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Mains Harmonics REO (UK) LTD, Units 2 - 4 Callow Hill Road, Craven Arms Business Park, Craven Arms, Shropshire SY7 8NT UK Tel: 01588 673411 Fax: 01588 672718 REO UK LTD Email: sales@ reo.co.uk W ebsite: www.reo.co.uk Contents W hat are mains harmonics? 1 2 Mains harmonics are voltages and/or Harmonics with numbers that are divisible W hat are mains harmonics? currents that occur in an AC mains by three (3rd, 6th, 9th, 12th, 15th, etc.) are 2 electricity power supply at multiples of the called zero sequence harmonics, because nominal mains frequency. 'Even-order' the fields they cause in a three-phase AC harmonic frequencies are those that occur motor are stationary 2 they do not rotate. How do mains harmonics occur? 3 at even-numbered multiples of the Odd-numbered 'zero-sequence' harmonics nominal mains frequency, whereas 'odd- (3rd, 9th, 15th, etc.) are called triplens. order' harmonics occur at odd-numbered W hy are harmonics an increasingly important issue? 7 multiples, as shown in Table 1. Table 1 Some examples of harmonics for four common AC power supply frequencies How mains harmonics cause harm 9 Harmonic Number Even Odd 16.667Hz 50Hz 60Hz 400Hz Order Order Relevant standards and codes on mains harmonics 28 1 (the fundamental 16.667Hz 50Hz 60Hz 400Hz mains frequency) Likely sources of harmonic interference 33 2 33.333 100 120 800 3 50 150 180 1.2kHz The influence of the mains distribution systems impedance 35 4 66.667 200 240 1.6kHz 5 83.333 250 300 2kHz 6 100 300 360 2.4kHz How can harmonics be detected and measured 37 7 116.667 350 420 2.8kHz 8 133.333 400 480 3.2kHz Testing for immunity to harmonically distorted mains supplies 42 9 150 450 540 3.6kHz 10 166.667 500 600 4kHz Prevention and avoidance measures 42 11 183.333 550 660 4.4kHz 12 200 600 720 4.8kHz 13 216.667 650 780 5.2kHz Harmonic mitigation products from REO 61 14 233.333 700 840 5.6kHz 15 250 750 900 6kHz References and further reading 63 ...etc... ...etc... ...etc... ...etc... ...etc... 100 1.6667kHz 5kHz 6kHz 40kHz 101 1.6833kHz 5.05kHz 6.06kHz 40.4kHz ...etc... ...etc... ...etc... ...etc... ...etc... How do mains harmonics occur? 3 4 W hen supplied with pure sinewave mains There are many kinds of non-linear loads Figure 1 A single-phase rectifier-capacitor AC-DC converter voltages at the nominal frequency, linear on the mains electricity supply, but the one loads consume pure sinewave currents at that is becoming the most significant is the only the nominal frequency (also known as rectifier-capacitor input AC-DC power the fundamental), and at no other converter, shown in Figure 1. frequencies. Linear loads consist of resistance and/or inductance and/or The input circuit of Figure 1 is very widely capacitance, and contain no used in mains AC-DC or AC-DC-AC ('AC- semiconductors, thermionic valves or gas- AC') power converters whether they discharge devices. employ 'linear' or switch-mode technologies. As Figure 2 shows, the However, non-linear loads also exist, and capacitor that follows the rectifier in such when they are supplied with pure sinewave circuits only 'tops up' its stored charge mains voltages they consume non-linear near the peaks of the AC mains supply currents. In addition to consuming current voltage waveform. As a result, its at the fundamental mains frequency, non- consumption of mains current is linear loads consume currents at its discontinuous, non-sinewave, and rich in harmonics. harmonic currents, as shown by Figure 3. Because electricity generators only Another common non-linear load is the produce AC at the fundamental frequency, phase-angle power controller, used in and cannot supply current at other lighting dimmers and a variety of other frequencies, harmonic currents spread residential, commercial and industrial throughout mains distribution networks, power control applications. Figure 4 shows Figure 2 Non-linear currents in a rectifier-capacitor type ac-dc converter creating a number of problems as they do an example of the current waveform in so (see later). such loads, and its spectrum. The actual waveform and spectrum varies depending (Although the non-linear loads consume on the phase angle that the controller is harmonic currents, these currents are set to, and the nature of the load. usually described in harmonic test standards as being 'emissions' of harmonic Figures 3 and 4 represent very simple and currents from the loads to their mains 2 to some extent idealised 2 examples distribution networks.) of harmonic current consumption ('emissions'). In real life, some types of Because of the continuing growth in non- equipment have very complex mains linear loading on mains distribution current spectra, such as the large motor networks, it is now necessary for all drive shown in Figure 5, which is taken equipment designers and electrical from Part 4 of [1]. The motor drive was a installation and distribution network variable frequency inverter driving a large designers to understand the issues three-phase AC motor, and the motor was associated with mains harmonics; and being driven at 39.4Hz. design accordingly. This guide is intended to provide the necessary background. 