Passive Harmonic Filters (Power System)

  • Thread starter EEstudent90
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  • #1
EEstudent90
Hi again, got a few questions (marked with numbers) for passive filters used to filter harmonic currents in the power system.

  1. Look at my one-line diagram below. Let's say I have a passive filter connected in shunt with respect to the load and the passive filter "diverts" the harmonic currents it is tuned for (let's say 5th harmonic) to ground. If my non-linear load is not connected to ground, would the passive filter work? We do not have any closed path for the harmonics to circulate in (if we neglect the capacitance between the load and earth), so I don't see how it could work. But I could use a delta connected passive filter, to provide the closed loop right? (Question 3 is somewhat related to this aswell).
  2. The main idea of a shunt passive filter is to provide a low impedance path for the tuned harmonic, so if the current has to flow through ground as shown in my picture below, this would be a relatively high impedance path right? Would we have to install a conductor to reduce this impedance?
upload_2017-9-24_12-8-4.png


Next question:
3. I have seen that you can connect the passive filters in either wye or delta. If I chose a delta connected passive filter, and I would want to provide a closed low impedance path for the third harmonic this would not work right? As the third harmonic is a so called zero sequence current, and the delta connection would look like a dead end, i.e. not a closed loop.

So if I wanted to compensate the third harmonics using a passive filter, I would need a similar setup as shown in my paint drawing above?


Sorry if my post is a bit messy, I tried my best. Thanks for any help.
Best regards​
 

Answers and Replies

  • #2
NascentOxygen
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If my non-linear load is not connected to ground,
If the load doesn't have a return wire then it draws no current so is not a load. The "earth" wire you want to not have is just a convenient way to represent the essential second wire for current that powers the load. I think it should be termed the "neutral". The purpose of the filter is to provide a path that bypasses the harmonic current around the load, so the passive filter needs to correctly connect in shunt with the load. The connections need to be low resistance, so will be copper wires (even if one is named "earth").
 
  • #3
Svein
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MainsFilterCircuits.gif
 
  • #4
EEstudent90
If the load doesn't have a return wire then it draws no current so is not a load. The "earth" wire you want to not have is just a convenient way to represent the essential second wire for current that powers the load. I think it should be termed the "neutral". The purpose of the filter is to provide a path that bypasses the harmonic current around the load, so the passive filter needs to correctly connect in shunt with the load. The connections need to be low resistance, so will be copper wires (even if one is named "earth").

I realize now that Q1 and Q2 might have been "stupid" :), I guess I got confused when I read a pdf I was studying saying "harmonic currents are diverted to ground" and should have thought about it before posting.
 

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