Those Pesky In-Between Harmonics
They can overheat your system and make your fluorescent lights flicker, and they're often caused by the instruments you need to improve power quality. Interharmonics are those odd frequencies that lurk between the fundamental and its typical multiples, and they'll make you yearn for the simpler problems of run-of-the-mill harmonics. Between stops on EC&M's Code Change Conference tour, Editorial Director John DeDad took some time to address the topic.
Q. In the past couple of years, I've had the “joy” of dealing with interharmonics on occasion. As a result, I've gained a meager understanding of what they are, what causes them, and how they affect power quality, but I could still use a little clarification. Can you help me better understand this issue?
DeDad's answer: Basically, interharmonics are harmonic voltages or currents that don't have frequency components that are integer multiples of the fundamental (50 Hz or 60 Hz). Power line carrier signals can also be considered interharmonics.
Suppose we took two or more pure sine waves with different amplitudes and frequencies that weren't integer multiples of the fundamental frequency and added them together (Fourier analysis). The result wouldn't necessarily be a periodic waveform, as you would expect from waveforms with frequencies that are integer multiples of the fundamental.
The origins and effects of interharmonics were relatively unknown just 10 years or so ago. Today we have a much better understanding of this phenomenon. You can generally say that interharmonics stem from frequency conversion. According to the book Electrical Power Systems Quality, Second Edition, published by McGraw-Hill, the primary source of interharmonics is the widespread use of electronic power converter loads that can produce current distortion over a whole range of frequencies. Adjustable speed drives (ASDs) in industrial applications, PWM inverters in UPS applications, active filters, and custom power conditioning equipment are some of the sources of interharmonics. Others include static frequency converters, induction furnaces, and most arcing devices.
Let's look at ASDs first. As shown in Fig. 1, the front end of an ASD is typically a diode rectifier, which converts an incoming AC voltage to a DC voltage. An inverter then converts the DC voltage to variable AC voltage with variable frequency. When the inverter uses an asynchronous switching scheme, it can produce harmonics in current. (According to the Wiley Publishers book Power Electronics: Converters, Applications, and Design, asynchronous switching occurs when the ratio of the switching frequency of the power electronic switches is an integer multiple of the fundamental frequency of the inverter voltage output.) If the harmonic current passes through the DC link and proceeds to propagate into the supply system, it's possible you'll have interharmonic problems.
Induction furnaces are sources of interharmonics because of their rapidly changing load current characteristics. This rapid fluctuation causes sideband frequencies to appear around the fundamental or harmonic frequencies. These furnaces use electronic power converters to supply a variable frequency to the furnace induction coil (Fig. 2). The frequency at the melting coil varies to match the type of material being melted and the amount of material in the furnace.
Like harmonics, interharmonics can create resonances in power distribution systems, but certainly more severe. This happens when the varying interharmonic frequency becomes coincident with the natural frequencies of the power system.
Because interharmonics can be any values between harmonic frequencies, you must use an interharmonic spectrum with sufficient frequency resolution. Because it only provides a frequency resolution of 50 Hz or 60 Hz, a single-cycle waveform sample isn't enough to compute an interharmonic spectrum. Any frequency that's between harmonic frequencies will be lost. You can use a one-cycle waveform because no frequency exists between harmonic frequencies. However, your best bet is to use a 12- or 10-cycle waveform to get higher frequency resolution, with the resulting resolution of 5 Hz.
Typical symptoms of interharmonics include filter overloading, overheating, power line carrier interference, voltage fluctuation, and visual flicker in fluorescent lighting, other arc lighting, and computer display devices.
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