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Calibrating Energy Meters
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Calibrating Energy Meters
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Calibrating Energy Meters

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Published
Published April 4, 2023
Author
Apoorva Singh

Objective

Can we build a low cost device to calibrate our energy meters?
Image obtained from [2]
Image obtained from [2]

Research

There are primarily two ways to calibrate an energy meter:
  1. Accurate Source - Have an unknown load and a reference meter, use the pulse from the reference meter to calibrate your device under test. In this case the root of trust is in the reference meter, that the values that it shows are correct.
  1. Reference Meter - Have a known source and connect your device under test directly to this source and calibrate it. In this case the root of trust is in the source, meaning the current and voltage that it generates is accurate.
One additional way to calibrate an energy meter is to use a calibration service. These services are typically offered by specialized companies or laboratories that have the necessary equipment and expertise to calibrate energy meters to a high degree of accuracy. However, this can be expensive and may not be a feasible option for those looking for a low-cost solution.
 
 
 

Requirement

Our objective is to calibrate the following parameters:
  1. Voltage - Constant voltage source
  1. Current - Constant current source
  1. Active Power - Constant power source
  1. Energy - Reference meter

Design 1

This design is fairly simple and makes use of a reference meter with any kind of constant current load.
notion image
As long as the AC load is fairly stable in consuming the same amount of power, we can calibrate the energy meter. We can use a resistive heating rod as our load.
 
But there are a few problems with this design:
  1. No Voltage Coefficient - The voltage on the line might change over time. We cannot calculate a proper voltage coefficient.
  1. No Current Coefficient - The current on the line might change over time. We cannot calculate a proper current coefficient.
  1. No Power Coefficient - Active and Apparent Power might also change over time. We cannot accurately calculate the power coefficient.
  1. Fixed Power Factor - We only have one power factor to test the meter with. We would also need to change the phase difference.
 
One way of fixing point 1, 2 and 3 is to simply accept the fact that the meter readings for voltage, current and power will have more error since it wasn’t calibrated from a stable source. This will have no effect on the accuracy of energy measurement.
 
Point 4 will have a detrimental effect on the accuracy of energy measurement. We need some sort of a circuity to create phase difference.
 
We are also not attaching an isolation transformer at the source, this would have increased the safety of the system. But to keep the costs low and the design simple, we’ll omit that.
 

Ideal Design

Image obtained from [2]
Image obtained from [2]
This setup has both, an AC Power Generator which can provide an accurate voltage and current for coefficient calculation and a Reference Meter for calibrating the energy consumption.
If I can get my hands on an AC Power Generator, then the entire setup would be very easy to build as reference meters are readily available.
💡
The AC Power Generator use a clever design. So the thing is that the meter under test can be cheated to operate at any programmed voltage and current without actually connecting to a real load. Therefore, the 240 VAC voltage input supplies only a minimal current, usually less than 10 mA (RMS). It’s testing current is also supplied at a very low voltage. This this equipment setup is capable of testing the meter at 45A (RMS) and 240 V(RMS) while consuming less than 200 Watt. MAGIC! ✨
 
One design that are used by AC Power Generator is the following:
Image obtained from [2]
Image obtained from [2]
Image obtained from [2]
Image obtained from [2]
Here a 2 channel Digital to Analog convertor (DAC) is used in conjunction with a microcontroller to generate a sine wave of required frequency and phase difference. Then each of the channel is fed into a voltage amplifier and a voltage-to-current amplifier which then gives us our required outputs.

Terms

  • Autotransformer - It provides a voltage-adjustable source of alternating current (AC) electricity. People also call it VARIAC, but it’s a brand name.
notion image
notion image
 
 

References

  1. Design and Implementation of a Low Cost Electricity Meter Testing Bench
  1. A design for programmable AC voltage and current generators for testing energy meters
  1. New means for testing energy meters
  1. AN-950: Calibrating a Single-Phase Energy Meter Based on the ADE71xx/ADE75xx Family
  1. Meco’s Calibrating Equipment Setup Guide
  1. Zeal Multi Function Calibrator (5 1/2 Digit)
  1. Zeal Single Phase Power Energy Meter Calibrator
  1. Zeal Three Phase Power/Energy Meter Calibrator - Implements the same design as mentioned under the Ideal Design section
  1. TI - Op Amps for Everyone
  1. Digitally Synthesised Power Calibration Source - Transformerless design of voltage amplifier
  1. A Precision Stabilized Sine-Wave Source for AC Power Measurements
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