Harmonic Analyzer for Supply Frequencies

HARMONIC ANALYZER FOR SUPPLY FREQUENCIES B. Wehrli Camille Bauer Measuring Instruments Ltd., CH· 5610 Wohlen/Switzerland

voltage networks having non-linear load charakteristics. Three cases are distinguished, depending on the gate power taken, the pulse number of the static converter, the harmonic level already existing at the point of common coupling, and the short-circuit power. For each of these groups the maximum harmonic currents taken and/or the harmonic voltages generated at the point of common coupling are laid down. This development shows clearly that the measurement of supply harmonics is extremely important, whether for analyzing supply networks, for continuous monitoring at critical network points, or in production control when manufacturing equipment and systems embodying stati c converters.

INTRODUCTION In recent years, static converter technology has found a wide application field. The main reasons for this are the high reliability, freedom from maintenance and particularly high economy. However, there is one fundamental disadvantage: the current taken by a static converter is generally nonsinusoidal to the highest degree, which requires more reactive power in the first place and also causes harmonic voltages through the network impedance. These in turn may affect other users detrimentally, especially the operation of electrical machines, capacitors, meters, centralized telecontrol systems, computers and other el ectron ic equipment. On account of this, standards and guidelines have been laid down, specifying the maximum harmoni cs that may be generated.

EQUIPMENT SPECIFICATION There are a number of different analysis procedures for measuring network harmonics. Depending on the requirement, one or the other method will be better suited for the specific task. In particular the following questions must be answered first:

STANDARDS The unclear situation with regard to reactions on the supply system has prompted CENELEC (European Committee for Electrotechnical Standardization) to draw up a standard which has been introduced with identical wording in all the eleven European member-countries (Ref. 1). However, it applies only to electrical domestic appliances, and therefore takes the form of an approval test. Among other things it states the maximum harmonics which an appl iance may generate with a standardized supply impedance. It also includes exact regulations for the measuring instrument used. These deal with the frequency range to be measured, the maximum error admissible, the measuring bandwidth, time response and maximum overswing. Since this CENELEC standard is at present the most important and widerspread ruling on supply harmonics, it has the character of a guideline for major industrial installations and for regulations in supply contracts between power uti Iities and major consumers. Presently the IEC (International Electrotechnical Commission) is also engaged in establishing standards with wording similar to the CENELEC standard. In September 1976 in Britain there appeared an Engineering Recommendation (Ref. 2) as a supplement to CENELEC standard EN 50.006, dealing with all cases of major installations in low and medium

- Is the information on both amplitude and phase required, or on the amplitude only? - Does the entire spectrum have to be measurable simultaneously, or is a sequential measurement of the desired harmonics sufficient? - Does the instrument have to be portable or not? - Is the instrument intended more for laboratory duty or more for field work? - How much is it allowed to cost? In the present case the aim was to achieve a real operational measuring instrument with the following capabilities: - Ampl itude measurement wi thout phase information. - Suitable for prolonged duty without supervision too, especially with supply frequency fluctuations. - Accommodation of the demands of power engineering with regard to input level and isolation of the input. - Operation as simple, practical and foalproof as possible. - Easy to transport. It was also to satisfy all the requirements of the European standard EN 50.006. 981

B. Wehrli

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The measuring system adopted works on the heterodyne principle. With suitable conception, this method of analysis meets 011 the above demands

in elegant manner, guaranteeing high dependability and accuracy for reasonable outlay. The finished instrument is shown in Fig. 1.

