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What is Total Harmonic Distortion(THD)

Aug 23, 2019

What is Total Harmonic Distortion(THD)?


Total Harmonic Distortion (THD) is a function-frequency relationship that helps quantify the extent to which the system outputs a copy input. . It is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. This will relate to the power supplies only and they are the only component that generates any kind of frequency. The lower the THD value, the less noise or distortion in the system output.


For each test frequency, the value of THD is between 0 and 1:

ZERO - A value close to zero means the output has low harmonic distortion. The output sine wave has a frequency component similar to the input.

ONE - A value close to 1 means there is a lot of harmonic distortion in the signal. Almost all frequency content in the signal is different from the frequency of the input signal.

THD can also be expressed as a percentage, from 0 to 100%, where 100% corresponds to 1.


In many applications, low THD is required. Low THD means that the system output is similar to the system input with minimal distortion.


Why is it so important?


First, as a definition, harmonics are voltages or currents whose frequency is a multiple of the fundamental frequency, and Australia is 50 Hz: 100, 150, 200 Hz, etc. Total Harmonic Distortion (THD) is the sum of all harmonic components for the fundamental frequency present in nonlinear electrical and electronic equipment.


LED drivers are electronic power sources in LED luminaires that contain inductive devices (reactance and capacitive components). They are non-linear devices because they modify the waveform of the current drawn from the supplied voltage signal and appear to be less sinusoidal.


Most LED drivers also include a diode bridge for rectifying the AC input signal to operate the DC LED module. The switching operation of these diode bridges produces a discontinuous current that eventually distort the sine wave.


Therefore, when the LED driver is connected to the main power system, it generates harmonic currents that distort the supply voltage. And the more luminaires (with non-linear LED drivers) in the circuit, the greater the interference to the power distribution system, making it inefficient, affecting the performance of other devices and overheating the wiring.


This is basically why the electrical specifications of the lighting equipment in new installations usually require that the maximum THD of the luminaire be less than 15%.