# Re: volt and current harmonics

#12035
Alen Matanovic
Participant

Good thing is first for you to understand the difference between
those two.. Voltage harmonics are caused by Current hamonics which are
generated by nonlinear loads.. Second thing that should be on mind is
that FACTORS THDu and THDi,, of Voltage and Current harmonic distortion
respectively,, are not allways the best solution for expressing the real
situation of distortion in the sistem – More appropriate would be use
of the TDD (Tottal Demand Distortion) factor, especially for Current
harmonic distortion – here's why – if you have “light load condition” in
provide high THD factor,, and it may seem like you have problems with
harmonic distortion which is probably not true (valid only at “light load
condition”) because this Current distortion won't cause anywhat serious
Voltage distortion.. There for, it is better to use TDD istead of THD
for Current harmonic distortion because it shows “real distortion
factor” depending on the available sistem capacity at that moment.. This
is why it's better for use for Current distortion bacause THDi can
varry in a wide range for various types of loads – but it's not
necessairy for Voltage distortion because it does not varries in that
wide range,, so it's still expressed in THDu at many chases (but it doesn't mean that there can not be TDD used)..

Speaking about numbers, here's the table from IEEE 519 std. valid in the US and recommended worldwide:

http://ecmweb.com/images/804ecmIPQfig3.jpg

Voltage distortion is said not to be higher than 8% at the PCC (Point Of Common Coupling – point were the object is connected to the grid, or where other customer could be connected to distribution too)..

Effects of Current harmonics are: heating of the loads, cabels, sistem equipment, transformer windings (the more harmonic current this transformer has to supply, the less
fundamental current it can provide for powering loads. In other words,
because the harmonic current doesn't deliver any power, its presence
simply uses up system capacity and reduces the number of loads that can
be powered),, than nuisance tripping of protective devices,, than least wanted failure or damage of sensitive loads such as computer equipment or PLC loads,, the effect of current distortion on distribution systems can be
serious, primarily because of the increased current flowing in the
system,, and least but not last is Voltage distortion..

Effects of Voltage distortion are: this distortion will have minimal effect on a distribution system, but
unlike current distortion, it isn't path dependent. So harmonic voltages
generated in one part of a facility will appear on common buses within
that facility.. Voltage distortion can cause misoperation of devices that use zero crossing as relevant for taking actions or
measurements (because there might be multiply zero crossing in a single
periode of sinus).. 5th harmonic voltage distortion can cause serious problems for 3-phase motors,, it's a negative sequence harmonic, and when supplied to an induction motor it produces a negative torque..

Feel free to ask for more you want to know ;)..

Cheers,, Alen..