[IEEE 2012 Conference on Precision Electromagnetic Measurements (CPEM 2012) - Washington, DC, USA (2012.07.1-2012.07.6)] 2012 Conference on Precision electromagnetic Measurements - Calibration of wattmeters for standby power testing to IEC 62301

Calibration of Wattmeters for Standby Power Testing to IEC 62301 I. F. Budovsky and D. Georgakopoulos National Measurement Institute, Bradfield Road, Lindfield NSW 2070, Australia Abstract — A system for the calibration of wideband wattmeters used in the measurement of standby electrical power in accordance with IEC 62301 has been developed at the National Measurement Institute, Australia (NMIA). The system has been tested with current waveforms having crest factors up to 20 and has uncertainties significantly lower than those required by the standard. Index Terms — standby power, wideband wattmeters, power standards, measurement techniques, measurement uncertainty. I. INTRODUCTION The standard 62301 of the International Electrotechnical Commission (IEC) [1] defines the standby mode of an appliance as “the lowest power consumption mode which cannot be switched off (influenced) by the user and that may persist for an indefinite time when an appliance is connected to the main electricity supply and used in accordance with the manufacturer’s instructions”. Although the average power consumed by an appliance in standby mode is small, the total energy consumption by a large number of appliances operating over prolonged times is significant. Therefore, testing of household appliance power consumption in standby mode is being introduced in many countries. IEC 62301 describes the measurement of standby power for such testing and specifies the requirements for wideband wattmeters used for this purpose. A wattmeter must be capable of measuring electrical power with crest factors of the current waveform, defined as the ratio of the maximum to the rms value of the current waveform, being up to ten. Fig. 1 shows, as an example, the measured standby current of a battery charger. The crest factor of this waveform is 4.6. The measurement uncertainty must be less than 2% for measured power above 0.5 W and less than 0.01 W for measured power below 0.5 W. At NMIA we have developed a calibration system to traceably verify the performance of wattmeters for the specific conditions described in IEC 62301. -0.03 -0.02 -0.01 0 0.01 0.02 0.03 -0.12 -0.07 -0.02 0.03 0.08 Tim e (s ) C ur re nt (A ) Fig. 1. Measured standby current of a battery charger. II. CALIBRATION WAVEFORMS IEC 62301 recommends essentially sinewave voltage and distorted current specified in terms of its crest factor. This specification does not uniquely define the waveform in the time (or the frequency) domain. We have chosen the pulsed current waveform shown in Fig. 2. It resembles typical current waveforms of appliances, such as the one in Fig. 1, allows to easily achieve large crest factors, and produces the largest power for a given crest factor. The particular waveform in Fig. 2 is for unity power factor (zero phase shift between the fundamentals of voltage and current). The peaks of the pulse are centered on T/4 and 3T/4, where T is the period of the test signal. Different values of power and power factor are achieved by varying the magnitude of I1 and its phase relative to the sinusoidal voltage waveform. t1 t2 t3 t4 T t i(t) Im -Im 3412 43 21 24 3 24 3 2424 ttttt tTttTt tTttTt p pp pp −=−= +=−= +=−= t1 t2 t3 t4 T t i(t) Im -Im 3412 43 21 24 3 24 3 2424 ttttt tTttTt tTttTt p pp pp −=−= +=−= +=−= Fig. 2. Current excitation waveform for the calibration of power meters to IEC 62301. With reference to Fig. 2, it can be shown that the rms value and the crest factor of the current waveform are: T t II pmrms 2= , prms m t T I I CF 2 == . (1) . For sinusoidal voltage in phase with the current waveform, the average active power is: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ = 2)(2 sin2 CF IVP mm π π , (2) where Vm is the amplitude of the sinusoidal voltage and Im is the peak of the pulsed current waveform. 