Harmonics Content on the Distribution System

[XEngineer12]

I came across a situation during a power quality investigation on a 13.8KV, 4-wire multi-grounded, distribution feeder.

The data showed large 3rd and 5th voltage harmonic content. The current harmonics were low on the 3rd and 5th, but high on 9th and 11th. This trend was uniform across the feeder.

I am perplexed as to why the high content voltage and current harmonics components do not correlate. I have spoken with someone that, while rare he said, has seen this a couple of times in his long career, but has never found an explanation.

Has anyone seen this in the past? What might cause this?

Thank you!

 

[Skogsgurra]

How did you mesure? VTs or HV probe? L-L or L-GND?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

The voltage harmonic will be the harmonic on the distribution system including your loads.
The current harmonic will be for your feeder loads.
What loads do you have that will generate 9th and 11th harmonics?
There are probably loads on the distribution system that are generating a lot of 3rd and 5th harmonics. You will not see these currents, but you will see the effect that they cause on the system voltage.
Your loads may be generating 9th and 11th, and you will see the currents but on a stiff system your harmonic currents may not be able to influence the system voltage very much.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[XEngineer12]

The measurements were taken on the 13.8kV primary. The device used was a Megger MDP3 - one hangs on each conductor, no neutral involved.

Is perhaps the voltage distortion being caused by adjacent feeders off the same bus, and the current harmonic content on the feeder of interest, while significant on this feeder, does not have a significant impact on the substation distribution transformer low-side bus voltage harmonic content as much as other sources? Is it possible resonance playing a role?

Pat

 

[waross]

Harmonics start as currents. This current in feeders causes a voltage drop, through the system impedances.
The greatest voltage drop will be in the feeders and the supply transformer. If the harmonic currents are great enough voltage drops will be created also on the distribution lines along with harmonic contribution from other users on that distribution line.
All the loads on that distribution line contribute to the harmonic voltages. Ampmeters on a feeder will see harmonic currents on that feeder only.
The source of the 9th and 11th harmonic voltages may be any of your feeders, any other loads on the distribution circuit or loads on other distribution circuits on the same power transformer at the sub station.
What are your loads?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

How much is 'high'? Some data could be useful.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[PHovnanian]

Got any capacitors on that feeder? Caps will provide a low impedance path through which harmonics can circulate. Moreso at higher frequencies (higher harmonics).

 

[XEngineer12]

The feeder of interest has industiral, commercial, and residential loads. This is fed from a 70MVA transformer and serves a metro area. There are three capacitor banks each of these feeders.


Pat

 

[jghrist]

Is perhaps the voltage distortion being caused by adjacent feeders off the same bus, and the current harmonic content on the feeder of interest, while significant on this feeder, does not have a significant impact on the substation distribution transformer low-side bus voltage harmonic content as much as other sources? Is it possible resonance playing a role?

You are probably correct on the other feeders being the source of harmonic current. Resonance could be a factor as well. If there is resonance between capacitors on the system and the source impedance near the 3rd and 5th harmonic, a small harmonic current could cause a high harmonic voltage. Resonance at higher frequencies with capacitors on the measured feeder could result in high 9th and 11th harmonic currents in the measured feeder.

Can you measure the total substation current harmonics?