page_title: Harmonics (electrical power)
section_title: Current harmonics
hierarchical_section_title: Harmonics (electrical power) / Current harmonics
caption_attribution_description: English: Same as CFL Positive Power.png but with current clamp reversed. Indicated power is now -29 watts, but this instrument still shows the power factor as 0.61.
context_page_description: In an electric power system, a harmonic is a voltage or current at a multiple of the fundamental frequency of the system, produced by the action of non-linear loads such as rectifiers, discharge lighting, or saturated magnetic devices. Harmonic frequencies in the power grid are a frequent cause of power quality problems. Harmonics in power systems result in increased heating in the equipment and conductors, misfiring in variable speed drives, and torque pulsations in motors.
context_section_description: In a normal alternating current power system, the current varies sinusoidally at a specific frequency, usually 50 Hz or 60 Hz hertz. 
  When a linear electrical load is connected to the system, it draws a sinusoidal current at the same frequency as the voltage (though usually not in phase with the voltage).
  Current harmonics are caused by non-linear loads. When a non-linear load, such as a rectifier is connected to the system, it draws a current that is not necessarily sinusoidal. The current waveform distortion can be quite complex, depending on the type of load and its interaction with other components of the system. Regardless of how complex the current waveform becomes, the Fourier series transform makes it possible to deconstruct the complex waveform into a series of simple sinusoids, which start at the power system fundamental frequency and occur at integer multiples of the fundamental frequency.
  Further examples of non-linear loads include common office equipment such as computers and printers, Fluorescent lighting, battery chargers and also variable-speed drives.
  In power systems, harmonics are defined as positive integer multiples of the fundamental frequency. Thus, the third harmonic is the third multiple of the fundamental frequency.
  Harmonics in power systems are generated by non-linear loads. Semiconductor devices like transistors, IGBTs, MOSFETS, diodes etc are all non-linear loads. Further examples of non-linear loads include common office equipment such as computers and printers, Fluorescent lighting, battery chargers and also variable-speed drives.  Electric motors do not normally contribute significantly to harmonic generation. Both motors and transformers will however create harmonics when they are over-fluxed or saturated. 
  Non-linear load currents create distortion in the pure sinusoidal voltage waveform supplied by the utility, and this may result in resonance. The even harmonics do not normally exist in power system due to symmetry between the positive- and negative- halves of a cycle.  Further, if the waveforms of the three phases are symmetrical, the harmonic multiples of three are suppressed by delta (Δ) connection of transformers and motors as described below.
  If we focus for example on only the third harmonic, we can see how all harmonics with a multiple of three behaves in powers systems.
  Power is supplied by a three phase system, where each phase is 120 degrees apart. This is done for two reasons: mainly because three-phase generators and motors are simpler to construct due to constant torque developed across the three phase phases; and secondly, if the three phases are balanced, they sum to zero, and the size of neutral conductors can be reduced or even omitted in some cases. Both these measures results in significant costs savings to utility companies. However, the balanced third harmonic current will not add to zero in the neutral. As seen in the figure, the 3rd harmonic will add constructively across the three phases. This leads to a current in the neutral wire at three times the fundamental frequency, which can cause problems if the system is not designed for it, (i.e. conductors sized only for normal operation.) To reduce the effect of the third order harmonics delta connections are used as attenuators, or third harmonic shorts as the current circulates in the delta the connection instead of flowing in the neutral of a wye connection.
caption: A compact fluorescent lamp is one example of an electrical load with a non-linear characteristic, due to the rectifier circuit it uses. The current waveform, blue, is highly distorted.
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