DSP-based Meters Offer High Performance Options for Utilities
By Jim Frawley and Andy Soukup, Texas Instruments Inc.
Industry deregulation and changing customer needs are putting the squeeze on utility profits these days. The result of these changes is that utility companies will no longer have a monopoly on supplying power to a specific geographic region. As competition heats up, utilities will have to streamline operations in order to compete. In the past, for example, utilities could simply tack onto the consumer`s bill the costs of reading meters in hard-to-reach places, such as rural locations or high-crime areas. With the advent of competition between power providers, utilities will find it a lot more difficult to recoup meter-reading costs.
Customers, too, are putting the squeeze on power companies. Not only do they want lower rates, but they are also less tolerant of power outages, want higher quality power, and in some cases, are becoming interested in advanced features like remote power control. The power-use measuring equipment currently in the field is simply not functionally designed for the market demands of the coming decades.
The key to providing these services while simultaneously reducing the cost of delivering them is better information, and AMR is one way to collect this information.
AMR systems reduce meter-reading costs, but utility companies also need meters that do more than simply report on power usage. They need meters that gather information about power quality and provide auxiliary functions. To provide these functions, some meter manufacturers are now building meters using special-purpose microprocessors called digital signal processors (DSPs). DSP-based meters have enough computing horsepower to digitally monitor power usage and power quality, as well as communicate with a utility`s data-gathering network and provide auxiliary features that some utilities` customers are demanding.
DSP-based meters do this at a reasonable cost, usually around $150 per unit. The reason for this low price is that DSPs are highly-integrated electronic components that pack a lot of power into a very small package. (For more information on DSPs, see the sidebar, “What is DSP Anyway?”.) While AMR systems can be designed using conventional microcontrollers, custom or semicustom integrated circuits, or discrete electronic components, only DSPs provide the unique combination of high performance, low cost and complete re-programmability.
DSP Meters Measure Accurately
Standard electric meters measure power usage mechanically. A coil across the load, called the potential coil, generates an electric field proportional to the voltage across the load. A coil in series with the load, called the current coil, generates a field proportional to the current the load is drawing. These fields generate eddy currents in the disk or rotor, causing it to spin. Meter manufacturers calibrate the meter so that the speed at which the rotor spins is proportional to the amount of power being used. The rotor is mechanically coupled to the register, which are usually dials that keep a running total of the kilowatt hours used (Figure 1).
DSP-based meters measure power usage in a completely different way. They take periodic measurements of the voltage and current, digitizing the measurements and storing them in computer memory. Next, the meter multiplies these values to get the instantaneous power and then multiplies this value by the time period between samples to obtain the kWh used during that time period. If the period is very small, the voltage and current will change little during the period, and the power usage calculated for that period will be very accurate. The meter can record these periodic measurements, along with a time stamp, or total them up for a measurement of total power usage.
Over the years, the accuracy of electromechanical meters that can be produced at a practical cost has improved from around 5 percent to about 2 percent, but has reached the limits of electromechanical design. In a regulated industry, this level of accuracy was and still is accepted and adequate. Electronic meters, however, already are available at 0.2 percent accuracy, far closer to meeting the needs of today`s deregulated billing environment (Figure 2). Electronic meters won`t drift in accuracy over time, either. The aging process is kinder to semiconductors than it is to mechanical devices and built-in capabilities like self-test, self-diagnosis, self-monitoring and auto-calibration are typical functions much easier to implement in electronic designs.
Collecting the Data
For meters to be read automatically, they must connect to some type of data-gathering network. There are several schemes for collecting data from automated electric meters. Some systems use wireless networks to gather the data, while others use the phone system.
Wireless systems generally use spread-spectrum, digital radio technology which operate in the 900 MHz range. In many applications, the limited range of wireless systems makes them unsuitable. Larry Bailey, Util-LINK LLC president, points to one example in New Mexico, where ranches can be very large and very far apart. In this case, it took one meter reader two days to read meters on six different ranches. To read these meters automatically, the utility opted to install meters that use the phone lines to report on power usage.
Util-LINK`s meter–The LINK–uses a Texas Instrument (TI) DSP chip to measure power usage and to communicate the results over the phone lines. To communicate over phone lines, the DSP is programmed to be a modem, decoding control signals from the utility, and encoding data signals to send back to the utility. DSPs are particularly good at this kind of processing.