238978-1-4673-0442-9/12/$31.00 ©2012 IEEE III. CALIBRATION SYSTEM The measurements are based on a high frequency thermal power comparator (TPC) developed at NMIA [2]. Fig. 3 shows the block diagram of the system. Dual Voltage UUTGPIB DCS DVM R GPIB DVM VD GPIB U ACV M UDC1 1 S 2 3 TPC DCS UDC2 2 DVM GPIB1 UuI ACI DCV DCI TCA Channel Voltage Source Amplifier ~ ~ Ui Fig. 3. Block diagram of the calibration system. The dual channel voltage source contains two arbitrary waveform generators programmed to produce required voltage and current waveforms. The outputs of these generators are connected, respectively, to a low noise voltage amplifier and to a transconductance amplifier (TCA) with 100 kHz bandwidth. The TPC uses multijunction thermal voltage converters and is, therefore, free of the accuracy limitations due to aliasing of the current and voltage waveforms that are common to sampling wattmeters. The output of the TPC, measured by digital voltmeter DVM1, is proportional to the difference between the unknown ac power applied to inputs ACV and ACI of the TPC, and the known dc power generated by two dc sources (DCS1 and DCS2 in Fig. 3) and measured by DVM2 and DVM3. The ac voltage applied to the unit under test, UUT, is converted to the input level of the TPC using a voltage divider, VD. The VD can be either inductive or resistive. The current is converted to voltage using wideband current shunts of NMIA design [3], [4]. The measurement system has been characterized for amplitude and phase up to 200 kHz and is traceable to the Australian national standards of voltage, current, resistance, electrical power and frequency. The ac-dc difference and the phase shift of the current shunts were measured at frequencies up to 1 MHz and 200 kHz, respectively, using the techniques described in [4] and [5]. The inductive voltage divider was calibrated using the method described in [6]. The typical in- phase and quadrature errors in the output voltage at power frequencies are of the order of 0.01 μV/V and 0.1 μrad, respectively. The resistive voltage divider can be used to measure voltages up to 240 V at frequencies up to 200 kHz [7]. Table 1 shows an example of typical uncertainties for the calibration of a commercial wattmeter at different crest factors and phase angles between the fundamentals of voltage and current. The uncertainties required by IEC 62301 for the corresponding power levels are included. Table 1. Typical Calibration Results Applied Current Phase Angle Nominal Power Uncertainty Uncertainty required by IEC 62301 (Apk) (degrees) (W) (W) (W) 3 0 22.513 ± 0.012 ± 0.45 3 + 60 11.256 ± 0.006 ± 0.23 3 - 60 11.256 ± 0.006 ± 0.23 5 0 8.141 ± 0.005 ± 0.16 10 0 2.036 ± 0.012 ± 0.04 20 0 0.509 ± 0.006 ± 0.01 3 0 0.188 ± 0.0005 ± 0.01 3 + 60 0.094 ± 0.0005 ± 0.01 3 - 60 0.094 ± 0.0005 ± 0.01 5 0 0.068 ± 0.0005 ± 0.01 5 + 90 0 ± 0.0005 ± 0.01 5 - 90 0 ± 0.0005 ± 0.01 10 0 0.017 ± 0.0005 ± 0.01 20 0 0.004 ± 0.0005 ± 0.01 0.005 0.6 Current Crest Factor VI. CONCLUSION A traceable measurement system for the calibration of wideband wattmeters used in the measurement of standby electrical power in accordance with IEC 62301 has been developed. The system has been tested with current waveforms having crest factors up to 20 and has uncertainties significantly lower than those required by the standard. REFERENCES [1] IEC 62301, “Household electrical appliances – Measurement of standby power,” ed. 1.0, 2005. [2] I. Budovsky, A. M. Gibbes and D. C. Arthur, “A high-frequency thermal power comparator,” IEEE Trans. Instrum. Meas., vol 48, pp. 427 – 430, 1999. [3] I. Budovsky, “Standard of electrical power at frequencies up to 200 kHz,” IEEE Trans. Instrum. Meas., vol 58, pp. 1010 – 1016, 2009. [4] I. Budovsky, “A micropotentiometer-based system for low- voltage calibration of alternating voltage current standards,” IEEE Trans. Instrum. Meas., vol 46, pp. 356 – 360, 1997. [5] I. Budovsky, “Measurement of phase angle errors of precision current shunts in the frequency range from 40 Hz to 200 kHz,” IEEE Trans. Instrum. Meas., vol 56, pp. 284 – 288, 2007. [6] I. F. Budovsky, G. H. Small, A. M. Gibbes and J. R. Fiander, “Calibration of 1000 V / 50 Hz inductive voltage dividers and ratio transformers,” CPEM 2004 Conf. Digest, London, pp. 322 – 323, June 2004. [7] I. Budovsky, A. M. Gibbes and G. M. Hammond, “Voltage divider characterization at frequencies up to 200 kHz,” CPEM 2000 Conf. 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