Power Test System for Standby Power

Y.T. Park, and W.M.S.Wijesinghe

Korea Research Institute of Standard and Science (KRISS)

pyt@kriss.re.kr

Abstract – In this paper, a standby power test system for the measuring low level power with non-sinusoidal signals, which utilizes two synchronized high precision digital voltmeters, is described. The signals of normal voltage and low level current waveforms were computed from digital samples and compared with the values obtained from thermal converters. An electronically aided low-current multistage current transformer for precise measurements of standby current was used. The system can be used as a reference system for calibrating standby power analyzers at power frequencies.

Index Terms — Standby power, synchronized high precision digital voltmeters.

I. INTRODUCTION

Standby power is electricity used by household electrical appliances and equipment while they are switched off or not performing their primary function. That power use is consumed by power supplies and the power is caused by circuits that continue to be energized even when the device is off. The standby power including harmonic component is small amounts compare with normal operating electricity. Several types of power analyzers designed for the accurate measurement of power are available commercially. These power analyzers are used by industry for standby power measurements. But because of limit of low current measuring ranges, standby power measurement is difficult by general power meter, or power analyzers. Standard IEC62301 suggests general conditions for measurement of standby power [1]. This standard requires to measure below 0.1 watt at normal voltage. Necessity for the accurate standby power measurement of household electrical appliances has grown, and the demands on NMI to provide calibration and traceability for such test instruments have increased. The power measurement system using sampling technique introduced in this paper is to be used to measure standby power of household electrical appliances and equipment and to calibrate power meters. Sampling techniques can be applied when calculating the electrical parameters of voltage and current sources. However, the use of sampling techniques for distorted signals is somewhat different than for sinusoidal signals.

Increasing the sampling interval rate tends to lower the aperture time, which is an important parameter of the sampling voltmeter, and affects the gain error for the measurements. Therefore, gain correction should include harmonic computation errors. The scientific literature provides some examples of harmonic [2] and non-sinusoidal power measurements [3] using sampling techniques. In determining

the ratio errors for standby power signals, it is also important to evaluate the precision inductive voltage divider (HIVD) and wideband low current transformer (WLCT). Usually, the frequency characteristics of a HIVD or WLCT are evaluated by applying a sinusoidal signal at various frequencies. However, their characteristics for periodic 60- or 50-Hz signals with harmonic components should be evaluated, particularly for HVDs. These characteristics have been evaluated using thermal converters (TCs), and the algorithm suggests how to compensate for gain errors in the data acquisition system. In this paper, the main approach is the plug-and-play reference standard solution for the measurement of harmonics and standby power in calibrating power analyzers. Therefore, a highly accurate digital sampling system with a sophisticated software algorithm that compensates for all system correction factors has been assembled. This system analyzes the fundamental and up to the 50th harmonic of non-sinusoidal waveforms and low power, such as standby power.

II. Measurement System The basic system circuit is shown in Fig. 1. The source LPS

for the system generates the voltage and low current waveforms at the same frequency. The waveforms are variable in amplitude and with phase differences that can be varied from 0 to ±π rad. The source can be programmed for up to the 99th harmonic, with phase variations between harmonics of 0 and ±π rad. The data acquisition system comprises two identical high precision digital voltmeters (SDVMs) used as integrated analog-to-digital converters. One of the two SDVMs is supplied via a PIVD, which is a precision inductive voltage divider, to acquire the voltage signal. The second SDVM, connected via a WLCT, is used for the acquisition of the low current signal. For the WLCT, an amplifier-aided wideband current transformer to measure high frequency very low current, together with an ac/dc current shunt or shunt resistor suited to the current range, can be used. The precision inductive voltage divider (PIVD) is designed for use in precision measurement of electrical power and other metrological applications requiring accurate scaling of voltages up to 1000 V rms to voltages in the range where the most precise measurements may be carried out. At power frequencies the ratio errors are of the order of 1×10-7 (in-phase) and 1×10-6 (quadrature), and the uncertainty of their determination is an order of magnitude less. An electronically aided two-stage current transformer WLCT which greatly increases the apparent permeability of the magnetic core of a current transformer, without a galvanic connection to either of

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the usual primary or secondary circuits was used. The two SDVMs were used at the same time only for

standby power measurement. Otherwise, only one SDVM is operated. The clock generated by the source for the data acquisition system was controlled by the program and was coupled to the external trigger input of both SDVMs. The sample and phase references were provided by the source and could be controlled by general-purpose interface bus (GPIB) commands. The error calculator EC was specially designed in our laboratory to calibrate standby power energy meter with the reference standard Wh function in this system and to compare two energy quantities. The devolved software converts the power values into frequency for the assigned meter constant and the pulse directed to a high-precision pulse generator PG to produce the pulse stream.

Fig.1. the basic system

III. Test method

The sample reference appears only when the positive zero

crossing of the phase reference signal in the GPIB is ‘‘ON’’ and triggers both DVMs to start sampling the waveform data, phase locked to the master clock of the source. With this synchronization technique, no time delay occurs differently from a master---slave synchronization method. However, a small time delay is observed and corrected via the program using a data reconstructing procedure. The time delay may not be exactly zero since a small jitter between the two SDVMs does occur, which can be only minimized via the program with a known level of uncertainty. The uncertainty of phase difference of the fundamental measured by the two DVMs is 3 μ rad. Each channel takes 256 samples per cycle,

synchronized to the source signals, and yields 2560 samples in ten cycles, which are then averaged to obtain accurate results.

IV. CONCLUSION

The efficient low power measurement system has been

developed to evaluate the performance of low current standby power measurement equipment and to test standby power of household electrical appliances. This system is based on a digital sampling algorithm that uses two high resolution integrating voltmeters. The developed system is fully automated and allows standby power, energy measurements and calibration of its measurement instruments.

REFERENCES [1] IEC 62301 Household electrical appliances-measurement of

standby power., CEIIEC 62301:2005. [2] S. Svensson, ‘‘Verification of a calibration system for power

quality instruments,’’ IEEE Trans. Instrum. Meas., vol. 47, no. 5, pp. 1391---1394, Oct. 1998.

[3] R. Arseneau, P. S. Filippsky, and J. J. Zelle, ‘‘An improved three phase digital recorder system for calibrating power instrumentation,’’ IEEE Trans. Instrum. Meas., vol. 46, no. 2, pp. 399---402, Apr. 1997.

LPS

MUT

PIVDWLCT

SDVM SDVM

PCGPIB

A V

EC PG

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