Implementing magnetic tamper detection in electricity meters

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Each year, utilities lose billions of dollars from energy theft; one common nonintrusive way to steal electricity is to apply a strong magnet near the electricity meter. Mekre Mesganaw from Texas Instruments explains how 3D linear Hall-effect sensors can help mitigate this.

If the electricity meter uses a current transformer (CT) current sensor, the placement of a magnet could reduce the current reading and thus reduce the sensed active power. Reduced active power leads to a reduction in sensed active energy, leading to a discrepancy between what the utility charges the consumer and what they actually consumed.

It is possible to deal with the magnetic susceptibility of CTs by shielding them; however, there are limits to the maximum protection that shielding can provide. Shielding can reduce the susceptibility of the CTs to magnetic tampering, but it cannot make the CT completely magnetically immune.

Magnetic tampering can also affect transformers in power supplies, making it difficult to power the meter from the main AC/DC and requiring meter operation on a backup battery or previously charged supercapacitor.

Figure 1: 3D tamper detection using three 1D out-ofplane
Hall-effect sensors.

Using iron-powder-core transformers instead of the typical ferritecore transformers decreases the susceptibility of power-supply transformers to magnetic tampering. However, they are not completely magnetically immune.

Transformerless capacitive-drop power supplies can serve as a magnetically immune alternative to transformer-based AC/DC power supplies, but they have limited current drive, and are not feasible for meters with power-hungry communication modules.

Depending on the requirements, it may not be possible to design an electricity meter that is completely magnetically immune. Given their susceptibility to magnetic tampering, electricity meters often include magnetic sensors designed to detect external magnetic fields and take appropriate action, such as disconnecting services to the electricity meter or applying a penalty fee for tampering.

One method uses two or three out-of-plane one-dimensional (1D) Hall-effect switches to detect strong magnetic fields in three directions. Out-of-plane sensors detect the magnetic field perpendicular to the die. But in order to detect the magnetic field in three directions and ensure the detection of magnet placements at different orientations with respect to the electricity meter case, you would need two through-hole sensors and one surface-mount sensor, as shown in Figure 1, where three 1D Hall-effect sensors detect three magnet-to-case orientations.

When using Hall-effect switches for magnetic tamper detection, each Hall-effect switch must have the appropriate operating point of the sensor (BOP), which is the threshold magnetic flux density that would cause the switch to change its output, indicating magnetic tampering. The necessary BOP depends on the distance from the Hall-effect sensor to the magnet (determined by the dimensions of the electricity meter case) and the specifications of the magnets being detected. If BOP is too large (low sensitivity), the Hall-effect switch cannot detect the presence of the magnet. On the other hand, if BOP is too small (high sensitivity), nearby interference may cause false positives and erroneous penalty charges. Shielding may also be added around the Hall-effect sensors to further reduce their sensitivity to help prevent false positives.

Although electricity meters primarily run off AC mains, they will need to run off a backup power supply when there is a power outage or if the meter has been tampered with such that the main AC/DC is not functional. Selecting low-power magnetic sensing devices can help enable magnetic tamper detection while maximizing the backup power supply’s lifetime. The ability to run at low voltages enables magnetic tamper detection over a longer time period as the backup power supply voltage drops over time and use.

Instead of using three 1D Hall-effect sensors, an alternative approach to magnetic tamper detection is to use one 3D linear Hall-effect sensor, which has three mutually orthogonal Hall elements in a single package. In addition to an out-of-plane sensor, 3D Hall-effect sensors also have two in-plane sensors integrated, where the in-plane sensors detect the magnetic field parallel to their die.


Consequently, 3D linear Hall-effect sensors can detect any magnet-to-case orientation with one surface-mount integrated circuit (IC), as shown in Figure 2.

Figure 2: 3D tamper detection using one 3D Hall-effect sensor – Texas Instruments
Figure 3: 3D-Hall-effect-sensor-Texas-Instruments

Figure 3 is a block diagram of the TMAG5273, a 3D linear Hall-effect sensor ideal for electricity meters from Texas Instruments. For this device, the Z sensor is the out-of-plane sensor and the X and Y sensors are the in-plane sensors.

This Hall-effect sensor has a precision analog signal chain, along with an integrated analogtodigital converter, to digitise the measured analog magnetic field values for each axis.

Result registers store the measured magnetic field values. A communication interface (I2C) communicates with a microcontroller so that it can retrieve the sensed magnetic field values.

Using 3D linear Hall-effect sensors offers these benefits:

Flexibility for defining a magnetic tampering threshold. Since 3D linear Hall-effect sensors provide information about the actual sensed magnetic flux density value, it is possible to select the magnetic tampering threshold of each axis to anything within the magnetic sensing range of the 3D linear Hall-effect sensor.

This facilitates customization of the magnetic tampering threshold based on the magnets to detect and the dimensions of the meter case. This type of flexibility is not possible for Hall-effect switches with fixed BOP specifications. In addition, many 3D linear Hall-effect sensor devices can sense large magnetic flux densities, which enables designers to set a large magnetic switching point (if necessary) for preventing false positives.

Ease of assembly. Hall-effect sensors are not as fragile as reed switches, the latter of which may break during assembly.

Requires only one surface-mount IC. Sensing in 3D dimensions requires only one IC for 3D linear Hall-effect sensors instead of the three ICs needed for 1D Hall-effect sensors. 3D linear Hall-effect sensors can therefore enable a more compact printed circuit board (PCB) layout. In addition, having a surface mount-only implementation can reduce PCB manufacturing costs compared to a through-hole device.

Ability to change between multiple device power modes. 3D linear Hall-effect sensors can support switching between multiple power modes, including an active mode for taking measurements, a sleep mode for minimizing current consumption and a duty-cycle mode that automatically switches between the two modes.

GPIO pin interrupts when detecting magnetic tampering. Some 3D Hall-effect sensor such as the TMAG5273 have the ability to set an interrupt pin when the sensed magnetic flux density of any axis goes beyond its user-defined magnetic switching threshold.

Figure 4: Interrupts based on magnetic flux density thresholds

Figure 4 shows an example of this functionality, where the interrupt pin is set when the absolute value of the X-channel’s magnetic field goes above a user-defined “X Ch Threshold.”

The comparison uses the absolute value of the field to detect both the South and North poles of the magnet, which is necessary because both sides of the magnet could affect the meter. Since the Hall-effect sensor’s interrupt pin can wake up the microcontroller when it is in low-power mode, and since the microcontroller doesn’t have to read the Hall-effect sensor to determine when the magnetic threshold has been surpassed, it can go to low-power mode when running off a backup power supply until woken up by the Hall-effect sensor’s interrupt pin. Used simultaneously, the general-purpose input/output (GPIO) pin interrupt feature and duty-cycle power mode can reduce system current consumption and extend the lifetime of the backup power supply.

Once the Hall-effect sensor’s GPIO pin wakes up the microcontroller, it can then retrieve the value of the sensed magnetic field reading that caused the interrupt.

Detection of AC magnets. AC magnets don’t just affect current They can also affect shunt and Rogowski coil current sensors. A 3D linear Halleffect sensor can detect AC magnets, but such detection requires a fast enough sample rate and a small-enough sleep time to properly capture samples along a cycle of the AC magnet waveform. Since linear Hall-effect sensors provide information about actual sensed magnetic flux densities, they are more suited to detect AC magnets than low-sample-rate Hall-effect switches.

Conclusion

To reduce the impact of energy theft on utilities, electricity meters must be able to detect magnetic tampering. 3D linear Halleffect sensors such as the TMAG5273 enable a flexible, compact and low-power solution for magnetic tamper detection.