Harmonics

Harmonics

Fig.: Grid distortion effects through frequency converters
Fig.: Grid distortion effects through frequency converters

The constantly rising number of non-linear loads in our power networks is causing increasing "noise on the grid". One also speaks of grid distortion effects, similar to those that arise in the environment due to water and air pollution. Generators ideally produce purely sinusoidal form current at the output terminals. This sinusoidal current form is considered the ideal alternating current form and any deviation from this is designated mains interference.

An increasing number of loads are extracting non-sinusoidal current from the grid.The FFT-Fast-Fourier-Transformation of this "noisy" current form results in a broad spectrum of harmonic frequencies - often also referred to as harmonics.

Harmonics are damaging to electrical networks, sometimes even dangerous, and connected loads are harmed by these; in a similar way to the unhealthy effect that polluted water has on the human body. This results in overloads, reduced service lives and in some cases even the early failure of electrical and electronic loads.

Fig.: Harmonics analysis (FFT)
Fig.: Harmonics analysis (FFT)

Harmonic loads are the main cause of invisible power quality problems and result in massive maintenance and investment costs for the replacement of defective devices. Grid distortion effects of an impermissible high level and the resultant poor power quality can therefore lead to problems in production processes and even to production downtimes.

Harmonics are currents or voltages whose frequency lies above the 50/60-Hz mains frequency, and which are many times this mains frequency. Current harmonics have no portion of the effective power, they only cause a thermal load on the network. Because harmonic currents flow in addition to "active" sinusoidal oscillations, they cause electrical losses within the electrical installation. This can lead to thermal overloads. Additionally, losses in the load lead to heating up or overheating, and therefore to a reduction in the service life.

The assessment of harmonic loads usually takes place at the connection or transition point to the public mains supply network of the respective energy supplier. One speaks in this case of a Point of Common Coupling (PCC). Under certain circumstances it may also be important to determine and analyse the harmonic load through individual operating equipment or equipment groups, in order to indicate internal power quality problems and possibly determine their causes.

The following parameters are used to assess harmonic loads:


Fig.: Capacitors destroyed due to harmonics
Fig.: Capacitors destroyed due to harmonics

Total Harmonic Distortion (THD)

Total Harmonic Distortion (THD) is a means of quantifying the proportion of distortion arising due to the non-linear distortion of an electrical signal. It therefore gives the ratio of the effective value of all harmonics to the effective value of the mains frequency.TheTHD value is used in low, medium and high voltage systems. Conventionally, THDi is used for the distortion of current, and THDu for the distortion of voltage.


THD for voltage

  • M = Ordinal number of harmonics
  • M = 40 (UMG 604, UMG 508, UMG 96RM)
  • M = 63 (UMG 605, UMG 511)
  • Mains frequency fund equals n = 1

THD for current

  • M = Ordinal number of harmonics
  • M = 40 (UMG 604, UMG 508, UMG 96RM)
  • M = 63 (UMG 605, UMG 511)
  • Mains frequency fund equals n = 1

Total Demand Distortion (TDD)

In North America in particular, the expression TDD is commonly used in conjunction with the issue of harmonics. It is a figure that refers to THDi, although in this case the total harmonic distortion is related to the fundamental oscillation portion of the nominal current value. The TDD therefore gives the relationship between the current harmonics (analogous to the THDi) and the effective current value under full load conditions that arises within a certain interval. Standard intervals are 15 or 30 minutes.

TDD (I)

  • TDD gives the relationship between the current harmonics (THDi) and the effective current value with a full load.
  • IL = Full load current
  • M = 40 (UMG 604, UMG 508, UMG 96RM)
  • M = 63 (UMG 605, UMG 511)

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