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The establishment of power/energy standard at China Electric Power
Research Institute and its comparison with NRC
Bai, Jingfen; Yang, Lin; Zhao, Sha; Zong, Jianhua; So, Eddy
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https://nrc-publications.canada.ca/eng/view/object/?id=9492f674-7f78-4c38-8cd6-02efcd1df232 https://publications-cnrc.canada.ca/fra/voir/objet/?id=9492f674-7f78-4c38-8cd6-02efcd1df232275 Monday Tuesday W ednesday Thursday Friday
2010 Conference on Precision Electromagnetic Measurements
June 13-18, 2010, Daejeon Convention Center, Daejeon, Korea
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THE ESTABLISHMENT OF POWER/ENERGY STANDARD AT
CHINA ELECTRIC POWER RESEARCH INSTITUTE AND ITS COMPARISON WITH NRC
Jingfen Bai1, Lin Yang1, Sha Zhao1, Jianhua Zong1, and Eddy So2 1
CEPRI, China Electric Power Research Institute
No 15 Xiaoying East Road, Qinghe, Haidian District, Beijing, China 2
NRC, National Research Council of Canada, Ottawa, Canada
Abstract
This paper describes the establishment of a single-phase power/energy standard at CEPRI. It is based on a computer-controlled commercial version of a NRC developed current-comparator-based power/energy bridge. The bridge allows the calibration of single-phase active/reactive power and energy meters at voltages up to 600 V, currents up to 100 A, any power factor from zero lag through unity to zero lead, and test frequencies of 45 Hz to 65 Hz. It has an estimated uncertainty (k=2) of less than 25 W/VA. A comparison of power/energy meter calibration systems between China Electric Power Research Institute and the National Research Council of Canada will also be presented and discussed.
Introduction
A unique standard for the unit of power does not exist. The measurement of power must rely on a reference that is derived from the standards of voltage and resistance. Various techniques have been developed for combining these two standards in such a way as to provide a practical and convenient power standard. The power bridge, based on the current comparator, is one way of achieving his objective. At CEPRI the power/energy standard is based on a computer-controlled commercial version of the NRC developed current-comparator-based power bridge (CCBPB) [1]. The CCBPB calibration system has been in operation since 1998 at CEPRI providing calibrations of power/energy meters. It has continuously been updated to reduce the system uncertainty. Traceability of the CCBPB calibration system has been established through traceable equipment/measurement systems comprising of direct (dc) voltage, alternating (ac) reference resistor, reference capacitor and an ac/dc thermal transfer standard in 2002. At present, the CCBPB calibration system at CEPRI is used to calibrate single-phase active/reactive power and energy meters at voltage ranges 60V to 600V, current ranges 0.1A to 100A, power factor ranges from zero lag (0 L) through unity to zero lead (0 C), and test frequencies of 45 Hz to 65Hz with an estimated uncertainty (k=2) of less than
25 W/VA. The performance of the CEPRI CCBPB calibration system was eventually confirmed by a comparison with the NRC CCBPB calibration system in 2009.
System description
The CCBPB calibration system is shown in Figure 1.
Figure 1. Current-Comparator-Based Power Bridge The current in the meter-under-test (UUT) is compared with currents derived from the test voltage through a reference resistor R and a reference capacitor C in the current comparator. The reference capacitor C, which is solid-dielectric, is a current-comparator-based active capacitor automatically compensated for its instability and dielectric losses [2]. The ampere-turn unbalanced current is detected by the detection winding Nd with the corresponding Detector/Amplifier. A computer controls a feedback circuit from the Detector/Amplifier to the trans-conductance amplifier using analog-to digital-converters and digital-to-analog digital-converters. The control is such that the output current from the trans-conductance amplifier that drives the UUT current circuit and Nx winding would allow the current comparator, not only to achieve an ampere-turn balance condition, but also to maintain the balance through out the calibration cycle. At ampere-turn balance condition, the active and reactive components are given by:
276 Monday Tuesday W ednesday Thursday Friday 2 2 1 r x c x N U UI jN U C N R
Z
§ · r ¨ ¸ © ¹(1)
Thus, the in-phase and quadrature components of the current, and hence the test condition, are set by the turnsNr, Nc, and Nx
.
The ac voltage source that provides the test voltage has a short-term stability of better than 1 V/V. The DC voltage standard used in the CCBPB calibration system for setting up the appropriate test ac voltage through an ac/dc thermal transfer standard is traceable to the dc voltage primary standard maintained at CEPRI. During calibration, the ac test voltage in conjunction with a capacitive voltage divider is continuously compared to the dc reference through the ac-dc transfer standard. The dc voltage source has a short-term stability of 0.2 V/V, while the ac-dc difference of the ac/dc transfer standard is less than 4 V/V. Further detailed explanation of the CCBPB calibration system at CEPRI, including sources of error and uncertainty analysis will be presented and discussed.
Comparison with NRC
The comparison was implemented using a commercial time-division multiplier power converter as a transfer standard which belongs to CEPRI. The stability of the transfer standard has been monitored for more than 10 years at CEPRI. The comparison calibration was done at 50 Hz, 100 V, 5 A, at power factors of 1.0, 0.5 lead (C) and 0.5 lag (L). The transfer standard was calibrated at NRC in October 2009, and then calibrated at CEPRI in November 2009. The calibration results obtained by the two laboratories and their differences are shown in Table 1.
Range condition Applied Test PF CEPRI (ȝW/VA) NRC (ȝW/VA) CEPRI - NRC (ȝW/VA) 1.0 4 -4 8 0.5 L -21 -12 - 9 120 V 5A 100V 5A 0.5 C 27 12 15
Table 1. Calibration results at CEPRI and NRC
Considering that at CEPRI the estimated uncertainty (k=2) of the CCBPB calibration system is less than 25 W/VA and that at NRC is less than 10 W/VA (improved since [1] was published in 1983), the differences of not more than 15 W/VA between the two calibration systems at CEPRI and NRC indicated good agreement within the uncertainty of measurements, and that there are no significant differences between the CCBPB calibration systems at both laboratories.
Conclusions
The comparison results with NRC indicated good agreement and confirmed the good performance characteristics of the CCBPB calibration systems at CEPRI. Further improvement in the system estimated uncertainty to achieve lower than 20 W/VA will continue. Also, further investigations to expand other system calibration functions to facilitate the calibrations of reactive power meters will proceed. Expanding the CCBPB calibration system capabilities would enhance the calibration ability of CEPRI on active/reactive power and energy meters.
Acknowledgement
The authors would like to thank Mr. Dave Angelo from NRC for his assistance in performing the calibration of CEPRI’s transfer standard.
References
[1] W. J. Moore and E. So, “A Current-Comparator-Based System for Calibrating Active/Reactive Power and Energy Meters”, IEEE Trans. Instum. Meas., Vol. IM-32, No1, pp. 147-149, March 1983.
[2] E. So and W. J. Moore, “A Stable and Accurate Current-Comparator-Based Quadrature-Current Reference for Power Frequencies”, IEEE Trans. Instrum. Meas., Vol. IM-31, No. 1, pp. 43-46, March 1982.
