Harmonics and intermodulation distortions

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Information about Harmonics and intermodulation distortions

Published on March 3, 2014

Author: anjanikyadav19941




WHAT IS DISTORTION IN ELECTRICAL SYSTEMS? Alteration of the original shape (or other characteristic) of sound or waveform.


WHAT CAUSES HARMONICS ? NON-LINEAR LOAD ::: CURRENT IS NOT PROPOTIONAL TO VOLTAGE e.g . DIODE , TRANSISTOR , NON-LINEAR AMPLIFIERS ETC. Some devices and loads: Converters, Devices which includes semi-conductor elements, Generators, Motors, Computers, Electronic ballasts, Switching power supplies, Welding machines, Control circuits, HVDC transmission systems, Electrical Communication systems. Transformers


TOTAL HARMONIC DISTORTION (THD)  Total Harmonic Distortion(THD): ------ Measure of amount of harmonics  THD of the load should be less than 7% for acceptable performance  THD for intermodulation distortion can also be calculated in a similar way.

INTERMODULATION DISTORTION distortion (IMD) is the amplitude Intermodulation or intermodulation modulation of signals containing two or more different frequencies in a system with nonlinearities Intermodulation occurs when the input to a non-linear system is composed of two or more frequencies. Consider an input signal that contains three frequency components at fa ,fb and fc which may be expressed as :

Output signal Y(t) = G(x(t)) Y(t) will contain three frequencies fa ,fb and fc of the input signal as well as number of linear combinations of these fundamental frequencies: Kafa+Kbfb +Kcfc Where Ka Kb and Kc are arbitrary integers which can assume positive or negative values. These are the intermodulation products (or IMPs). Intermodulation order: The order O of a given intermodulation product is the sum of the absolute values of the coefficients O= |Ka|+|Kb|+|Kc| In our example 3rd order intermodulation products occur where

The performance of an ideal amplifier can be represented by the transfer function: V out A0 A1V in An amplifier with some distortion due to nonlinearities can be expressed by a transfer function in the form of a power series expansion: V out A0 A1V in A 2V in An input signal with two frequencies The first order term A0 A 2V 2 A 2V 12 A 2V 2 A 2V1 2 2 2 DC terms and A3V in 2 3 A 4V in 2 4 .... V in may be shown as: A1V in gives the fundamental products The second order term A 2V in 2 in 1 2 V out V1 cos( A0 1 t) V 2 cos( A1V1 cos( 1 t) t 2 cos( t) A1V 2 cos( determines the second order products: 2 2 cos( 2 t) 1 A 2V 2 cos( 2 2 2nd harmonic terms t) 2 A 2V1V 2 2 [cos( 1 2 t) 2nd order IMD terms 1 t 2 t )] 2 t)

The third order term A 3V in A3V 3 in 3 A3 V1V 2 A3V13 2 2 cos( 3 4 V1 3 determines the third order products: 3 cos( 2 1 t) A3V 23 1 t) 3 A3 2 cos( 3 4 2 3 V2 2 cos V1 V 2 4 2 2 Fundamental frequency terms t) 3rd harmonic terms t) 2 3 A3V1 V 2 cos( 2 [cos( 2 1 t 2 t) cos( 2 1 t 2 t )] 3 A3V1 V 2 4 [cos( 2 2 t 1 t) cos( 2 2 t 1 t )] 3rd order IMD terms – The troublemakers

THIRD ORDER INTERCEPT POINT The 3rd order products will be the largest (loudest) of the intermodulation products. As a general rule, the 3rd order products will increase (grow) 3-times faster than the fundamental signal (the signal of interest)

DISADVANTAGE S Increased losses on the distribution system due to increase in the effective rms current Overloads, vibration and premature ageing of the generators, transformers and motors as well as increase in the noise level. Distortion of the supply voltage that can disturb the operation of the sensitive loads Disturbances in the communication networks and telephone lines SOLUTIONS  Selective filters for low-order harmonics.  Active power filters before non-linear loads.  THD based power tarrifs methods.


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