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I'm a student of Politecnico di Milano and today my prof during the lesson told me that it's important to limit current harmonics into the grid and not voltage harmonics because these last do not create problems. I asked the reason to prof but he didn't answer me. He just told me the problems that current harmonics bring, that is current harmonics are critical because they create torque harmonics in motors but also increase ohmic losses. But what problems voltage harmonics in the grid may cause and why is not important to not decrease them? And should voltage harmonics generate also current harmonics?

Samuele Benito Di Gioia
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    What sort of scenario would produce bad voltage harmonics do you think? – Andy aka Nov 05 '21 at 12:30
  • @Andyaka I think voltage flickers for the lamp (but that is also due to current harmonics right?) and in general not proper working conditions for loads. Another problem could be the failure of circuit breakers I think. Are they right? – Samuele Benito Di Gioia Nov 05 '21 at 13:01
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    Well, the voltage supply (the sinewave) is a really "hard" signal produced by the generating stations that can deliver tens of thousands of amps without hardly any distortion being inflicted onto the waveform due to the most extreme non-linear loading condition. On the other hand the current taken from this hard signal is totally shaped by any non-linear loading irrespective of the supply being a sinewave and barely distorting under the most adverse conditions. – Andy aka Nov 05 '21 at 13:39
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    So, you tell me what can cause this very hard tough signal to become significantly distorted (compared to a distorted current (easily produced). – Andy aka Nov 05 '21 at 13:40
  • @Andyaka ah okok thank you! And what about the negative effects that voltage harmonics can cause? Which are these? – Samuele Benito Di Gioia Nov 05 '21 at 18:07
  • You tell me how voltage harmonics can arise based on what I've told you and I might be interested in thinking about what you request. – Andy aka Nov 05 '21 at 18:26
  • As Charles said, non linear loads create current harmonics which cause voltage harmonic drops. So even if we have good voltages across the generators, non linear components will create voltage harmonics @Andyaka – Samuele Benito Di Gioia Nov 05 '21 at 18:30
  • Then I'm sure Charles will answer that. – Andy aka Nov 05 '21 at 18:32

2 Answers2

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Harmonic current in the grid is caused by nonlinear loads. Harmonic current is like reactive current. It does not result in useful power delivered to the load, but it causes losses in the generation, transmission and distribution system. Harmonic current can not easily be mitigated by adding capacitors, the remedies are a bit more complex.

Harmonic voltage in the grid is caused by harmonic voltage drops in the transmission components due to the flow of harmonic current. Since Harmonic voltage is caused by harmonic current flowing through impedance, percentage harmonic voltage is the highest near the source of the harmonic current, where the harmonic current is the highest percentage of the total current. The extent to which an entire grid experiences harmonic voltage distortion is determined by the extent to which users with harmonic loads are distributed throughout the grid.

Look at IEEE 519-2014 - IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems.

Also look at questions tagged [harmonics] such as what causes voltage harmonics and Harmonics are bad... But how bad?

  • First of all, thank you for your help! So, if I understood well, we try to reduce mainly current harmonics rather than voltage harmonics because in this way we reduce also voltage harmonics right?? And which negative effects may voltage harmonics cause? – Samuele Benito Di Gioia Nov 05 '21 at 18:06
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    Those who have loads that draw harmonic currents try to reduce the harmonic currents because they have access to the source of potential voltage harmonics. Equipment manufacturers are also encouraged or required to do that. Negative effects are described in an answer to the second of the two questions for which I provided links. –  Nov 05 '21 at 18:40
  • Thank you very much Charles!! You clarified a great doubt that I had! Thank you again! – Samuele Benito Di Gioia Nov 05 '21 at 18:48
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In a Power Transmission Grid voltage and current should be always pure sine waves without harmonics.

Voltage is delivered to homes, firms, and offices in this mathematical form:

V = A * sin (2 * π * f * t)

f = 50 Hz in Europe and 60 Hz in USA

Voltage V is measure between LINE (black or gray wire) and NEUTRAL (blue wire).

Appliances contain non linear components like diodes, diode bridges, and transistors that introduce locally (inside homes, inside offices, ..) non linearities in the current sinked.

Current non linearities are estimated and valued by measuring how much power is wasted in the harmonics of f by special instruments.

In Europe and in the US there are regulations that manufacturers must follow in order not to waste power in harmonics.

Voltage measured across devices is almost always quite linear because it is imposed/stated by the Power Grid Transmission system which is the SOURCE.

Appliances, which are considered LOAD, can't easily modify the shape of the voltage imposed/stated by the SOURCE unless the current sinked is huge.

Enrico Migliore
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