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  • Single-Phase Anti-Tamper Energy Meter

  • Created: Jun 04, 2017

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Summary

Energy tampering is one general cause of fire and illegal used of electric energy in residential or commercial building. The unwanted consuming of energy can cause much resources for electric company provider to lose. A common method of tampering with electric meters is by attaching magnets to the outside of the meter. These magnets prevent the alternating current from forming eddy currents in the rotor, by saturating the coils or current transformers. The reference design is an single-phase anti-tampering energy meter that calculates active and reactive energy, active, reactive and apparent power, power factor, RMS current, RMS voltage and the line frequency.


The meter will turn on when the line connection has 220V connected. The LCD display will show the total energy accumulated. It is a fully functional single-phase meter with enhanced capabilities such as battery backup, RTC and anti-tamper features. The process of detecting the tamper condition starts by monitoring and comparing both currents (neutral and high-voltage line). There are two current channels to be measured, connected to the external 16-bit two-channel ADC (MCP3911). Channel 0 measures the voltage on the shunt, while channel 1 measures the output voltage of the current transformer. The difference between RMS currents measured simultaneously on the two current channels. Anti-aliasing low-pass filters are placed on both current channels and also on the voltage channel. The design used a current transformer and a shunt resistor to measure the current. The PIC18F85K90 microcontroller directly drives the LCD and communicates via UART with the MCP2200, offering an isolated USB connection for meter calibration and access to the device power calculations. The reference design also implements USB connection, IR transmitter and receiver, and isolated pulse outputs for reactive and active energy. Three magnetic sensors are placed in the middle of the board for detecting a perturbing exterior magnetic field. A 9V DC input jack connector for powering the meter is ready incase main supply is not available. Also, a three non-isolated ICD programming header and three push button switches is utilized in the design. Switch 1 is used for tamper switch, while switch two is not used in this firmware implementation. Switch 3 is the MCU hardware reset.


The anti-tamper features are designed to detect five exterior conditions that can alter the functionality of the meter. The difference between RMS currents measured simultaneously on the two current channels, thus exceeding the threshold value of 1.5625% of the value measured by the main current channel) will trigger a tamper condition. Current circuit reversal refers to the monitoring of both channels for the sign of instantaneous active power, thus mismatch of the two channel signs trigger a current reversal tamper condition. Magnetic field changes which is being monitored by magnetic sensors on the board can also trigger a tamper condition. Switch opening detection on SW1 can generate a tamper event. Lastly, frequency disturbance in which the line frequency is outside the acceptable range of 45 to 65hz can generate a tamper condition.

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