Harmonics Question

[wirenut1980]

Hello, I have a situation on a transmission line fed from 2 sources. When the stronger of the two sources is switched out, there is high voltage distortion (between 5 and 7% voltage THD) on the transmission. Most of the voltage distortion is 17th harmonic. When both sources are switched in, voltage distortion is low (1-2% voltage THD), and the 17th harmonic is not the dominant harmonic present, more typical 3rd, 5th, 7th, etc are dominant. I have determined a large transmission fed customer is responsible for the 17th harmonic current and they are exceeding IEEE 519 current distortion limits under any switching scenario.

At first I thought I had a resonance around the 17th harmonic, but I could not get my ETAP model to agree with me :blink:

My question is would the higher source impedance (due to the stronger source being switched out) cause higher order harmonics to distort the voltage more than lower order harmonics? For example, if I measured the same amount of 5th harmonic current as 17th harmonic current, would there be more 17th voltage distortion than 5th? I think the answer is yes since the voltage distortion would be dependent on source impedance, and the inducatance part of source Z increases as frequency increases. But is that increase in voltage distortion linear as the harmonic frequency increases? Or is it a larger effect/increase?

Or do you think there is a resonance? There are no capacitors on the 138 KV line, although there are multiple distribution capacitors fed from substations along the 138 KV line.

Thanks!

 

[Bugman1400]

You didn't say what kind of customer or load they had but, I've seen chemical plants and rock quarries have similar harmonics from using large VFDs for drive motors. I've also seen a similar issue on the distribution side with a hospital that had an arc boiler.

You are correct in that a larger source impedance where reactance increases that harmonics are more of an issue. This also happens for large X/R applications. However, just because you have harmonics doesn't mean you have resonance. Typically, a resonant system will result in large voltage transients and can sometime cause lightning arrestors to fire or fail as well as other devices such as breakers, transformers, and capacitors because the insulation level can be exceed during the transient. If resonance is an issue, many times TRV capacitors are installed in an effort to change the "tuning" of the system and also shunt the transients.

 

[wirenut1980]

Hey bugman, I am talking more about a parallel resonance. I think what you are describing is ferro-resonance.

I am not sure at this time what the load is, I imagine it could be a power supply for an induction furnace given the customer. The customer does have 12 KV harmonic filter tuned to the 5th and 11th harmonics, which does help. But they still have issues with the 7th and 17th harmonics especially.

 

[Bugman1400]

Well, it seems to me that if you switch out the stronger source that you are effectively changing the tuned circuit. This new tuned circuit is apparently more resonant with the 17th harmonic than it is with the 7th.