Fig. 1: Front view of the unit

MODE OF OPERATION As already mentioned, the harmonics ore anaIyzed on the heterodyne principle. This offers the advantage of constant absolute measuring bandwidth throughout the entire frequency range. Fig. 2 shows the block diagram for the instrument. The measuring signal, whether current or vol tage, passes first into 0 matching circuit, where it is brought to 0 level suitable for further processing. From here it posses on the one hand into 0 50 Hz bandpass filter, which eliminates 011 harmonics. After subsequent rectifi cation an impressed vol tage is available, proportional to the fundamental oscillation of the input signal. On the other hand, from the matching circuit the measuring signal posses into a hole filter, which attenuates the fundamental oscillation so strongly that it has about the some ampl itude os the harmoni cs. The low-pass filter following eliminates any very high frequency components still present, lying outside the frequency range to be measured. In the mixer A, the input spectrum is transposed into a higher frequency range by means of a suitable oscillator signal, as shown in Fig. 3. The mixer is able to form the sum and the difference of two input frequencies. In this case the entire input spectrum is mixed with a fixed oscillator frequency, giving rise to an upper and lower sideband. If the oscillator frequency is altered, the two sidebands move with it. This makes it possible to employ a fixed intermediate frequency filter with constant bandwidth, and to control the selection of the input frequencies to be measured

with a suitable oscillator frequency. Filtering the intermediate frequency is performed by a crystal filter with suitable bandwidth. The signal is then amp Iified in steps (measuring range selection) and rectified. The result is an impressed voltage proportional to the amplitude of the harmonic to be measured. For absol ute measurement of the harmonics, this signal is employed directly for the indication. If a relative measurement is required, os 0 percentage of the fundamental oscillation, the signal posses as numerator into a division stage. The denominator signal is provided by the dc voltage proportional to the fundamental oscillation mentioned above. This ensures that with this mode of operation a genuine percentage measurement of the harmonics is possible related to the fundamental oscillation, even with fluctuating input levels. The measuring frequency for the analyzer is selected by adjusting a suitable oscillator frequency. This is processed in a Phase-Locked Loop so that operating is restricted to a minimum, and the measurement is independent of the exact frequency of the fundamental oscillation. The following frequency must be supplied to the mixer A:

oscillator frequency for mixer A centre frequency of intermediate frequency filter fHARM:frequency of the harmonic to be measured

Harmonic analyzer for supply frequencies

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: frequency of the fundamental oscillation of the measuring signal

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ordinal of the harmonic to be measured

This is generated in a voltage-controlled oscillator (YeO). Besides mixer A it is also led into a mixer B, where the two signals result: f f f

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output frequency of mixer B

is simulated in a crystal oscillator lF followed by a divider, and thus has the same stability as the intermediate frequency filter itself. The low-pass filter following mixer B eliminates the sum signal f , By combining equations (I) and MBI (3) the difference signal may also be written: The signal f

(5) This signal is led to a programmable divider, for which the division factor can be adjusted on the front panel. If the ordinal N is put in as division factor, the frequency fFUND appears as the output. A phase detector now compares this signal with the fundamental oscillation obtained from the measuring signal. If there is a phase difference or even a frequency difference, the phase detector controls the integral controller in the appropriate sen5e, and the frequency of the veo changes until the phase condition is satisfied. With that the control loop is closed, and the oscillator signal always has the right frequency, regardless of the frequency of the fundamental oscillation. The harmonic is selected simply by putting the ordinal number into the programmable divider. From the block diagram in Fig. 2 it will be clear that the plug-in E I and the printed circuit board P 4 are isolated from the rest of the electronics.

A INTERMEDIATE FREQUENCY FILTER INPUT SIGNAL SPECTRUM

LOWER SIDEBAND

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Fig. 3: Spectrum of mixer output

UPPER SIDEBAND

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B. Wehrli

This makes it quite possible to perform also measurements between two phases connected to the input. And so this concept fulfills all the requirements stipulated at the outset. In porticular, measuring is independent of variations in the input frequency and operation is very simple, because only the ordinal of the harmonic sought needs to be selected. To obtain maximum flexibility, the matching of the input signals is accommodated in a plug-in (Fig. 2, El). One plug-in is provided for measuring current harmonics and one for voltage harmonics. The entire PLL oscillator is also accommodated on a plug-in (E 2), so that other applications might be considered, like measuring the levels of central ized tel econtrol systems. The data sheet following gives the principal technical particulars of the analyzers.