5 6 Figure 3 Rectifier-capacitor input AC-DC Figure 5 Example of real harmonic and interharmonic converter current waveforms and spectra currents produced by a large inverter-fed motor Figure 5 shows the drive's consumption of Harmonic currents can never be in-phase with their mains voltage's sine waveform Figure 4 Phase-angle power control current waveforms and spectra mains current at the fundamental frequency and its harmonics, plus its at its fundamental frequency 2 so can consumption at 39.4Hz and its harmonics, have no relationship with an equipment's plus currents at other frequencies that real power consumption in W atts 2 arise due to intermodulation between but they do affect its consumption of these two sets of frequencies. Currents 'W attless Power', or VAR (reactive VA). that are not related to the mains supply The ratio of an equipment's consumption frequency are known as 'interharmonics', of W atts to its overall VA is known as its and in Figure 5 the frequencies related to Power Factor (PF), so where an 39.4Hz and all of the intermodulation equipment consumes more harmonic frequencies are interharmonics. current, it tends to have a lower PF. Only the current that is in-phase with the A PF of 1.0 means that the VA equals the sine voltage waveform of the fundamental W atts consumed by an equipment, in mains frequency can produce real power which case it appears to the supply as a (measured in W atts), real energy pure resistive, linear load. Rectifier- (measured in Joules or W att-Hours) and capacitor input electronic equipment with do real work. This means that it is only an no harmonic reduction techniques tend to equipment's consumption of current at the have PFs of around 0.6. Magnetically- fundamental frequency of its mains supply ballasted fluorescent lamps (running at 50 that is associated with its real power or 60Hz) can have a PF as low as 0.3. consumption in W atts. Electronic techniques that reduce an W hy are harmonics an increasingly important issue? 7 8 equipment's consumption of mains In decades past the maJority of the load on Another significant proportion of the total Overheating supply networks can trip-out, harmonic currents also improve its PF, so mains power distribution networks was load on mains distribution networks is causing power cuts at great cost to they are often called Power Factor linear, but the proportion of non-linear lighting equipment. Decades ago, all business, industry and the community as a Correction (PFC) techniques. loads has been increasing as electronic lighting was supplied by filament lamps whole, so harmonics must be controlled to power control and conversion has become (linear) and fluorescent or discharge help maintain security of supply. W here It is important not to confuse the more common. For most mains distribution lamps (both non-linear). Energy-saving supply networks are running close to their comprehensive definition of Power Factor networks, non-linear loads now represent initiatives are encouraging the maximum thermal ratings, the choice is (= W atts/VA) with the 'power factor' term a significant proportion of their total load. replacement of filament lamps by 'low- between spending money to upgrade them that is traditionally used by electricity The Climate Change Levy (in the UK) and energy compact fluorescents' (non-linear). to cope with the harmonics 2 and supply engineers 2 simply the cosine of similar energy-saving initiatives by other W here low-voltage halogen lamps (linear) spending money to reduce the harmonics the angle between the supply voltage national governments are increasing the are used for reasons of light quality, they themselves. Usually both approaches are sinewave and the load current, traditionally commercial incentives to save energy (to are increasingly powered from 'electronic needed. adJusted to be closer to unity by adding help 'save the planet') by replacing many transformers' (non-linear, actually For more information on the importance of either shunt capacitance or inductance to a 'traditional' linear loads with non-linear electronic AC-DC or AC-AC converters) mains harmonics, and what should be power line. The traditional 'cosf' PF only electronic ones, because of their higher instead of ordinary wound transformers done about them, there is a lot of useful applies where the voltages and currents energy efficiencies. (linear), to save space and weight.