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.