DC Metering and Traceability
- 1. 1 Prepared by Tom Lawton and William H. Hardy, Ph.D. for the AEIC Measurement Technologies Working Group EEI Metering Track – Virtual Fall Conference 7 October 2020 DC Metering and Traceability
- 2. 2 DC – EDISON MAY WIN IN THE END ♦ Edison thought DC was the winning form of electricity. ♦ Maybe in the end he will be right. ♦ Today almost everything we use could, WITHOUT MODIFICATION, be run from DC
- 3. 3 DC – EDISON MAY WIN IN THE END ♦ Who would have imagined 10 years ago the cars powered by DC electricity would be the future of transportation.
- 4. 4 DC – EDISON MAY WIN IN THE END ♦ DC micro-grids prove effective for shared cogeneration and storage
- 5. 5 DC METERS – TOMORROW’s CASH REGISTERS ♦ If you are going to sell it, you have to measure it. ♦ Real needs are driving the urgency for DC metering ♦ California has passed legislation requiring DC metering standards by December 1, 2020 ♦ Yesterday ANSI SC32 voted out a DC Metering Standard
- 6. 6 BACKGROUND ♦ DC Energy – the definition ■ Power 𝑊 𝑡 = 𝑉 𝑡 ∙ 𝐼 𝑡 ■ Energy 𝐸 𝑡0 + 𝑇 = 𝑡0 𝑡0+𝑇 𝑊 𝑡 𝑑𝑡 = 𝑡0 𝑡0+𝑇 𝑉 𝑡 ∙ 𝐼 𝑡 𝑑𝑡 Four quantities to measure: Voltage, current, time, simultaneity
- 7. 7 CHALLENGES ■ Same definition as AC energy ■ Voltage ●For DC voltages we are limited to resistive dividers to get from system voltages to ADC levels ▪ Generally this is not an issue. ■ Current ●DC meters may use shunts, Hall effect sensors, or more sophisticated devices such as zero flux transducers ●Both high current and low current applications present issues at high accuracy
- 8. 8 CHALLENGES ■ Time Interval ●Neither measurement nor accuracy is an issue ■ Simultaneity ●Modern sampling ADCs make true simultaneity easy ■ So why is DC metering considered so difficult? ●EVSE manufacturers demanded a relaxation of accuracy requirements to 5% and a delay of up to a decade to comply.
- 9. 9 CHALLENGES ■ DC may not really be DC ■ There may be 10% of AC ripple on top of the voltage at frequencies in the 60Hz to 360Hz range ■ There may be high frequency saw-tooth waves in the 20kHz to 250kHz range ■ Loads may vary rapidly including large instantaneous steps
- 10. 10 DEVIL IS IN THE DETAILS ■ Modern AC meters use a variety of signal processing techniques to enhance accuracy. ● Any DC content of the signals is removed ● Algorithms such as FFTs and digital transforms make use of the AC nature of the signals to improve accuracy ● For AC, small, inexpensive CTs are available with inherent accuracies of ±0.02 percent
- 11. 11 DC CURRENT MEASUREMENT ■ SHUNTS ● For high currents shunts are generally large and produce very low signal levels ● A 1mΩ shunt in a 100A meter produces 10W with only a 0.1V signal ● At 1.5 amps the signal is only 0.0015 volts ● DC offsets can easily be larger than the signal and there is no simple signal processing trick to eliminate them ● Shunts generally have fairly high temperature coefficients
- 12. 12 CURRENT MEASUREMENT ■ Hall effect sensors ● Hall effect sensors are generally in the 1% to 5% accuracy range ● Closed loop sensors may be better than 1% ● Cost is generally high compared to shunts ● Temperature dependence may be an issue
- 13. 13 CURRENT MEASUREMENT ■ Zero flux transformers/flux gates ● Can be very accurate <0.1% over a wide temperature range ● Measure AC and DC ● Are VERY expensive compared to all other techniques ● Few sources, large in size
- 14. 14 ESTABLISHING TRACEABILITY ■ Normally one starts by sending a measurement device to NIST for calibration as a primary reference standard ■ Unfortunately, NIST has no primary DC Energy Standard and does not offer that service ■ So if you want a traceable measurement you have to start from basic measurements and qualify your own primary standard
- 15. 15 ESTABLISHING TRACEABILITY ■ The Validation Process ● A primary reference standard candidate is constructed using multiple current sensors with AC/DC measurement capabilities ▪ Precision zero flux transformer for high current 10 – 1000A ▪ Precision shunt 0.1 ohm, 1 ppm TC with integrated PT100 temperature sensor
- 16. 16 ESTABLISHING TRACEABILITY ■ Current Measurement is validated in multiple ways ● AC accuracy is validated against a Fluke 6105A with basic uncertainty of <50ppm ● DC accuracy is validated against the Fluke 6105A up to 10A ● Above 10A a pair of precision shunts are used ▪ An 0.100 Ohm shunt for 0.01 to 21 amps ▪ A 100uOhm shunt for 10 to 1000A
- 17. 17 ESTABLISHING TRACEABILITY ■ Voltage Measurement is validated directly with the Fluke 6105A in both AC and DC modes ● The Fluke 6105A has a basic uncertainty of <50ppm ■ Because the system is fully calibrated in AC and DC modes it is also capable of mixed signal measurement of any kind ● AC validation resolves any issues of timing and simultaneity
- 18. 18 TESTING DC METERS ■ TESCO is currently testing a production meter for compliance with ANSI C12.32 (Draft of 10/6/2020 ■ TESCO is currently testing a production meter for compliance with ANSI C12.32 Draft of 10/6/2020 ● DC 120V-250V ● 80A Self-contained ● Accuracy class 1%
- 19. 19 EXAMPLE RESULTS ■ Accuracy testing Nominal Voltage Nominal Current Test Time (sec) Standard WHrs Meter WHrs % Error 250 80 180 989.67 989.458 -0.021% 250 50 288 995.84 997.334 0.150% 250 30 480 998.17 999.744 0.157% 250 20 720 999.74 999.946 0.020% 250 10 1440 1,005.28 1005.47 0.019% 250 5 2880 1,012.52 1008.48 -0.399% 250 1 14400 1,063.83 1054.1 -0.914% 120 80 375 994.61 996.006 0.140% 120 50 600 997.60 998.525 0.092% 120 30 1000 1,000.55 1001.56 0.101% 120 20 1500 1,002.01 1003.21 0.120% 120 10 3000 1,006.28 1006.72 0.044% 120 5 6000 1,013.61 1007.04 -0.648% 120 1 30000 1,061.62 1054.2 -0.699% ACCURACY TESTS (METER SN M2200000007)
- 20. 20 EXAMPLE RESULTS ■ Temperature Testing ■ Results well within the requirements for an AC or DC meter of Accuracy Class 1%
- 21. 21 CONCLUSION ■ DC Metering is here today ■ Today’s technology can deliver cost effective DC metering with accuracies and environmental performance similar to AC meters. Edison is smiling today!!!!
- 22. 22 Questions and Discussion William H. Hardy, Ph.D. CTO Bill.Hardy@tescometering.com TESCO – The Eastern Specialty Company Bristol, PA 215-228-0500 This presentation can also be found under Meter Conferences and Schools on the TESCO website: www.tescometering.com ISO 9001:2015 Certified Quality Company ISO 17025:2017 Accredited Laboratory