and load control in both the transmission and distribution of electricity, gas and water.
In the case of hard-to-read meters, a solution for utilities has been found by the close co-operation between satellite manufacturers and the providers of communications and tracking systems. Here the subscriber communicator (SC) is designed to provide cost effective, two way messaging for utility automation applications and fixed site monitoring using the satellite system.
A single compact remote unit contains the SC complete with the electronics, antennae, and software necessary to collect, package, transmit and receive data from virtually any type of meter or sensor. The built-in applications allow the SC to operate in a number of modes, including residential, time-of-use, load profiling and demand metering. AMR applications also include loadcontrol. thermostat control, power outage monitoring, tariff scheduling and customer information display.
Utilities in the USA are offered cost effective, round the clock , two way messaging, message routing, message processing and communicator status information. A simple but cost effective PC-based operations terminal gives the utility a single view of its field activities and can be used to initiate value added services.
Telephone line communications -
Telephone-based AMR technology has been in existence for over 25 years. Meter pulses or encoding signals are collected by a telemetry interface unit (TIU) installed at the site. The signals are then either transmitted, or logged and transmitted, at a programmed call-in time over the telephone line.
Because telephone lines allow two-way communication, consumption data can be transported back and forth from site to receptor and between customers. Data transmission times can be reprogrammed at the site, or alternatively reprogramming can be done remotely, by providing the new call-in schedule for the TIU when it is transmitting readings.
Dial-inbound systems (from meter site to destination) are the most affordable and widely used, because the telephone line already exists. This means it is operated and maintained by the local telephone company, so installation and monthly service charges can be avoided. Dial-inbound telecommunications are suitable for residential, commercial and industrial customers.
Two methods of on-demand meter readings can be provided. If the local public switched telephone network (PSTN) has incorporated "no ring" (or UTS) technology, demand-read calls can be made directly to the site without the telephone ringing. If UTS is not available, the customer will be asked not to lift the receiver off the hook while a preset number of rings occur. This particular number of rings signals the TIU to begin transmission of the reading.
Dial-outbound systems (from data receiver to the meter site) have limited applications and involve the purchase of dedicated lines. This technology is preferred when most of the information is to be collected by on-demand reads during business hours. Outbound systems operate without interfering with customer telephone usage, and are compatible with either "no test" or utility telemetry trunking (UTT) switch technologies, where they are available.
A NON-INTRUSIVE SYSTEM
Telephone-based AMR system communications are non-intrusive; they operate with no interference to customer telephone use. An off-hook detector is used which recognises when the line is engaged or when a customer lifts the receiver off the hook. If the system is transmitting data as the customer engages the line, it automatically releases the line and re-initiates contact later.
The cost-effectiveness of deploying a telephone-based AMR system can be demonstrated in several ways. First of all, the high costs of constructing supportive infra structure for radio-frequency, fibre-optic, cable and other systems are avoided, because the telephone-based system uses existing telecommunications networks.
One of the most important advantages of a telephone-based AMR system is the ability to install automated equipment at a particular site without having to consider installation at an adjacent site. Initial system installations are usually at commercial or industrial sites, where surrounding areas are not scheduled for immediate deployment . Telephone-based communications thus avoid the need to build infrastructures to support dense meter populations – usually a costly telecommunications option.
UNB power line carrier by Paul Hunt.
Satellite communications by David Wigglesworth.
Telephone line communications by Sandy Fernstrom