The continued integration of electricity meter functionality into a single chip solution promises to increase and expand the feature set of the meter as well as to help control manufacturing costs. This newfound functionality lends such electricity meter architecture implementations to not only raw power line voltage and current measurements for energy metering, but also provides a flexible and feature rich platform for extended functions. One key element of newer meter designs is the addition of automated meter reading technology, or AMR.
In the case of electronic electricity meters, or e-meters, there are a wide array of design methodologies, mainly driven by the technology most prevalent at the time of design’s origination. This technology availability influences not only the meter function, but also the AMR implementation.
Integration cuts costs
Of course, it goes without saying that the first three requirements for a meter manufacturer are cost, cost and cost. It is not only the migration of mechanical to electronic meters that inherently enable low cost designs and advance AMR capabilities, but also the high level of integration those electronic elements provide means developers can offer more flexible solutions at attractive prices. Figure 1 illustrates the electronic e-meter integration into a single chip solution.
As shown in Figure 1, there are a few main elements that enable the electronic e-meter’s functionality.
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Figure 1: The e-meter components are combined into the TI MSP430FE42x single-chip AMR solution.
At the heart of the meter is a microcontroller (MCU) responsible for handling all metering functions, ranging from control of the analog front end measuring current and voltage, to calculating power and energy to keeping time to, ultimately, displaying all of this information to the user.
If that were not enough, new metering initiatives across the world are driving designs that are capable of performing higher level functions including AMR. Not only does the addition of AMR capabilities lower costs to utilities by eliminating the need for personal visits by the power company each month, but these capabilities also provide consumers with powerful options in billing and energy costs.
Common AMR implementations
Automated meter reading extends to any method one can imagine to provide communication between the meter and the utility or user. Figure 2 shows just a few common implementations of AMR, all made easier through semiconductor integration.
At a high level, Figure 2 illustrates the meter as the customer sees it: a liquid crystal display (LCD) connected to the power line connection, or mains connection, at the home.
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Figure 2: Automated meter reading methodologies using e-meter integration.
Beyond the meter itself, there is often a communication path back to the utility establishing the AMR connection. At its simplest, the power company can always directly read the meter on-site.
While this reading can be done via the LCD, a common reading method is to use a digital link between the meter and the technician’s reader. Due to isolation concerns—not only electrical but also environmental—a common approach is infra-red communication. Essentially like a two-way TV remote, the reader can interrogate the meter for billing information, as well as perform meter diagnostics and software updating all automatically.
This communication can be taken a step further using a direct connection back to the utility, eliminating the need for that on-site visit. Using technology similar to a PC-modem connection, this type of AMR meter piggy-backs digital data onto the power lines where it is received at a central location for access by the utility.
With new advances in wireless communication, standards such as IEEE 802.15.4 open the door to flexible networks of meters transmitting and receiving their data wirelessly. Wireless communication platforms must provide a secure channel for communication to be feasible. However, once secure, wireless meters can create dynamic mesh networks where each meter can communicate with its nearest neighbors in a ripple effect, transmitting consumption and status information eventually back to the utility.
Automated meter reading doesn’t have to be limited to communication between the meter and utility. In some applications, meters can be enabled with card reader technology, creating a sort of pre-pay meter for customers. In such cases, customers can make advance purchase of energy from the utility and apply the credit to the meter via a credit card.
The ability to integrate all of the necessary functionality into a single piece of silicon is a fundamental advantage of an e-metering solution and is provided by new semiconductor devices such as the ultra-low power MSP430FE42x MCU family from Texas Instruments. At the center of the device is an efficient and low power 16-bit CPU. Built around the CPU are the elements needed for the metering functions along with those elements helpful in AMR. Of key importance is the analog front-end responsible for accurate energy measurement. Figure 3 represents the metering architecture and analog front end of the MSP430FE42x e-metering microcontroller.
Independent inputs
Figure 3 shows three independent analog-to-digital conversion inputs for line measurement.
