TIDUEM7A April   2019  – February 2021

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
    1. 1.1 End Equipment
      1. 1.1.1 Electricity Meter
    2. 1.2 Key System Specifications
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Highlighted Products
      1. 2.2.1 ADS131M04
      2. 2.2.2 TPS7A78
      3. 2.2.3 MSP432P4111
      4. 2.2.4 TPS3840
      5. 2.2.5 THVD1500
      6. 2.2.6 ISO7731B
      7. 2.2.7 TRS3232E
      8. 2.2.8 TPS709
      9. 2.2.9 ISO7720
    3. 2.3 Design Considerations
      1. 2.3.1 Design Hardware Implementation
        1. 2.3.1.1 TPS7A78 Cap-Drop Supply
        2. 2.3.1.2 TPS3840 SVS
        3. 2.3.1.3 Analog Inputs
          1. 2.3.1.3.1 Voltage Measurement Analog Front End
          2. 2.3.1.3.2 Current Measurement Analog Front End
      2. 2.3.2 Current-Detection Mode
        1. 2.3.2.1 ADS131M04 Current-Detection Procedure
        2. 2.3.2.2 Using an MCU to Trigger Current-Detection Mode
          1. 2.3.2.2.1 Using a Timer to Trigger Current-Detection Mode Regularly
          2. 2.3.2.2.2 MCU Procedure for Entering and Exiting Current-Detection Mode
        3. 2.3.2.3 How to Implement Software for Metrology Testing
          1. 2.3.2.3.1 Setup
            1. 2.3.2.3.1.1 Clock
            2. 2.3.2.3.1.2 Port Map
            3. 2.3.2.3.1.3 UART Setup for GUI Communication
            4. 2.3.2.3.1.4 Real-Time Clock (RTC)
            5. 2.3.2.3.1.5 LCD Controller
            6. 2.3.2.3.1.6 Direct Memory Access (DMA)
            7. 2.3.2.3.1.7 ADC Setup
          2. 2.3.2.3.2 Foreground Process
            1. 2.3.2.3.2.1 Formulas
          3. 2.3.2.3.3 Background Process
            1. 2.3.2.3.3.1 per_sample_dsp()
              1. 2.3.2.3.3.1.1 Voltage and Current Signals
              2. 2.3.2.3.3.1.2 Frequency Measurement and Cycle Tracking
            2. 2.3.2.3.3.2 LED Pulse Generation
            3. 2.3.2.3.3.3 Phase Compensation
    4. 2.4 Hardware, Software, Testing Requirements, and Test Results
      1. 2.4.1 Required Hardware and Software
        1. 2.4.1.1 Cautions and Warnings
        2. 2.4.1.2 Hardware
          1. 2.4.1.2.1 Connections to the Test Setup
          2. 2.4.1.2.2 Power Supply Options and Jumper Settings
        3. 2.4.1.3 Software
      2. 2.4.2 Testing and Results
        1. 2.4.2.1 Test Setup
          1. 2.4.2.1.1 SVS and Cap-Drop Functionality Testing
          2. 2.4.2.1.2 Electricity Meter Metrology Accuracy Testing
          3. 2.4.2.1.3 Current-Detection Mode Testing
          4. 2.4.2.1.4 Viewing Metrology Readings and Calibration
            1. 2.4.2.1.4.1 Viewing Results From LCD
            2. 2.4.2.1.4.2 Calibrating and Viewing Results From PC
              1. 2.4.2.1.4.2.1 Viewing Results
              2. 2.4.2.1.4.2.2 Calibration
                1. 2.4.2.1.4.2.2.1 Gain Calibration
                  1. 4.2.1.4.2.2.1.1 Voltage and Current Gain Calibration
                  2. 4.2.1.4.2.2.1.2 Active Power Gain Calibration
                2. 2.4.2.1.4.2.2.2 Offset Calibration
                3. 2.4.2.1.4.2.2.3 Phase Calibration
        2. 2.4.2.2 Test Results
          1. 2.4.2.2.1 SVS and TPS7A78 Functionality Testing Results
          2. 2.4.2.2.2 Electricity Meter Metrology Accuracy Results
          3. 2.4.2.2.3 Current-Detection Mode Results
  8. 3Design Files
    1. 3.1 Schematics
    2. 3.2 Bill of Materials
    3. 3.3 PCB Layout Recommendations
      1. 3.3.1 Layout Prints
    4. 3.4 Altium Project
    5. 3.5 Gerber Files
    6. 3.6 Assembly Drawings
  9. 4Related Documentation
    1. 4.1 Trademarks
  10. 5About the Author
  11. 6Revision History

1.1.1 Electricity Meter

Each year, billions of dollars are lost by utilities due to nontechnical losses. One form of nontechnical loss for electricity utility providers is electricity meter tampering, where individuals hack meters to slow or stop the accumulation of energy usage statistics, thereby stealing electricity. One of the most common ways someone tries to tamper with an electricity meter is to apply a magnet on it. This magnet paralyzes transformers in power supplies as well as current transformer current sensors, thereby enabling electricity theft. Since magnets can affect current transformers(CT), shunts are often used as a current sensor for one-phase meters. The output voltage produced by shunts at low currents is small, especially when compared to the output voltage produced by current transformers over the same low input current range. As a result, for shunt-based high-accuracy meters, an accurate ADC is needed to sense the low output voltages from shunts to accurately bill utility customers.

