Detect and harden against non-intrusive tampering


In part 2 of this series, Mekre Mesganaw examines some of the elements of magnetic tampering.

To maximise efficiency, electric utility providers must minimise losses in energy between electricity generation and customer distribution. A part of these losses includes nontechnical losses, such as losses from energy theft. Some of the most prevalent methods of energy theft involve tampering with the electricity meter (e-meter) since meters are relatively accessible.

There are multiple ways to tamper with a meter. In addition to intrusive tampering methods, it’s also possible to tamper with an e-meter non-intrusively without opening the meter case.

One of the most common forms of nonintrusive tampering is magnetic tampering, where an individual places a strong magnet near the meter. A strong magnet could cause nearby transformers to saturate, thereby paralysing them. Specifically, a strong magnet could paralyse a transformer in the power supply or a current transformer’s current sensor, which could cause utility customers to be charged less for electricity than what they should actually be charged.

To deal with magnetic tampering, countermeasures include trying to detect the presence of a magnetic field with a Hall-effect sensor, as well as hardening the meter against magnetic tampering attacks. To detect magnetic tampering, three Hall-effect sensors can detect the presence of a strong magnet in all three dimensions. It is important for the average current consumption of the Hall-effect sensors to be low for when the system runs off of a backup power supply. It’s possible to achieve low average current consumption from Hall-effect sensors by externally duty cycling them or selecting Hall-effect sensors with this duty cycling integrated.

To harden a transformer in a power supply against magnetic tampering, one option is to shield the transformer; however, this is only effective to a certain extent. A second option is to choose a transformer that is either completely magnetically immune or magnetically tolerant enough for an expected magnetic tampering attack. For systems that do not draw too much current, a third option is to use a capacitive-drop power supply that does not have any magnetic components.

Similar to transformers in power supplies, to harden a current transformer against magnetic tampering, one option is to shield the current transformer. However, again, this is only effective to a certain extent. The best way to obtain magnetically immune current sensing is to use shunt current sensors instead of current transformers. Using a shunt for a single-phase meter is relatively simple: just reference the system with respect to the shunt. For polyphase meters, using shunts as sensors is more complicated. Since shunts do not have inherent isolation, external isolation is necessary in order to prevent large, damaging differential voltages on the device connected to the shunts.

Figure 1 shows the functional components of a three-phase system with isolated shunt sensors. In this architecture, one individual device per phase measures the voltage across the shunt sensors. These devices could be isolated delta-sigma modulators or metrology analogue front end (AFE) microcontrollers (MCUs). Since the shunt sensing devices are isolated, you must have an individual power supply for each device.

Functional components of a polyphase system with isolated shunt sensors

Select the back-end device (shown in Figure 1) based on its ability to communicate with the shunt sensing device. For example, if you’re using an isolated modulator as the shunt sensing device, then select a backend device with digital filters. These digital filters can be part of a standalone device or integrated within the metrology MCU. Alternatively, if you’re using a metrology AFE as the shunt sensing device, then select a back-end device with Serial Peripheral Interface or a universal asynchronous receiver transmitter interface.

To calculate the active energy, it is necessary to measure the mains voltage in addition to the current of the customer’s load. A resistor divider typically translates the mains voltage to a range that the analogue-to-digital converter can sense. In a polyphase system with isolated shunt sensors, you could implement the mains voltage sensing on the same device that senses the voltage across the shunt, or on the back-end device if that device’s voltage sensing is synchronised with shunt sensing. If the back-end device is sensing the voltage, isolation is not necessary, since it’s still possible to measure the voltage on multiple phases without a large, damaging voltage on the back-end device.

In order to prevent hazardous voltages on back-end devices (because shunts don’t inherently have isolation), it is necessary to isolate the communication from the shunt sensing device to the back-end device. This isolation can be integrated within the shunt sensing device or it can be a separate digital isolator device.

There are two approaches to implementing isolated shunt current sensing. The first approach, shown in Figure 2, involves using a metrology AFE. In this approach, the metrology AFE calculates the primary metrology (voltage, current, power, etc) instead of having the back-end device perform these calculations. Calculating these parameters on the shunt sensing device reduces the processing that the back-end device needs and provides good separation between metrology and host functionalities.

Isolated shunt sensors using a metrology AFE

The second approach to isolated shunt sensing is to have the shunt sensing device only sense current and have the metrology MCU perform the metrology calculations. Figure 3 shows an example of this approach. The advantage of this architecture is that it more easily allows parameter calculations between phases, such as measuring the angle between different phases.

Isolated shunt sensors using isolated modulators


It is possible to design a magnetically immune e-meter using shunt current sensors and capacitive-drop supplies.

By following these anti-tampering techniques, it’s possible to thwart or at least mitigate meter tampering, thereby reducing inefficiencies when supplying electricity to utility customers. SEI