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
2.4.2.1.2 Electricity Meter Metrology Accuracy Testing

To test for metrology accuracy, a source generator was used to provide the voltage and current to the system at the proper locations mentioned in Section 2.4.1.2.1. Additionally, a nominal voltage of 230 V, calibration current of 10 A, and nominal frequency of 50 Hz are used. During all of the tests, the board is powered directly using the TPS7A78-based power supply by placing a jumper between the LDO_OUT and DVCC positions on J3. In addition, the system and shunt is referenced with respect to the neutral for most of the tests.

When the voltage and current are applied to the system, the system outputs the active energy pulses and reactive energy pulses at a rate of 6400 pulses/kWh. The pulse output is fed into a reference meter (in the test equipment for this reference design, this pulse output is integrated in the same equipment used for the source generator) that determines the energy % error based on the actual energy provided to the system and the measured energy as determined by the active and reactive energy output pulse of the system. In this reference design, active energy error testing, reactive energy error testing, voltage variation testing, and frequency variation testing are performed after performing energy gain calibration, phase calibration, and energy offset calibration as described in Section 2.4.2.1.4.2.2.

Reactive energy testing is performed using a 200-µΩ shunt for the current readings. Active energy testing is performed using both the shunt and CT channel on the board for the current readings. Since the default channel used for active energy pulses is the shunt channel, the software was modified to use the CT channel as the default channel for pulse outputs to test the active energy error when using the CT channel. For the active energy tests performed using the shunt channel, both a 200-µΩ shunt and a 100-µΩ shunt were tested.

For the active energy error and reactive energy error testing on the shunt channel, current is varied from 50 mA to 90 A when testing using the shunt for measuring current. When performing the CT active energy test, conversely, current is varied from 50 mA to 100 A . For active energy error testing, a phase shift of 0°, 60°, and −60° is applied between the voltage and current waveforms fed to the reference design. Based on the error from the active energy output pulse, a plot of active energy % error versus current is created for 0°, 60°, and –60° phase shifts. For reactive energy error testing, a similar process is followed except that 30°, 60°, –30°, and –60° phase shifts are used and reactive energy error is plotted instead of active energy error.

In performing metrology tests, two sets of voltage tests were also performed, both of which were performed with a 200-µΩ shunt. In the first test, the 230-V nominal voltage was varied by ±10% at different currents and power factors. The resulting active energy error at each test point was then logged. For the second test, the active energy error was plotted when voltage was varied over a larger voltage range at unity power factor. Specifically, voltage was varied from 75 to 270 V. Testing beyond 270 V can also be done; however, this requires the 275-V varistors to be removed from the design and replaced with varistors that are rated for a higher voltage.

Another set of tests performed are frequency variation tests, which are performed using a 200-µΩ shunt. For this test, the frequency is varied by ±2 Hz from its 50-Hz nominal frequency. This test is conducted at 0.5 A and 10 A at phase shifts of 0°, 60°, and −60°. The resulting active energy error under these conditions are logged.