# what causes voltage harmonics

People keep telling me that Nonlinear loads causes current harmonics and when i ask them about what causes voltage harmonics, they tell me wherever there is current harmonics there is voltage harmonics.

So i did the experiment myself, powered a rectifier and DC motor using AC, and monitored using the Fluke PQ analyzer, the current THD was 32% but the voltage THD was almost 0% !

Could you please explain to me how harmonic voltages are created ? and Does it happen in the case of inverters only ?

Just add a current-to-voltage-translator (somethimes called "resistor") in series with the non-linear load and you will see also voltage harmonics across the load (and also across the resistor).

By definition the voltage across a (ideal) voltage source cannot be changed (as also noted in another answer). So if the voltage source provides pure sinusoidal voltage (i.e. no harmonics) no matter what load you have (linear or non-linear) the voltage will be pure sinusoidal. This changes as soon as the voltage source has a series resistance. simulate this circuit – Schematic created using CircuitLab

Left circuit: current throuh non-linear element (diode) will have harmonics. Voltage across non-linear element will not (cannot) have harmonics because V1 is an ideal voltage source.

Right circuit: current throuh non-linear element (diode) will have harmonics. Voltage across resistor will be proportional to non-linear current through resistor, i.e. will have harmonics. Also voltage across non-linear element (diode) will be pure sinusoidal minus voltage with harmonics, i.e. will also have harmonics.

• Thank you very much, in fact i did this experiment at the Lab and you were right ! – KADEM Mohammed Dec 22 '18 at 0:06

A true voltage source cannot be altered by load currents of any type (harmonic, transient or otherwise). This is because an ideal voltage source has zero output impedance.

If a voltage source has non-zero output impedance (and harmonic or transient currents are drawn from the source), then there will be remnants of these disturbances seen on the output terminals of the voltage source. This is due to the non-zero impedance of the source.

The current passes through that series impedance and creates a volt drop that adds/subtracts to the original immutable voltage source.

Voltage harmonics result from voltage drops across the source impedance due to current harmonics. If the current harmonics are low in comparison to the short circuit current capacity of the source, the voltage harmonics will be low.

Any nonlinear load will have current harmonics. Nonlinear loads include rectifiers, controlled rectifiers, voltage converters etc. Saturation of magnetic circuits also causes harmonic currents. Inverters that supply power to the grid can cause harmonic currents. Inverters that are powered by grid power and supply powers for variable speed motor control can cause harmonic currents in the motors. That type of inverter receives grid power through a rectifier, so the rectifier can cause harmonic currents. All loads that cause harmonic currents can have the harmonic currents reduced by mitigation measures in the load device. Harmonic currents are also reduced by adding source impedance.

Harmonics are a result of our mathematics, where correlation transfers energy into narrow-band filters or basis-functions of the Fourier math.

Fast rise and fall times will correlate with anything; a reoccurrence of the fast edge, of the same polarity, but exactly one period (of the narrow band filter) later will produce an interesting behavior.

Here is 0.31MHZ squarewave into a 1.000MHz filter with Q = 100 