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Abstract— IEEE Standard 519 is presently under revision

and is expected to recommend that harmonic and interharmonic measurements be performed according to the method defined in IEC 61000-4-7 [1]. This summary paper and the associated presentation describe important aspects of the 61000-4-7 standard in the context of its intended use in the draft IEEE 519 standards.

Index Terms — Power quality, harmonics, standards.

I. INTRODUCTION

ower quality standards are developed in IEC by Study Committee SC77A. Working Group 1 (Harmonics and

other low-frequency disturbance) is responsible for harmonic and interharmonic emission standards that apply to individual pieces of equipment connected to the supply network. This same group is also responsible for defining the method of measurement which has been standardized in IEC 61000-4-7. In the IEEE, power quality standards are generally developed in working groups and task forces under the Power Quality Subcommittee of the T&D Committee and are coordinated across IEEE societies by the IEEE SCC-22 Power Quality Standards Coordinating Committee. The IEEE harmonic control standard – IEEE 519 – is a jointly sponsored standard between the T&D Committee in the Power Engineering Society and the Industrial Power Converter Committee of the Industry Application Society. IEEE 519 is currently being revised and the current draft recommends that harmonic measurements be performed according to the methods defined in 61000-4-7. This summary paper outlines some important aspects of 61000-4-7 and areas that are the focus in the 519 revision.

II. SCOPE OF IEC 61000-4-7

61000-4-7 is “applicable to instrumentation intended for measuring spectral components in the frequency range up to 9 kHz which are superimposed on the fundamental of the power supply systems at 50 Hz and 60 Hz. For practical considerations, this standard distinguishes between harmonics, interharmonics and other components above the harmonic frequency range, up to 9 kHz.”

E. Gunther (erich@enernex.com) is with EnerNex Corporation in Knoxville,

TN.

The standard defines the instrumentation to be used for emissions testing for individual pieces of equipment as well as for the overall measurement of harmonic/interharmonic voltages and currents in supply systems.

III. DEFINITIONS AND INDICES

The standard defines several fundamental relationships and indices (e.g. total harmonic distortion) that are all derived from the Fourier series using the following specific notations:

Note that to avoid confusion when reading the 61000-4-7

standard, equations in this summary paper will utilize the numbering defined in 61000-4-7.

IV. BASIC INSTRUMENT DESIGN

There are four main components of an instrument that complies with 61000-4-7:

1. Input circuits with anti-aliasing filter 2. A/D converter with sample-and-hold 3. Main synchronization (e.g. phase locked loop) 4. DFT-processor providing the Fourier coefficients

Harmonic and Interharmonic Measurement According to IEEE 519 and IEC 61000-4-7

E. Gunther, Senior Member, IEEE

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One of the most important aspects of the standard is the

definition of the time windows to be used to sample the waveform prior to analyzing using the Fourier transform. In

order to facilitate an instrument design that operates in

basically the same way for 50 Hz and 60 Hz systems, the window width is defined as 10 cycles for 50 Hz systems and 12 cycles for 60 Hz systems – basically a 200 ms window.

This results in a frequency resolution at the output of the Fourier Transform of 5 Hz for both 50 Hz and 60 Hz systems.

The general structure of the instrument is depicted in

Figure 1. This reference design provides for several outputs that are defined in the standard. Output 1 is basically the raw

output of the Fourier transform (the 5 Hz resolution frequency

components at 200 ms intervals). In order to simplify the analysis and facilitate repeatability of results, other outputs

are defined that group spectra into blocks and then apply a smoothing function.

Figure 1 – General Instrument Structure

For harmonic analysis, the raw data from Output 1 is

grouped to be the sum of the squared spectral lines (the 5 Hz

bins) between adjacent harmonics centered on a specified harmonic according to equation (8) in the standard which is

repeated here.

For interharmonic analysis, the raw data is grouped in a

similar fashion – this time just the spectral components in

between each harmonic line. Figure 2 illustrates this grouping

method.

Figure 2 – Illustration of harmonic and interharmonic

groups. In order to facilitate the analysis when the mains signal is

fluctuating, an alternative grouping method is defined by equation (9) and depicted in Figure 3.

Figure 3 – Illustration of a harmonic subgroup and an

interharmonic centered subgroup. A smoothing function is applied after the grouping

algorithms to provide a stable output which facilitates getting comparable measurements between two instruments connected to the same mains signal. This smoothing function is a simple 1 pole low pass filter with the general form illustrated in Figure 4. The coefficients for the filter are dependent on the power system frequency and are defined in the standard.

Figure 4 – Digital, low-pass smoothing filter In addition to the basic calculation and analysis

methodology, IEC 61000-4-7 also defined the desired accuracy of the instrument. Two accuracy classes are defined – Class I for cases involving contract compliance, resolving disputes and other cases where high precision is required; Class II for general measurement purposes. An important principal for Class I instruments is that any two compliant

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instruments when connected to the same signals produce matching results within the specified accuracy shown in Table 1.

Table 1 – Accuracy requirements for current, voltage and

power measurements

V. IMPLEMENTATION ISSUES

The omission of the required anti-aliasing filter defined in the standard is one of the most often observed reasons why harmonic / interharmonic measurements (especially current measurements near power electronic devices) fail to compare with a known good reference instrument. The mains synchronization mechanism is another common source of measurement error when not properly implemented (or omitted).

VI. USAGE IN IEEE 519

The current draft of the IEEE 519 revision specifies the use of the 61000-4-7 measurement methodology. 519 augments the methodology by specifying how the individual 200 ms measurement blocks are to be aggregated to provide data for “very short time” (Tvs) and “short time” (Tsh) harmonic measurements. The Tvs interval data is obtained by calculating the square root of the sum of the squares of the 200 ms samples over a 3 second interval. The Tsh interval data is obtained by calculating the square root of the sum of the squares of 200 consecutive Tvs values.

The IEEE 519 draft also specifies how the data obtained

using the 61000-4-7 method and aggregated as described above is to be evaluated statistically. The Tvs and Tsh values should be accumulated over periods of one day and one week, respectively. For Tvs interval measurements, the 99th

percentile value (i.e., that value which is exceeded for 1% of the measurement period) should be calculated for each 24 hour period for comparison with the recommend limits in the revision. For the Tsh interval measurements, the 95th and 99th percentile values (i.e., those value which are exceeded for 5% and 1% of the measurement period) are calculated for each 7 day period for comparison with the recommended limits. These statistics should be used for both voltage and current harmonics and interharmonics with the exception that the 99th percentile Tsh value is not recommended for use with voltage harmonics.

VII. REFERENCES [1] IEC 61000 4-7, Electromagnetic compatibility (EMC) – Part 4-7: Testing

and measurement techniques – General guide on harmonics and interharmonics measurements and instrumentation, for power supply systems and equipment connected thereto.

[2] IEEE Std. 519-2000 Draft 0 - Recommend Practices and Requirements for Harmonic Control in Electric Power Systems.