Powering up to new European standards

01 May 2007

The rapid development of high-efficiency power semiconductors has increased their usage, but also caused problems for the electricity supply system. To address these, two new European standards have been introduced

The generation of harmonic currents and the voltage distortions that sometimes result, can cause complications for all those who are plugged in to the grid. In order to address these concerns, the European standards covering electromagnetic compatibility now include EN61000-3-2, which specifies limits on harmonic current, and EN61000-3-3, which specifies limits on voltage flicker.

Harmonic limits
EN61000-3-2 defines the limits of the harmonic current of all electrical and electronic equipment that has an input current of up to 16A per phase and is suitable for connection to the low-voltage AC public mains distribution network. For the purpose of specifying harmonic current limits, equipment is grouped into four classifications. Class A includes balanced 3-phase equipment, household appliances (excluding equipment identified as Class D), tools (excluding portable tools), dimmers for incandescent lamps, audio equipment, and any other equipment not otherwise classified. Class B consists of portable tools. Class C includes lighting equipment. Finally, Class D incorporates PCs, PC monitors and TV receivers under 600W, and equipment that has an input current with a ‘special waveform shape’ defined in an earlier regulation.

This system of classification reflects, the number of items of equipment in use, duration and simultaneity of use, power consumption, and harmonic spectrum (including phase). For example, a single satellite set-top box with 10W consumption is unlikely to cause any problems in normal use, but if a million sets are switched on at the same moment to watch a particular programme the resultant 10mW can cause a significant effect, even to the extent of it being measurable on the national grid. The actual harmonic current limits are different for each class of equipment and their calculation involves operations such as discrete Fourier transforms. Their evaluation also depends on the type of equipment behaviour: quasi-stationary, short cyclic (less than 2.5 minutes), long cyclic (more than 2.5 minutes) or random.

Measurement requirements In order to ensure that their products meet the relevant requirements, equipment manufacturers need to use the right type of test equipment to carry out the necessary evaluation: specifically a power meter, which these days means a digital power meter. The requirements of test equipment for harmonics measurement are specified in the EN61000-4-7 standard, and cover parameters such as measurement error, input impedance, time constant, measurement period and interharmonics. There are also requirements on the parameters used in the discrete Fourier transform operation.
These requirements are addressed by the latest generation of precision power analysers, which incorporate powerful computation functions and which can be used in conjunction with harmonic measurement software to carry out compliance testing to the IEC 61000-3-2 standard. Instruments can offer a basic power accuracy of ±0.02% of reading, DC and 0.1Hz to 1MHz measurement bandwidth, and up to four input elements. An advanced calculation function provides wide bandwidth harmonic measurement, IEC compliant harmonic measurement (with the appropriate software), FFT calculation, waveform calculation functions, and the saving of waveform sampling data.

Harmonic measurements Using the instrument’s wide bandwidth harmonic measurement mode, harmonic measurements can be carried out over a frequency range from 0.1Hz to 2.6kHz. This includes measurements up to the 50th- order harmonic at a fundamental frequency of 1kHz. More significantly, users can perform tests conforming to the latest versions of the IEC standards. In particular, the IEC harmonic measurement mode meets the window width requirement of the latest IEC harmonic standard (10 cycles of 50Hz and 12 cycles of 60Hz). This mode can also be used with harmonic measurement software to perform tests conforming to IEC 61000-3-2 rev. 2.2. The instrument displays a harmonic current measurement value list and bar graph, enabling ‘pass/fail’ evaluations of harmonic measurement results in line with the standard A, B, C and D class divisions. The harmonic measurement results can be printed as value lists and graphs, and image data can be saved for use in automatically generated reports. An FFT (Fast Fourier Transform) calculation mode provides more detailed frequency analysis than is available with the harmonic measurement mode.

Two FFT calculations can be performed simultaneously on waveform data of measured voltage and current, and the user can select an FFT resolution of 1Hz or 10Hz. FFT analyses at up to 100kHz can be performed. Monitoring of instantaneous power waveforms is facilitated by the waveform calculation mode. Up to two waveform calculations and other functions can be used simultaneously. It is possible to create a formula that multiplies voltage and current waveforms, allowing the user to confirm an instantaneous power waveform on screen. Waveform calculation data can be saved in CSV or WVF format. All captured voltage and current waveform data, calculated waveforms and FFT calculated waveforms can be saved. Again, it is possible to save in CSV or WVF format, and to save to PC card or USB memory.

Flicker limits
EN61000-3-3 defines the limits of voltage fluctuation and flicker of all electrical and electronic equipment rated at up to and including 16A per phase. Flicker is the unstable impression felt by the human eye due to the stimulation of the light caused by changes in brightness and/or spectral distribution, and normally results from voltage drops caused by large current flows. Even a small voltage change can cause a noticeable amount of flicker because the brightness of a lamp or LED is proportional to the square of the voltage. In order to evaluate flicker, it is necessary to look at a number of parameters, including the relative steady-state voltage change, the maximum relative voltage change and the relative voltage change. These are then correlated with the short-term flicker indicator (Pst) and the long-term flicker indicator (Plt): parameters that are measured with a flicker meter and based on statistical evaluation of the measurements over specific observation periods.

The EN61000-3-3 standard specifies that the relative steady-state voltage change should not exceed 3.3%, the maximum relative voltage change should not exceed 4% (with higher limits for certain product categories), and the relative voltage change should not exceed 3.3% for more than 500msec. Pst has to be no greater than 1.0, and Plt no greater than 0.65. The precision power analyser can be used to carry out voltage fluctuation and flicker tests for single-phase and three-phase instruments and to determine whether the results fall within limit values. When fitted with the voltage fluctuation/flicker option, the instrument shows the limits, distribution curve, maximum deviation, perceptibility (Pst) and other values, and determines pass or fail criteria for each observation period. A further option for dealing with fluctuating voltages is a cycle-by-cycle measurement function which is effective in system interconnection tests, motor evaluation and other applications where there is a requirement to capture rapidly changing phenomena. The cycle-by-cycle measurement function takes measurements of parameters such as voltage, current and active power for each cycle, and then lists the data on screen in a time series.

SANDER VAN DASSELAAR is product marketing manager, power meters & hand--held instruments, Yokogawa Europe

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