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Figure 3: MSP430FE42x analog front-end architecture shows three independent A-to-D conversion inputs.
Two of these inputs are for line current and line voltage, respectively. The third is optional, depending on the overall meter architecture, and can be used for tamper detection of the meter.
The energy processor is essentially a fixed function processor responsible for all tasks related to the actual energy metering. This includes peak and RMS I/V measurements, active, apparent and reactive power calculations, line frequency monitoring and tamper detection. All of this information and data is made available to the CPU through a shared memory architecture that allows for simultaneous data access.
In a less integrated system, the MCU’s central processing unit (CPU) is normally responsible for controlling the analog front-end and making all necessary energy calculations. Integrating the energy processor offloads the analog front-end control, metering and computational tasks to provide the CPU with more precious bandwidth for higher level display and AMR communication functions.
Stepping outside of the integrated function of the device, the three-channel architecture of the analog front-end provides a metering solution that is compatible across a wide range of power line interfaces. Common two-wire and three-wire, single phase meters are supported easily by this three channel analog implementation. Figure 4 shows the mains connection for a two-wire single phase implementation.
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Figure 4: Two-wire, single phase connects with tamper detect, made easier to implement by three independent channels.
In this instance, a current transformer, or CT, is used to measure the line current into the residential load. This could also be done using a shunt resistor, which is also supported. Line voltage is measured directly through a simple resistor divider. A key feature of the analog front end is the ability to allow for input voltages to fall below the ground level of the integrated MCU.
The third channel can be used for tamper detection when required by the application. Typically using a shunt resistor to save cost, the tamper input monitors the current through the neutral connection looking for deviations between it and the line current as an indicator of connection failure, potentially caused by tampering.
While the two-wire connection is common in many parts of the world, the three-wire single phase mains connection shown in Figure 5 is standard in the US. This is provided by the desire to have 120V and 240V inside the home.
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Figure 5: Three-wire, single phase connection is made up of two line connections and is standard in the U.S.
A three-wire standard
Instead of a line and neutral mains connection, the three-wire standard is made up of two line connections, L1 & L2, each 120V with respect to the neutral (not shown) and 180 out of phase with each other. This provides for two 120V legs at the load along with one 240V leg. In addition the neutral need not be connected through the meter, eliminating this third wire connection.
Assuming that the 120V lines are balanced, the meter can simply monitor the 240V voltage input along with the combination of L1 and L2 currents through the coupled CT as shown. The coupled CT sums the two line currents providing a complete picture of the energy consumption at all loads.
While three independent channels are not explicitly required for single phase metering, without it, tamper detection support as shown in Figure 4—which is a key feature required in countries such as India and China—cannot be easily implemented. In countries like the U.S. that do not require this functionality, only two channels are necessary.
While the fundamental drive for AMR capability in e-meters is to streamline energy reading and billing by automatically transmitting consumption data to the utility, this communication path cannot always be unidirectional, further complicating the design and CPU task management. When advanced meter functions are designed into the meter, the utility often must send data back to the meter.
As increasing peak loads on utility grids continue to strain electricity infrastructure, government and utilities are developing and implementing new ways to counter such spikes and smooth out the demand. At the core of such capabilities lies the electronic e-meter, which must not only measure energy, but must also support a wide variety of AMR and advanced metering technologies.
Through the increased integration of both analog and digital functionality such as multiple 16-bit sigma-delta ADCs and PGAs, a dedicated energy processing engine, low-power 16-bit CPU and flexible communication peripherals, modern meters can enable more advanced functions for manufacturers, utilities and consumers. This integrated approach to e-metering design makes adding AMR technologies easier than ever. Meter communication schemes ranging from simple infrared to wireless mesh networking and wired communication are all made easier through these advances in architecture and integration.
About the Author
Zack Albus is MSP430 Microcontroller Applications Engineer at Texas Instruments, Inc., www.ti.com
