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A typical installation is

Load - 400 V net - Transformer 1:25 - 10 kV net

If the load is for example a SPS, the 400 V net can contain harmonic currents and thus have a considerable THDi.

How can the transmission of harmonics or disturbance from 400 V net via Transformer til 10 kV net be described?

Some thoughts:

  • The Transformer is a galvanic isolation, so no direct current can pass
  • Via induction and magnetic fields, effect is transmitted, i.e. also disturbances
  • Turns ratio needs to be considered
  • The transformer will heat up due to harmonics if not it is a specially designed "K-factor" transformer. This affects the transmission behaviour?
  • Dependance on "stiffness" of HV net. If a power producer sits right next to the installation, it is hard to insert harmonics. Can this be described via the nets impedance? Power Quality
Irenaius
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1 Answers1

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As mentioned in the link you posted, the voltage harmonics occur due to the voltage drop in series resistances of the circuit. The current harmonic components (caused by non-linear loads) generate these voltage drops. The voltage at the transformer terminals then becomes the original voltage added to these voltage drops (which contains harmonics), becoming a "distorted signal", so to speak. About your thoughts:

The Transformer is a galvanic isolation, so no direct current can pass

Exactly.

Via induction and magnetic fields, effect is transmitted, i.e. also disturbances

Yes. Thinking of the periodic signal in terms of FT (Fourier Transform), each component of the signal is transmitted by voltage induction. For three-phase transformers with delta connection or arrangements of several connection groups (12 pulse transformers, for example) some harmonic components cancel each other and this is often used to reduce the harmonic content generated by non-linear loads (rectifying bridges, in general).

Turns ratio needs to be considered

Yes. The voltage induction for each component follows the transformation ratio.

The transformer will heat up due to harmonics if not it is a specially designed "K-factor" transformer. This affects the transmission behaviour?

Even a "K-factor" transformer will heat up, but this transformer is designed to withstand this heating without exceeding the maximum temperature allowed by the insulating materials. Part of the losses generated by the load current depend on the frequency squared, so components of high harmonic order have can greatly increase these losses, manufacturers usually take some action in the design to reduce that part of the losses that increase with frequency, for example, using conductors with smaller dimensions (of course this implies using more conductors in parallel). The voltage harmonics do not affect the transformer so strongly, in general they flow through the core, but their impact is inversely proportional to the square of the frequency.

Dependance on "stiffness" of HV net. If a power producer sits right next to the installation, it is hard to insert harmonics. Can this be described via the nets impedance?

That's right. The lower the series impedance of the grid, the lower will be the voltage drops caused by current harmonic components.

Edit---- Finally, answering the question: Each harmonic pass to the other side according with the transformer ratio. So, if you have 10% of 3rd harmonic in LV, you will have it in HV too. If 10% is significant, then you have a problem. In single phase transformers, there is no cancelling of harmonic components, it occurs only in three phase arrangements. The common practice is to filter it in LV side

Luiz Oliveira
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  • Thanks a lot for the fine comment. However, it does not answer my main question: "How can the transmission of harmonics or disturbance from 400 V net via Transformer til 10 kV net be described?" What I am trying to find out is how much harmonics couple over to the HV side of the transformer. You wrote something about some harmonics canceling out, that is interesting. Could you specifiy that? With a coupling ratio of 1:25, even small voltage harmonics distortions would be significant on the HV side? And that means, if I have a distorted LV net, then I better filter the heck out of it on the LV? – Irenaius Mar 19 '21 at 07:15
  • Each harmonic pass to the other side according with the transformer ratio. So, if you have 10% of 3rd harmonic in LV, you will have it in HV too. If 10% is significant, then you have a problem. In single phase transformers, there is no cancelling of harmonic components, it occurs only in three phase arrangements. The common practice is to filter it in LV side. – Luiz Oliveira Mar 19 '21 at 10:16
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    Can you update your answer, then I can accept it. Thanks Luiz – Irenaius Mar 19 '21 at 11:00
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    Besides transformer ratio we have its stray inductance (corresponding to the short circuit reactance) and stray capacitance, both across a winding and between windings. So the transmission is not exactly the turns ratio, and there may be amplification due to resonance (self capacitance and capacitance of connecting cables). Then, please, consider that harmonics have phase sequence, positive or negative, so from a three-phase viewpoint there is a change passing through the transformer. – andrea Mar 19 '21 at 20:16
  • I agree, this extends the accepted answer. – Irenaius Mar 20 '21 at 11:10
  • The question title is "transformer harmonics "- not "load harmonics". so the statement "the voltage harmonics occur due to the voltage drop in series resistances of the circuit. The current harmonic components (caused by non-linear loads) generate these voltage drops." is misleading - it implies that all distortion is caused by load non-linearity, when in fact a saturated core is also a significant source of transformer harmonics – N.G. near Mar 25 '21 at 06:39