With more electricity connections being metered worldwide, and the cost of electricity rising, the likelihood that an electricity meter will be tampered with has increased.
The newest generation of single phase metering SOCs (system-on-chips) is ideally suited to detecting tampering. In a stand-alone meter, without long distance communications, a tamper event can be stored in memory to be reported during the meter’s monthly polling. The value of the tamper detection increases tremendously if the meter is part of an AMM (automatic metering management) network, where the event can be reported quickly and appropriate action can rapidly be taken to investigate and correct the condition.
In smart meters, one common way to detect tampering is to install a case-open switch. The ability to monitor this switch when line power is off is critical, as this is when tampering is most likely, due to the reduced threat of injury. Continuously monitoring these switches when the line is off requires a very low power consumption mode that can wake up when the case-open switch is activated and record the state of the meter.
In some countries it is also common practice to use two current sensors in a single phase design to detect tampering. One current sensor is used to detect phase current and the other to monitor return current, to ensure they do not differ by more than a specified amount. This allows the meter to record the proper energy under earth fault and partial earth fault tamper conditions.
A more sinister method of tampering is shown in Figure 1. The missing neutral tamper condition is a special type that occurs when the neutral is disconnected from the power meter. Without a voltage reference, the meter’s board ground floats to the line voltage and the voltage sampling input is invalid. As Figure 2 shows, when the neutral line is cut at the two points marked #1, the electric potential at points #2 and #3 is identical.
Because there is no neutral reference in the meter, no output can be generated from a normal capacitor or transformer-based power supply, and the meter will go into its power down condition. But, if a load is applied, there will be a valid input signal on current channel and power will be consumed. How can the meter calculate power when there is no voltage signal input?
First, let’s examine theory: the definition of power is P = V*I, but under the missing neutral condition the voltage input is zero and therefore P = 0. But, if during this condition the voltage is assumed to be fixed at a known amplitude and phase, a constant can be substituted for the voltage and the formula becomes P = k*I. If k*I is calibrated to produce the same power output as V*I, when the voltage is at its nominal value, then the billing can be estimated when the missing neutral arises. This can be implemented simply by setting the power output to be proportional to the Irms measurement and adjusting the gain of the Irms such that k*I = V*I. The measurement has changed from Watt-hours to Amp-hours that have been scaled to match the ideal voltage condition.
Because a constant is being substituted for the voltage input it is not possible to respond to any changes in the power factor (PF) or the amplitude of the voltage channel input, therefore, errors will occur if the power factor varies from the calibration point of PF = 1 and if the voltage varies from the nominal amplitude. While these errors are normally unacceptable, this method is more accurate than the result of P = 0, which would otherwise be measured in a meter without a missing neutral billing mode.
The next problem to consider is that with only one potential entering the meter, all standard ways of developing a DC power supply will not work. Therefore the 3.3 V supply must be developed only from the current travelling through the meter. As shown in Figure 3, a power CT (current transformer) is used to output current on its secondary side, which can be used to develop the required power supply voltage.
How is the missing neutral power supply designed? With a current transformer with a turns ratio of 200:1 and 2 A passing through the primary side, there will be 10 mA in the secondary side. This 10 mA current can be used to power the meter by placing a full bridge rectifier and a low quiescent current regulator, such as the ADP3330, after the CT.
One issue is that if the primary side current continues to increase, the secondary current will follow. If the secondary current becomes too large it can exceed the rated current of the components, and therefore the secondary side currents must be limited by causing the CT to saturate. The saturation of the CT is determined by the choice of core material and the number of windings on its two sides. An inexpensive CrGO core works well in this application. One limitation of this type of power supply is that if the load current (Ip) is too small then there will not be enough current available on the secondary side for the meter’s power needs.
Selection of metrology IC
Now consider the selection of the main metrology IC. A basic requirement is that the energy measurement is very accurate and stable over time, temperature and input dynamic range. The second criterion for selection is long term reliability as the meters are likely to be installed for 10 years or more. While initial accuracy measurements are easy to obtain, determining the long term accuracy and reliability are significantly more difficult, making the choice of energy measurement vendor harder. This makes the vendor’s reputation in the market important, but there are also long term reliability tests like High Temperature Operating Life that can be performed on the IC to test reliability and change in accuracy over time. As time to market pressures are increasing, having a metering SOC that contains the full feature set is also important.
The ADE71xx family of energy metering SOCs contains a highly accurate and stable energy measurement engine. In addition to the dedicated energy measurement circuitry with built-in tamper detection, they integrate an 8052 microcontroller with flash memory, an LCD driver, real time clock (RTC) and intelligent battery management. The power management circuitry monitors the main supply voltage and the battery, and automatically chooses between normal operation mode and an ultra-low power 1.5 µA battery mode. This enables easy monitoring of the case-open switch and meets specifications in some countries that the LCD display must remain active for several hours after power is lost.
Most smart meters have an LCD display to show energy consumption, billing cycle, time, date, RMS voltage and current values. The ADE71xx contain a 104- segment LCD driver, which can control the display contrast. As electricity meters are typically placed outdoors, the temperature of the device may vary wildly from over 60°C to well below 0°C, which can make the LCD display difficult to read. The ADE71xx contain a temperature sensor that is used to measure the temperature of the device and a digital-to-analogue converter (DAC) that can vary the voltage amplitude of LCD signals. An integrated hardware real time clock (RTC) has base frequency and temperature compensation, which allows a 2 ppm accuracy after calibration.
In addition to the high accuracy and stability of the meteorology section, the ADE71xx devices offer an integrated two current channel sampling ADC, with automatic channel selection based on their amplitude. This feature enables an easy way to implement tamper detection for the earth fault condition. For the missing neutral tamper detection, the SAG detection interrupt can be used to detect if the neutral has been removed and the ADE71xx can then be internally configured to record the energy based only on the Irms measurement. The Irms_GAIN can be adjusted to provide the proper k value.
To control revenue losses, utilities worldwide need to detect and continue billing accurately when tampering has occurred. The ADE71xx family of energy metering SOCs provides an excellent solution to implement multiple layers of tamper detection, including using its low power mode to monitor the case open switch when the line power is off, using the two current channel ADC monitor for earth fault tampering and using the SAG detection and Irms to easily implement the missing neutral tamper detection.