Today energy theft is a worldwide problem that contributes heavily to revenue losses. Consumers have been found manipulating their electric meters, causing them to stop, under-register or even bypassing the meter, effectively using power without paying for it.
This article discusses vulnerabilities, challenges and techniques to prevent tampering in an energy meter.
1.0 Introduction to Energy Meters
An energy meter is a device that measures the amount of electrical energy supplied to a residential or commercial building. The most common unit of measurement made by a meter is the kilowatt hour, which is equal to the amount of energy used by a load of one kilowatt in one hour.
Figure 1 shows a system block diagram for a three-phase energy meter. As shown the energy meter hardware includes a power supply, an analog front end, a microcontroller section, and an interface section. The analog front end is the part that interfaces to the high voltage lines. It converts high voltages and high currents to voltages sufficiently small to be measured directly by the ADC (Analog/Digital Converter) of the microcontroller.
Figure 1: System Block Diagram for three phase Energy Meter
Voltage measurement is done with a shunt resister (shown as "Load"), while the current measurements require more precise measurement and thus are done by Current Transformer (CT) on all phases along with current measurement on neutral. Meter manufacturers often integrate gain amplifiers in order to amplify voltage as well as current measurements in the range supported by the ADC. The amount of amplification required depends on the ADC resolution as well as the Class accuracy (0.1, 0.2. 1.0 etc.) required for a three-phase meter.
A typical energy meter also requires a Real Time Clock (RTC) for tariff information. The RTC required for a metering application needs to be very accurate (< 5ppm) for Time of Day (TOD), which involves dividing the day, month and year into tariff slots. Higher rates are applied at peak load periods and lower tariff rates at off-peak load periods.
The heart of the meter is the firmware, which calculates Active, reactive energy based on voltage and current measurement. The firmware also includes tamper detection algorithms, data logging and protocols like DLMS and Power Line Modem communication protocol for Automatic Meter Reading (AMR).
The energy meter also needs to be calibrated before it can be used and that is done in a digital domain for an electronic meter. Digital calibration is fast, efficient and can be automated, removing the time-consuming manual trimming required in traditional, electromechanical meters. Calibration coefficients are safely stored in an EEPROM that can be either internal or external.
An energy pulse output (EP) is an indication of active power, as registered by the meter; the frequency of the pulse is directly proportional to active power.