According to Bailey, the use of TI`s digital signal processor was a key factor in providing The LINK with the necessary functions, while at the same time keeping costs low. The LINK costs utilities only $150 per unit. While this cost is approximately $100 to $125 more than the cost of a standard electric meter, Bailey notes that utilities can easily justify the extra equipment cost when they consider the cost of reading meters in remote areas. And when the benefits of increased accuracy and auxiliary functions that these meters provide are added in, DSP-based meters could be a real boom to utilities servicing rural areas.
The most popular auxiliary feature of The LINK is its ability to provide power-quality monitoring functions like voltage readings. The LINK includes voltage measurement data with each usage report sent to the utility company, making it possible to continuously monitor the quality of the power being delivered to each customer. Real-time alerts also can be programmed to immediately report under- or over-voltage conditions.
In urban areas, The LINK is designed to interface with other telecommunications media including packet-switched radio, cable television, low-earth orbit satellites and various power-line carrier methodologies. The inherent flexibility of the DSP enables The LINK to be re-programmed to interface with a variety of telecommunications methods in single hybrid communications platform.
Auxiliary Functions Also a Plus
Adding a processor to the electric meter also allows utilities to offer features not possible with standard electric meters. One such feature is time-of-use rates. Because the meter time-stamps its measurements, utilities know how much power is being used at a particular time of day and can charge accordingly. Consumers will also have access to this information so that they can adjust their usage and save money on their utility bills. Another popular function is the ability to detect and report power outages and other types of abnormal power conditions, such as brownouts.
A feature that is bound to be popular with power providers is reconfigurability. When a consumer decides to change power providers, it`s important that the meter be re-programmable so that power usage data gets reported to the correct company. TI`s family of DSPs includes several models with integrated flash memory, which is quickly and easily re-programmable. These parts can even be re-programmed remotely, saving the cost of sending a technician out to replace or re-program the meter.
DSP-based meters provide utilities with a distinct and definable set of advantages in the changing utility business. They not only improve meter accuracy and reduce meter-reading costs, they provide customers with the advanced features they are looking for. And they do so in a compact, low cost package that should help utilities remain competitive for years to come.
Jim Frawley is North American Industrial Segment Sales and Marketing Manager for Texas Instruments` Semiconductor Group, based in Niskayuna, N.Y. He is responsible for developing the use of DSPs and other TI technologies in new applications in the industrial marketplace. He received his bachelor`s of science degree in electrical engineering from the Rochester Institute of Technology.
Andrew Soukup is a DSP Product Manager with Texas Instruments` Semiconductor Group, based in Houston. He is responsible for expanding the use of DSPs in new applications by developing and marketing products that meet customers` needs. He received his bachelor`s of science degree in electrical engineering from Clarkson University and a master`s degree in business administration from Cornell University.
What is DSP Anyway?
Digital signal processing (DSP) is a technology that makes low cost electronic meters possible. In general, a DSP-based device has the following characteristics:
The analog signals, voltage, motion, sound, temperature, etc., being measured or manipulated are represented in digital format (as a series of discrete values). To convert analog signals into a series of discrete values, DSP-based devices use an analog to digital converter. To convert a series of discrete values into an analog signal, DSP-based devices use a digital-to-analog converter. The signals are processed in real time.
DSP-based devices often use digital signal processors, which are microprocessors with specialized hardware and instruction sets which designers have optimized to perform signal processing quickly. In addition, DSPs usually have on-chip memory, both RAM and ROM and FLASH, which speeds up operation by reducing the time needed to fetch program and data from external memory (Figure 3).
DSP-based devices offer several advantages over other devices that use other methods to perform signal processing functions. With a DSP, designers can implement any signal-processing function, including scaling, filtering, signal analysis and generation, and extraction of frequency information. To perform these functions with analog components, you would need a separate circuit for each function. Devices which must perform several functions would be far larger and cost more than a DSP-based device, and analog circuits are more susceptible to aging and drift, making them more error prone.
DSP-based devices are also much more flexible than hard-wired analog circuits. To change the operation of an analog circuit, you need to change the values of the resistors and capacitors in the circuit. To change the operation of a DSP-based device, you simply have to change the program.
Other approaches have limitations, too. Microcontrollers have long been popular for metering solutions, offering re-programming and flexibility. But because of their limited computational performance, microcontrollers had to be coupled with special chips or DSP co-processors anyway. Likewise, customized chips, called ASICs (application specific integrated circuits) promise a single-chip digital solution, but offer limited or no re-programmability, the longest product development time and the lowest post-design reconfigurability.