Harmonics?

ryanryan · Oct 14, 2009

has anyone had any experience with harmonics, measuring them, analizing the results and correcting them?

I am experiencing 36v N-E on a 400A sub main.... my first thought was a loose Neautral, so out ame the thermal iimager (to no avail). shut down and checked all connections (some a little loose),problem still there, when a 200A machine (with VSD) is running, dissappears when isolated.

So harmonics i thought?

anyone?

quince80 · Oct 15, 2009

there was one instance i came across but i didn't know what it was and, like you, thought it was a loose connection. but when electrical consultant came out he explained it was due to the large amount of, in this example, computers being used on the installation. the air con units where pretty hi-tec too and that made it even worse. how to measure or solve it is beyond me. in fact looking back i don't know why i didn't ask the guy, i guess i wanted him to think i knew what it was all along. sorry mate thats not going to help you much but its all i got.

Guest123 · Oct 16, 2009

Hi

Sounds like harmonics for sure.

Basically harmonics are caused by lots of electronic control circuits and equipment.

This equipment causes load currents to be out of frequency with our 50Hz system if you were to look at them through an oscilloscope instead of just the usual sine wave you would see another set on top.

These 3rd harmonics as they're known are what causes the humming sound you hear at mains positions and DB's etc on large installations.

Normal balanced 50 Hz 3 phase loads cancel each other out in the neutral but, 3rd line harmonics i.e not 50Hz but multiples of, dont they add up and become load current in the neutral conductor.

This obviously affects things like volt drop, cable sizing and OCPD's.

ryanryan · Oct 17, 2009

thanks lenny,

any ideas on the rectification of these little demons?

and i assume that you are using more useless power, so the removal of harmonics would cost you less?

i have also read that harmonics can drastically reduce the life of HF flourescent control gear? (another reality of my situation!)

MikeCDF · Oct 21, 2009

Hi Ryanryan,

In a 3-phase installation I usually measure harmonics as the current in the neutral as the 3-phases should cancel out. Normal checks I would make are:
Am I using screened cable from motor to VSD
Is screen of cable securely tied to earth
Is VSD driving motor at high frequencies
Is motor good enough for connection to VSD (sometimes I've had to change motors because the windings are rubbish)
How old/good is VSD - Can you change capacitors? Are thyristors OK?
Is VSD correctly rated for load and ambient temperature?
Is Earthing of VSD in place?
Is cable length to motor within limits for VSD?

It's a chore but in my experience well worth checking the above. If you still have a problem put a (harmonics) filter on the VSD - make sure it is the same maunfacturer as VSD. If all else fails look at replacing VSD and/or motor.

Could also make this machine on its own dedicated circuit with MCB from main DB then any harmonics shouldn't effect other equipment. Just make sure you can auto-disconnect in 5s for faults etc.

Hope this helps.