Ensuring accuracy in standby power evaluation

For more than a decade, energy consumption in low-power and standby modes has been recognized as an important issue. Standby power measurement is a challenge because most power meters on the market today do not have the accuracy to measure low power values accurately - especially at high crest factors and low power factors.

High-precision power meters and analyzers are now available with standby power consumption measurement software that complies with all the stringent conditions, testing methods and resolution of the test instruments specified in the IEC 62301 Ed.2.0 and EN50564:2011 standards.

Let’s examine the results of tests on one of these instruments to show the levels of accuracy available: in some cases at least 15 times better than the uncertainty requirements of the international standby standards.  In these tests, the Yokogawa WT310 digital power meter (see Figure 1) performance was evaluated against the requirements of the IEC62301 Ed.2.0 and EN50564:2011 standards at VSL (the National Metrology Institute of the Netherlands, providing direct traceability of measurement results to internationally accepted measurement standards) and at the Yokogawa European Standards Laboratory.

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Fig.1.     The Yokogawa WT310 power meter

The Yokogawa laboratory is presented as the only industrial (i.e. non-government or national) organization in Europe to offer traceable power calibration, to national and international standards, at frequencies up to 100 kHz at all power factors from zero to one.

IEC and EN standard uncertainty condition

The IEC and EN standard defines the maximum current ratio (MCR) as the ratio between crest factor and power factor. The significance of MCR is in the creation of test conditions where high crest factors are combined with low power factors. The standards state that, when the MCR is less than or equal to 10, for power values greater or equal to 1.0 W then the uncertainty should be less than or equal to 2% of the measured value; and for measured power values less than 1.0 W, the uncertainty should be less than or equal to 0.02 W. If the MCR is greater than 10, the uncertainty is calculated using the equation:

U_pc = 0.02 × [1 + (0.08 × {MCR - 10})]

which means that the tolerance range of the uncertainty value becomes broader as the MCR value increases.

Test setup

The tests on the Yokogawa WT310 were carried out using a calibration system based on a high-speed sampling wattmeter, which calibrates power at frequencies from DC to 100 kHz.  This system is excellent for calibrating high frequency power, distorted waveforms and harmonics.

The generator for the signals is a Yokogawa FG320 two-channel arbitrary waveform generator whose output drives wideband amplifiers (DC to 1 MHz), which in turn generate the reference voltage and current. The total harmonic distortion of the voltage signal is smaller than 0.1% at 230 V. The current amplifier is able to drive from 0 to 5 A.

The front end of the test system is the part that converts the voltage and current to the 1 V signal that the sampling system uses for the A/D input. The 230 V AC mains input is converted to a 1 V signal by using a capacitor-compensated resistance divider. To convert the current into a 1 V signal, a special wideband shunt was developed. The 1 V signal is fed to a differential amplifier to avoid common-mode effects.

The core of the sampling system is the NI-5922 high-speed high-resolution A/D converter giving a resolution of 24 bits at 500 kS/s or 16 bits at the maximum speed of 15 MS/s. There are two input channels using the same clock source.

Measured parameters such as voltage, current and power are derived using software-based calculations, allowing the power meter to be checked against the sampling system. The accuracy of the total system is many times higher than is needed for the IEC 62301 Ed.2.0 standard.

Measurements

The Yokogawa standby power software was used to read out the values measured by the Yokogawa WT310 power meter. A minimum of five minutes was set and the averaged value was taken. During the five minutes, the applied signals were stable. The voltage was a sine wave of 230 V, and the current range was set to “auto”. The WT310 measures the peak current and selects the best current range.

The results of the evaluation of the power meter with various “challenging” signals (i.e. signals with high crest factors and low power factors) are shown in Table 1.

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Table 1: WT310 Evaluation results

From these results, it is clear that the Yokogawa WT310 digital power meter not only fulfils the uncertainty requirements of the IEC 62301 Ed. 2.0 and EN50564:2011 standards, but also exceeds them by far.

Certification

The WT310 has recently been certified for standby power measurement by the VSL as well as by Yokogawa’s Standards Laboratory. The VSL certificate (see Figure 2) has certain limitations. To date, for example, it offers accredited calibration for sine-wave signals only. Therefore the VSL certificate is only relevant for specific calibration points on the sine wave. The VSL certificate also includes the comment: “This measurement is not within the formal VSL scope of accreditation for power measurements. They are, however, within the capabilities of VSL and are directly traceable to the Dutch National Standards.” 

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Fig. 2    Certificate issued by VSL (Netherlands National Metrology Institute) covering standby power measurements using the Yokogawa WT310 power meter

In order to overcome these limitations, further tests were carried out in the Yokogawa laboratory. As indicated above, in practical scenarios it is important to have measurements done not only with sine waves but also with distorted waveforms. In order to show the superiority of the standby power software and the WT310 power meter, these additional tests were carried out with on both sine-wave and distorted waveforms with high crest factors and low power factors (which are mentioned in the Yokogawa certificate). It is also important to note that the power calibration capabilities of Yokogawa are also traceable to national and international standards.

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