In addition to ensuring accurate customer billing, it is the responsibility of the utility company to guarantee good power quality to their customers. However, it is possible for current harmonics from a utility customer’s load to induce voltage harmonics, which may effect multiple utility customers. By performing harmonic analysis, utility providers may be able to identify customer loads that negatively impact power quality. Adding harmonic analysis capabilities to an electricity meter may require an increase in the sample rate of the meter to capture the desired frequency range. The increase in sample frequency many times has to be done without compromising on accuracy or even while simultaneously increasing accuracy. The high sample rate, in turn, also requires more processing.

As the accuracy and amount of processing expected from electricity meters increases, it becomes more difficult to find a metrology SoC that fulfills both the processing and accuracy requirements of an electricity meter. To address this limitation, a standalone ADC can be used with a host microcontroller (MCU) to simultaneously overcome the processing and accuracy limitations of electricity meter SoCs. Using an accurate standalone ADC typically has the following advantages:

  • It enables meeting the most stringent of accuracy requirements
  • It enables meeting minimum sample rate requirements(without compromising on accuracy) that may not be obtainable with applications-specific products or metrology SoCs
  • It enables flexibility in selecting the host MCU since you are not limited to selecting host MCUs that have accurate ADCs. The host MCU can be selected solely based on application requirements, such as processing capability, minimum RAM and Flash storage for logging energy usage, and MCU security features for ensuring meter data security.

In this reference design, Class 0.5 one-phase shunt-based energy measurement is implemented by using a standalone ADC device. The standalone ADC senses the Mains voltage and current. For sensing the current, the design measures both the line and neutral current by using a shunt and CT. By measuring both currents, metrology parameters can be properly sensed in case someone tries to tamper with the meter by bypassing the sensed line or neutral current in an attempt to have the meter register a smaller energy consumption than what is actually consumed.

When there are new ADC samples available from the standalone ADC, the host MCU communicates to the standalone ADC using SPI to get the new samples. The host MCU uses the new ADC samples from the standalone ADC to calculate metrology parameters. In addition to calculating the metrology parameters, the host MCU also drives the liquid crystal display (LCD) of the board and communicates to a personal computer (PC) graphical user interface (GUI) through either the isolated RS-232 circuitry or isolated RS-485 circuitry on the board. As an additional safeguard, an external SVS device is added to the design to reset the host MCU when the supplied voltage to power the host MCU is not sufficient. In general, using an external SVS provides more security than the internal SVS on a host MCU.

In this design, the test software specifically supports calculation of various metrology parameters for one-phase energy measurement. These parameters can be viewed either from the calibration GUI or LCD. The key parameters calculated during energy measurements are:

  • Active, reactive, apparent power and energy
  • RMS line current, RMS neutral current, and RMS voltage
  • Power factor
  • Line frequency

In addition to affecting current transformer current sensors, magnetic tampering could affect a transformer in the power supply as well. To deal with magnetic tamper attacks affecting the power supply of the meter, one option is to use cap-drop supplies, which do not use a magnetically-susceptible transformer. However, one disadvantage of cap-drop supplies is their small maximum output current. To increase the maximum output current from a cap-drop supply without increasing the capacitor size of the power supply, a buck converter could be used with the cap-drop supply instead of the LDO that is used in traditional cap-drop supplies; however, using a buck converter would require adding an inductor, which may be affected, like a CT, by an external magnet. In this design, an AC voltage regulator is used to create a compact, cap-drop power supply that can provide more output current than conventional cap-drop supplies without having to use magnetic components, thereby making the power supply magnetically immune.

Another technique used to tamper with an electricity meter is to remove the neutral wire from the meter. If the neutral is disconnected, the voltage measured is 0 V, which in turn leads to a 0 W measured value for the active power. With the neutral missing, the main AC/DC is not functional so a backup power supply like a battery or powering CT must be used to power the meter. For this tamper technique, although the active power reading is 0 W because of the 0 V reading, there is still current flowing through the line wire that could be sensed. The standalone ADC used in this design has a current-detection mode that could be used to detect the presence of current for this tamper scenario. In this mode, the ADC runs off an internal oscillator and provides an interrupt to the MCU if a user-configurable number of samples have surpassed a user-defined ADC threshold value, which may indicate tampering. Since the ADC is doing this current detection, the MCU can enter a sleep mode until it is alerted that current has been detected by the standalone ADC. This current-detection mode is low-power, which allows the mode to be entered periodically without significantly draining the backup power supply that the meter is running on. The AC/DC power supply of the design provides an early alert of AC supply failure, which can be either be from an actual power outage condition or from the removal of the neutral connection, so that the standalone ADC could be triggered to enter this current-detection mode. When the meter sees AC mains again after power has been restored from a power outage, the AC/DC in the design provides an alert that can be used to exit current-detection mode.