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I am need of help in giving direction to others on a project where a 29.4kv 600 amp transmission is feeding variable frequency drives (VFD) over unknown (by me) mileage (an oil field). As the main load are these VFD which are poor quality, they generate a large amount of harmonics.

General direction (assumed Federal policy on public lands) requires lines under 33kv to be buried.

I have read in an IEEE article about how the harmonics cause premature failure mostly due to additive voltages, but also affected by the slope of the wave and my questions are as follows:

Is anyone aware of such a Federal policy on Federal lands?

What effect would harmonic filters have on the transmission line?

Based on some info given me the peak voltage due to harmonics on a test was about 75% above normal, would design based on 200%*1.25 insulation be adequate?

Will this effect continue to increase over larger and larger distances?

What ball park effect on the total cost per mileage be?

My stake in this is 3rd party, based on questions posed by fellow engineer, asking him if it is reasonable to waive the burial requirement. I could only give a minimalistic opinion as it is outside my expertise. I will be passing the answers on as a courtesy which would be weighed against asthetic issues.

A few very general observations/opinions…
  • Unlike low-voltage systems, a limiting factor is usually voltage drop and not cable thermal capacity
  • Source characteristics — Usually 600 amperes is the limit for an entire/single MV distribution feeder. 600 amps is equivalent to ~29,000hp
  • Regarding drive front ends — Will they have 6-, 12-, 18- or 24 pulse rectification? You can look at harmonic mitigation from a purely economic standpoint.
  • UG versus OH cable characteristics {and losses} make them respond differently to harmonics.
  • Will there be any foreseeable loads in addition to VFDs/motors
  • Suggest that a distribution engineer experienced in industrial applications be retained to conduct loadflow studies for various line arrangements/loads.

[This message has been edited by Bjarney (edited 07-29-2004).]
Thankyou for your reply.

My fellow engineer is basically the regulatory agency and has the ability to tell the power company underground or no permit. He is operating from limited information and my understanding is that this line will be feeding several locations all involving oil, gas, or coal producing facilities.

The VFD's are poor quality per his information and so I assume 6 pulse. This is the basis for my guess at the 200% insulation level to avoid premature insulation failure. I hope that I am in the ballpark even though this seems very high.

I am sorry that I was not clear in giving you a ballpark starting point. I know costs per mile of typical overhead of this nature at $55k and that the claimed cost underground (UG) is $260k. I am also trying to find out if the UG is inflated unreasonably, or if it would be typical for the 74kv level (using my assumptions on the additive harmonics and that there seems to be limited other loads), or if the line costs are for the 37kv (29.4*1.25 noting that I do not know insulation values at these voltages) level insulation.

I am not concerned about the overhead line and it's response to harmonics, nor is the other engineer. We could always pi analyze them but it would serve no purpose to either of us other than sharpen our math skills and would be again heavily dependent upon many assumptions (or cause us to pull out what hair we may have left, neither of us has interest in purchasing software to do this). We may choose to review the design by the power company. However, we are interested in general or ballpark answers. Such as:

Would the use of harmonic filtering at the loads mitigate the need for my assumption of double the insulation level to avoid premature insulation failure?

If left unmitigated would the harmonic problems be much greater in a line 150 miles long versus one only 50 miles long (the practical circuit limits using a pi or T equivalent, just ballpark value from experience)?

In any case the line design will be done through the owner of the lines engineering group.

I hope that I clarified my questions.

No one wants to spend much money at the conceptual stage, but you may have to, by farming out a basic loadflow study after some (admittedly, educated-guess) ‘hard’ numbers are assigned. Characteristics that may only be provided only by the serving utility are source-impedance parameters; else, what is the change in voltage at the supplier’s end of the line when loads are fired up?

Determine if it’s cheaper to reduce harmonics at the source {load} than to increase line-harmonic tolerance [id est, line losses resulting where harmonic effects are turned into heat.]

Contact an electrical PE familiar with utility-distribution practices and costs, for preliminary help in developing an RFP for line design
Interesting that I am still giving the impression that we have a stake in this other than approval for how we require these lines to cross our lands. A bummer I never learned how to communicate and just say things outright.

Well, until Monday.

Query is unanswerable without more information and consultation with others. Again, that is an opinion.
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