Is that true that underground cables with no power factor correction have resonances in range of frequencies which leads to multiple zero crossing distorted waveform?

But why? Is that is because it work as a band-pass filter?

From the book Electrical Power Systems Quality:

While they may cause interference with low-power electronic devices, they are usually not damaging to the power system. It is also difficult to collect sufficiently accurate data to model power systems at these frequencies. Acommon exception to this occurs when there are system resonances in the range of frequencies. These resonances can be excited by notching or switching transients in electronic power converters. This causes voltage waveforms with multiple zero crossings which disrupt timing circuits. These resonances generally occur on systems with underground cable but no power factor correction capacitors.

  • \$\begingroup\$ Where did you hear this? Can you provide links to reference material on this? \$\endgroup\$ – Dave Tweed Jan 7 '13 at 12:39
  • \$\begingroup\$ In a book,Electrical power system quality. I'll update quoted text. \$\endgroup\$ – Standard Sandun Jan 7 '13 at 13:48

I general ALL power-lines exhibit resonances that vary with timeframes that are both long and short.

To see why all you have to do is realize that the power-line is subjected to many different loads, some of which are Inductive (motors), Capacitance (Fluorescent lights), resistive (heaters) to list just a few. Longer tem variations arise from people turning on loads, say a washing machine or lights and indeed even a change in the mechanical load seen by a motor will change what electrical load it presents to the power-lines. On shorter time frames, there is variation even on e a cycle to cycle basis as the sinusoidal waveform interacts with rectifiers, chopper circuits and switching power supplies.

You can see where a C load in parallel with a L load might form an easy resonance. This resonance might exist for long time frames, or the power-line may only see the C art of the resonance during part of the sinusoidal cycle. These L'and R' and C's are also distributed spatially thorough out a neighbourhood, so this ends up to be a seething complex time varying mess.

Buried cables, due primarily to the fact that the line and neutral wires are closer together tend to have higher capacitances than an air/pole mounted power-lines. That means that the resonances can be enhanced, depending upon the primary inductance that are present. Although it must be mentioned that it is possible that a buried cable can have fewer resonances also because of the increased capacitance. It all depends upon the mix of equipment and parasitic impedances.

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  • \$\begingroup\$ so in other words because it have high capacitance? \$\endgroup\$ – Standard Sandun Feb 28 '13 at 18:34
  • \$\begingroup\$ IF you are comparing the two (air vs buried) in an unloaded state, then yes the buried will have a high line/neutral capacitance. \$\endgroup\$ – placeholder Feb 28 '13 at 18:36
  • \$\begingroup\$ then why unloaded state? \$\endgroup\$ – Standard Sandun Feb 28 '13 at 18:41
  • 1
    \$\begingroup\$ because that is the only way to compare them, no two power-lines environments are the same. It's entirely possible to have a power pole type environment feeding a house with tons of florescence lights (capacitance) that would have more C than the buried in the next block over. You can only speak about trends. \$\endgroup\$ – placeholder Feb 28 '13 at 18:44

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