12th Harmonic Resonance?!?!

Status
Not open for further replies.
D

diakonos1984

Member
I'm dealing with a power system that is appearing to have resonance problems at the 12th harmonic. I know that would be highly unusual, but it has been confirmed by both math and audio analysis.
The system is medium voltage fed by a 7.5MVA transformer. It is a small system with very large loads (pipeline compressors). The MV motors are on solid-state softstarts, but they are bypassed when the motors are running, and the resonance occurs even when the motors are not running. There are power factor capacitors (hence the resonance).
The medium voltage feeds a small 480V transformer (1000 kVA I think) that feeds a small MCC with a variety of small motors on it and a few PowerFlex VFDs.

What on earth could produce 12th harmonics?!?! :blink:
 
diakonos1984 said:
I'm dealing with a power system that is appearing to have resonance problems at the 12th harmonic. I know that would be highly unusual, but it has been confirmed by both math and audio analysis.
Click to expand...
Have any direct electrical measurements been carried out? Is that what the mathematical analysis was based on? Audio would give you sound frequencies and magnitudes but it wouldn't tell you whether it's source is mechanical or electrical.
diakonos1984 said:
The system is medium voltage fed by a 7.5MVA transformer. It is a small system with very large loads (pipeline compressors). The MV motors are on solid-state softstarts, but they are bypassed when the motors are running, and the resonance occurs even when the motors are not running. There are power factor capacitors (hence the resonance).

What on earth could produce 12th harmonics?!?! :blink:
Click to expand...
Well, If you have a circuit with Ls and Cs in it, there is always going to be a possibility of resonance at some frequency, I suppose.
Do the PFC capacitors take excessive current?
Do they have detuning reactors? It's becoming more common to see that done here (UK). Usually this is designed for just below 5th harmonic.
 
There is some connection between weird harmonics and electrical trees in MV cables and transformers, I think I have an IEEE paper somewhere that discusses it, will see if I can find it.
 
Thanks for jumping on this guys!

There were direct electrical measurements taken, which indicated more traditional harmonics in the 5th, 7th, 13th, 17th, and 19ths. However, they were taken at a different time than the audio sample and under different system conditions. Calculations indicate possible resonances at 12th, 13th, 17th and 19th under various system conditions (including with only half of the cap bank in service (to answer the question about steps)).

Unfortunately, while I have all the metering capability to find out exactly where this is coming from and answer questions about capacitor current draw (which I'm sure is a yes, since they're blowing fuses and caps), I'm being held at arms length by the customer at the moment. They have taken their own readings, but not as methodically as I would like to. They are interested in any ideas I have, but not in paying me to come out with a meter just yet.

To encourage them, I was hoping to come up with some ideas regarding the unusual 12th harmonics. I'm very interested to see what you've got about electrical trees, Zog. The plant is only a couple years old.

We'll be looking at filters, but I really hate to proceed with that until we know what the source of the harmonics is.
 
http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/ia02607001e.pdf

At a given harmonic frequency in any system where a capacitor exists, there will be a crossover point where the inductive and capacitive reactances are equal. This crossover point, called the parallel resonant point, is where the power system has coincidental similarity of system impedances. Every system with a capacitor has a parallel resonant point.
 
diakonos1984 said:
I'm very interested to see what you've got about electrical trees, Zog. The plant is only a couple years old.
Click to expand...

Which is usually when PD shows up due to lack of proper installation or improper commisioning procedures (Like using a DC hipot on XLPE cables for example)
 
Status
Not open for further replies.
Top