Input for vol tage or current (optional)

Voltage module: 500/250/100/50/25/10V rms Input resistance 200 kQ Current module: 25/10/5/2,5/1 A

rms Inbuilt low-induction shunt approx. 16 mQ for all ranges

Isolation:

2 kV

Frequency range:

Fundamental oscillation {ordinal number 1),50 Hz, optionally 60 Hz, harmonics of ordinal number 2 - 49 directly selectable by thumbwheel switch

Measuring range:

rms

Selectable between 100% and 0,1 % (FSD of instrument)

Attenuation against neighbouring harmonics: ~ 60 dB Accuracy:

Inbuilt instrument

Calibration:

Relative, in percent of the momentary fundamental osci lIation or absolute value, in percent of the selected input voltage or current by selector switch

Output:

One connection on front and one on bock for fundamental osci Ilation and selected harmonic 0 - 10 Vdc impressed voltage, external burden ~ 2 kQ

Power supply: 110/220 V, + 15 %, 50/60 Hz, approx. 25 VA Dimensions:

Approx. 437 x 133 x 361 mm

Weight:

Approx. 10 kg

APPLICATIONS

TECHNICAL DATA Input:

Indication:

+ 5 % referred to momentarily measured value (absolute value)

All users of harmonic analyzers, such as power utilities, major consumers, manufacturers of static converter equipment etc., hove their specific measuring problems. Four typi cal appl ications are given below as examples. 1. Direct measurement of the instantaneous value of the harmon ics in the network, whether tracing faulty equipment, at the point of common coupling between generating station and major consumer, testing new systems in the laboratory, at final inspection in production or acceptance tests of entire installations. The very straightforward and foolproof operation is a big advantage here, especially the direct selection of the harmonic ordinal instead of the frequency. 2. Continuous measuring for monitoring and fault annunciation, on stati c converter systems for example. In some cases it is no straightforward matter to establ ish whether all components in a system are functi on ing proper! y. Often the malfunction reveals itself only by increased harmoni cs on the secondary side. 3. Use of the harmonic analyzer in conjunction with a recorder for plotting the contents over days and weeks. The instrument needs no attention or maintenance, because it is independent

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Fig. 4: Plot of the 3rd harmonic in a 220 V network

985

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Iy from the power utilities, because they naturally want to be able to offer all their consumers a supply satisfying certain minimum qual ity standards. Generally it is they too who have to cover the increased reactive power demand and the higher transmission losses (Ref. 3). Power electronics are now in a position to offer circuitries with low network reaction, whether by increasing the pulse number, by employng choppers or by similar circuitry artifices. In any case, existing and additionally generated harmonics will have to be taken into account in the future, both in the development of equipment and in the control and supervision of supply networks. Here the measurement and recording of supply harmonics assumes crucial importance.

of the momentary supply frequency. Fig. 4 shows a plot of the 3rd harmonic in a 220 V network as an example. Very clear is the rise in the harmonic content in the morning, and the drop over midday. 4. Use of the analyzer with a high-speed recorder. This will often pinpoint individual faulty equipment, because the switch-on time can be compared with the plot. Fig. 5 shows a recording of a brief disturbance, in this case a photocopier which set up a considerable amount of harmonic voltages every time a copy went through. The copying machine was about 30 metres from the measuring location, and the analyzer was not even connected to the same ma ins feeders. CONCLUSION The demand that something should be done about the problem of network harmonics, including the laying down of maximum values, originates certain-

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References: (1) The Limitation of disturbonces in electricity supply networks caused by domestic and similar appliances equipped with electronic devices. European Standard EN 50.006 CENELEC (2) Limits for harmonics in the United Kingdom Electricity Supply System Engeneering Recommendation G. 5/3 System Design and Development Committee (3) Kl::lntje, C Die wirtschaftl iche Auswirkung phasen'anschnittgesteuerter Verbraucher in Niederspannungsnetzen Dissertation (75) RWTH Aachen, Deutschland