Calculating additional Increase in Current due to Harmonics in the capacitor bank

Status
Not open for further replies.
A

adityalagudu3291

Member
Location
Austin
Occupation
Electrical Engineer
Our wind turbines experienced abnormal amount of turbine faults and after investigating identified that the faults were caused by high levels of harmonics while the capacitor banks were in service. After harmonic study analysis , identified resonant point at or near 5th harmonics and have high individual harmonic distortion at the 5th , 7th and/or 11th harmonic. So sized and installed a single tuned bandpass filter tuned to 5th harmonic on one of our capacitor bank. See attached for reference.

After installing the filter also, we frequently seen a blown fuses on capacitor banks. So what is the methodology to calculate additional increase in current due to harmonics other than 5th in these capacitor banks , so we can size the fuses and disconnect switches correctly?

Each capacitor bank is 17.5MVAR, rated @34.5KV and we have 4 of them on each bus.
 

Attachments

  • Capture.PNG
    Capture.PNG
    137.1 KB · Views: 15
  • Capture2.PNG
    Capture2.PNG
    79.2 KB · Views: 16
Were measurements made after the bandpass filter was installed to see if there were any significant higher order harmonic currents present that could be due to resonances above the 5th harmonic? Of course, this would be influenced by how many caps are switched in.
The fuses on capacitor banks should be sized as needed to protect them. If they are blowing I would check first for resonant currents that might be causing this to happen.

This paper discusses resonances that are not adequately damped when there's a a single bandpass filter on a bus with multiple capacitor banks:
https://nepsi.com/resource/Mixing Banks.pdf

Also, from page 5 of the document at the link below:
"In general it is not a good idea to combine filter banks with unfiltered banks in the same system. The unfiltered capacitors create additional parallel resonances and may become troublesome."
https://www.qualtecheng.com/docs/harmonics/QT-614.pdf
 
adityalagudu3291 said:
So what is the methodology to calculate additional increase in current due to harmonics other than 5th in these capacitor banks , so we can size the fuses and disconnect switches correctly?
Click to expand...
That should just be standard AC analysis. You should be able to figure the voltage magnitude of each harmonic, and the reactance of the capacitor bank at each harmonic, and just use ohms law.

Fourier analysis or la place transforms might be a way to speed up the repeated calculations, but its been so long since I've done those I can't say for sure, much less give you the exact process off the top of my head.

But I'm wondering why is there so much harmonic content? Do the wind turbines generate a DC voltage that is then converted to AC?
 
steve66 said:
That should just be standard AC analysis. You should be able to figure the voltage magnitude of each harmonic, and the reactance of the capacitor bank at each harmonic, and just use ohms law.

Fourier analysis or la place transforms might be a way to speed up the repeated calculations, but its been so long since I've done those I can't say for sure, much less give you the exact process off the top of my head.

But I'm wondering why is there so much harmonic content? Do the wind turbines generate a DC voltage that is then converted to AC?
Click to expand...
How can I get voltage magnitude of each harmonic? I know I can use following formulas to measure the harmonic current at each order. I will get back on the reason for this harmonic content.


Capture4.PNGCapture5.PNG
 
steve66 said:
But I'm wondering why is there so much harmonic content? Do the wind turbines generate a DC voltage that is then converted to AC?
Click to expand...
That would be my guess. Wind turbines are not my forte but did a lot of variable speed drives, mostly Kramers (slip recovery systems). We installed harmonic filters, 5th, 7th, 11th, and 13th. The magnitude of the harmonics was proportional to power of the variable speed drives.
 
adityalagudu3291 said:
They installed the filter based on THD but didn't provide the study after installing the filter. Is there a way to manually calculate harmonic currents?
Click to expand...
We did a frequency sweep measurement on our system just last year, that is how we knew the harmonic contents of our power system. We determined that the THDi was 17% and the dominant contributor was the 5th harmonics.
For starters, determine your power factor and use that data to do PFC corrections. Then use that capacitance value as the capacitance in series with a reactor to filter out the worrisome dominant harmonics level. Reactors available are in 2, 3, or 5%.
 
Status
Not open for further replies.
Top