As buildings and other facilities work toward achieving a net-zero future, utility meters such as electricity meters and water meters must provide more functionality than just recording usage data. The meters must become part of the infrastructure of an integrated structure, as shown in Figure 1 for a building and Figure 2 for a residential home.
The latest generation of “smart” electricity, water, and gas meters offers commercial and residential customers the information they need to use these resources more wisely. They also allow utility companies to monitor usage remotely, essentially eliminating the need for manual readings. These intelligent meters also enable the management of grid power peaks and tampering identification.
Figure 1. Building electric and electronic systems to help reach net-zero goals.
The switch from traditional electromechanical to smart meters presents various challenges for electronics designers as they strive to develop solutions compatible with Advanced Metering Infrastructure (AMI). This approach allows for integrating smart meters into the fast-growing Internet of Things (IoT), which supports remote communication and fault detection.
However, one thing that has remained the same is that the utility companies that install these meters need them to be robust enough to operate reliably for decades and provide accurate measurements over their lifetimes.
To do that, they must incorporate a growing array of circuit protection, sensing, and power control components. This article offers an overview of the various component options and how utilizing them in the design can improve the operation of the meters.
Figure 2. Smart metering accumulates information on energy generation and consumption. (Click image to enlarge)
Smart Meter Overview
The sophisticated electronics in today’s smart meters require even better protection from electrical transients such as electrostatic discharge (ESD), power surges (such as from lightning strikes), and other occurrences than in earlier designs. Figure 3 shows a smart electricity meter example and presents a high-level block diagram of the subsystems within the meter.
Figure 3. Smart electricity meter, its circuit blocks, and recommended component options. (Click image to enlarge)
Figure 4 illustrates a smart gas or water meter and its block diagram. The smart electricity, gas, and water meters have similar functional blocks. Each Figure highlights multiple component options for protection, sensing, and efficient control of the smart meter circuit blocks.
Figure 4. Smart gas and water meter, circuit blocks, and recommended component options. (Click image to enlarge)
Protecting the Electricity Meter
Electricity meters draw their power from the electrical grid, which is subject to high energy transients resulting from lightning, inductive load switching, or capacitor bank switching. The AC power lines have, by necessity, high current capacity to support the loads in a home or building. A fuse and a metal oxide varistor (MOV) placed at the AC power input can be the first defense against an overcurrent condition and high energy transients.
A fuse provides protection from the high current capacity AC line that can flow if an internal short circuit develops. Recommended fuses can be either a fast-acting or a time-lag type. A time-lag fuse will be less susceptible to opening due to in-rush current and will avoid nuisance shutdowns.
In addition to sizing the fuse for the smart meter’s average current draw, ensure the voltage rating exceeds the line voltage. Also, use a fuse with a high interrupting rating of at least 1000 A so failure in a shorted state is avoided even under extreme conditions. Always use UL and CSA-approved fuses to save standards compliance test costs and time.
An alternative to a one-time fuse is a polymer resettable fuse. These fuses offer fast trip times with UL and CSA certification.
The MOV in Action
When an overvoltage transient impinges on the meter, the MOV quickly clamps the transient to a suitable voltage level. However, if the MOV is subjected to a sustained, low current overvoltage, the MOV can go into thermal runaway, resulting in overheating, gaseous emission, and potentially fire.
In cases where this is likely to occur, such as in electrical meters, the designer can choose a thermally protected varistor (TMOV, Figure 5), which integrates a thermally responsive element within the body of the device that will open-circuit the varistor in case of overheating. A version of this type of MOV shown in Figure 5 includes a third terminal that can connect to a circuit that displays the status of the MOV.
Figure 5. Indicating TMOV with an annunciator circuit
The required level of surge immunity dictates the rating and size of the MOV. For essential surge protection between 2 kV and 4 kV, a 14 mm MOV can be adequate. However, for protection levels of 20 kV, a larger MOV or TMOV would be needed. Furthermore, MOVs have a finite number of surges they can absorb, and once the MOV has reached its end-of-life, it can no longer protect the meter. Therefore, derating the MOV is essential to ensure that the MOV remains operational during the specified lifetime of the smart meter.
Designers can supplement the MOV with a transient voltage suppression (TVS) diode for further protection. TVS diodes offer secondary protection for sensitive downstream electronic components. They have under-picosecond response to voltage overloads and do not have lifetimes dictated by the number of surge strikes. These devices clamp the fast-rising transients to low voltages while the front-end MOV absorbs the bulk of the high energy in those transients.
SIngle Device Solution
As an alternative to discrete fuses and TVS diodes at the smart meter input, Littelfuse has a single-device solution for overcurrent and transient voltage conditions. In addition to current overload and input voltage surge protection, the chip protects against inrush currents, excessive temperature, undervoltage, and reverse currents. These Littelfuse components are known as eFuses.
The recommended eFuse shown in Figure 6 shows the source-to-load pathway controlled by a MOSFET with ultra-low RDS(ON) resistance of 24 mΩ for minimal power loss during normal operation. The chip works in circuits with operating voltages ranging from 3 V to 24 V, and the eFuse allows a programmable current limit up to 6 A.
Figure 6. eFuse multiple protection function IC. (Click image to enlarge)
Discharge control lines limit the voltage rise on the input and output lines. With all this capability, an eFuse can save designers development time and printed circuit board (PCB) space while providing robust protection.
Protecting the Microcontroller
A microcontroller unit (MCU) controls a smart meter’s energy measurement and processing functions. The MCU is very sensitive to transients. Fast-rising transients can sometimes pass through the input protection and the filters on input signal lines.
