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I'm doing a study of harmonic distortion in a system. The system contain several large VSDs and other non-linear loads. There are no filters in the system, and all cables are relatively short.

Not surprisingly, the results show that the harmonic distortion is low close to large generators, while it is very high far from the stiff part of the network (given that there are non-linear loads in that part of the system).

My question is: What exactly is it that determine the level of distortion, except for the harmonic contributing equipment?

What influence does the different factors have:

  1. Voltage level
  2. Load at the bus
  3. Short circuit current capacity

I understand that if the non-linear loads are 1/1000th of the total load, the THD will be low, and if the load is completely non-linear the THD will be high, but is it the most important factor? And supposing the load level is the same at two buses, what other factors will determine the THD levels? Will the distortion level be different if the loads at the bus are passive linear loads, compared to for instance motors?

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  • \$\begingroup\$ It's a simple impedance-divider problem. The generator, transmission line, and load all have some impedance. The load is basically injecting noise (the harmonics) at its location with some voltage and impedance. The rest is a divider problem to determine how those harmonic signals are attenuated at various points in the network. \$\endgroup\$ – Olin Lathrop Oct 20 '13 at 13:18
  • \$\begingroup\$ @OlinLathrop: I agree, harmonics must of course follow KCL / KVL, thus the distortion levels will be a function of impedances etc. But, assuming there exist a "typical motor", and typical "passive loads". If you turn off a motor and turn on a gigantic light bulb drawing the same power, will it affect the harmonics? (I'm aware of the fact that this depends on specific impedances etc., but is there a general "rule of thumb"?) \$\endgroup\$ – Stewie Griffin Oct 20 '13 at 13:37
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The only general rule of thumb I can think of is to balance loading whenever possible.

In a three phase system, when harmonics are generated at any point, every mitigating measure is some variation of getting them to dissipate as harmlessly as possibly, through or across some kind of reactance. One way of getting them to dissipate is by simply allowing them to flow through devices that are closer to the non linear load than the service entrance. The load creating a harmonic can only supply it so much power, and by sinking that through another load, it doesn't make it's way back to the mains as well. Some harmonics negate entirely when balanced. Occasionally facilities that get in trouble for harmonic generation or effective power factor can get back within limits simply by balancing their phase loading.

Of course, if this is the strategy, then the linear loads near harmonic generators will have to be rated to handle the extra heat generated.

And it seems like a higher per unit impedance would attenuate harmonics leaving or entering a facility; so generally speaking, I suppose it is safe to say that a higher short circuit capacity would allow higher amplitude harmonics simply by not stopping them.

As far as your gigantic light bulb theory, we can reason it out. If they draw the same amount of instantaneous power, then they draw the same amount of instantaneous current, which suggests they have the same impedance at the fundamental frequency. Of course, that impedance will be higher for higher order harmonics, making the motor less effective at harmonic sinking than the resistive light bulb. In reality, though, that light bulb is enormous, AND you need three of them - and you probably don't need that much light. But, maybe this makes a case for placing lighting loads alongside noisy loads?

I suppose the next question would have to be, can a halogen luminaire bulb handle the harmonics?

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