Total Harmonic Distortion

Total Harmonic Distortion (THD ) and Total Harmonic Distortion is an indication to quantify the size of the shares caused by nonlinear distortions of an electric or acoustic signal. This is similar to the usual harmonic distortion and the distortion.

THD is defined as the ratio of the summed service pH of all harmonics to the output of the fundamental wave P1. ( A square wave of 50 kHz, for example, contains a sinusoidal fundamental wave of 50 kHz and harmonic at the 3 -, 5 -, 7 -, 9 -fold, etc., of the fundamental frequency ( Fourier analysis → ) ).

The information may be given in% of the ratio of the two benefits, ie

Or as a ratio of the powers in dB,

Other calculations for amplitudes, voltages, currents, etc. are equivalent. For a voltage signal, for example, the ratio of the RMS voltages is the energy ratio equivalent:

In this calculation, Un is the rms voltage Vrms of the harmonics n

Alternatively, the amplitude ratios instead of performance ratios, especially in the field of audio engineering, and set in relation referred to as THD. This leads to the following included in the above definition of definition:

It is also common to specify the THD N, where N is noise ( noise) is.

Here the sum of the interference powers PINTERF = interference power of the harmonics plus noise power Ph is compared to the noise Prausch with the performance of the overall signal Ptot.

Or

The THD is also determined in electric power grids. Electrical devices with non-linear characteristic and consumers with semiconductor elements ( switching power supply, inverter, dimmer with phase control, etc.) do not cause a sinusoidal current in the power supply network. This can also be considered as emission of harmonics which, according to the network impedances distort the line voltage, which can lead to interference among consumers and increases the losses in the power grid. A low THD of the mains voltage therefore corresponds to a good power quality in the network. It is in Europe comply with the terms defined in the standard EN - 61000 noise level.

In power engineering, the THD of voltage is according to IEEE Standard 1459-2010 defines as

V = RMS voltage and V1 = rms value of the fundamental.

Also applies to the current: