volt and current harmonics

[jatin333]

up what extent voltage and current harmonics acceptable?

what are effects of current and volt harmonics in system?

[Alen Matanovic]

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
your sistem, and you're using highly nonlinear loads,, they would still
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..

[Spir Georges GHALI]

jatin333 said:

up what extent voltage and current harmonics acceptable?

what are effects of current and volt harmonics in system?

---

Dear Mr. Jatin333 ;

 

refer to your question and the reply of Mr. Alen, I want to clarify some points :

1 – We know all that the path of any current either Fundamental or Hamonic current is from the load to the source, that means there's no any direct effects from the harmonic currents on the other loads, but, the harmonic currents with the harmonic inpedance of network are reponsable to have a ” Voltage Distrotion ” in the network, and that voltage distortion has a direct effets on the loads even the linear-loads. So, the effetcs of the harmonics current is, as Mr. Alem said, on the : Cables, Transformers, Generators, Motors, Capacitors, ext. where there's many solution for that.

 

2 – Normally, as the direct effects on the load is from ” Voltage Distortion ” we should look to the percantage of Voltage Harmonic that means ” THD U  or  THD V “, and when these values exeed some levels, we should find the suitable solution for the network, else we will have problems with some sensitive equipments. Noting that when the ” THD I ” incraese the ” THD U ' will increase also, but be carfull even if we have the same percentages of ” THD I ” in 2 different networks it's not necessary to have the same percantage of ” THD U ” for both, because it dependes on the value of the ” Harmonic Impedance ” of each network.

 

3 – In some time when the level of Voltage distortion is smaller than standard or definded value, we can accept, as THD I, till ” 40-45% “.

 

4 – For information, the ” IEC 61000,2 ” defined the percantage of Harmonic Voltage for each harmonic ” Odd & Even harmonics ” in LV networks, and also in some countries the Electricity Co. define the ” THD U ” for their :V & MV networks.

 

Regards.

 

3 –

[Anonymous]

I need to find out if there is a way to identify the flow direction of the Current harmonics. I need this so that to identify the source which is creating the harmonics. For example i need to know if the current harmonics enter via the Main grid line that is incoming or is it generating within the building which is will be outgoing. From what i have read no one has emphasize on this subject of direction and only focused on the magnitude.

Looking forward in hearing from you all.

Thanking you.

With regards,

Irfan.

[Author]

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