To prevent damage to the microcontroller, use TVS diodes on the signal input lines. In addition, the microcontroller I/O communication lines require surge suppression, typically from ESD. TVS diodes, such as the Littelfuse SMAJ or SMBJ, can ensure the necessary level of protection. These diodes can respond in under 1 ps and withstand ESD strikes up to 30 kV.
To further enhance the microcontroller’s protection, designers can use Littelfuse opto-isolated solid-state relays for galvanic isolation between the microcontroller and external circuitry. An optically coupled solid-state relay isolates the pulse-out signal between the MCU and the M-Bus. These components have low drive power requirements for minimum power consumption and do not generate any electromagnetic interference (EMI).
Industrial electricity meters employ optically isolated solid-state relays (SSRs) for control by programmable week calendars, dynamic commands, or Active Information Management Systems. Other smart meter applications of SSRs include:
- Custom load output
- Active/reactive power
- Tariff switching
- Alarm outputs
- Limiting power consumption
- Energy-direction contact
Protecting the Communication Interfaces and Auxiliary I/O Lines
The meter uses wireless communication ports with either the GSM cellular protocol or the Zigbee mesh network protocol to transfer usage and other data back to the utility company. These communication ports interface with the external environment and need protection from ESD due to transients such as lightning. TVS diode arrays can provide the necessary protection from ESD.
In addition, components with low capacitance will minimize distortion to the transmitted and received RF signals. TVS diode arrays such as the SC1205-01ETG (Figure 7) can absorb ±30 KV ESD strikes and safely dissipate a 7A 8/20us surge event as defined in IEC 61000-4-5. It safely clamps transient voltages to, typically, 10 V and has a maximum capacitance of 9 pF.
Figure 7. SC1205-01ETG bidirectional TVS diode array for ESD protection on transceiver lines
Protecting the Battery
Smart gas and water meters are usually powered by a fixed internal battery pack designed to last 10 years. The most used battery chemistry is lithium-ion (Li-ion). These battery packs have significant capacities; the most important is protecting the battery against short-circuit failures caused by faults in the pack or the meter’s circuitry.
A PCB space-saving, surface-mount resettable fuse can quickly remove the battery pack from its load in case of an internal or external failure. Miniature resettable fuses are available with low internal resistance under 150 mΩ and fast trip times.
Efficient Sensing Components
One use for sensors in utility meters is flow measurement. Reed switches like the Littelfuse MDSR-10 series offer an accurate and proven technology for flow counting and offer the advantage of not drawing any power.
Unfortunately, there will always be a certain percentage of utility customers who want to “game the system” by tampering with their meters to manipulate the readings and reduce (or eliminate) the amount they will be charged. Smart meter designs include a variety of tamper-detection strategies to help combat the problem.
The most common tampering method is simply to open the meter cover and damage the circuitry. Using a detect switch in the design allows the meter to determine when the cover is opened and send a signal to the microcontroller. Versions of detect switches can have a very low activation force, either under 40 grams or 75 grams.
These switches allow 2 mm of overtravel, a valuable feature for detection applications. Side-activated detect models can have a minimal profile of 2 mm above the PCB. In low voltage 1.8 V logic circuits, versions can switch current as low as 10 µA.
Another method of meter tampering involves bringing a powerful electromagnet close to the meter’s body, which can cause a magnetic transformer to saturate or affect other components. However, by employing a sensitive tunneling magnetoresistance (TMR)/CMOS logic sensor in the design, the meter can detect a small magnetic field and send a trigger to the MCU to record the tampering event.
With either tampering method, after recognizing the attempt, the microcontroller notifies the monitoring support staff at the utility company, which can schedule an on-site follow-up and potentially impose a tampering penalty on the user.
Efficient Power Control Components
Many smart meter designers are turning to power MOSFETs for power conversion functions like input voltage pre-regulation and step-down conversion. Typically, the internal power supplies that feed the meter electronics use simple, inexpensive flyback converters. However, the latest high-voltage silicon carbide power MOSFETs allow power supplies to work with an extensive input voltage range from 85 VAC to 440 VRMS or even higher for polyphase meters.
SiC MOSFETs improve the reliability and robustness of the power converter. Power MOSFETs with voltage ratings of more than 600 V are essential to accommodate wide AC input voltage ranges combined with the primary transformer’s reflected voltage and lightning surges coming from the grid.
Regardless of the application, smart meters require manual interfaces. For a reliable switch, look for sealed switches that prevent moisture from entering the switches’ internal structure. Also, investigate switches with a long operating life. The Littelfuse C&K line of switches offers several options for environmentally protected and long-operating-life switches.
Applicable Standards
Table 1 below lists the national and international standards defining electricity meters’ safety requirements. Table 2 covers safety standards for many of the components used in smart meters. These tables can help designers determine the standards their product must comply with based on the markets where it is intended for sale.
Table 1. Safety standards for electricity meters. (Click image to enlarge)
Table 2. Safety standards for smart meter components
Support for Protection, Control, and Power Components
Designers can save precious development time and costs by consulting with component manufacturers’ application engineers. They can help with:
- Appropriate component selection
- Identification of the appropriate standards with which the product must comply
- Pre-compliance testing services are offered by some manufacturers (such as Littelfuse)
With applications engineering assistance, designers can optimize their design for rugged, reliable, and efficient performance. To learn more about emerging circuit protection, sensing, and power control technologies shaping the next generation of smart meters, visit the Littelfuse website. Also, check our Building and Home Automation Application Guide.
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