Calibration of high-frequency wattmeters used for standby power testing to IEC August 2011 I. Budovsky and D. Georgakopoulos National Measurement Institute, Australia Low Frequency Electrical Standards

Overview IEC definition of standby power Motivation for this work Calibration waveforms System description Traceability Experimental results Summary

Standby power definition according to IEC 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 manufacturers instructions.

Motivation Electrical appliances operating in standby mode consume small amount of electrical power The current in standby mode can take the form of pulses or spikes Required measurement uncertainty by IEC W < 0.01 W, measured power < 0.5 W The IEC standard specifies current waveforms with CF up to 10 CF=1.41 for a sinewave CF=1.00 for a square wave

… Motivation Because of the high CF specified by the IEC standard, power meters for standby power cannot be calibrated with sinusoidal waveforms Hence there is a need to develop techniques to traceably verify power meters for the specific conditions described in IEC 62301

Calibration waveforms Voltage – sinewave with maximum distortion of 2% (up to and including the 13th harmonic) –crest factor from 1.34 to 1.49 – use of a sinewave (THD <0.02%) for calibration is good enough Current – specification only for CF (up to 10) – no distortion specified – recommends the instrument must measure up to at least the 50 th harmonic

Current waveforms Same CF and fundamental magnitude But different harmonic content peak values and rms values when the harmonics are included

Selecting current waveform We are looking for a current waveform to test the current channel of a power meter which: satisfies the IEC requirements or recommendations –CF requirement –contain pulses or spikes –high bandwidth is physically realisable In our calibration system we use a pulsed current

Advantages of using pulsed current A pulse is a wideband signal and is a more challenging test for the measurement system I-V converter AmplifierLPFS/HADC Sampling frequency > 2 maximum signal frequency The channel frequency response known for the signal frequencies A crest factor of 10 is more easily achieved with a pulsed current waveform while maximizing the measured power Satisfies the 50 th harmonic recommendation of the IEC standard Simple relation of the CF with the power FsFs -f x LPF, F s >2×f max fxfx FsFs f f H(j ) f max

Measured power as a function of CF Zero power factor t p : pulse width T: pulse period Harmonics CFT/t p

Calibration waveforms (… continued) The power for the selected waveforms is concentrated only on the fundamental The calibration system must be characterized at power frequencies 0˚0˚ 90˚45˚

Calibration system Power Generation Power Measurement System description High frequency Thermal Power Comparator (TPC) Measures the difference between the unknown ac power (ACV and ACI) and the known dc (DS1 and DS2) TPC: multi-junction thermal converters, no aliasing UUT RsRs VD TPC DCS 1 DCS 2 ACV ACI DCV DCI Dual Channel Voltage Source Voltage Amplifier TCA 10 MHz i(t) t TCA minimum Distortion, high bandwidth

Thermal Power Comparator

Traceability Thermal Power Comparator Current shunts Voltage divider Voltage measurement error (dc only) Frequency (negligible) Each of these components has been characterized and is traceable to the NMIA standards of voltage, current, resistance, frequency and electrical power Magnitude and Phase errors Power measurement

Uncertainty budget

Typical uncertainties of commercial wattmeters compared to IEC requirements

Summary A traceable measurement system for calibration of wattmeters used in standby electrical power testing has been developed The measurement system is based on a thermal power comparator, precision current shunts, inductive and resistive voltage dividers and precision amplifiers Each of these components has been evaluated in a traceable way The expanded uncertainty of the system is better than 100 W/VA for a crest factor of 3 The system has been tested for current waveforms with crest factors up to 20

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