Could 9th Harmonic Be Causing Noise on Local Telephone Company's System?

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We have a local telephone company complaining to our utility that they have noise on their system. We have investigated and found a fairly significant 9th harmonic on the power line. This seems to play havoc with phone systems.

What is a likely cause of a 9th harmonic? We are a rural utility with little industrial load. The area in question, however, has a lot of pipelines and cathodic protection.
I understand there are filters readily available. Where are they to be found and how much are they likely to cost?
 

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What is a likely cause of a 9th harmonic? We are a rural utility with little industrial load.
What types of industrial load do you have. I would think it's coming from some type of equipment.
I understand there are filters readily available. Where are they to be found and how much are they likely to cost?
http://www.nepsi.com/pmfilt.htm"
 
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The only industrial load is a 1200 Hp gas line pump, which does not appear to be the culprit (i.e., the harmonic still exists when the pump is down). The rest of the area is rural load.
The link was good, but I was looking for something the phone company could put on their equipment.
 
If you are very rural, the power company may have installed a line conditioner (step-up transformer) to combat voltage drop over long runs. There is one 1/10th of a mile from me. The power company had been trying to serve this entire road with a single-phase hot line and a ground, but the boom in housing out here made that a problem. I don't know if such equipment could induce such a harmonic, but that would be place to start.
 
Is the telephone equipment in a substation? You might to be able to use some type of optical isolation equipment but you are going to have to share a common ground. If the equipment is in a substation, have you looked at how well you're grounding?