Wednesday, June 13, 2012

Where does harmonic voltage distortion come from? PQM_101


Voltage distortion is caused when harmonic currents flow through system impedance.  Simply put,  when a piece of electronic equipment draws harmonic current,  the harmonic currents flow though the power source impedance (XL)  and voltage results at each harmonic frequency based on Ohm’s Law (amps times ohms). The sources of harmonic voltage distortion are those electrical loads the draw non-sinusoidal current. 

On a 60 Hz power system, 5th harmonic current is current flowing at 300 (5 x 60) Hz.  Following ohms law, when harmonic current flows though upstream impedance it will cause a voltage drop at the same frequency.  5th harmonic current will cause 5th harmonic voltage on the power system that supplies this load.  It is common to all loads connected to the system. 

Utilities generate sinusoidal voltage, but power electronic loads in customer facilities distort both current and voltage. To reduce power system harmonic voltage distortion, one must typically identify the source of harmonic current and minimize harmonic current distortion at this point.

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