What causes a 9th harmonic on the power line?

[wfowfo]

My utility is trying to help the local phone company find the source of a 9th harmonic which is causing noise on their system. The area is almost exclusively rural load with two exceptions; a lot of pipelines in the area (cathodic protection) and one large pipeline pump (1200 Hp).
We've ruled the pump out as the harmonic exists whether the pump is running or not. We've also temporarily killed out some of the cathodic protection with no lessening of the harmonic.
We've suggested a separate ground and/or 9th harmonic filters to the phone company, neither of which do they seem willing to invest in.
Any ideas for a source of that particular harmonic?
Any source for a filter suitable for a phone pedestal?

 

[ghostbuster7]

Usually if the 9th harmonic is present,the 3rd harmonic is also there.Do you have a frequency profile of this interference in order to determine possible sources?

 

[Skogsgurra]

Yes, there are probably also a lot of third harmonics (180 Hz). But, since telephone BW usually is between 300 and 3400 Hz, the 180 Hz is suppressed. The ninth harmonic (540 Hz) is within the phone's passband and also where the ear has a rather high sensitivity.

So, my guess is that you have a rather normal harmonics profile. And then, just about anything can produce that interference, TVs, hi effificency lamps, computers, VFDs, thyristor controlled heating - anything. Just go there with a current clamp and an analyzer to trace the source.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Scrummer4]

Silly question, which the utility should already know the answer to: It's not the carrier frequency for some sort of ripple control signalling, is it ? They shouldn't have chosen a line harmonic frequency, but stranger things have happened !

 

[wfowfo]

We have two PLC AMR systems, but neither are close to this frequency.
We can see the problem in the substation (although, strangely enough, there is no significant 3rd harmonic). The problem worsens the closer you get to the end of the line (some 5 miles away) at one particular phone company pedestal.

We have systematically dropped various loads around the pedestal to no effect. We transfer to different substation source and the harmonic disappears, so it is specific to the sub in question and not the pedestal equipment itself.

 

[mls1]

Are there any cap banks on the feeder? If so there could be a resonance with the line reactance that shifts away from a zero sequence component when you switch the feeder. If it's a capacitor bank resonance you might actually find the magnitude drops as load increases due to increased damping.